Tire Pressure

Tire pressure is the fine adjustment element of a bike's suspension. It also plays a part in traction, rolling resistance, and efficiency. While I don't pay much attention to it on my road bike, I like to have it dialled in on the mountain bikes.

A couple months ago two homework problems got me thinking about front vs rear tire pressure and it's effects on contact patch and traction. The first homework problem was for Fluid Mechanics and involved calculating the contact area of a tire given the weight of the vehicle. The second problem was for Dynamics and involved calculating the maximum acceleration of a car with front, rear, and all-wheel drives.

From basic physics, I learned that the force of friction is equal to the friction coefficient times the normal force (f = μN). From the "real" world, however, I know that tread patterns, contact area, and tire pressure all effect the friction between a tire and the ground. It turns out that the formula from physics makes some assumptions that don't hold true for tires as seen here.

So, while I was bored in class one day, I decided to calculate what my front tire pressure needs to be relative to the rear to have equal contact patches and therefore equal traction. One assumption I made is that my weight is centered over the bottom bracket. On my singlespeed this is reasonable as I tend to unweight my handlebars often (corners, bumpy sections, etc). I measured the distances from the center of the bottom bracket to each axle and calculated the fraction of my weight on each wheel. Calculating the weight on each wheel was kind of a round-about way of doing things; all I really needed was the lengths. Alternatively, a more accurate way to do this is to use a bathroom scale to measure the actual weight transmitted through each wheel. From there I found that my front tire pressure should be 68% of the pressure in the rear tire (for the singlespeed; 62% for the full suspension) for an equal contact patch.

The result was a bit surprising. The difference in pressures was much greater than I anticipated. I used to run a couple psi higher in the rear just because it seemed to make sense, but 7psi seemed like a lot. I decided to try it out anyways to see how it rode and ended up really liking it. My method is to find the minimum acceptable front pressure and adjust the rear accordingly. For the singlespeed, this is 14psi front, 21psi rear.

One question that came up is 'Do I want equal traction front and rear?' Another was 'Is there anything in practice that would make this not work?'

For the former, if anything, it's better to have more traction in the front. The rear tire sliding out isn't a big deal, but the front sliding out will put you on the ground in a hurry. Running near equal tire pressures means you're either giving up front wheel traction or increasing rolling resistance in the rear unnecessarily or both. It's noteworthy that I use the same tires front and rear. Front traction can also be increased by using a bigger and/or knobbier tire which could complicate things.

For the latter question, it can be argued that the front wheel is subject to higher impacts since the energy it absorbs is absent when the rear wheel hits the obstacle. This could necessitate a higher front tire pressure relative to what the calculations show. On the rigid singlespeed, this hasn't been an issue. The full suspension is still in testing.