tire pressure Q

[madisongrrl]

In the ideal gas law, P is linear with T, so a temperature change from 70F to 120F is only a 10% increase (convert T to Kelvin).

10% can be very significant...especially if you are racing at multisport events which generally start in the wee hours of the morning.

A perfect example would be the Green Bay Duathlon that is held in Wisconsin in early October. You set up in the transition area between 5-6:30 in the morning. It's sucks, it's cold, there is stinking frost on the ground (I absolutely hate fall!) and the temps in the low 30's. You pump up your tires to the pressure that is correct for you....based upon your weight, bike, tires, course etc. The race starts at 7pm, but your run wave doesn't go off until 7:30. (I generally finish up the couse before 10, though there are plenty of people who finish up at 11:30-noon+...you really have to applaud them for being out on the couse so long). Now the temp is topping out in the low 70's (this happend in 2004).

If you run your tires at a 115 psi, even a 5% increase puts them up at 121 psi. Your bike will ride differently (harsher), you might be more prone to flatting and your traction will be reduced. I have noticed that my Cervelo rides differently even when I change the tire pressure 5 psi. The first year I had that bike I ran the pressure way higher than it should have been and gave myself a harser ride than was unnecessary (and when riding in the aero position.....boy does that hurt the bits and pieces). Lessons learned...

I don't think we need to obsess over tire pressure, but just the same its an important consideration on race day. I'm generally not worried about pressure and temp when I do shorter mountain bike races. But I still can't figure out which tires and pressure I should be running based upon my bike, my weight and course conditions. It seems like the longer I'm at this sport, the more I realize that I have to figure out.

Best,