Under Pressure

This summer I helped out at a ITU World Tri event helping athletes get their bike tuned up before their races. It was a really fun experience: I got to play with bikes all day and meet all sorts of interesting people from around the world. Most of the fixes we minor (brake rub and shift indexing) but a few were much more serious (be careful how you pack your bikes!)

While getting some elite age groupers' bikes ready for a training ride, we got on the topic of air pressure and I was surprised to find how little thought they put into the topic. Many simply looked at the tires rated max pressure and pumped them up somewhere around that, because more is better, right?

When it comes to tire pressures, this is hardly ever the case.

The truth is that there are many factors that should be taken into account when looking for the ideal tire pressure. It all comes down to weight and width.

Weight

System weight (bike+rider)

Some charts use the total system weight in calculating pressure. Simple add your weight to the weight of your bike. Or, stand on a scale while holding your bike. Note: both bike and rider should be in "ready to ride condition" with all necessary equipment

Weight distribution

Alternatively, some charts use the "wheel load" as their x-axis. This looks at the distribution of weight over the whole bike/rider system. Unless you have 50/50 distribution, your front and rear tires should have different pressures.

Weight distribution is related to bike fit. A triathlete on the aerobars is going to have a much different distribution that a roadie on the hoods. A simple way of determining your own wheel load is to use a bathroom scale. Place the front tire in the scale, and the rear on a couple books the same thickness as the scale. Sit on the bike (ride ready) and record the results. Repeat for the rear wheel.

width

Tire Width

The trend is for people to be running wider tires. Even the pros are getting into it! So much has been written on wider tires already that I am not going to get into it too much here. In simplest terms, wider tires are faster at the same pressure, or equally as fast at lower pressures (with increased comfort).

Read more here, and here

Rim width

Rims are getting wider as well. Wider rims spread out tire for better cornering grip and comfort. Slowtwitch has the details here and here. Wide rims increase the volume of the tire (similar to wider tires) without the extra weight of the added rubber.

Official info from: Mavic  and Schwalbe

On your road bike, your tires are your suspension system. With every bump you hit, there is an upward and rearward force on the bike. If your tire does not deflect (high pressure) to absorb that force, it slows you down. Lennard Zinn explains it here.

So how do you find the right pressure for you? There is a wealth of information out there:

  • Tour Magazine says that your should drop 0.5 bar of pressure for ever tire size up. And another 0.5 bar if you have wide rims.
  • Of course, there is the ubiquitous Sheldon Brown with a page dedicated to all things tire related.
  • One of my favorite methods (that I have only recently come across) is called the 15% drop method, You can read about it here.

The point is: tire pressure is complicated but it doesn't have to be hard. Experiment a little. You my be surprised by what a little less air can do for your ride.