Re: Effect of extra pressure on wide tires?

What is the effect on tire wear, track performance, and daily performance from having say 5 lbs extra air pressure in our wide tires?

In the "old days", too much pressure equaled wear on the tread center. Low pressure, the tires wore on both edges. But, with these super wide tires and the negative camber, particularly in the rear, what does this extra pressure do?

Peace
Bruce in Philly

If not, its nto extra pressure,. They are designed to run that way.

I am told by tire engineers (Pirelli, if you are interested) that modern tires do not have nearly the same reaction as older tirs to inflation changes. They are largely NON PNEUMATIC with regard to shape. So center wear if oftne not only an nidicator of over-inflation.

now, you also asked about camber. Yes, negative camber places more wear on the inner edge. In my Audi i have found the effect top be smaller than most would imagine. On the other hand, it reduces outer edge wear in hard use, so the next effect onlife may be positive. It is for me.

Grant

Grant

gee-lenahan-at-gee-mail-dot-com

The reason I asked this was twofold:

1) I will be doing tons of highway miles coming up soon and I was curious if there was something I could do to tire pressure to lengthen the life of my tires. Since I won't be blasting through country curves..... why not save a few bucks and get extra tread life? A few years ago... um maybe 10(?).. I heard from a few sources that raising tire pressures by a few pounds will lengthen the life of these radial tires.
2) My last set of rears wore out on the inside at what appeared to me to be at a very disproportional rate. I was totally unaware that I had some exposed belts!!! The outside and centers looked OK to me.... about time to change.... then on the rack I saw the insides were stupidly worn away.

I got an alignment and had them take out some of the negative camber in the rear... not by much.. but now they are -1.32. The tech said that this should allow the tire to wear more normally and he said these really wide tires 295/30 19" are sensitive to wear with just small differences in camber. True?

So with this a maybe running a few pounds more (manual state 40 lbs in rear), I can get more and longer tread wear.

Peace
Bruce in Philly

maybe we should beat each other up, for good measure.

I'm embarrassed and mortified to say that, I too, unknowingly allowed the inside tread of the rear tires to wear down to the steel belts, on my BMW wagon. It was discovered when the car was on a hoist to sort out a suspension noise in the midst of a long trip.

In my defence, before starting on the trip, I'd taken the car to an independent (who let me down not once, but twice - two strikes and you're out), because I was concerned about the noise (the source of which I couldn't find). He couldn't locate the source of the noise, nor did he notice the worn tires.

Moral of the story - get the car up on a hoist, or stands, or whatever, and CHECK THOSE TIRES!. Tread wear can be (and often is) uneven and what looks to be a good tire on a cursory examination, may be anything but, on closer examination. And don't be content to rely on someone else - in the end, it's your safety.

I'd add that I change my tires (mounted on separate wheels), using my own two hands, in the spring and fall (winters/summers) and had checked them in the spring. But clearly that wasn't often, nor good, enough.

Oh, and the noise? Worn sway bar links. The sound is now permanently stored in my 'bad car sounds' memory bank.

Quote
Bruce In Philly (2000 S Boxster, now '09 C2S)
The reason I asked this was twofold:

1) I will be doing tons of highway miles coming up soon and I was curious if there was something I could do to tire pressure to lengthen the life of my tires. Since I won't be blasting through country curves..... why not save a few bucks and get extra tread life? A few years ago... um maybe 10(?).. I heard from a few sources that raising tire pressures by a few pounds will lengthen the life of these radial tires.
2) My last set of rears wore out on the inside at what appeared to me to be at a very disproportional rate. I was totally unaware that I had some exposed belts!!! The outside and centers looked OK to me.... about time to change.... then on the rack I saw the insides were stupidly worn away.

I got an alignment and had them take out some of the negative camber in the rear... not by much.. but now they are -1.32. The tech said that this should allow the tire to wear more normally and he said these really wide tires 295/30 19" are sensitive to wear with just small differences in camber. True?

So with this a maybe running a few pounds more (manual state 40 lbs in rear), I can get more and longer tread wear.

Peace
Bruce in Philly

One is be sure the car is properly aligned. I can't stress this enough. My experience over uncounted sets of tires is bad alignment will eat rear tires in sometimes around 8K miles while by way of comparison a good alignment on the same car with the same brand of tires and me driving the car the same way can have that tire life up to over 20K miles with in one case enough tread depth left (3mm) to qualify one tire that picked up a nail be covered by tire hazard insurance.

