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. So we designed a roll-down test to see if that was the case. 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 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 were 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.
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.
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…
- Further reading:
- Bicycle Quarterly Vol. 5, No. 1: Our first big tire test with performance numbers for many tires, different pressures, details on our testing methods…
- Bicycle Quarterly Vol. 5, No. 3: More tires tested, statistical analysis of our tire test data showing which tires were significantly different.
- Bicycle Quarterly Vol. 8, No. 1: Suspension losses measured, more tires tested on rough and smooth surfaces at different pressures.
