Bicycles Stack Exchange is a question and answer site for people who build and repair bicycles, people who train cycling, or commute on bicycles. Join them; it only takes a minute:

Sign up
Here's how it works:
  1. Anybody can ask a question
  2. Anybody can answer
  3. The best answers are voted up and rise to the top

Is there a way to estimate contact patch size given tire size plus tire volume and rider weight?

Specifically, I've tried to build a unicycle that maximizes contact patch for riding on soft snow. I have a 26"x3.7" tire with a 65mm rim that I have at 12psi, how can I calculate the resulting contact patch?

For extra credit, I'd like to calculate the contact patch size required to ride across newly fallen snow for a particular rider weight.

This question was previously asked as a comment on my previous question, but I feel it needs its own question...

share|improve this question
You are assuming "newly fallen snow" is a static variable. It is not. Snow falling at 30F will behave much differently than snow falling at -10F. They will both also be greatly affected by what is underneath them (another variable). – Suspended User Mar 18 at 20:37
up vote 5 down vote accepted

Basically: weight/psi = square inches.

Has to be combined weight of rider and cycle. And it's approximate. And much easier with a unicyle: with a bicycle you need to know weight distribution, which can change during a ride.

Minor things that make it only an estimate:

  • Compression from weight increases psi, making contact patch smaller than estimate.
  • thickness of material could increase contact patch size. In fact, I think this math gives you the area of the part of the interior surface of the inner tube that's pressed against the part of the tire pressed against the ground
  • stiffness of tire materials could decrease contact patch size
  • tread features (knobs, grooves, etc) could do all sorts of things to contact patch size.
  • bumps, jumps, etc momentarily increase "weight" (downward force on wheel or upward force on contact patch), contact patch size and psi.

No idea about new fallen snow.

share|improve this answer

Your Answer


By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.