Skinny higher pressure tires make a difference

[Decrepidude]

As the inflation pressure divided by inner area of the tyre means the force acting on the tyre wall. it is obvious the lower pressure in thicker tyres will produce the same force as higher pressure in skinnier ones. (Or, am I getting that wrong?) In other words, same air force exerted on the tyre walls means the same tyre stiffness, or the same rolling resistance.

What, however, about the fact thicker tyre has bigger contact area with the surface than the skinnier one? Is it not reflected in increased friction? (It is a question because I don't know the answer).

There also is another factor: The tyre tread. I can easily compare 1.75" Smart Sam (a slightly aggressive multi-terrain tyre) to 2" Electrak (slick but soft rubber tyre). Under the same road conditions, I'm achieving high speed on Electraks way easier than on Smart Sams! The difference is quite dramatic. (I use Smart Sams just because they allow me riding in light off-road while Electraks are road only tyres).

I believe the "nubbiness" and extra weight of the Sams wastes a lot of energy. They feel squidgy when riding on pavement, and all that "traction" affects rolling resistance.

 

[Sierratim]

I believe the "nubbiness" and extra weight of the Sams wastes a lot of energy. They feel squidgy when riding on pavement, and all that "traction" affects rolling resistance.

I would agree that switching from smoother tread to the Smart Sams does increase rolling resistance, but it wasn't what I would call "a lot". There was a bit of extra noise when on pavement, which is lost energy slowing you down. On the downhill from my house my unassisted peak speed dropped from ~34mph to ~32mph. I didn't notice a drop in range, though there must have been a few miles lost.

I ended up going with a Smart Sam on the front tire for added control on gravel and dirt and with a smoother tread on the rear. This is the OEM Specialized Trigger Sport for now. I ride with them both at ~50psi. Seems to be a good combination of low-ish rolling resistence and good steering control. If the great majority of your riding is on pavement this setup probably isn't for you, but it's working well for me.

 

[Decrepidude]

I would agree that switching from smoother tread to the Smart Sams does increase rolling resistance, but it wasn't what I would call "a lot". There was a bit of extra noise when on pavement, which is lost energy slowing you down. On the downhill from my house my unassisted peak speed dropped from ~34mph to ~32mph. I didn't notice a drop in range, though there must have been a few miles lost.

I ended up going with a Smart Sam on the front tire for added control on gravel and dirt and with a smoother tread on the rear. This is the OEM Specialized Trigger Sport for now. I ride with them both at ~50psi. Seems to be a good combination of low-ish rolling resistence and good steering control. If the great majority of your riding is on pavement this setup probably isn't for you, but it's working well for me.

That is exactly right... I really don't need alot of off-road traction, and just feel like response on pavement or cement is sluggish, it's harder to propel without PAS, plus I also want to save weight where I can by using a narrower (29 x 2.10) size with lower profile. What actually bothers me the most is how the tread kicks up a huge rooster tail in bad weather, spraying crap all over the place. I transitioned from a hybrid with skinny tires and hardly any spray...

 

[Marcela]

There is a happy medium for psi. Re the relationship of the crown or center of a tire with the outside of the contact patch, if a tire is over inflated the outside is always trying to play catch up with the larger diameter of the inside crown of the tire (might be reversed), and this will also develop more heat into the tire which equates to more drag, as race care drivers know an overinflated tire will develop heat quicker than correctly inflated tire. An underinflated tire is vice versa and develops more drag also.

May not apply in all situations of course as there are tradeoffs when the tire gets so skinny it's running on a very small contact patch and all the above becomes moot.

 

[PedalUma]

There is a happy medium for psi. Re the relationship of the crown or center of a tire with the outside of the contact patch, if a tire is over inflated the outside is always trying to play catch up with the larger diameter of the inside crown of the tire (might be reversed), and this will also develop more heat into the tire which equates to more drag, as race care drivers know an overinflated tire will develop heat quicker than correctly inflated tire. An underinflated tire is vice versa and develops more drag also.

May not apply in all situations of course as there are tradeoffs when the tire gets so skinny it's running on a very small contact patch and all the above becomes moot.

I am crazy about my Big Apple 29'x60's. I run them at about 55-60psi. They are fast on broken pavement, tarmac, and gravel.