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Tires

The Tire Pressure Revolution

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Of all our research on tires, the most revolutionary finding is this: Tire pressure has almost no effect on a tire’s speed. We did not believe it at first, either, so we’ve tested it numerous times. It’s been confirmed numerous times, with different methodologies.
The real revolution is not how you use your pump… What has totally changed our riding are the wide, supple tires, which only work because of this new insight.
track_tire_test
First, let’s look at the data. Here is one experiment: We ran three different 25 mm tires (a supple clincher, a supple tubular and a harsher-riding clincher) at pressures from 4.5 and 9 bar (65 and 130 psi). These tests were done on very smooth asphalt (above), a surface where high pressures should offer the greatest advantages. The graphs below show the power required to ride the bike (above) with the tires at a constant speed of 27.8 km/h, but with different tire pressures.
tire_pressure
There is no relationship between tire pressure and performance in the tested range. (Lower and higher pressures are unsafe to ride.) The graph above shows some variation in power output (lower is better), but there is no trend. The CX tubular rolls fastest at 5.5 bar, the CX clincher is a little faster at 6 bar, while the Rubino is fastest at 9 bar, but almost as fast at 6.5 bar.
Take-home message: Don’t stress about tire pressure!
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This finding has revolutionized our understanding of tires. In the past, we all thought that higher tire pressures made tires roll faster. And that presented a problem for wide tires: A wider tire puts greater loads on the casing than a narrow one. To compensate, you have two choices:

  1. Beef up the casing, which makes the tire less supple and slower.
  2. Lower the pressure, which we thought made the tire slower.

No matter which route you took, then-available science predicted that your wider tire would be slower. It was a catch-22, and for the best performance, you stuck with narrow tires, where you could have a supple casing and high pressure at the same time.
But after realizing that tire pressure doesn’t matter for performance, we were able to explore new possibilities. If lowering the pressure does not make tires slower, you can make supple, wide tires. You run them at lower pressures, and you don’t give up any performance on smooth roads. On rough roads, you gain speed, because the tire (and you) bounce less. And on all roads, you are more comfortable. Instead of a catch-22, you have a win-win-win situation.
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It’s this research that has led professional racers to adopt wider tires. They are up to 25 mm now. (Wider ones won’t fit on their bikes!) For the rest of us, there is no reason not to go wider. I now ride 42 mm tires at 3 bar (43 psi), knowing that they roll as fast as a 25 mm tire at 6 bar (85 psi) – or 9 bar (130 psi), for that matter.
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To get the most benefit out of these lower pressures, you need supple tires. A stiff sidewall takes more energy to flex, so the tire will be slower. And since the sidewall is stiffer, it also will be less comfortable. You could call it a “lose-lose” situation.
Professional racers have known this all along: As much as their equipment has changed over time, they’ve always ridden supple tires. They usually ride hand-made tubulars (above), but for the rest of us, supple, wide clincher tires now make it possible to enjoy the ride and speed of supple tires on any road.

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TPI and Tire Performance

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Tires with supple casings are faster, more comfortable and simply more fun to ride. Most cyclists know this, but how do you measure “suppleness”?
A measure that often is used to describe the quality of tires is “threads per inch” (TPI). The idea is that tires with higher thread counts usually have finer weaves that make these tires more supple.
The reality is more complex, and TPI is of limited use when comparing tires. Here is why:
1. How do you measure? Ideally, you look at the TPI of the casing fabric before it is made into the tire. Casing fabrics vary between 15 TPI for coarse utility tires to 120 TPI for very high-end tires.
What about the tires with 300 TPI or more? These makers count every layer of the tire. Most tires have three layers of overlapping casing, so by that method of counting, a 100 TPI fabric will make a 300 TPI tire. And if you added a fourth layer for added puncture protection, you’d make the tire slower, but you’d bump up the TPI to a record-setting 400! So if a tire makers claims a TPI of more than 130, you have to divide the number by 3 to get the TPI of the fabric.
2. What is the diameter of the threads? The reason high-TPI tires usually are more supple is that the threads are thinner. If you keep all things equal, thinner threads will mean more threads per inch. However, if you make your weave denser, you also get more threads per inch, but actually a stiffer casing.
Panaracer, who makes our Compass tires, offers a 120 TPI casing. However, they found that if they use the same super-fine threads, but space them out a little further, they get an even more supple, and even faster, tire. So the Compass Extralight tires use that casing, which only has 90 TPI.
If you go by TPI alone, the Extralight casing looks inferior, but it’s in fact the more supple, faster casing.
3. How much rubber? Fabrics with very thin threads are fragile. They have to be handled very carefully during production. Some makers of budget tires compensate for this by covering the fabric with more rubber, which protects the threads. Of course, this makes the casing stiffer, and thus less performing. So one maker’s 120 TPI casing may be a lot less supple than another maker’s 120 TPI casing.
4. What material is used for the threads? With hand-made FMB tubulars, you get a choice of cotton or silk threads. The silk is much more supple than the cotton (which already is more supple than most polyesters). Even among polyesters, there are great differences in the thread materials. It makes no sense to claim that a 90 TPI silk casing is less supple than a 100 TPI cotton casing.
These are just a few of the factors that determine the tire’s suppleness. Let’s compare two hypothetical tires:
Tire 1 uses a stiff and relatively large-diameter thread. The fabric has a super-dense weave and is slathered with rubber. The maker counts every layer of the casing, and thus arrives at a 300 TPI tire.
Tire 2 uses a supple, superfine thread, woven into a relatively loose weave. The manufacturer keeps the rubber coating to a minimum. They report the TPI of the casing fabric, and arrive at a 90 TPI tire.
It’s easy to see that the Tire 2 above is superior to Tire 1, even though it has less than 1/3 the TPI. Suppleness, like so many important things, is hard to quantify, but you’ll notice it when you ride the tires.
Acknowledgments: Thank you to the engineers from Panaracer, Francois Marie of FMB tires, and Challenge Tires for the information contained in this post.

