Here’s the thing, as rubber ages, it interacts with the air in the atmosphere, causing oxidation. Oxidation causes the rubber to harden and become brittle.
The rubber will wear away much more slowly but will offer significantly lower levels of grip.
Oxidation also shows up as cracks or “checking” on the sidewall. Cracking on the sidewall, or in between the traction blocks, indicates severe oxidation, and it’s a sign that the tires should be replaced.
This brings us to the third reason why a tire wears out.
Heat Cycles
Heat increases the rate of oxidization, so if your bike lives in a hot garage, or you live in a hot environment, your tire will lose its grip more rapidly because the heat accelerates the rate of decomposition of the rubber.
What’s more, every time your tire warms up and cools down, it goes through a heat cycle, which also compromises the grip the tire can provide.
When a tire has gone through so many heat cycles that it can no longer provide adequate levels of grip, that tire is “baked-out.”
And not all tires deal with heat cycles the same way. A race tire or even a DOT race tire that can withstand extreme heat punishment on the racetrack may only be able to withstand two or three heat cycles before the level of grip is significantly diminished.
While a sport touring tire may not be able to withstand the harsh punishment and heat like a race tire can, it can typically survive a lifetime of heat cycles from everyday riding stresses before grip decreases.
However, tires intended for cooler, wetter climates are less capable of dealing with too much heat when used in hot, dry climates and may bake out long before the tread is worn away.
This is part of the reason racers and track day riders use tire warmers. Not only to keep the tire warm and grippy but to reduce the number of heat cycles that the tire will have to endure.
This is also why it’s a good idea to try to replace your tires and sets. Your front tire is going through just as many heat cycles as your rear tire and is baking out at the same rate.
We’ve seen a lot of riders who will replace the rear tire, and then a few months later replace the front tire, and then a few months after that, replace the rear tire again.
So they are always chasing a new set of tires. And nothing makes your bike handle like new like a fresh set of tires. So, again, if you can swing it, it’s always best to replace your tires as a set. But if you can’t, for whatever reason, then at least stick with matched tires.
Knowing how old your tires are, or how many heat cycles they’ve endured, maybe a more significant indication as to how worn your tires are than how much tread remains.