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Old 11-08-2011, 04:02 PM   #1
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Tire Pressure Questions

Ok, one thing I have noticed on here is how every says to inflate your tires to the sidewall maximum. This is bugging me because I've always heard that this is not the thing to do, the center of the tires will wear out quicker than the outsides. Why would y'all pump the tires to their sidewall max if you are going to be replacing tires more often?

And what would the procedure be for putting wider than OEM tires on stock rims and getting a flat contact patch? I've got 10.5" wide tires and plan on going up to 12.5"
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Old 11-08-2011, 04:50 PM   #2
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Re: Tire Pressure Questions

It is commonly said, but mostly incorrect. Here's the lowdown, from theory and from lots of experience:

- With extreme overinflation, it happens a little bit...but actual tread life isn't reduced. Let's say you were going to get 40,000 miles before your evenly worn tread was down to 2/32". Instead, overinflated, you're getting a little center wear; 40,000 miles later your center is down to 2/32" and the rest of the tire is at 3/32". Less rolling resistance means less wear.

- In most cases it just doesn't happen. Radials can't stretch much, the steel belts make a hard limit. Bias ply tires probably could.

- Looking at tires as you walk down a parking lot, you'll notice lots of worn edges, lots of even wear, and very few worn centers. Extra inflation helps prevent edge wear from cornering too fast (something I do a lot of, for fun and for saving fuel).

- Even if you DID get severe center wear, it'd be pretty harmless. Tread needs to channel water to the sides; a ridge of baldness down the center wouldn't affect that. When you go to turn, it's the edges that need to bite the road.

I had a brand new 2008 VW Rabbit with Continental P195/65-15 tires. I ran them at 51psi instead of the 30 that VW specified. I got more miles from them than other Rabbit drivers reported. At the end, the center was down to the wear bars, the edges were barely above the wear bar, and I only replaced them because at the end of the lease they would have charged me more for tires than I paid on my own. It's not like the center was bald and the edges were above the wear bars.

I have been through a few sets of tires in my truck's 200,000 mile history. I've never had any significant center wear. I don't go all the way up to the 80psi max in the rear because, with the bed empty and rain on the road, traction is reduced at 80. 70 works fine rear, and 80 is good in front. GM specifies 35.

My experience is similar in various other vehicles.

When it's all said and done, you need to be comfortable with your choice. If you're not comfortable with experimenting with high pressure, don't do it. We promote it so heavily because it is free (guaranteed to pay for itself), effective, and easy.
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Old 11-08-2011, 08:20 PM   #3
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Re: Tire Pressure Questions

wider tires on the same car will get less traction and more mpg usage... you have a bigger surface area with the same weight pushing down so less contact patch IE rain and snow you will slide everywhere.

Cow i agree with what you've said and what you've personally noted - whats your feelings on bumping up SUV (2 door s10 blazer) tire PSI with a rather large sidewall? (235/75/r15) on 6" wide rims (not factory rims but same 15" diameter and backspacing is nearly identical, farther out by 1/4" and rims narrower by 1"

it rides different but im wondering if i should bump the PSI up. I have em at i THINK 32 33 now, maybe 35 psi.
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Old 11-09-2011, 04:23 AM   #4
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Re: Tire Pressure Questions

First, I am not a big fan of using higher inflation pressures - that is, more than a few psi over the vehicle tire placard. So with that disclosure here is my take on the subject:

1) One of the problems with citing the sidewall maximum as a usage pressure is that it isn't founded on anything consistent. While the law requires that it be there, the law doesn't say how it is to be derived (or calculated or tested for, etc.) As a result, there is a wide range of values for what should be very similar products.

2) Center wear? While it is true that a tire with higher inflation pressure tends to bulge more in the center of the tread, observing the effects of this is clouded by the fact that this is but one of many things that affect tire wear. Some things - like alignment - have greater affects, so it is difficult to judge what affect inflation pressure is having. There have been reports of center wear with folks using elevated inflation pressure, so it appears the affect is there.

3) Wider tires on the stock rim? Every tire size has an allowable rim width range - so you need to check this. If you use a rim that is too narrow, then the tread will tend to arch and that will cause center wear as well as directional stability issues - that is, the tire will tend to follow the grooves and ruts in the road - not to mention be sensitive to cross winds.

4) Speaking of directional stability, using elevated inflation pressures tends to make tires 'tramline' - sometimes referred to a 'groove wander'. This again is not a 100% thing as the effect is tied to both the spacing of the grooves in the tire and the spacing of the grooves in the road. It's less clear, but there may be reports of this phenomenon occuring in rutted roads a well.

5) Traction? This is also hard to sort out as well. Larger footprints obviously help traction in pretty much all conditions, with a couple of exceptions: hydroplaning, and in snow where the tire can penetrate the snow layer to get down to the pavement. (Pavement always has more traction than snow) If you are using a tire that is too wide for the rim, you've got a problem that can't be solved with inflation pressure.

That answers the questions that you asked. Like was said earlier, you need to make an informed choice - and part of the problem is that there is a lot of mis-information out there.
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Old 11-09-2011, 06:11 AM   #5
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Re: Tire Pressure Questions

^On those comments:
1: Agreed. The way I see it, that number is merely a good maximum limit for your experiments. Beyond it you may be exposed to legal liability in a crash. As for how it's derived, there actually is some consistency; the Rubber Manufacturers Association and Tire And Rim Association set standards for maximum inflation, load-inflation tables, etc.