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Optimizing Tire Tread

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Most tire manufacturers agree that supple sidewalls and a thin tread make a tire fast, but the role of the tread pattern remains poorly understood. Most modern tires have either a completely smooth tread (slicks) or a coarse tread pattern similar to car tires. Many high-performance tires are smooth with just a few large sipes. None of these tread patterns are optimized.
Car tires have tread mostly to prevent hydroplaning. With their wide, square profile, a layer of water can form between tire and road surface. The tread pattern forms channels so the water can be pushed out of the tire/road interface.
Bicycle tires do not hydroplane. Their contact patch is too small and too round for that. This means that car-inspired tread patterns are not necessary on bicycle tires. Does this mean that no tread pattern at all – a slick tire – is best? Any tread pattern reduces the amount of rubber on the road surface… In the lab, it does work that way: Slick tires grip best on smooth steel drums.
Real roads are not as smooth as steel drums. An optimized tire tread interlocks with the irregularities of the road surface to provide more grip than the pure friction between asphalt and rubber. This is especially noticeable in wet conditions, when the coefficient of friction is reduced by half, yet you can corner with about 70-80% of the speed you use on dry roads. (Unless the road surface is greasy…)
The ideal tire tread has as many interlocking points with the road surface as possible. The “file tread” found on many classic racing tires does this. The ribs are angled so they don’t deflect under the loads of cornering or braking.
Why do race cars use slick tires, and not a file tread? The reason is simple: It would be abraded the first time the car accelerates. However, bicycle tires don’t wear significantly on their shoulders – the part that touches the ground when you corner hard – so we can use a tread pattern that is optimized for grip without worrying about wear.
Each Compass tires has three distinct tread patterns, each designed for a specific purpose.

  • Center: Fine ribs serve as wear indicators. When the lines disappear, the tire is about half-worn. (The tread of our narrower tires is not wide enough for ribs, so small dots are used instead.)
  • Shoulders: When the bike leans over as you corner, the tire rolls on it shoulders. A chevron or “fine file” tread pattern optimized grip.
  • Edges: This part never touches the road (unless you crash). They serve only to protect the casing from punctures, so they don’t need any tread.

TireProfile-hi
Supple casings make tires faster, but a supple casing is of little use when it’s covered by thick tread rubber. The fastest tire would have just a minimal layer of tread rubber, and many “event” tires are made that way. Unfortunately, that means that they don’t have much rubber to wear down until they are too thin to use. At Compass Bicycles, we call these tires “pre-worn”.
Compass tires have a slightly thicker tread in the center. A little more material there doubles or even triples the life of your tire, while adding minimal weight and resistance. (On our widest 650B x 42 mm tire, the added tread weighs less than 50 grams.) Once you have ridden the tires for a few thousand miles, they’ll be as light as the “event tires”.
On the shoulders and edges, the tread does not wear. So we made it much thinner to keep the tire supple and reduce its weight. The tread extends far enough down the sidewall that the casing is protect when seen from above. Extending the tread further adds little protection, but makes the tire less supple and thus less comfortable and slower.
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Another important factor is the tread rubber. This is an area where incredible progress has been made in recent decades. In the past, you could either have good grip or good durability. I used to ride Michelin’s Hi-Lite tires, which gripped well, but rarely lasted even 1000 miles (1600 km).
Compass tires use Panaracer’s best tread rubber, which is amazing. Our tires are among the grippiest you can find, yet I just got an e-mail from a 230-pound rider who got 3786 miles (6093 km) out of a set of our 26 mm-wide Cayuse Pass tires. The wider tires spread the wear over more rubber, so they last significantly longer. (Don’t try to set wear records, but replace your tires once they get thin. The risk of flats, or worse, blowouts, is not worth getting an extra few hundred miles out of a worn tire.)
Tread color is another important consideration. Modern colored treads no longer are the “death traps” they used to be, but especially in wet conditions, the grip of tires with colored treads – including the Grand Bois Hetres we sell – is not quite as good as that of black treads. That is why we offer only black tread.
Click here for more information about Compass tires.
Update 9/25/14: Roadbikerider.com just published a review of the Compass tires. Click here to read it.
 
 

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The Actual Width of Tires?

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Sometimes we get an e-mail or a phone call from a customer asking, “I bought the Compass 32 mm tires, but they only measure 28.5 mm on my rims. Why is this?”
Decades ago, some tire makers cheated when stating tire widths. Why? To make their tires appear lighter than they really were. By selling a 25 mm tire as a 28 mm, they made the tire seem lighter than the competition’s tires, which actually were 3 mm wider.
That was long ago, and it’s not what is going on here. We label our tires as close to their actual width as possible. Here is why different people report different widths for their tires:

  1. It can be difficult to accurately measure the width of a supple tire.
  2. The casing of supple tires stretches for a few weeks or even months after they have been installed.
  3. Tire width depends on tire pressure and rim width. That means the actual width can be a little narrower or wider than the nominal width.

38_squeezed
I recently installed a set of Compass Barlow Pass Extralight 700C x 38 mm tires on a Bicycle Quarterly test bike. How wide are they really?
When you measure metal with calipers, you squeeze the calipers until they won’t go any further, and then read your measurement. If you compress the calipers on a rubber tire, the tire will deflect. In this case, I measured 34 mm. But that isn’t the actual width of the tire: If you tried to fit the tire into a frame with just enough clearance for 34 mm tires, it would rub.
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Here is how you measure tire width: Open up your calipers in 0.5 mm increments. Check whether there is “play” between the caliper jaws and the tire. In the photo above, I am already at 35.5 mm, and the calipers still fit snugly on the tire.
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At 36.5 mm, I am finally getting some wiggle. This means that the tire is just over 36 mm wide. That is the width when it’s new.
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Two weeks later, I measured the tire again. It has stretched to 36.5 mm. I was surprised that it was still so narrow, until I checked the tire pressure. I had let the pressure drop to about 30 psi. How wide would the tire be at its maximum pressure?
38_final_width
I inflated the tire to 75 psi, and lo and behold, it now measured 37.5 mm. It probably will stretch a little more, and achieve its full 38 mm width before long. Of course, I wouldn’t ride it at that pressure (unless I put it on a tandem), so at the pressures I usually ride, the tire will be a tad narrower than its nominal width.
Should I inflate my tires to a higher pressure to make them wider? No, that doesn’t make sense. Your tire’s comfort and performance is determined by the tire width at the contact patch, which gets larger at lower pressure. Putting more air than necessary into the tire defeats the purpose, even if it makes the tire wider where it does not touch the ground.
For narrower tires, rim width also plays a role. The Compass Stampede Pass tires measure about 31 mm on a 20 mm-wide rim, like a Mavic MA-2, but 33 mm wide on a 23 mm-wide rim, like a Grand Bois rim. For wider tires, this is less of a factor, since all the rims we use are narrow when compared to the wide tire.
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In any case, our testing has shown that the material and construction of the casing are more important for comfort and speed than a millimeter or two in width. When you put a set of supple Compass tires on your bike, you’ll notice a huge difference in how the bike feels and performs.
And when you buy your next bike, make sure to spec a frame that provides ample tire width. On my own bikes, I am not too concerned whether the tires measure 39 or 41.5 mm. Either is ample for most of the riding I do.
Further reading: How Wide a Tire Can I Run?

