Friday, October 23, 2009

100 psi

While we are on the subject of numbers, I really wonder if people have forgotten what air-filled tires are all about? I was reminded of a crash I saw where one guy's tire blew up because it was over pressure. He wanted to claim a free lap but was scolded by the referee for endangering other people. A fight ensued and the cyclist left the scene angry even though the evidence was solidly against him.

It turns out roadies are a bit better at this than triathletes, who seem obsessed with high pressure tires. Eager to get rid of that last bit of rolling resistance, triathletes love to put 150+ psi in their tires. Frequently tires give out -you can often hear them popping in transition- but nobody seems to make the connection.

A tire is there first and foremost to provide traction (i.e. friction) and comfort. If these did not matter we would all ride on cast-iron wheels. I think you agree that this would be most dangerous and unpleasant. While it is true friction means rolling resistance it is also necessary to provide traction so one can corner and accelerate, and brake when needed (think ice if you doubt this). In the rain and on slippery surfaces a larger contact patch (read lower pressure) is desirable.

Furthermore, since a bike has no suspension, the tire is supposed to give a bit and provide some measure of shock absorption. All these desirable properties disappear when you pump your tires as hard as a rock. It may not matter as much in a one hour effort, but riding rock solid tires in an Ironman event will take its toll. You will be more fatigued than you need to be.

Additionally, as the tire heats up for a variety of reasons, the pressure inside increases. So if you start up with a tire near bursting pressure, you are asking for trouble when temperatures rise throughout the day or when you have to brake often (or both). Popping a tire on a long descent is not unheard of and it can be quite dangerous. More dangerous than a pinch flat due to insufficient pressure.

That is why 100 psi is the recommended tire pressure for most people. And when it is very wet and slippery, 80-90 is better. Will you lose some effort because of higher rolling resistance? No doubt, but the amount is minimal and the cost benefit is simply not there.

One should be even more careful in a road race or criterium because a blown tire may cause others to crash as well. That is less of an issue in triathlon where you can't (or at least aren't supposed to) draft. But triathlons tend to last a long time and so there is plenty of time for the pressure to build up.

Today I rode 40 miles on the KOM. Nice day, comfortable temperature and sunny. Tomorrow, track.

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