PSI for road bike tires?

[Eden]

What Sheldon Brown has to say about rolling resistance: http://sheldonbrown.com/tires.html#rolling (cause his stuff is usually pretty well researched)

I weigh about 105. I tend to notice it when my tires get down to about 80psi. Feels a bit logy. I generally run about 105psi, but I'm lazy and don't pump them up every ride.... I've had very few pinch flats - only one I specifically remember and I went over a *nasty* sharp curb with an under inflated tire.

[Seajay]

Good One Eden. Sheldon strikes a nice balance. Regarding his comment
" the rolling resistance at correct inflation pressure is already so low that the infinitesimal reductions gained are more than outweighed by the trade-offs"

I take this to mean that if you are somewhere in the ballpark of the correct pressure...the changes in rolling resistance are so small...that "we" are making up any differences in our heads. THAT...sounds like the late Sheldon Brown

The rest of the article is a nice summary as well.

[aicabsolut]

I'm not trying to attack anyone either--just continuing the discussion.

PSI also has a lot to do with the tire and what it's max is. I'd blow out some tires at 120, whereas 120 feels fine in others. And then there are tubulars (which most of the pros use), and varying pressure in tubulars has some grip advantages, but again it depends on the tire (and the rim).

I have no physics data, just personal experience.

There are some people who ride tires narrower than 700x23. There are some who ride lighter tires. Think of the Michelin Pro3 Light tire in 700x20. The narrower tires generally require higher PSI for the rider weight, according to the charts I've seen. These are often used by triathletes who want a thin, light tire that is good for riding in a straight line. The narrower tires can get a bit tricky to ride in tight cornering situations (but lots of people like using them for crits too), and the lighter tire means it has less flat protection. So there are practical drawbacks to using this arguably faster tire.

Because we're still talking about a road bike tire (even at high pressure) and not a MTB or CX tire and not a hard rubber disc, the amount of energy lost bouncing vertically is probably adequately absorbed by the tire, fork, and rider such that it's still going to be a faster tire. It will be less comfortable the more rigid the tire, and, depending on the tire, there may be less grip at times you'd want more grip (i.e., not going in a straight line on a relatively smooth road). At some point, "comfortable" turns to a squishy, sluggish feel, and then you get to a point where (with clinchers), you hit some pretty minor holes and you get pinch flats.

[Seajay]

the amount of energy lost bouncing vertically is probably adequately absorbed by the tire, fork, and rider such that it's still going to be a faster tire.

Yes, Fun conversation. Thank you for continuing it.
I will stand by my theory that the energy the fork, frame and rider absorb (convert) is wasted. Yes, you can run too soft... but with an appropriately inflated tire, energy taken up by the front of the contact patch and returned at the aft is not wasted....thus a faster more efficient set up.
It would be interesting to do the same road with a powermeter and a speedometer. How much power does it take to hold X mph with a range of tire pressures?
We need a tire engineer to jump in here.

[aicabsolut]

Yeah, sure it's wasted energy, but I think that it's negligible compared to the vertical shock absorption being done by the tire/fork/frame/rider with a softer tire in light of the differences in propelling forward. The more I think about it, I think you are right that you've got to measure this in terms of power output by the rider instead of just rolling resistance numbers for a tire since travel in the vertical direction would be hard to measure. The fewer watts it takes to go X mph over different terrain with different pressures under the same wind conditions would mean that the tire setup is "faster" because the ride would be more efficient.

I'm not sure how many watts we'd be talking, though. I would guess not that many, based on my assumption that this topic eventually boils down to personal preference for comfort, the road "feel" for a given tire, and the odds of getting a pinch flat due to ease of compressing the tire.