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Continental Vertical (courtesy, REI)
Gear Guy

Do tires with bigger knobbies grip better in mud?

I'm doing a science project and need to know if tires with bigger knobbies grip better in mud. Jason San Ron, California

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Well, Jason, if you’re doing a science project, then you should apply scientific methods. Develop a hypothesis—say, that tires with big knobbies grip better than ones with small knobbies—and test it. That shouldn’t be hard to do. Find a muddy slope with a known (or at least estimated) coefficient of friction, reduce as many variables as possible by sending the same rider up the hill with the same bike, turning the same gear. And then simply swap out two tires. OK, so the test will require a small investment, but you only need to worry about the back tire, and you likely can find some candidate tires on sale for $20 or so. Then, send your test-cyclist up the hill, turning the cranks at a measured rate, and determine how far he/she gets up the slope before the tire slips. It’s not a perfect test—that would entail putting the bike in some sort of “mud tunnel” where you can mechanically measure the downward pressure on the tire, the rate of turn, and so on. But it’ll do.

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So, you can go ahead and start working away at that, or you can read my answer here: Generally speaking, tires with fewer, bigger knobs work better in mud than those with many small knobs. The reason is fairly simple: Mud packs into the gaps between the more numerous knobs, and then you don’t have a treaded tire, you have a mud tire. Less clear is the benefit of long knobs that run across the tire, from side to side, like “paddles.” The theory is that they dig into the soft earth and give the bike some grip. But I think they also cause mud to build up, and reduce traction rather than aiding it. Mind you, the current tires on both my mountain bikes—which shall remain nameless—have the paddle-style design and I think they work fairly well. But next winter I’ll probably ride on tires with knobs.

Tires with lots of knobs run more smoothly, so are faster on hard surfaces. It’s also true—or at least obvious to me—that big, widely spaced knobs reduce grip on some surfaces because they break up and soften the top soil layer. But, every tire maker has its own design, and its own rationale for using it. So trial and error, to take into account your own riding style and the trails you visit, has to be taken into account.

Need more than scientific musings? Get totally inspired by 2006’s best new bikes in the April 2006 issue of Outside, on newsstands now, where science and technology meet sleek, irrefutable style.

From Outside Magazine, April/May 2021 Lead Photo: courtesy, REI