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How much faster are supple tires?

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Improving your tires can make the biggest impact in the speed of your bike (apart from changing the motor!). The difference is especially pronounced for slower riders, whose wind resistance is less than that of faster riders.
Most cyclists know that supple tires make you faster on your bike. But so do ceramic bearings in your derailleur pulleys. The important question is: “How much faster?” For ceramic bearings, the difference is too small to notice on the road, because standard ball bearings already have close to zero resistance.
For supple tires, the difference is much greater. If you have a hard time staying with a group, changing your tires to a faster model may help you avoid getting dropped. And if you get close to the time limit in brevets, faster tires can provide you with a significant time cushion, so that a flat tire or a slight detour due to misreading the route sheet no longer results in a DNF.
Here is a comparison between three tires from Bicycle Quarterly‘s tire tests. These are all marketed as performance tires, and none of them have puncture-proof layers that would further slow them down.

  • Vittoria Open CX Corsa 700C x 25 mm
  • Grand Bois Cyprès (standard casing) 700C x 32 mm
  • Rivendell Rolly-Poly 700C x 27 mm

rolldown
Our first tests were rolldown tests on relatively rough pavement, like you typically find on American backroads. The speed was between 23 and 25 km/h (13.5 – 15.5 mph).
On this surface, the fastest tire rolls 13.5% faster than the slowest. That is a huge difference. Imagine going 15 mph with the slower tire, and on your next ride, after changing your tires, riding at 17 mph with the same effort. During a century ride, you’d be 45 minutes faster!
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We also tested these (and many other) tires on a very smooth asphalt surface at constant speed with a Power Meter. The speed was higher (27.9 km/h; 17.3 mph), and the ultra-smooth surface reduced the vibrations. However, even under these “ideal” conditions, the rider on the slowest tire had to put out 13.5% more power to keep up with the rider on the fastest tire. That can make the difference between “hanging with a group” and getting dropped within a few miles.
If you calculate the speed difference for the same power output, it’s 5%. (Wind resistance going up exponentially with speed, so you need 13.5% more power to increase your speed by 5% to stay with the rider on the faster tires.)
As you can see, supple tires make the greatest difference on rougher surfaces, and at lower speeds. But even at high speeds, make the largest difference in the performance of your bike. For comparison, aero wheels make you about 1% faster. And when you are drafting, your wind resistance goes down, so rolling resistance becomes even more important. That is why the pros always have ridden supple tires.
Does this mean we all should ride Vittoria CX tires? Not exactly. The CX is optimized for ultimate performance, and it has a very thin tread. This means it will wear out quickly and suffer more punctures on the way. If you are racing or riding a timed event, these compromises may be worth making. For everyday use, it often makes sense to give up some speed to obtain twice as much service life and fewer punctures.
The Grand Bois Cyprès is designed as an all-round tire. It has a thicker tread that will last thousands of miles. It will get faster as it wears. The Grand Bois also has a sturdier casing that resists sidewall cuts better. As a result, it rolls a little slower. (Disclosure: Compass Bicycles sells Grand Bois tires.)
The Rivendell Rolly-Poly has an ultra-tough casing that provides peace of mind when you ride through debris and are afraid of cutting your tire’s sidewalls. This may be overkill for most riders. The more rigid casing slows the tire down significantly.
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When we designed our Compass tires, we started with the Grand Bois tires, and then optimized the performance further, without making the tires into “event” tires that are not very suitable for everyday riding. We reduced the tread thickness on the shoulders of the tire, where it does not wear out, but kept it the same in the middle, where it wears.
For the Extralight models, we used a casing that is significantly more supple than the “standard” casing shown in the test results above. We haven’t measured the performance under controlled conditions yet, but our (and others’) on-the-road experience suggests that they are significantly faster than the standard models.
Tires really make a big difference. When I switched from tires with stiff sidewalls to supple ones, not only did I set many personal bests on long rides, but I also found that I could rest while drafting, whereas before, I was working hard just to hang on.
Take our Flèche team in the photo at the top: If one of us had significantly slower tires than the others, he would have to be much stronger just to keep up. We’d rather have the stronger rider take longer pulls at the front!
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What if you don’t care about speed? Supple tires also are much more comfortable. And they just feel different, making cycling much more fun. To me, that is the most important difference, and why I ride them on all my bikes.
Further reading:

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The Art of Compromise

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It may be popular to talk about “no-compromise” products, but the reality is that the best products involve a careful balance of features and properties. Take our new Compass tires…
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We could have made them lighter!
The only place to remove material is in the middle of the tread. We might save up to 50 grams on the 650B x 42 mm Babyshoe Pass, but the tire would wear out much faster.
So we removed all the weight we could, but left just the right amount of tread to provide a long service life.
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We could have made them faster!
A thinner tread flexes less, reducing the rolling resistance slightly. If we had reduced the tread thickness in the center, we might have increased the speed by up to 1%. The difference is very small, and it comes at the expense of longevity and puncture resistance.
We already use the most supple casing available. Our research has shown that the casing, more than anything else, influences the speed and comfort of a tire.
Our tires will be as fast and as light as “event” tires once you have ridden them for a few thousand miles. A friend of mine calls other companies’ super-thin event tires “pre-worn.”
mtb_trail
We could have made them sturdier!
Reinforcing the tire sidewalls, adding puncture-proof belts or making the tread thicker all will make the tire sturdier. The downside is that the tires would ride harshly and roll slower.
We decided that our tires needed to hold up in most off-pavement conditions. We have tested them on gravel roads and even moderate mountain bike trails (above) without problems. For us, that makes them sturdy enough.
Hint: Wider tires are inflated to lower pressures, so they roll over debris that would puncture a narrower tire. You get less flats that way.
worn_tire
We could have made them last longer!
A thicker tread gives you more rubber to wear down before you have to replace the tire. However, after a while, the tire becomes “squared off” and no longer corners well. The thicker tread also increases the tire’s rolling resistance. (The Grand Bois Hetre in the photo above may look squared off, but it’s actually still nice and round after about 10,000 km/6000 miles.)
We decided to make our tread thick enough that it will last thousands of miles, but not so thick that it will square off before wearing out. We feel that is a good compromise.
Hint: Wider tires spread the wear over more rubber and last longer.
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We could have made them more colorful!
Our on-the-road experience has shown that colored rubber does not grip as well as black rubber, especially on wet roads. So our Compass tires use black tread rubber for optimum handling and safety, but the sidewalls are available in both tan and black, depending on your taste.
Fortunately, there are some things where compromise is not necessary. Handling is one of them.
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We could NOT have made them corner better!
We spent a lot of time researching tire treads, before selecting a pattern that offers optimum grip in wet and dry conditions. The tread pattern along with the grippy yet durable rubber make our tires corner better than any tire we have tried.
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We made many compromises when we designed our tires. We think they are the right compromises to provide you with tires that offer a maximum of performance, comfort and fun, while being suitable for everyday riding, commuting and even gravel roads. We are proud of the result, and we are glad to hear that others are enjoying these tires as much as we do.
Click here to find out more about our Compass tires.

