Why We Don’t Like Shoulder KnobsJan Heine
When we developed our first knobby tires, we wanted tires that could handle all conditions, not just mud and snow. We wanted tires that rolled fast on the road, yet offered tenacious grip when conditions got really tough. We wanted a tire for adventures where we don’t know what lies ahead: pavement, gravel, mud, and maybe even snow.
There’s a standard way to make a fast-rolling gravel tire: Give it a smooth (or almost smooth) center tread and add corner knobs. The idea is that you roll on the smooth part most of the time, but you get added traction when you lean into a corner and need more grip. At first sight, it makes sense. And indeed, these tires roll fine on smooth surfaces, as long as you go straight. That’s no surprise, since you’re riding a tire with minimal tread.
It’s in the corners where these tires disappoint: The shoulder knobs don’t really help. When you’re on slippery surfaces, you simply can’t lean far enough to engage the shoulder knobs. You’ll start sliding while you’re still on the ‘semi-slick’ portion, and by the time the shoulder knobs engage, you’re already going down.
The opposite is the case on pavement. There you can lean much further, but once you get onto the shoulder knobs, they fold over and the bike suddenly breaks away. It’s not a good feeling… To add to the woes, when it gets slippery, you don’t get much traction, because the part of the tire that has the most pressure on the ground doesn’t have meaningful tread.
You don’t have to take my word for it. Gerard Vroomen from OPEN Cycles recently made the same assessment. He wrote in his newsletter about a semi-slick tire:
“The feature we don’t really like are the side knobs. In general we find side knobs superfluous on gravel tires. If you’re hanging at such an acute angle you’re about to hit the ground anyway. Even worse, the only terrain where you could achieve such an angle without falling is on asphalt, and the last thing you want is huge side knobs hitting the road instead of smoother rubber.”
Is there a way to make a better dual-purpose tire that works well on smooth and rough surfaces? What if you started with a slick tire and cut away the tread as much as possible? So instead of thinking of knobs, we thought about the rubber surface that we need on the road for speed and cornering grip.
You still need knobs, of course, to cut into the ground when it’s slippery. The big question is whether you can make those knobs big enough that they don’t squirm, yet small enough to dig into loose surfaces.
Our first calculations indicated this seemed possible. And since the large knobs don’t squirm, we could make them taller for even more traction without losing speed. That left the last question: Could we space the knobs far enough that the tread wouldn’t clog up with mud, but still offer enough support for fast cornering on pavement? Again, our calculations were promising.
Thinking about the tire in terms of rubber on the road also eliminated another problem with many knobbies: As you turn and lean the bike, you first roll on a line of knobs, and everything seems fine. When you lean just a little bit further, you get off the knobs and suddenly have much less rubber on the road. We placed the knobs so that you always have the same amount of rubber on the road, no matter your lean angle. On our most recent models, we slightly increase the rubber surface on the shoulder knobs to give you a little extra traction when cornering hard on pavement.
All calculations only take you so far – the proof lies on the road. Before you can get even a single tire for testing, you need to invest into a very expensive tire mold. It’s always exciting when the first prototypes of a new tire model arrive. After hours, when the production staff is gone, the engineers at the tire factory heat up the huge, brand-new tire mold to make just a very small batch of totally hand-crafted tires. As soon as they come out of the mold, they are airshipped to Seattle. They still smell of fresh rubber when we open the parcel. There’s no packaging, and they usually don’t have stickers yet. When we mount them on our wheels, we see the tire tread for the first time in real life, rather than just on a screen or on a sheet of paper.
And then it’s time for the first test ride. I’m happy to report that our dual-purpose knobbies exceeded expectations. They excel in mud, but that was to be expected. They hook up great in gravel – but so do most all-road tires. They work well on dirt and hardpack, too.
We wondered mostly about their on-pavement performance. And there the tires exceeded our – very high – expectations. It wasn’t just the cornering – although you wouldn’t want to lean most knobbies into a corner like this. (Sorry for the out-of-focus photo – the camera had a hard time catching up on the typical Seattle day when we first tested the tires.)
The biggest surprise was the speed. The tires felt fast. On familiar terrain, I was riding in the same gears that I use with our smooth all-road tires.
I took a bike with our first knobbies to an all-paved BQ team ride and asked Mark to switch bikes. We’re almost clones of each other with the same height, weight, build, power output and pedaling style, which is ideal for testing the relative performance of two bikes/tires/other components.
On this morning, he looked at me, puzzled: “Why would I ride knobbies on a paved ride like this?” We never share our impressions before a test is complete (to prevent what scientists call it ‘confirmation bias’), so I just said: “I want to get a feel for how much speed we lose when riding knobbies.”
We switched bikes several times during the ride. Mark was running our Extralight all-road tires, which are known to be among the very fastest tires you’ll find anywhere. Between his bike and the one shod with the knobbies, there was no noticeable difference in performance. Whoever was on the knobbies had no trouble keeping up.
Sorry, I have no photos from that ride. We were going all-out, and there was no time to pull out a camera! Mark later commented that only the tire noise reminded him that he was on the bike with knobbies – and during a fast, twisty descent, when the wind noise drowned the hum from the knobs, he forgot that he was riding knobbies altogether.
From looking at our dual-purpose tires with their big knobs, you might think of them as clunky, heavy and slow on smooth ground. The tread looks almost like a mountain bike tire. But that’s a misunderstanding – these aren’t knobbies in the conventional sense, but slick tires with much of the tread cut away. You could ride them on a fast-paced road ride like we did, and have no trouble keeping up. They are faster than many ‘road’ tires.
So why don’t we ride knobbies all the time? The noise is an obvious drawback, although we are working on that. They have a little less directional stability, because the tread isn’t continuous –there is less of what experts call ‘pneumatic trail.’ They are a little heavier, too. And on a bike with fenders, you need more clearance, since the knobs can pick up stones and sticks that could get stuck in the fenders.
Those drawbacks are minor: For a ride where the conditions ahead are uncertain, even if it’s 90% paved and only 10% gravel and mud, I’d pick our dual-purpose knobbies every time.
Photo credits: Natsuko Hirose (Photo 1), Heidi Franz (Photo 5), Ryan Hamilton (Photo 6), Rugile Kaladyte (Photo 7).