What tire pressure should I run?

[JockoT]

The grip on my wee FIAT was mainly down to the very soft compound. Just wore out far too quickly, and cost 3 times the price of ordinary tyres. Eventually changed the wheels and tyres for the normal ones fitted to the 126.

[CapriRacer]

Originally Posted by Old Tele man

A 'wide' tyre spreads the weight across (perpendicular to) the direction of travel.

A 'narrow' type, still has some width, but tends to spread the weight more fore/aft (in the direction of) the direction of travel.

There's a 'cross-over' point where the AREA (width×length) of the tyre's 'contact patch' with the road surface reaches a minimum value: That's good for LRR but bad for braking.

Overall, a physically wide tyre presents a larger AREA of resistance than does a narrow tyre; so aerodynamically, a narrow type is better than a wide tyre of equal diameter.

Considering BOTH aero- and pavement (frictional) losses, the narrower tyre presents a smaller aerodynamic and frictional loss than does an overly wide (ala' drag racing slicks) tyre; but, without moving upward to a larger diameter wheel to regain equality in the total fore/aft contact area, the risk of reduced braking ability becomes a problem.

I like this. Let's see how it plays out with the data we have.

If you will recall, I took some RRC data for the same tire model for different sizes and derived an equation:

Barry's Tire Tech

So if I take 3 different sizes: P235/75R15. P245/70R15, and P255/65R15, all 105 load index, then what ought to happen is the wider, lower aspect ratio tires ought to have a higher RRC.

And just the opposite happens. The wider, lower aspect ratio sizes are ever so slightly worse: 0.15% and 0.3%. Yes, those are really small numbers.

Maybe that is within the error range, and maybe you could call those the same, but I don't think this supports the contention.

Do I have a theory as to why this is so? Perhaps it is because the width of the tread (THEE largest contributor to RR) is some fraction of the overall width of the tire - and that translates into a tiny bit smaller amount of tread rubber in wider tires.

I wish we had more data than this single study. More data would help refine the regression - and might even change it enough to support the above, but that's the data we've got.

[stefanv]

Originally Posted by Old Tele man

There's a 'cross-over' point where the AREA (width×length) of the tyre's 'contact patch' with the road surface reaches a minimum value:

Just thinking out loud, but ...

Wouldn't the area of the contact patch always be equal to the weight the tire is supporting divided by the tire pressure, regardless of the geometry of the tire (and ignoring the contribution of the tire's ability to support weight on its own, without relying on the air pressure inside it)?

During braking, I would expect most of the friction to come from the front of the contact patch, and during cornering, most of it to come from the sides of the contact patch. Thus, wider tires would be better for braking, and narrower tires better for cornering.

We have a few automotive engineers working at my company, at least one of whom has expertise in tire dynamics, so I may need to ask them.

[gone-ot]

Due to the physical ply orientation inside the tyre rubber, the tyres ability to "flex" is more in one direction than the other, and "flexing" takes energy, and generated energy is a power loss, ie: resistance.

[ThermionicScott]

Originally Posted by JockoT

From what I have read, increasing the tyre pressures reduce the grip and the braking ability. By increasing the inflation pressure you reduce the tyre's footprint which is your only contact with the road. Anyone who has had a puncture and tried to brake will know how the deflated tyre grips and pulls you to that side. Okay, higher pressures increases the steering response but again that is because it has reduced the grip. It is the reduced grip that gives you the increased fuel figure.
Here in the UK there is a lot written about over-inflated tyres increasing wear in the centre of the tread.

That picture looks super fishy to me. It's artifically blurred down the center, and you don't see a great reduction in depth at the top. Almost like they couldn't easily find evidence of radial tires wearing out in the center to go along with their article, so they had to make something up.

[gumby79]

This is 70k on a 50k tire adjusting air to suite the current load. 5k rotations. 65-80psi front 50-80 rear first # was my empty seting 2nd is an up to based on load weight balance.

No funny business it is ~ 20k mi extra wear in the centers.

I ran the air at 80 front (sidewall max/recommend psi) and only the centers were part of the contact patch. Shows here.

100mi @70mph @80psi on 3800lbs front.
And more obvious on the rear @70psi 3000lbs

Notice the mold injection tits are warn off completely yet the sholder block tits on both sides are intact , shows what is touching the road with how much froce/heat generation . The sholder tapers rabidly as indacated by the verried leangth of the tits , shortest were the closest to contact closer to the center tread. The color also shows the transition from contact to Airborne.

[Prophecy99]

Originally Posted by ThermionicScott

That picture looks super fishy to me. It's artifically blurred down the center, and you don't see a great reduction in depth at the top. Almost like they couldn't easily find evidence of radial tires wearing out in the center to go along with their article, so they had to make something up.

hahaha your funny, yes thats photo shop but simple a demonstration, they dont need a real picture for the proof.


+1 to gumby79 , i had lowprofiles run really bad center wear from 43 psi (50 psi max on side wall), so much that it was about 2/32 in center and 4-5/32 on outside, i didnt pass inspections because of it.

[stefanv]

Originally Posted by Prophecy99

hahaha your funny, yes thats photo shop but simple a demonstration, they dont need a real picture for the proof.

Ah yes, this post-evidence society we live in. :-)

[Frank Lee]

Center tread wear from over-inflation can happen; I bet these have a nice low r.r.!

[nemo]

Your just not cornering hard enough.