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Feedback on the new Compass Tires

barlow_pass_in_britain
As we get the first feedback from riders about our new Compass tires, it’s nice to hear that others enjoy them as much as we do. Here are a few samples of unsolicited feedback:
“I used the new Barlow Pass tyres on a route that included miles of rough railway trackbed. They make a big difference! A lovely ride both on and off road, I realise what I’ve been missing after years of heavy-duty Schwalbes.” – Tim Bird, who took the photo above.
“Thanks for making available the finest clincher tires I’ve ridden on in my 45 years of cycling!” – a customer who placed an order for another 8 tires!
“These new tires feel like they were designed for racing. It seems like they have better rolling resistance (as if I were running in a lower gear) and with better cornering that the Hetre XLs.” – a customer who wondered whether we’d discovered an extra-grippy rubber compound for the tread.
“Recently I have been looking at everyone’s rubber trying to find that perfect tire. You have pretty much delivered everything I had in mind.” – a former tire developer for a big U.S bike company.
“I have ridden about 130 miles so far on the 700×38 Barlow Pass Superlight tires. They have made my mid-1980’s Miyata 1000 touring bike much more enjoyable and fun to ride. Thanks.”
“I have been commuting on the Barlow pass tires for a week or two and am really happy with them. They roll faster and smooth out the bumps more than my previous set of Schwalbe Marathon Racers.”
“Oh my gosh I just love those tires!”
It’s nice to see that others share our enthusiasm and appreciate all the research and testing that went into designing these tires. Click here to find out more about our Compass tires.

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Tires: How Wide is too Wide?

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How wide a tire is too wide for optimum performance? Our research shows that wider tires don’t give up anything on smooth roads, and gain a significant advantage on rough roads. This has been shown for tires up to 31 mm wide.
It’s now a well-established fact that wider tires roll faster than narrow ones. Professional racers now use 25 mm tires, which are 20% wider than the tires that most racers used just 20 years ago. Will this trend continue? Can we expect racers to be on 30 mm tires in the future? No matter what the pros do – they are influenced by many factors that have little to do with science – the real question is: Up to what point are wider tires faster?
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It is obvious that the tires in the photo above will not roll very fast. Clearly, at some point, the performance benefits of wider tires (shorter contact patch and thus smaller hysteretic losses; reduced suspension losses) will be outweighed by the disadvantages of extra weight and increased wind resistance.
Test_Woodland
In our original tire tests (above), we tested the same tires in 21, 23 and 25 mm widths on a moderately rough “backroad” surface. The results were clear: The 21 mm tires were slowest, 23 mm was in the middle, and 25 mm tires were fastest. The speed difference between 21 and 25 mm tires amounted to about 2.5%. Over a typical 200 km brevet, I would gain about 11 minutes. It’s not huge, but significant. These results appear to have prompted the current trend of racers using wider tires.
What about tires that are wider than 25 mm?
RumbleStrip
Our testing on rumble strips showed that on very rough surfaces (the equivalent of cobblestones), 42 mm tires are faster than 25 mm tires. However, few of us ride all the time on cobblestones, and what we want to know is whether we give up anything on smooth roads when riding wider tires.
To determine this, we tested Grand Bois tires in 26, 29 and 31 mm widths on a super-smooth asphalt surface (see photo at the top of the post). The results were the same for all three tires. On the smoothest asphalt, you don’t gain anything by going to tires wider than 25 mm, but you also don’t give up anything.
Those tests were run at 25 km/h (16 mph). At higher speeds, the aerodynamic disadvantages of wider tires might be greater. Does that mean that 31 mm tires are a fine choice for riding at moderate speeds, but that you would be better off on 25 mm tires when you go faster?
drafting
We tested both 25 and 31 mm-wide tires in the wind tunnel. The result: The raw data showed a 1% increase in wind resistance for the wider tires, but the results weren’t statistically significant. Even if we accept them at face value, the added wind resistance is too small to make a noticeable difference. For example, at a very high speed of 40 km/h, decreasing your wind resistance by 1% only adds 0.4% (or 0.14 km/h) to your speed.
What about the heavier weight of wider tires blunting the acceleration of your bike? That doesn’t appear to be a major factor either, since wheel weight is less important than many riders believe. (See this recent post for a discussion of wheel weight on professional racers’ bikes.) If you use smaller 650B wheels, you make up some of the greater weight of a wider tire through a lighter rim.
All this data shows that 31 mm tires roll as fast as 25 mm tires, even on very smooth roads. And when the roads get rougher, the wider tires roll faster.
rando_ti
What about even wider tires? Our on-the-road experience suggests that even 42 mm-wide tires do not roll slower than 25 mm tires (above), but without rigorous testing under controlled conditions, we can not say for sure. We hope to test this soon.
Of course, there are other reasons beyond performance to ride wider tires. You gain comfort. You will incur fewer flats, since you run wider tires at lower pressures, so they roll over obstacles that would get hammered into narrower tires. You’ll be safer, since a wider tire will be less affected by small cracks and railroad tracks.
Most of all, you’ll be enticed to go on small roads that have great scenery and little traffic – roads you might have avoided with narrow tires because the pavement tends to be rough. With more comfortable tires, you can even enjoy roads with no pavement at all!
To answer the question in the headline: Even 42 mm does not yet appear to be “too wide.” Tires wider than that are hard to fit into the rear triangle of a bike without compromising performance (tread/Q factor, chainstay length), so perhaps frame design more than other factors limit the maximum tire width on a performance bike.
Wide tires are one of the few things with a lot of advantages, but very few disadvantages. (There are some downsides to wide tires, which we’ve mentioned here.)
For all our tests, we used tires that had the same casing material, tread pattern, etc., to isolate the effects of tire width. Of course, there are many other factors that influence tire performance, and width is only one important factor. (A wide “touring” tire with a stiff puncture-resistant casing is much slower than a narrow “performance” tire with a supple casing.)
This post is just a summary of the research. The original data and much more detail were published in Bicycle Quarterly. Here are a few resources for further reading:

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Tire Wipers

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Many cyclists are tempted by the performance and comfort of supple tires, but they are afraid that they might get too many flats without puncture-resistant belts and reinforced sidewalls. Tire Wipers improve the flat resistance of your tires without detracting from their comfort and performance.
Few foreign objects are so large and so sharp that they puncture the tire upon first impact. Most require several revolutions of the wheel to work their way through the tire. When you roll over debris, you often hear the “pock, pock, pock” as the debris gets pushed into the tire with each wheel revolution, followed by a “pshhhh” as the tube deflates.
If you could get rid of the debris after it is picked up, but before it gets hammered into the tire, you could prevent a good number of flats. Enter Tire Wipers – small wires that lightly rub your tires and remove debris before it gets lodged in the tire.
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Do they work? Flat tires are so random that this is hard to quantify, but the general consensus is that they do prevent many, if not all, punctures.
Tire Wipers have been hard to find in recent years, but Scott Gabriel is making them again. Compass Bicycles carries them, in two models. One attaches to the brakes of bikes without fenders, the other is installed at the exit (front edge) of your fenders (above). Click here for more information.

