Compass has long championed the use of “road” tires on gravel. More and more gravel racers agree: When gravel is sliding on gravel, knobbies are of little use.
So then why does Compass offer a knobby tire, the Steilacoom 700C x 38 mm? Knobs are useful in mud. They dig into the surface, and since the mud is viscous (gooey), it provides something for the knobs to push against. That is why cyclocross bikes use knobby tires.
It’s important is to space the knobs widely, so the mud is ejected as the tire rotates. Otherwise, the tire just clogs up, and soon you are riding on slick tires again, except that their tread is made of mud instead of rubber. What you want is a muddy bike, but clean tires (above) – the tires pick up mud only briefly before it is flung off.
Snow is a different story again. Depending on your speed, it behaves differently. At high speeds, you glide through the snow almost as if you were skiing, and tread patterns make little difference. At low speeds, you compact the snow and create the surface on which you ride. Knobs dig into that surface and give you extra grip. Even a herringbone tread works OK. Slick tires or longitudinal ribs act like the runners of a sled – they just slide and offer very little traction.
What about ice? Ice is too hard for rubber tread to dig into. You need metal studs that bore into the ice to find traction. Sometimes, snow compacts to ice (above). I prefer to walk rather than risk a fall when I see ice on the road. (Unfortunately, I don’t know of a good method to see “black ice” before it’s too late.)
Back to mud, where knobbies make the biggest difference: Designing a good mud tire isn’t hard – space your knobs widely, and the tire will self-clean as it rotates. The downside is that it’ll be buzzy and slow on pavement. I love the FMB Super Mud tires (above) on my old ‘cross bike (our Steilacooms don’t fit!), but their secret isn’t in the tread pattern – the extra-supple casing makes them wonderfully fast and contributes to their great traction. The tread is incredibly buzzy on pavement. It’s good that most ‘cross courses include no more than a few meters on pavement.
The knob shape itself makes little difference. “It’s all about ‘design'” a Panaracer engineer confided.
Designing a knobby tire that rolls OK on pavement is not too hard, either. Space your knobs closely, and the tire will roll fine. But when it gets muddy, the tire will clog up, depriving you of the advantages of a knobby tire. You get only the disadvantages of knobbies, without many of the benefits.
Designing a tire that rolls well on pavement and grips well in mud is much harder. If you also want the tire to corner well without knobs folding over and suddenly losing traction, it seems almost impossible. And yet… with the engineers at Panaracer, we spent a lot of time analyzing and testing knob designs during the development of our Compass Steilacoom 700C x 38 mm knobbies. We found a few things that can greatly improve a knobby’s performance on pavement, without detracting from its ability in mud. More about that in a future post…
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Photo credit: Wade Schultz (bottom photo)