Camber plays no role in uneven tire wear. Toe in is the critical setting. Sure, camber wants to be right but even though these cars have a lot of negative rear camber and this looks like it causes inside rear tire wear it does not. Observe a Porsche going down the road. The rear tires are flat and parallel to the road surface.

Ok wheel/tire alignment is critical.

Next comes tire inflation. I have found with my Boxster using the factory recommended inflation pressure for the front and rear tires to give the best ride, handling, with no compromise in tire life. The 996 recommendation is a bit tricky as the inflation pressures on the gas cap lid are for a fully loaded car and are too high for a car lightly loaded. So I drop those pressures by 4psi based on what the techs tell me. The ride is much less harsh and tire life is unaffected by the lower inflation pressure.

With a proper alignment and tire pressures tire life will be as good as it can be and all during your time in the car the steering, handling, and ride will be as good as they can be. And being a Porsche they can be really really good.

I always have my cars set up for near zero toe anyway. I do nto see the benefit of toe-in.

Grant

Grant

gee-lenahan-at-gee-mail-dot-com

the wheels/tires assume the proper and desired geometry. A bit of toe in with the car on the alignment rack turns in to less toe in maybe zero toe when the car is underway. No toe in can turn into toe out under the same conditions.

Firmer means less give and the bit of give may not result in that much toe out.

Besides in some cases toe out is desired to make the car a bit quicker to react to steering wheel input.

Then again, under braking when the front end drops some the front wheels/tires could then have zero to even a bit of toe in and this makes the car less twitchy under braking.

IOWs, alignment is a trade off of tire wear, car feel, handling and other things.

My best advise is to make sure the alignment is within spec and then drive the car. If in your opinion you want to move to a more aggressive alignment and possibly give up say tire life for quicker reaction that is your call.

I leave the details of implementation up to Chris. He also has the car set up for optimal geometry with one person - the driver - in the car.

Grant

Grant

gee-lenahan-at-gee-mail-dot-com

As to the sensitivity of modern tires to camber - re inner wear - true. They are very rigid and therefore more of the wear occurs on the inner edge. If they were perfectly rigid, only the inner ridge would touch the road and the rest would be (slightly) in the air. Think about it.

As noted, extra air does not make modern tires baloon out as much as you would think - in fact very little. But it might help *some*. Might not too.

That said, Porsche's reccomndations are desigend to dial in "safe" understeer. For my car that is 36R/29F. I run mroe like 34R/31F (cold) on the road and 35R/33F (hot) on the track

Grant

Grant

gee-lenahan-at-gee-mail-dot-com



Edited 1 time(s). Last edit at 08/29/2013 02:26PM by grant. (view changes)

Modern wide tires behave differently than older narrow ones. Wide tires had a concave tread when deflated and require extra pressure to flatten the tread. A lithe more may reduce edge wear.
Toe and camber will affect edge wear more.

Ed B

Both the Owner's Manual and the door sticker state 19" tires get 29/30 (at 68ºF*) but the TPMS, when reading those numbers show the Fill Info to be "-1." For much of the year I had them set to a nominal 30/31** and have noticed more wear to the centers of the rear tires. For the remainder of their lives I've been using 29/30.

* I believe the manual gives 68F as the reference temperature, but not sure why that is a factor because as far as I know, tires should be set to their nominal pressures when they're "cold," meaning before any usage heating.
** Something that bugs, me, and the dealer says they can't do anything about, is that the TPMS doesn't display 31 and 34 psi. I tried using "atmospheres" instead, but can't get used to the units. (I have the clock set for 24 hour display, though.)

wear more in the center. At first I thought they were overinflated and so tried a few PSI less air. No real difference in the wear

Then I thought well I'm using the car at high speeds (highway speeds) a lot and maybe the tires are growing from centrifugal force and more air would reduce this. It did not. But it was with more air I discovered the front tires introduce a vague wheel/tire unbalanced like vibration.

So I settled on the recommended cold tire inflation pressures and while the rear tires do wear a bit more in the center it is not enough to make a difference in tire life overall and tire life has been real good so I don't worry about it.

Oh, I forget now but after more than a few sets of tires and thousands and thousands of miles of monitoring tire wear and tire pressures and trying different pressures I did come across info that stated wear in the centers of the rear tires is normal even for tires properly inflated and aligned. And I also learned my high speed driving was no where near the speed at which the tires would grow.