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How Wide a Tire Can I Run?

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Many cyclists want to ride on wider tires, but are limited by the clearances of their bikes. So they want to use the widest tires their bike can fit. How do you determine your bike’s maximum tire width?

You need a set of calipers (below). If you don’t have any, bike shops have them. Please leave a generous tip if you ask a shop mechanic to help you with this!

calipers

1. Measure your current tires

Measure the width of your current tires. Be careful not to pinch the tire. Instead, increase the opening of the calipers by 0.5 mm increments, and check whether the opening has play when it is slipped over the tire. (The calipers “rattle” when you move them.) When this happens, the calipers are set just a tad larger than the actual width of your tires. Use this measurement as your current tire width.

If you pinch the tire with the calipers, you may get a measurement that is up to 2 mm narrower. Also don’t use the width that is printed on the side of your current tires. It’s often not accurate enough for this purpose.

chainstay_clearance

2. Check your clearances

Check all around your wheels how much clearance you have between tire and frame/fork/brakes. Most important are:

  • fork crown
  • brake calipers (front and rear): squeeze your brakes to get the clearance when braking
  • chainstays
  • seatstays can limit the clearances on some bikes.
  • fenders: often, you can adjust the fenders to eliminate a tight spot.

I recommend a minimum of 3 mm clearance between your tires and the closest part of your bike. Any less, and you risk having your tire rub under hard acceleration or if the wheel develops a slight wobble.

3. Calculate your maximum tire width

Start with the smallest clearance between your existing tire and bike. Then deduct the tightest clearance, 3 mm, to get the existing available clearance for your wider tire. Multiply this by 2 (you have that much room on both sides of the tire), add the current tire width, and you get the maximum tire width:

max. tire width = 2 x (tightest clearance – 3 mm) + current tire width

Example: Your current tires are 29 mm wide. You have 5 mm clearance at the tightest spot. That leaves 2 mm available clearance. You can run 4 mm wider tires than your current ones. Your maximum tire width is 33 mm:

max. tire width (example) = 2 x (5 mm – 3 mm) + 29 mm = 33 mm

This assumes that your new tires have a similar height-to-width ratio as your existing ones. Some inexpensive tires can be much taller than they are wide, but high-end tires usually are relatively round, and this formula works well.

If you find yourself between two available tire sizes, I suggest using the narrower tire. High-end tires tend to expand over time as the casing “relaxes,” and you don’t want your tire to rub every time you rise out of the saddle and flex your wheel. If you find that you still have extra clearance, you can go up one more size when your new tires wear out.

By the way, the bike in the photos does not appear to have extra clearance, so if your bike looks like that, you probably should stick with the tire width you have. You still can improve the ride and performance of your bike by using more supple tires with higher-quality casings.

Photo credits: Ernie Fong.

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Hutchinson 650B x 32 mm Tires


Compass Bicycles now carries the Hutchinson “Confrérie des 650” tire. The Confrérie des 650 was created by French riders who loved their 650B bikes. They were concerned that the wheel size might become extinct, leaving them without rims and tires for their bikes. So they formed the Confrérie and began working with manufacturers to offer 650B tires.
Since its inception, the Confrérie has focused on 32 mm-wide tires, because most French 650B bikes of the 1960s-80s used this size, for example, this lovely 1965 René Herse that recently sold on eBay. The first tire the Confrérie created was the Michelin Megamium. This was a utilitarian tire, but it was enough to keep many bikes on the road. In recent years, the increasing popularity of 650B tires in North America and Japan has brought many new and excellent 650B tires to the market, and there is no longer any risk of the wheel size becoming extinct. However, the Confrérie was wary of depending on others for their supplies. So when Michelin decided to stop making the 650B Megamium, the Confrérie worked with Hutchinson to make a replacement.
The result is the new Hutchinson 650B x 32 mm tire. Unlike the relatively narrow Megamium, it measures a true 32 mm wide. Hutchinson used their top-of-the-line racing casing for this tire, so it rolls very smoothly and absorbs shock very well. It is hard to estimate puncture resistance, but it appears to be fine in that respect. As a modern tire, the Hutchinson is black with reflective silver sidewalls. Whether you like it or not is a matter of taste – I prefer the more classic appearance of other 650B tires. It’s hard to dislike the light weight of the Hutchinson: At 267 g, it is the lightest 650B tire available today.
I feel that the Hutchinson is an excellent tire that adds significantly to the appeal of the 650B wheel size. To make it available in North America, Compass Bicycles now carries it in our tire program. More information is here.

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Mounting Tires on Rims with Deep Wells


Sometimes we get a call or an e-mail from a frustrated customer: “I have a brand-new set of your tires, and both wobble when I mount them on my rims.” In most cases, it is not the tires’ fault. Usually the problem stems from the difficulty of mounting tires on poorly designed rims. However, there are some tricks for mounting tires on these rims.
Above, you see a correctly mounted tire. Most tires made today have a line molded into the sidewall (arrows). This line must be visible all around the tire, and parallel to the rim edge. The line not only helps seat the tire, it also serves as a visual indicator that the tire is concentric with the rim. (Usually, the line is a little higher above the rim, but always parallel to the rim edge.)

Above are three 700C rims, which all have the same outer diameter. However, the cross sections show that they are very different on the inside. Rim 1 is a proven design. Rim 2 has a shallower well (the place where the tire mounts). Rim 3 has a very deep well. Tires seat differently on each of these rims.

