I can't get on Sheldon Brown to ask this, but I am curious as to why one would lover tire pressure by 10lbs, say from 120 to 110, for a wet ride.
1. I have read that slicks are as good as treaded tires in wet weather because the contact patch is so small that a slick won't hydroplane.
2. And because of that very "focused" contact patch, slicks have as as good a grip on the road surface as treaded tires, since the grip you gain from the tread is offset by the lack of contact in the voids.
3. Does a (slight?) reduction in tire pressure make a big difference in the size of the contact patch?
4. Is the goal of lowering tire pressure to make the sidewall more flexible, giving the rider more leeway before breaking the road grip/sliding the tires out?
5. Or does 1&2 make it unneccessary to reduce tire pressure?

Thanks for the input.
Hmmm,

What you've said about the contact patch surprises me. I'd have guessed that the whole idea was to increase the size of the contact patch for better road adhesion in the wet.

Another assumption I'd make is that, because tires are more prone to picking up debris and holding it in the wet (so that it ultimately digs into the tread and causes a puncture), debris might not dig into the tread as easily when there's more give behind the tread.

Bob C.
When you consider that the contact patch for a 23c tire at high pressure is about the size of the end of your thumb, a 10 psi drop in pressure will make a pretty big percentage increase in the size of the contact patch. Whether adding a several square millimeters to the contact patch makes a difference that the average cyclist can actually take advantage of is another matter.
"# Does a (slight?) reduction in tire pressure make a big difference in the size of the contact patch?"

The size should be directly proportional. If tires are 100 psi and rider + bike are 200 pounds then the surface contact area will be 2 square inches. Reducing pressure to 90 psi will result in 200/90 square inches of contact area.

I would think more surface area of contact = greater chance of hydroplaning. Maybe less chance of slipping sideways when cornering, though?
Bear in mind, bike tires, regardless of the size of the contact patch, don't lose traction because of hydroplaning, but boundry layer lubrication--so decreasing the contact patch to prevent 'hydroplaning' doesn't make them less likely to slip out in the wet.

I don't know why tires are less likely to slide out in the wet, but I'm guessing that point # 4 is probably a good guess. I do know that when Brandt tested this, lean angle was better @ lower pressures (up to a point--80-90 psi, IIRC?).

I typically run my tires @ 90 psi anyway, so I've never felt the need to correct the pressure for wet....
maybe the added flex in the tires makes it that much more likely that they'll remain in contact with the road when higher psi wouldn't? On a perfectly smooth road with no braking I'd still run a normal psi (less hydroplaning risks), but on a bumpy road I think having the extra 'suspension' makes things marginally safer...

I often wonder, though, if all this is not some placebo effect at work, and it's just that lower psi tires make you slower and hence make you feel more in control (because you're going slower!)

--
A wise man can learn more from a foolish question than a fool can learn from a wise answer.
I can't imagine how it would even be possible to hydroplane something as narrow as a 23c tire... It is going to cut all the way through the water and down to the road. I don't even try to go around many puddles in races any more... Swerving around a puddle has got to be much more risky than just hammering right through the puddle...

The world age group du race in Corner Brook this year featured many large puddles maybe 30-40 mm (1.5") deep and aside from the extra spray, handling didn't seem affected at all at 40 km/h...

I only worry about wet roads when braking and cornering where the rubber doesn't have much grip with the road.

I don't know about lowering the pressure to help cornering. It's going to slow you down on the straight sections though...
When you consider that the contact patch for a 23c tire at high pressure is about the size of the end of your thumb, a 10 psi drop in pressure will make a pretty big percentage increase in the size of the contact patch. Whether adding a several square millimeters to the contact patch makes a difference that the average cyclist can actually take advantage of is another matter.
that "sounds" right...but i can also see where that would make an insignificant difference, since i can see how that wouldn't cause a linear change in contact size at the high end of tire pressure and normal, steady state riding pressures on the road (i.e. flat)...funny how a "given rule" is not easily answered.
My lbs guy, a long-time cat racer, is adamantly opposed to tire deflation in wet conditions. His position, and I agree from personal experience, is that the higher (regular) pressure is more likely to cut through the water and make contact with the road. The lower pressure will increase surface area, which is likely to increase the chance of hydroplaning.

Same factor applies to my car, which has 19 inch, low profile, high performance tires. In the winter, I switch to 16 inch all weather tires. The reduced surface area allows the tires to cut through the snow and make contact with the pavement. Running my low profile tires in the snow is like riding on hockey pucks.

If there are no dogs in Heaven, then when I die I want to go where they went. - Will Rogers

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