Tires and pressure

[booooda]

Does this make sense: Higher the TPI (threads per inch) the lower the rolling resistance or better the tire? And, the higher the pressure, the faster the bike. I have 120 psi tires now, so if I went to 150 psi or higher, I should, go faster, correct?

 

[artmichalek]

Yes and no. Mostly no. About the TPI, in theory a higher count will make for a more flexible tire, which gives a smoother ride and loses less energy going over small imperfections in the road. There is however some variation in how this is measured. Some manufacturers give the threads per inch of a single ply and some give the total for all of the plies. Unless you know exactly how it's measured, this number isn't very helpful. As for pressure, the higher is faster rule only applies on a pretty much ideal surface, like a track. On the road, the optimum pressure depens on a compination of surface, tire casing stiffness, and rider weight. For most cases rolling resisance bottoms out around 110 psi. Any higher and resistance starts to go up again.

 

[booooda]

So, I'm 6'1" at 186 lbs. A 120 psi tire would have the same effect as a 150 psi tire? Or, from what I gathered from your comment, the 150 psi might perform worse. The way I view it is, the 150 psi wouldn't compress as much under my weight, leaving a smaller tire foot print on the ground, hence less rolling resistance. My 120 psi itres currently flatten slight under my wieght. Bigger footprint, more resistance. Thoughts?

Thanks for the TPI lesson. You see tires ranging from 170 tpi to 40 tpi and both are good tires.

 

[artmichalek]

The way I view it is, the 150 psi wouldn't compress as much under my weight, leaving a smaller tire foot print on the ground, hence less rolling resistance. My 120 psi itres currently flatten slight under my wieght. Bigger footprint, more resistance. Thoughts?

This has been picked apart here and elsewhere, so I'll give you the short answer. On an ideal surface, all of the rolling resistance is coming from hysteresis (internal friction in the rubber). So in that case, less deformation is faster. When you get out on the road the surface has a bunch of little bumps. At a lower pressure, your tire is deforming around these bumps. How efficiently it does this goes back to the flexibility of the casing. At a higher pressure, the tire isn't deforming much at all. So the tire is riding up and down over each of these bumps. There's a break point where the increase in resistance from going over every bump beats out the savings from not having the tire flex. As for the idea of a "120 psi tire", this is the recomended maximum. You don't have to inflate them all the way. Typically anywhere from 80-110% of the rated pressure is the safe range.

 

[booooda]

Thanks!!
I pretty much do everything to the max. So, 120 psi works for me. Your perspective on this is much different from how I would have/did view it. It makes sense though.

 

[531Aussie]

I'm about the same weight as you, and I like a little more pressure in the rear tyre; sometimes as much as 140 on smooth roads. If I'm grinding along, staying seated, moderate pressures are fine, but during "stomping" and sudden accelerations, I feel that a rear tyre at 110 or 120psi is too squishy

It's worth a try, but I recommend something like Vredestein TriComp tyres which are rated to 145psi. The high-end Vittorias also have high psi ratings, but they're usally more expensive

 

[domaindomain]

And lighter tyres would help too - less rotating weight and, something like the top end Veloflex tyres would tend to be better materials and construction too.

A 130g tyre with a latex tube does make a noticeable difference - not sure you can put a mph / time on it though!

Cheers