Brian Johnson

Interesting. I was just think about the issue of tire pressure this morning.

First let me make clear that I'm not an engineer, nor do I have the resources to test my hypotheses.

I do, however, have plenty of experience.

When you inflate your bike tires, unless you're riding a track bike on a track, you need your tires to compress a little when you load them with your weight. This gives you the optimum contact patch.

Here's my hypothesis, which has so fare been borne out by practical (racing) experience:
When you are riding off-road in dry conditions, you want your tire to deform or "squish" more so that the tread can envelop more of the terrain irregularities (contact patch) and thus preserve traction. An over-inflated tire will tend to skip and bounce more and have a smaller contact patch.

In wet, muddy conditions, the opposite will happen: the terrain will envelop your tire. Reducing tire pressure too much, beyond increasing the risk of pinch flats in a clincher setup, will not provide any benefit. If anything the under-inflated tire will only slip and slide in the soft terrain. A properly inflated tire will "cut through" the muck to find the firmer ground underneath. Over the years in various cross races I've experimented with tire pressures as low as 28psi (I weigh 150lbs) and as high as 50psi.

The lower tire pressures did not appear to significantly increase my traction and only made the bike feel like it was wallowing around. At higher pressures the handling felt a little more predictable as the tire wasn't deforming so much. At pressures approaching 50 psi I began to experience more "spin-outs" as the tire, while digging through to harder ground, wasn't deforming and gripping the terrain. Thus it would just break traction.

I found that the optimal pressure, for me (as a 150lb rider) was in fact the optimal pressure no matter if the weather was wet or dry: 35pis in the back and 30 psi in the front. (Plus or minus a pound of air or two.)

What DOES make a huge difference is the tire tread pattern itself. Open, cone or pyramid shaped knobs will shed mud better. Doing so will free the tread to grip the terrain. Closely spaced, blockier treads work better in dry conditions but will clog with mud and make it impossible to gain traction.

So far I have found the Maxxis Larsen Mimo CX to be the best all-around tire. It works great, wet or dry, grass, whatever.

As for sand? Yes, a tire with higher pressure that will cut through soft terrain will dig deeper into sand, which would indicate a lower pressure. However, you'll need to take into consideration the rest of the course. Still, I believe that there is a somewhat narrow range of optimal tire pressure based on a rider's weight. This will give you the best possible tire response/deformation no matter the terrain. If it gets too hard to pedal, jump off and run.

Here are a couple of links by folks that are smarter than me:

http://www.sheldonbrown.com/tires.html

http://www.bikequarterly.com/images/TireDrop.pdf

Good luck!
Brian J.

> Message: 37
> Date: Thu, 6 Oct 2011 21:15:44 -0700 (PDT)
> From: "Jason A. Skelton"
>
> In very wet, muddy conditions like we had at the Blind Date on Wednesday night, does lowering the tire pressure make it easier to ride through the mud? ?I know in sand lower tire pressure helps with traction and I wonder if there is a similar situation in mud.
>
> thanks
>
> Jason