5: As you noted in other parts of your post, there are often more variables. In the case of inflation between the car's number and the tire's maximum, it seems that footprint is usually not heavily affected. Additionally, footprint isn't the only issue; as you take a turn, the tire deflects sideways, depending on the outside edge for traction, and increased inflation increases traction under that circumstance (as does a reasonably short sidewall, a trend that has probably been taken way too far for cosmetic purposes).

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Originally Posted by VetteOwner View Post
wider tires on the same car will get less traction and more mpg usage... you have a bigger surface area with the same weight pushing down so less contact patch IE rain and snow you will slide everywhere.
If we use standard math for it, contact patch area should depend only on weight and inflation. Contact patch shape would change with width of the tire. Surface area of the tire outside of the contact patch is irrelevant.

Additional variables have been pointed out in previous discussions that make me think contact patch doesn't always obey that math, often limited by steel belts that will allow shrinkage but won't allow stretching.

Quote:
Cow i agree with what you've said and what you've personally noted - whats your feelings on bumping up SUV (2 door s10 blazer) tire PSI with a rather large sidewall? (235/75/r15) on 6" wide rims (not factory rims but same 15" diameter and backspacing is nearly identical, farther out by 1/4" and rims narrower by 1"
235's are pretty wide for 6" wide rims, I believe 235's specify 7 to 7.5 inch rims, don't they? Certainly the tall sidewalls mitigate the effect.

Ok, looked it up. A BFG Long Trail T/A Tour (common average tire) in that size specifies 6 to 8 inch rim width, so you ought to be fine.

Well, there's only one sure way to find out how increased pressure will affect them.

The great thing about experimenting with tire pressure is that it's free and (except extreme circumstances) harmless. Whatever your concerns are, you can find out easily enough.
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Old 11-09-2011, 11:51 AM   #6
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Re: Tire Pressure Questions

Quote:
1) One of the problems with citing the sidewall maximum as a usage pressure is that it isn't founded on anything consistent. While the law requires that it be there, the law doesn't say how it is to be derived (or calculated or tested for, etc.) As a result, there is a wide range of values for what should be very similar products.
There's more consistency and guidelines there than from the car's placard. The gen2 Prius would wear out the tire edges faster when Toyota's recommended psi numbers were followed with the OEM tires. If even wear isn't possible on factory tires at manufacturer pressures, how can you have faith in those numbers with a different tire model or brand.
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Old 11-09-2011, 12:17 PM   #7
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Re: Tire Pressure Questions

Data from the same sources I cited earlier is used by the manufacturers for that placard too. They aren't so concerned with wear pattern, though. They use the minimum pressure that won't cause tires to explode so they can sell cars with the smoothest ride possible.
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Old 11-09-2011, 12:23 PM   #8
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Re: Tire Pressure Questions

I always inflate my tires to the max sidewall. It makes tires last longer if I forget to check my pressure.

What really bugs me is grossly under inflated tires. Last week there was a van with a tire with the steel belts showing at the edges, with tread at the center. Obvious wear from under inflation. If I was a cop, I'd have written him up on the spot and called a tow truck.
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Old 11-09-2011, 07:47 PM   #9
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Re: Tire Pressure Questions

Quote:
If we use standard math for it, contact patch area should depend only on weight and inflation. Contact patch shape would change with width of the tire. Surface area of the tire outside of the contact patch is irrelevant.

Additional variables have been pointed out in previous discussions that make me think contact patch doesn't always obey that math, often limited by steel belts that will allow shrinkage but won't allow stretching.
yea, i had 235/70/r15 on my 2wd 4 banger s10 which factory is 205/70/r15 and god help me if it were wet, going around corners and give some gas (maybe 3/4) and the backend would start to wash out as if i were on snow. a few times id be at a stop sign with the front wheels slightly turned and it would slide forward (NOT turn) untill i let off. this is coming from a 96 hp to the ground s10...changed back to stock and rain it was as if i were on dry pavement again.

so yes the shape would change of the contact spot but generally have the same tire to surface area well area. but i think the shape of the contact spot greatly effects it. and yes on snow WITH PAVEMENT UNDERNEATH narrower is alot better snow with mud underneath wider is better (like snowshoes vs tennis shoes) chevette with its pizza pan tires could go anywhere while bigger tired cars would spin. I never had a traction problem with that car on snow at all (except when i would purposely try to fishtail and it would let the front tires slide haha)


Quote:
235's are pretty wide for 6" wide rims, I believe 235's specify 7 to 7.5 inch rims, don't they? Certainly the tall sidewalls mitigate the effect.

Ok, looked it up. A BFG Long Trail T/A Tour (common average tire) in that size specifies 6 to 8 inch rim width, so you ought to be fine.
yea thier on the bare minimum but seem fine. I bumped em up to all 36 from 32/33 tonight. seems to roll better and steer easier but could be placebo effect. we shall see when i run a full tank thru it. Ive got a good baseline in a log book i keep in it of it being 32.
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Old 11-11-2011, 08:58 AM   #10
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Re: Tire Pressure Questions

I asked this due to some premature center wear in my BFG at/ko's after running them at 43-45 psi front and back. They have a max psi of 50 , so it was puzzling to hear that y'all run max psi in your cars. The tires were manufactured in 2003, though, so I think that might have something to do with how they are wearing, ride, etc. Y'all are freaking amazing with the wealth of knowledge that I see on here, its just incredible!
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