Originally Posted by
grolby
I'm not sure I agree 100%; there is a point where lower pressure will be slower due to deformation. The crazy-low pressures we run in 'cross tubulars (example: I am 125 lbs and depending on the course will run as low as ~20f/22r in 32mm tires, plenty go even lower than that) are faster because they keep the tire in better contact with the ground, and because surface irregularities means the tire is doing a lot more work and softer makes for less work to deform it. It's just a completely different environment for the tire than you have on the road, where most of the deflection in a tire is from the weight on it, or from cornering forces. On the road, folding more rubber than you need to in order to keep the tire on the road is just wasting energy.
The other issue with lower pressure is the effect of cornering forces on tire shape. Lower pressure means the tire can't resist lateral forces as effectively, so it could fold a bit. In theory, this hurts cornering precision. In practice, I'm not sure how much of an issue it really is on the road. It's a real thing in cyclocross, where those sub-30 PSI pressures make tire folding a fact of life. But in cyclocross we deal with it because we basically have no choice, not because flopping around on soft tires feels great.
Overall, I agree that most people ride on much higher pressures than necessary. Over the years I've come down to something 80f/85 r for racing on 23mm clinchers and have not found that it slows me down at all. Even keeping in mind that I am light, I'm running tires at well below the pressures they are designed for, and could probably get another 5-10 PSI out of them before they started feeling too squishy. But then pinch flats become a risky. Eh. Tire pressure is an arcane subject. But I think the conventional wisdom of running road tubulars at slightly higher pressures than clinchers is basically sound.
many good points. i think we're tossing around slower and faster. i was referring to rolling resistance, but when we talk about "faster" overall, the performance of a tire in corners as you note could come into play. a tire that rolls faster is not faster overall it if folds underneath the rider and causes a crash.
a few years ago i tested pressure on real-world roads. i'd done testing Crr testing on rollers (before and since) to compare rolling resistance of various tires and tubes. in theory, the results from smooth rollers translate to roads, but i wanted to see.
for my test i varied pressure while riding similar power (and corrected for it). i was riding up a steep, protected grade to eliminate or at least minimize the effect of wind. i dropped pressure from 120 on down to 80. never went below 80, but as i went lower each run was faster/i went further, adjusted for power. perhaps there is a point where distance traveled would go back down, but i didn't see it or see any evidence of it. (well, surely at zero pressure it would happen...)
your point about cornering is a good one; at a certain point the characteristics of a tire change and this would come out in cornering, so there is a balance. my testing was centered around a hill climb/time trial, so that wasn't so much a factor.
as noted before,
re: the last point (tubulars @ higher pressure than clinchers), i know that sounds right, but two things come to mind:
1) my testing was done on tubulars, and the tubulars had lower Crr for lower pressures.
2) tubulars resist pinch flats better. higher pressure is a way to also reduce pinch flats (for clinchers). tubulars don't need to rely on that.
for similar construction, tubular sidewalls are more supple for the same pressure (better handling)...but this doesn't mean it's got lower rolling resistance. it's really the opposite of conventional thinking.
it's part of the reason why we can run lower pressures for tubulars in CX than we can if we were running tubes or tubeless. pinching/burping a tire (for tubes or tubeless) is more an issue at lower pressure. [of course for tubeless, the sidewall is structural and therefore stiffer, so there's always a difference in feel for a given pressure. this is particularly notable for MTB tubular vs tubeless.]
i appreciate the points in your post.
we're not even getting into differing construction of tires...