Friday, May 1, 2009


Dear internet, I have a question. On the road, tire pressure is easy. It says on the side of the tire what you pressure you need. Off the road, not as easy. Tires give a ball park, but that's it. It's there some kind of formula you can calculate with your weight? I don't like getting pinch flats, but I like to maintain control and not slide out on rocks. I weigh 150lbs and usually have about 25 or 27 lbs of bike under me. At present, I ride on the high pressure side, but it's pretty unforgiving. I know this is a rookie question, but I've never quire figured out the sweet spot for places with rocks, especially when I ride rigid.

Happy May Day!


murbike said...

Depends on the terrain and your tires.

I don't think there's any formal formula.

Use this:

Soft Surface (loose dirt, sand, snow) lower pressure.

Hard surface (pavement, rocks, hardpack) higher pressure.

Carry a pump/CO2 cartridge, and you should be able to adjust the pressure to suit your needs.

Brendan said...

I practice those principles, but there's still wiggle room. There's my problem. Rocks need higher pressure because of pinch flats, but that bounces you all over the place.

The formula I'm looking for is: what kind of impact can a tire and tube take (factoring in psi, rim width and rider/bike weight) before getting a pinch flat? Something that'll tell me to start riding at 37 psi if there's X kind of anticipate impact. Sheldon Brown, Jobst Brandt and other people always cool graphs and tables that speak to this kind of stuff, but I've never seen one like this.

Aaron said...

Way too many variables from riding style to bike geometry to rim width to take into account. The best method I heard was this:

Start out with a high-ish PSI (say, 45). drop about 5 PSI every 10 minutes. When you get a pinch-flat, go up 5 PSI. Stay there unless you get another pinch flat. Repeat.

Brendan said...

That's like how Calvin's dad explains how they come up with the weight restrictions for bridges: they keep driving heavier trucks over the bridge collapses. Then, they rebuild the bridge.

I guess no one can answer my question.