"Inflation pressure does not affect grip": Autospeed article.

[orange4boy]

It's also a truck tire which is not applicable here.

You guys are aguing about whether or not overinflation increases or decreases traction when the quality of the tire in the first place is going to have more of an effect.

Good point.

Another point is that higher inflation pressures (up to max sidewall) increase the life of the tire so even if your centres wear out a bit faster (and I seriously doubt the would) you should still be ahead of the game. If in doubt, buy a good tread depth gauge and keep an eye on them.

[PaleMelanesian]

I have 30,000 miles on my tires. They're inflated to above sidewall max. The tread is worn down to about half, and is completely even across the tire. There is no bulging out in the middle of the tread.

I saw the same on my van - 60,000 miles when I replaced the tires, and the tread was worn down evenly all the way across. Inflated at sidewall max pressure.

I think the LT part above is a significant part of why the tread bulged in the middle.

[TheEnemy]

Originally Posted by orange4boy

It's also a truck tire which is not applicable here.



Good point.

Another point is that higher inflation pressures (up to max sidewall) increase the life of the tire so even if your centres wear out a bit faster (and I seriously doubt the would) you should still be ahead of the game. If in doubt, buy a good tread depth gauge and keep an eye on them.

I wouldn't discount it because its a truck tire, I'm running a mild AT on my pickup and they roll nicely, and run cooler than the tires that came with it. That and they don't blow up when I hit a rock on a rough road. (Cough Firestone cough cough.)

I have seen tires wear faster in the middle, but they didn't wear that much faster to where it caused a problem.

[CapriRacer]

Originally Posted by TheEnemy

I wouldn't discount it because its a truck tire......

I posted the footprint of the Goodyear tire to show how bad some footprints can be. I wanted that to contrast against the 6 footprint matrix I posted earlier.

That matrix is the only time I have seen a set such as that. It was published by NHTSA in “The Pneumatic Tire”. Those are probably the best set of footprints I have ever seen – which is probably why they were published. Plus they do a good job of illustrating some principles:

1) That increasing the inflation pressure increases the pressure at the center of the tread while decreasing the pressure at the shoulders. – obviously there are implications about wear and handling.

2) That increasing the inflation pressure or decreasing the load does not necessarily lead to the shoulders losing contact with the road surface. – which means that the “chalk test” isn’t a good way to determine the inflation pressure for even wear.

3) That the size of the footprint is indeed a function of both the inflation pressure and the load.

And to answer another question that’s been posed:



This footprint is much more the norm – as footprints go - and obviously quite different than that Goodyear footprint.

[dcb]

Thx for the pix. So there is 1900 lbs on that one tire? And that is "the norm"?

[CapriRacer]

Originally Posted by dcb

Thx for the pix. So there is 1900 lbs on that one tire? And that is "the norm"?

Yes. The load carrying capacity of an LT235/85R16 at 44 psi is 1980# - and that's a standard way of looking at footprints.

[dcb]

yes, but it isn't "normal" to be driving around a passenger car that weighs 8000 lbs, even in the U.S. That footprint looks underinflated/overloaded, perhaps considering that ~1/2 of cars on the road are 6psi or more below placard, it is "normal".

[cfg83]

dcb -

Originally Posted by dcb

yes, but it isn't "normal" to be driving around a passenger car that weighs 8000 lbs, even in the U.S. That footprint looks underinflated/overloaded, perhaps considering that ~1/2 of cars on the road are 6psi or more below placard, it is "normal".

So we also need a set of contact-patch-pix of both tires with typical "real world" load weights. My car is ~2400 lbs. My tires are rated at 1356 lbs per tire at 51 max PSI. That's 5424 lbs max. I would say that with all my crap + my wife equals up to 2800 lbs, or 700 lbs per tire. That's a little over 50% of max lbs per tire.

If we do other calculations, I wonder if we'll get a similar 50% "real world" load per tire, at least for passenger cars that aren't hauling heavy stuff around.

CarloSW2

[dcb]

well you can make a pretty good guess looking at capris original image
it is a 205/60/16 tire with max=1356 lbs, before the truck tires started appearing

the top left is 44% load at 45 psi. You are a little heavier but with a bit more psi, so it will probably be similiar.

[CapriRacer]

Originally Posted by dcb

yes, but it isn't "normal" to be driving around a passenger car that weighs 8000 lbs, even in the U.S. That footprint looks underinflated/overloaded, perhaps considering that ~1/2 of cars on the road are 6psi or more below placard, it is "normal".

Well, for one thing, the footprint in question is a for a heavy duty pickup truck - one that can have a GVW over 13K. So the load is at least appropriate for that.

Second, if I want to be able to compare footprints from different tire sizes, I should pick something that can be replicated in a different size - particularly if I have machine limitations - and in this case it was both available time on the machine and a weight limitation of 2K.

BTW, the 2 individual footprints I posted date back to around 1997.

Third, the matrix of footprints is unusual, not only because these are the same tire tested at multiple conditions, but also because none of the conditions is a "standard" condition. It has been awhile since I looked this up, but I recall the conditions match the conditions for one of the standard rolling resistance tests.

A word of caution: DO NOT take these footprints as absolutes. Footprints vary considerably - as the 2 individual footprints demonstrate. Use the footprints for trends.

And I will say this again, because it is important. The matrix of footprints is probably the best set of footprints I have seen. Most tires have considerably more variability in pressure distribution between conditions - and that leads me to suspect the tire has a cap ply (perhaps 2!)