How much of an increase in mpg can I expect with overinflated tires?

[brucepick]

I've read that on modern steel belted tires additional pressure doesn't cause the center bulge as on non-belted tires.

The proof seems to be in the anectdotal reports of drivers who have run their tires at considerably more than max sidewall pressure for tens of thousands of miles with no uneven wear detected. In fact the additional pressure reduces rubber flexing. Flexing creates the heat that softens rubber and destroys tires, and those runnning higher pressure long term report tire life at least as long as expected if not longer.

[trebuchet03]

I've read that on modern steel belted tires additional pressure doesn't cause the center bulge as on non-belted tires.

Yep -- a few months ago, I remember seeing a picture of someone's tire with a broken belt.... The difference belted vs. no belt is VERY clear.

An easy test would be to roll your tire onto flexible measuring tape, then pump up and measure the diameter change

[skewbe]

One could also paint a stripe across the tread and drive around "normally" for 25 miles or so with them pumped up and see if they wear evenly or not.

[bowtieguy]

Originally Posted by skewbe

One could also paint a stripe across the tread and drive around "normally" for 25 miles or so with them pumped up and see if they wear evenly or not.

brilliant skewbe! just inflated mine to 60psi(from 44) and i'm gonna try that. thank you.

[trebuchet03]

Originally Posted by skewbe

One could also paint a stripe across the tread and drive around "normally" for 25 miles or so with them pumped up and see if they wear evenly or not.

ha, that would totally work too Actually, that's one method to check gear lash on really really big gears -- spray on some paint and then run the machine for awhile... Then check how the gears are meshing.

[garyhgaryh]

I would have to disagree.. Both overinflating and underinflating tires will cause premature wear. It's not just underinflating it.

I've seen pictures of over inflate tires and it's wear after 20k miles from tire companies. The middle budges out. Yes, the steel belts are there and the one in the picture was steel belted.

Gary

Originally Posted by skewbe

Overinflating is not really a cause of premature wear, low pressure is the major cause of that.

It may affect wet handling adversely, so practice. Dry handling effects are good, the sidewalls are nice and stiff and do not flex as much in the corners (them guys that drive on two wheels use like 100 psi).

Them steel belts keep things lined up pretty well so there's no appreciable bulging from overinflating.

[DarbyWalters]

The difference is not readily discernable to the eye...use a pyrometer and take temps across the tread...you WILL see a difference. The higher temps in the middle show an overinflated tire and uneven wear will result. You just have to weight the cost savings verses the wear factor...you have to assume that the vehicle is properly aligned

Higher pressures are used in Auto-X to keep tire squibb to a minimum...you dont' have time to build up heat on a run

Lower pressures are used in Road Racing...heat builds and psi increases

For the street you are pretty safe unless you run underinflated...BOOM!!!

[skewbe]

sigh, we can go through this all again, or we can re-read the previous debates:

http://www.gassavers.org/showthread.php?t=1727#6

People here are reporting even tire wear with lots of pressure and 10s of thousands of miles. Many of the overinflated tires here are pretty skinny to begin with and being steel belted radials they seem to hold their shape better with extra pressure.

You have steel belts going in every direction, if it were to bulge out in just one part of the middle tread it would have to pull in from another part of the middle tread, but that part of the tread also has extra pressure forcing it into the shape defined by the steel belts.

I know it seems counter-intuitive and unconventional, maybe the center bulge theorists are a hold over from before steel belted radials? I don't have enough miles to consider myself a first-hand expert, just trying to think it through in the mean time and make sense out of the reports of "overinflating works".

[Ernie Rogers]

Yes, "inflation stiffness" is greater.

When you push down on an uninflated wheel and tire, it won't even support your body weight-- you can neglect the stiffness of the rubber in determining the area of the contact patch, so--

F (load on the tire) = P (tire pressure) x A (contact patch area)

Let's try some numbers. A 195 wide tire is 195 /25.4 = 7.68 inches wide at the tread. The load on one tire is about 900 pounds. At 40 psi, the area of the contact patch is--

A = 900 lb /40 psi = 22.5 square inches.

Divide that by the tread width to get the other dimension--

22.5 /7.68 = 2.93 inches long.

To make 2.93 inches of curve flat on a 25 inch diameter tire, you have to deflect the center of it by 0.11 inches.

The harder it is to get the tread surface to move in that amount, the more energy the car loses as it rolls. That means soft rubber is better, and a thin tire is better. "Low friction" rubber and tire construction is also better, and that's what you are paying for in an "energy tire."

Ernie Rogers

Originally Posted by trebuchet03

The offs of a blowout while inflating are quite low (unless you've got a old crappy tire).... It's more of a blowout when you hit a pothole. In any case, the max pressure is based off of a minimum factor of safety.

So I did a quick search... and found a patent for bias ply tires... It makes a reference that says the steel cord safety factor should range between 4 and 11, 7 being a target. Now that's not directly related to inflation pressure - but should be related to hoop stress (P*D/2).... Again, this isn't really an equivalent - and I don't know if the mentioned factor of safety applies to traditional radially belted tires... Really, I doubt I'll find the actual number anywhere as it's probably trade secret.

----


So that makes sense and all...



However, (just thinking intuitively now) - shouldn't we be comparing the deflection delta? That is, a shorter (and stiffer) sidewall doesn't deflect as far when conforming to the road (stiff/bumpy ride) as compared to a larger sidewall with a lower k value...

I am also thinking of a case of a tire with wheel run out (out of round). So we basically have a tire in the shape of an ellipse (a very minute one though). The areas with the shorter sidewall will have more stiffness and translate that into the the vehicle suspension - as compared to the higher sidewall. So that, combined with the assumption that the normal force is constant leads me to the difference in taller versus shorter sidewall is the k. Thus the deflection for the stiffer (shorter sidewall) should be less than the taller sidewall...

So that paragraph above would mean that sidewall stiffness AND inflation pressure are variables that determine contact patch size (something tells me stiffness from the tire is much less than inflation stiffness)

Of course, this could be *** backwards - and x is constant and the force changes (although, intuitively - that seems wrong or I am missing a key point).





In any case, it doesn't matter for me either way. I have no intention to get a shorter sidewall I just like this sort of discussion

[omgwtfbyobbq]

IME it really depends on the tire. My passenger car tires have shown no wear after thousands of miles, but my light truck tires definitely showed the lower tread in the middle and alla that.