When you mount a tire, the tire beads need to go over the rim’s hook (above; the foam is used to hold the tire in shape). Tire beads are what holds the tire on the rim. They don’t stretch much – otherwise, the tire would just blow off the rim when you inflate it. The well of the rim has a curved bed. When you mount the tire, the tire beads drop into the center of the rim’s well bed. This provides enough slack to get the last bit of tire bead over the rim’s hook on the opposite side. As you inflate the tire, the beads slide up the rim’s curved well bed until they seat tightly underneath the rim’s hooks.
The photo above shows Rim 1. The tire’s beads fit perfectly onto the well bed and underneath the hook. The tire will seat concentrically by itself as you inflate it. The bead seat diameter is 622 mm, as industry standards specify (ETRTO). This is how rims should work.

Rim 2 has a shallower well. The bead seat diameter is 624 mm, which makes the well bed higher than the standard 622mm. To seat correctly, the tire has to stretch by 2 mm in diameter. This translates into 6.5 mm (1/4″) along the tire’s inner circumference, which is a lot of stretch for a tire bead. Often, the beads don’t stretch enough, and don’t quite reach the rim walls (arrow). Then the tire will wobble on the rim. Putting talc (baby powder) on the tire bead may help it slide into position. A very thin and slippery rim tape also can be helpful.
The high well bed also makes the tire difficult to remove: It is difficult to insert a tire lever underneath the tire bead, because it is stretched so tight onto the rim.

Rim 3 has a very deep well. The tire is not supported by the well bed at all (arrows). The tire has to float. When you inflate this tire, it cannot just slide into position on the well bed. You will have to manipulate the tire until it is seated correctly.
Unfortunately, several common rims for wider tires, including the Velocity Synergy, several Velo-Orange models, and the no-longer-available Grand Bois, have overly deep wells. This makes mounting tires difficult. The sole advantage is that the tires come off the rim easily.
On their 650B rims, Synergy tried to “fix” the problem of poorly seating tires by increasing the overall rim diameter. In my experience, this has made things worse, because now the hook is in the wrong place. There is nothing to locate the tire: The well still is too deep, so the tire cannot sit on the well bed. And the hook is too high, so the tire cannot sit underneath the hook.
If you have rims with wells that are too deep on your bike, there are some tricks for mounting tires on them. There even is a “fix” that can overcome the problem of the overly deep wells to a large degree.

On all rims, even well-designed ones, tires often don’t seat well at the valve. The tube is reinforced here, making it stiffer, and it sometimes gets caught under the tire bead. Above, you see how the molded-in line moves away from the rim at the valve. (Often, this is more pronounced.) Not only will this cause the tire to wobble, but if the tube is trapped under the beat, it can chafe until you get a flat tire.

With the tube barely inflated (~5 psi), push the valve stem inward as far as you can. This usually frees the part of the inner tube that is trapped.

Harder to fix is the problem shown above: The line that is molded into the tire sidewall disappears into the rim (arrow). This often happens on rims where the wells are too deep, such as the Synergy Velocity shown here. (The Grand Bois rims we used to sell unfortunately were not much better.) It also can happen if the well is too shallow, and the tire bead does not contact the rim sidewall.

Push the tire to get it into the right place. Inflate it to about 15 psi, and use both hands to push it away from you, until the molded line appears. Go around the tire on both sides until the molded line is visible everywhere and parallel to the rim edge. This takes patience. It can be frustrating, and it’s the last thing you want to do when you have a flat on the road, and all your friends are waiting for you to get back on the road.

To address the problem of the overly deep wells, you can add two layers of rim tape (or handlebar tape, which is the correct width for 23 mm-wide rims). This raises the bottom of the well. Now the tire should seat correctly without as much manipulation.
Do not ride a poorly seated tire! The tire could come off the rim and cause a crash.
Of course, it would be nice to have correctly designed rims, where the tires seat automatically as you inflate the tubes.
Update 3/19/2014: Grand Bois has redesigned their rims with a shallower well, so they fit the tires properly. The new rims are in stock.

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The Downsides of Wide Tires


Since we have published a post about “The Dangers of Narrow Tires,” it is only fair that we look at the other side of the coin. What are the disadvantages (or even dangers?) of wider tires? I can think of a few:
– Weight: A wider tire and a wider tube always will be heavier than a narrow tire. The scale above shows the difference. On the left is a 650B x 42 mm tire. It weighs 410 g. With the same casing, the 700C x 23 mm tire on the right weighs just 220 g.
If you take the difference (190 g), add the extra weight of the larger tube (37 g) and multiply by 2, you get a weight difference of 454 g, or almost exactly a pound. (You probably also will use a wider rim, but the smaller diameter of the 650B wheel actually makes for a slightly lighter wheel.)
A full pound sounds like a lot, but it is less than a waterbottle. Does adding a second water bottle make your bike noticeably slower?
What about the importance of rotating weight? Thanks to the smaller wheel diameter of a 650B wheel, the rotational inertia of the 42 mm-wide 650B wheel-and-tire is actually the same as that of a 700C x 28 mm wheel/tire. So the difference there is negligible.
[youtube http://www.youtube.com/watch?v=8pzc2aYVFEg?rel=0&w=425&h=349]
– Shimmy: Shimmy is a problem with many causes and solutions. One factor among many are wide and supple tires: They can exacerbate shimmy on some bikes.

However, there are many bikes with wide tires that do not shimmy. And some bikes with narrow and medium-width tires shimmy (see video above, the bike shimmied even without my slapping my thighs). Still, it’s harder to make a bike with wide tires that doesn’t shimmy.

– Appearance: Some bikes just look wrong with wide tires. The iconic 1980s Cinelli Supercorsa (above) would not look right with 42 mm-wide tires. Riding bikes is not just about performance, but also aesthetics, and many riders prefer the slim appearance of a classic racing bike. (Photo: speedbicycles.ch)
And if you show up to a group ride with wide tires, you’ll have many people give you their well-meaning advice: “You need narrow tires if you want to go fast.” If you’d rather fit in, a wide-tired bike may not be for you.

– Availability: Few performance bikes are designed for wide tires. The image above shows the “Performance Bike” categories from a big manufacturer. You have a choice between more than ten types of performance bikes, but all have narrow tires.
Unless you can afford a custom frame, your choices for wide-tire road bikes mostly are limited to touring, commuting and hybrid bikes with overbuilt frames that may not be ideal for spirited riding.
Update 06/2016: Wow, this has changed tremendously since this post was written in 2012: Today, there are dozens of “gravel” and “adventure” models available even in mainstream bike stores.

