Worn Mountain Bike Tires: Hooked Side Knobs

This past Tuesday, at the CRAMBA River Bends With Friends ride, I had a both-tires-slide-out fall on a tight, flat, sandy-over-hard trail that resulted in a pretty-decent scrape to my left calf. Why did it happen? Two reasons. One — the primary reason — is because I exceed my ability to handle the bike. Two, the bike’s ability to corner was reduced because I let the front tire get too worn out. So what was fine before, this time… wasn’t. And I didn’t adapt.

While one thinks of a tire wearing out, it’s typically the knobs getting smoothed out, especially along the center. On knobby mountain bike tires, it’s the side knobs which bite into dirt to maintain traction when cornering. And those same cornering forces eat away at the inner edge of the side knobs.

Many high quality tires, such as the Maxxis Rekon 29×2.4 3C/EXO/TR shown above, have harder rubber in the center than on the sides, meaning the center continues to look (and work) fine (in a straight line), even as the sides have failed. (Note the visible siping on the center knobs, still-vertical trailing edges, and even some of the flashing left over from the molding process.)

Eventually the side knobs become undercut, almost like hooked flaps as seen above, and no longer offer much support. Without close inspection, feeling with fingers and such, they often still look sharp or at least sufficient. This loss of support means radically-reduced cornering ability on soft surfaces, and the now-super-flexible knobs make for unpredictability on hard surfaces; almost like riding on loose bits of rubber.

I should have replaced this tire long ago, but I kept putting it off; “I’ll do it after this next ride” because it wasn’t too bad. Until it was.

Whenever I replace a tire I’m starkly reminded of how much better new tires with sharp, crisp knobs feel. It’s a bit annoying that this wear can be hard to see and more needs to be felt for (with fingers), and it also just sort-of sneaks up on you. A long ride with a bunch of paved sections or a trip somewhere a bit rocky can push tires over the edge, even while the other knobs might still look fine.