PSI for road bike tires?

[OakLeaf]

But... Where do you get the fact/opinion that charts are based only on preventing pinch flats?

It's a conclusion (I don't know whether that counts as "fact" or "opinion" ) based on three things:

* the charts don't take into account road conditions;
* the charts don't take into account rider preference for comfort vs. performance; and
* the pressures they give are well below what I need to minimize rolling resistance.

The only reason any road tire needs a "minimum" pressure is for pinch flat prevention. Other factors yield a range of pressures for a given rider weight. Tubeless tires are becoming popular mostly because they allow riders to run much lower pressures than tubed tires.

[Seajay]

Oakleaf...Thanks for the stimulating conversation on a boring day. I should be working.

Consider that a tire is designed to take up load/bumps (energy) at the front of the contact patch...and release that energy at the end of the contact patch.

If your tire pressure is above that on the engineered chart.... the energy will not be taken up by the tire...it will pass THROUGH the tire, into the fork, frame and YOU where you absorb it (convert it, actually) with your body (same as a tire with a very stiff sidewall (like an Armadillo) ....THUS never being returned to the road through the back of the contact patch = HIGHER rolling resistance not lower.

The other consequence here is that the weight of the bike and your body are forced to travel up/down/up/down with every imperfection in the road as opposed to that mass traveling on an even plane.
My turn for conjecture... it must take MUCH more energy to have you and the bike move up and down than it does to move it smoothly down the road.

[OakLeaf]

Try it out for yourself.

As I said before, I'm right around 120#, and if I put less than 110# in my tires, I may as well be riding my 35# commuter with 1-1/2" knobbies. It's like pushing a tank.

Once in a while I do get on roads that are so rough that it really will bounce and beat me up, and I know lower pressure would help me there, but I'm never on those kinds of roads long enough to make it worth changing the pressure.

[aicabsolut]

Oakleaf...Thanks for the stimulating conversation on a boring day. I should be working.

Consider that a tire is designed to take up load/bumps (energy) at the front of the contact patch...and release that energy at the end of the contact patch.

If your tire pressure is above that on the engineered chart.... the energy will not be taken up by the tire...it will pass THROUGH the tire, into the fork, frame and YOU where you absorb it (convert it, actually) with your body (same as a tire with a very stiff sidewall (like an Armadillo) ....THUS never being returned to the road through the back of the contact patch = HIGHER rolling resistance not lower.

The other consequence here is that the weight of the bike and your body are forced to travel up/down/up/down with every imperfection in the road as opposed to that mass traveling on an even plane.
My turn for conjecture... it must take MUCH more energy to have you and the bike move up and down than it does to move it smoothly down the road.

This is mostly true. If you run a higher pressure, the ride isn't going to feel as smooth, and some of the shock is going to be transferred to the fork/frame/rider instead of being diffused by the tire. On the road, this is pretty minimal, because you're talking about high pressure tires anyway. This isn't like in cyclocross where on the race course you'd run maybe 40psi in your fatter tubular tire (tubies to help avoid pinch flats you might get with clinchers running over a tree root), but on the road, you'd want to max that tire out at 60psi for a faster ride. The difference felt there is probably much greater than going from 100-120psi on a skinnier road tire. You also need to take into account that there is less rolling resistance (though this depends on the specific tire design) with higher pressure which may translate into a faster ride (make more of a difference in the forward direction) than diffusing some of the vertical shock.

I don't even reduce air pressure when it's wet. I know how my tires grip at their usual pressure, and so I stick with that. Perhaps if I had the cornering benefits of tubulars, I might change that, but then dry condition pressure for road tubulars is higher than what i run with clinchers anyway.

[Seajay]

. You also need to take into account that there is less rolling resistance (though this depends on the specific tire design) with higher pressure which may translate into a faster ride (make more of a difference in the forward direction) than diffusing some of the vertical shock.

Hi Aicabsolut,
Please keep in mind..I'm just having a conversation here....I know forums can seem a tad confrontational sometimes....this is not one of those times.
From the reasoning above...a completely rigid (but lightweight) tire would be fastest. ie something like a rubber coated rim. Yet no one does this... even in the straight line speed world of human powered speed records.
Speaking with Phil White of Cervelo cycles...he claims his Test Team ride 95 to 100 psi on their road bikes. Just experimenting, I've played around with pressures as low as 60psi and have never felt it was as draggy as you describe.
I guess I'm wondering what evidence you are citing to make your case.
Have a great day !

I guess I should add in that in the unlikely event "we" are riding on a brand new, glass smooth road it would be fastest to run as high a pressure as possible. But this only further makes my case that smooth is fast.

[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.