– Tire choice. Narrow high-performance tires are available at every good bicycle shop. They are made by numerous manufacturers. Wide high-performance tires can be harder to find.
All those disadvantages are real and worth considering. For us, the disadvantages of wider tires are outweighed by the advantages:

  • Higher speed on rough surfaces, equal speed on smooth surfaces.
  • Fewer flats because wider tires run at lower pressures.
  • Longer wear because the wear is distributed over a larger contact patch.
  • Greater safety as tracks, cracks and holes no longer pose a serious risk.
  • Greater comfort and enjoyment, especially when riding on poorly surfaced backroads.


Just putting wide “touring” tires on your bike won’t transform it into a capable “Allroad” machine. A few factors are key to a nimble, fast bike with wide tires:

  • Supple tires: Most wide tires on the market are utility tires. Their sidewalls are not supple, and they lack both speed and comfort.
  • 650B wheels: Reducing the wheel size retains the nimble handling of a good racing bike. With very wide 700C tires, a bike becomes too stable to offer the sensations of a good performance bike.
  • Needle-bearing headset: It acts like a steering damper and greatly reduces the risk that your bike will suffer from shimmy.
  • Performance frame: The frame and its flex characteristics affect how the bike feels. To get the performance and feel of a racing bike, you need a frame with similar flex characteristics, and not a hybrid or touring bike.
  • Optimized geometry: Wide tires affect a bike’s steering. The geometry should be adjusted to get a surefooted, yet nimble bike.

You can obtain many of the advantages of wide tires simply by putting the widest tires that fit on your existing bike. My old Alex Singer was designed for 25 mm-wide tires, but I managed to fit 32 mm tires with adequate clearances. (I had to replace the fenders with a wider model.)
32 mm is a good compromise, but not as wide as I would like. My new bike with its 42 mm-wide tires (above) has shown me that the advantages of wide tires are best enjoyed with a new bike designed specifically for the tires you want to use. I now regret not having made the switch sooner.
What are your thoughts about tire width? Which tires do you ride now, and which tires will your next bike use?

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The Dangers of Narrow Tires


It is unfortunate that most “road” and even “hybrid” bikes are sold with tires so narrow that you cannot cross streetcar tracks at an oblique angle without risking a fall.
The Seattle Times reported recently about a rail line that crosses a city street not far from my house. “A bicyclist falls there daily,” the article reports. A business owner at the tracks found that “bike wrecks are so constant he keeps a first-aid kit at his front door.”
Over the past decade, there were 66 crashes serious enough to call out the fire department. Then there are the accidents on Seattle’s drawbridges, where cyclists fall into one-inch-wide cracks, sometimes with horrific consequences. There has been much talk about what could be done to make these places safer for cyclists, but gaps and tracks simply are part of the urban landscape in which we cycle.

I know that a skilled rider can bunny-hop across tracks and cracks, no matter at what angle they run. However, the fact is that many people ride bikes who are not that skilled. Nor should they need to be.
With the 42 mm-wide tires of my Urban Bike, I have experimented (at low speed) with the gap next to the rails. Even when crossing the tracks at a very shallow angle, the tires just rolled over the gap.
Since we cannot eliminate tracks and cracks from all roads, why don’t we fit 42 mm tires to the bikes that are sold to most cyclists? Wider tires would make cycling much safer. Besides, the current research indicates that wide tires are at least as fast as narrow tires. In fact, I never felt that I was handicapped during Paris-Brest-Paris last week by the 42 mm tires on my new randonneur bike. Of course, bikes with narrow tires would still be available for enthusiasts who really like the look and feel of narrow tires – but they’d come with warning labels.

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Preventing Flat Tires


Every cyclist knows the sensation: You ride along, feeling at one with your bike and the world. Suddenly you notice your tire going soft, often accompanied by a hissing sound. Like Icarus, who flew too high and got singed by the sun, your euphoria is dashed as you come to a wobbling halt on the side of the road.
Unlike Icarus, who perished as he lost his feathers, flat tires are merely inconvenient. In the video below, Mark shows how to change a tire in less than 90 seconds.
[youtube http://www.youtube.com/watch?v=6bWNOLuEBag?rel=0&w=425&h=349]
Even though you need some additional time to find the debris that caused the flat in the first place, a flat tire needn’t keep you off the bike for long. Nonetheless, we all would like to avoid flat tires as much as possible.
There are two main causes for flat tires. The first are pinch-flats, when your tire deflects so much upon hitting an obstacle that the tube gets pinched between obstacle and rim. The tube punctures, and sometimes your rim dents as well. (The photo below is from Paris-Roubaix, where racers use tubular tires, which are less likely to pinch-flat.)

If you suffer from pinch flats, raise the pressure of your tires. If your pressure already is at the recommended value, then your tires are too narrow for your weight and the road surface. With wide tires, pinch flats no longer are a cause for concern.
The second, more common, cause for flat tires is a puncture. Sharp objects work their way through the tire tread until they puncture the tube inside. There are three ways to reduce the incidence of punctures:

  • Avoidance: Don’t ride over debris that will puncture your tires.
  • Removal: Remove debris before it can get lodged in your tires.
  • Barrier: Introduce barriers that prevent debris from puncturing your tires.

Avoidance is why some riders have far fewer flats than others, even on the same roads and with the same tires. Debris accumulates especially on the sides of major highways. In the main traffic lanes, cars displace the debris until it ends up in the places where cars rarely tread – usually the side of the road or the shoulder, but also some spots at intersections.
If you ride in the traffic lane, you automatically avoid most debris. On busy highways, this is not an option, but at night or on lightly trafficked roads, there is no need to ride on the shoulder. On backroads without shoulders, you ride in the traffic lanes anyhow, greatly reducing your risk of flat tires. Check your maps and see whether you can avoid major highways, not only to reduce your risk of flat tires, but also for a more enjoyable ride. If you have to ride on the highway, don’t ride on the shoulder unless there is significant traffic. In the city, don’t hug the curb, where debris accumulates.
No matter where you ride, scan the road ahead. When you see debris, give it a wide berth. Debris to avoid includes:

  • Gravel left over from winter snowfalls. Freshly crushed gravel contains sharp rock shards that can puncture most tires. Gravel roads rarely see flats, though, perhaps because the gravel’s sharp edges are worn off as it is moved around when the road is built.
  • Glass. Bottle glass is sharper than the glass from broken car windows.
  • Exploded truck tires. Their insidious steel wires will work their way through most tires.
  • Plants with thorns that overgrow the pavement.

If you see debris up ahead on the shoulder of a highway, check whether traffic is clear, and if it is safe, ride in the main traffic lane for a few hundred feet. Debris tends to spread “downstream” from its source, as it gets moved by vehicles and their slipstream.

