Tyre pressure and altitude

Paying attention to the inflation pressure of your tyres will give you a better riding experience. You'll get more traction, speed and control by choosing the right pressure. However, once you have found the pressure that works for you, another problem appears: how do you compensate for the effect of height?

When you pump up you tyre at home, what the manometer reads is not absolute pressure, but rather, the difference between the pressure inside and outside. It follows that the higher you climb, the bigger the difference, so potentially your tyres could blow up (that's why some companies ask you to deflate your tyres when carrying your bike on their planes).

This is more than a rethorical question: here it is perfectly normal to ride from sea level to a peak or pass at 1700 m (5000 ft) and down again in time for lunch (ok, late lunch).

Since we at Barcelona Mountain Bike are fond of our mountains, and since we are also technically inclined and not easily scared by a little spreadsheet work, we've given the matter some thought and this is what we came up with:

If you start with 30psi at sea level and measure again at the top (5000ft), your pump will read 37 psi. That means your tyres are bouncier, stiffer and less grippy when you least need it. In Kg/cm2, that is half a kilo more.

On a more positive note, however, it can be concluded that if you're not planning on that kind of climbing, air pressure change is not an issue and for most situations, your tyres won't probably feel too different.

Suspension and altitude

On a final note, remember that forks and air shocks do not suffer from changes in atmospheric pressure since the air is contained in a structurally stiff container that can withstand huge pressure differences without changing volume. The pressure you set at home is the pressure you have troughout the ride.

How about temperature?

Temperature, though, does affect shocks, tyres and brakes. Slow rebound speeds for forks, longer retraction times for brakes, and less pressure in your tyres are the consequences of sub zero riding (not to mention cold numb toes and fingers!)