Debris removal can prevent many flats. Many racers wipe their tires with their gloves after riding through debris. More effective are “tire wipers” (see photo above), which also work on bikes with fenders. The wires of tire wipers very lightly brush the tire tread. They scrape off debris before it gets hammered into the tire as it rotates. When I raced on tubular tires, I used tire wipers (also called “tire savers”), and had only one flat in 4 years. That flat occurred when the wire of my tire wiper had moved and no longer brushed the tire. I have been thinking of putting tire wipers on my bike, but I have so few flats that it hardly seems worth while.

Barriers can prevent some debris from penetrating your tires. Puncture-resistant belts are effective against glass, because the glass gets pulverized between the belt and the road, rather than hammered into the tube. Sharp flints do not give up so easily, and usually penetrate the “puncture-resistant” belt all the same, just taking a bit longer to puncture the tube. Some tire manufacturers have increased the thickness of their tread so much that many obstacles no longer can get through, but remain stuck inside (see above). Unfortunately, extra belts and thicker tread make tires less supple, greatly reducing their comfort and speed.
Another way of creating a barrier is making the tire rubber itself harder to puncture. Many professional racers “age” their tires before they use them. Lance Armstrong famously had a mechanic with a cellar full of tires. As the natural rubber used in many high-end tires cures more, it is supposed to get more puncture-resistant. Does it really work? It’s hard to say, because flats can be random. However, it appears that when I ride on tires that have been manufactured recently, I get more flats than if I ride tires from a batch made a year or two ago. For Paris-Brest-Paris, I have stashed away a few tires, just in case.
Conclusion
I recommend avoidance as a first line of defense, with tire wipers as a backup in case you have too many flats. Both methods do not detract from the performance and feel of your tires. Aging your tires for a year or two may make them more puncture-resistant, and it doesn’t hurt.
Barriers are a good idea only if you suffer from a very large number of flats, and everything else has failed. Icarus may have soared too high, but without wings, he would had stayed on the ground and not got anywhere, either. To me, riding on puncture-resistant tires is like trying to fly with clipped wings. I prefer to fix a flat every couple of months.
Update (7/1/2013): Compass Bicycles now sells Tire Wipers.

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Science and Bicycles 1: Tires and Pressure


Most cyclists are interested in improving their bike’s performance, because rolling along at considerable speed while expending relatively little effort is one of the great appeals of cycling. Before you can improve your bike’s performance, you need to know what makes your bike faster, and that is where science comes in. Science is a fascinating process. Here is how we determined that higher tire pressures (beyond a certain point) don’t make your bike faster.
Science usually starts with a hypothesis. In 2005, the German magazine TOUR published performance tests of racing tires, and found that at 50 km/h (31 mph), the differences between racing tires were relatively small. Looking at the data, I realized that at more moderate speeds, the differences in rolling resistance could be quite significant. We designed a roll-down test. Our preliminary results showed that some tires rolled much faster than others. We refined our test protocol, and started testing dozens of tires (see BQ Vol 5 No 1 for more details and complete results).
Science also is hard work: Mark Vande Kamp rode up and down the same hill about 300 times over the course of several months, always in the early morning, when the chances of zero wind are greatest. And several times, we got up at 4 a.m., set everything up, only to have a slight wind rise despite a forecast of perfect conditions… All we could do is go back home. (And because our test track was next to a BMX practice track, we had to sweep the pavement – all 245 m of our test hill – the evening before to create a clean surface for testing.)

We also tried to find out how much performance improved with higher tire pressures. We knew that higher pressures are less comfortable, so we wanted to find out just how much speed you give up for that added comfort. To our surprise, the answer was: “None.” We found that higher pressures beyond a certain point did not make the bike roll faster. This was counter to our and almost everybody else’s expectations…  To rule out that these results were just noise in the data, we did more tests of different tires at various pressures. The results were consistent with our previous tests. Statistical analysis showed us that the results were highly significant, that means, they are unlikely just noise in the data.
Suspension Losses
The next step was to develop a hypothesis that explained what we had observed: Suspension losses are caused by friction in the the rider’s body tissues as they are vibrating. Higher pressures cause more vibrations, and hence higher suspension losses. This appears to cancel any gains at higher pressures from reduced flexing of the tire (hysteretic losses), as the tire deforms less at the contact patch as the wheel rotates.
This hypothesis also allowed us to explain why the drum test results were different – by neglecting the suspension losses, they measured only one half of the equation.
To test this hypothesis, we had to establish that suspension losses really were a significant factor, rather than some theoretical construct. (OK, the U.S. army already had established this for vibrating tank seats, but we had to show that it happens on a bicycle, too.) We did this by testing power output at constant speed on a smooth and on a very rough surface, side by side (see photo below). The differences were huge. On the rough surface (rumble strip), our rider had to put out 290 Watts more than on the smooth surface (right next to the rumble strip). That means that 290 Watts were lost through vibrations of the bike and rider’s body.

The Army studies had shown that energy absorption in human bodies was directly correlated to discomfort. After having ridden up and down our test hill 300 times, Mark wasn’t keen on riding 11 miles on rumble strips. That task instead fell upon me. I was able to confirm the Army’s results on the discomfort of absorbing hundreds of Watts as your body vibrates. Did I mention that science is hard work?
As a side effect, the suspension loss tests confirmed once again that higher pressures don’t make the bike faster even on very smooth pavement. And this time, we tested with a power meter instead of a roll-down test. So we had confirmed the results with two different methodologies. (This is much more powerful than just reproducing our initial results, which simply means running the same tests again, and finding the same results. We have done that as well, multiple times.)

After all this testing, we now can say with great certainty that increasing your tire pressure (beyond a certain point) does not make your bike faster on road surfaces that range from very rough to very smooth. In fact, on very rough road surfaces, higher pressures are a lot slower than lower pressures, because the suspension losses are so great. On most surfaces, tire pressure (beyond a certain point) simply doesn’t make a difference in speed.
Optimized Tire Pressure
Our initial tests even established at what point the performance no longer increases with higher tire pressures. For most tires and on “average” roads, this point appears to be a little higher than the 15% tire drop measured by Frank Berto. Note that the loads are given per wheel, not for the entire bike.

This means that Berto’s values are a good starting point for experimenting with tire pressures. If you want to optimize performance, you may want to go a little higher. If you are mostly concerned about comfort, you might prefer a tad lower pressure.
As always in science, there remain open questions. Is this cut-off point the same for different tires? Or do stiff tires benefit from higher pressures more than those with supple sidewalls? After all, a stiffer sidewall takes more energy to flex, so reducing that flex by all means may be helpful, even if it makes the bike vibrate more. Or is it the opposite, that stiff tires vibrate so much that running them at lower pressures is better, even if it increases the losses due to tire flex? Rest assured, we are working on this…

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