tire wear and all wheel drive

Hi all

Just had to get tire repaired due to nail in same('02 Forester S). Tire guy expressed some concern about my swapping spare for the repaired tire because of difference in tread depth/wear affecting all wheel drive performance. I remember some posts about this issue some time ago, but don't remember what the consensus of opinion was. I personally don't see the point in replacing all 4 tires if one happens to go flat, nor do I have the $$ to do so. Please advise.

Andy

Reply to
Andy K
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On Tue, 12 Aug 2003 23:21:48 GMT, "Andy K" wrote in news:gEe_a.89974$ snipped-for-privacy@rwcrnsc52.ops.asp.att.net:

You shouldn't really be making decisions based on usenet opinions.

The issue is a difference of rotation rates when wheels are not the same circumference. I think it has been shown that front to rear rotation rates should be kept as close as possible, but side to side differences are less important, as the centre differential has more potential for premature wear.

There are two fairly distinct camps of thinking.

One believes the edict of Subaru in the U.S. which states cirumferences should be kept within ½ inch or so. That's only about 0.6% difference.

Another, of which I am a member, thinks that normal tire wear balanced with regular rotations will be just fine. Replacing all four tires is likely unnecessary when only one is damaged. Wheels travel different distances while driving in the city around corners than any small difference in circumference is likely to make.

I have not seen any concrete examples of premature centre differential failure on a Subaru AWD which can be shown to be caused by tire sizes. Given the number of owners who talk about the hundreds of thousands of kilometers they have on their cars, it should be showing up somewhere.

Reply to
Dave Null Sr.

It all depends on the wear of the other tires. I had to replaced a tire at

15k and have 97k on the car now. I do not think my next car will be AWD or 4WD too much of chance of screwing up the system.

Gary

Reply to
GC

On Wed, 13 Aug 2003 00:26:52 GMT, "GC" wrote in news: snipped-for-privacy@comcast.com:

The advantages far outweigh the disadvantages since the risk of "screwing up the system" must be quite small, or we would have lots of truly pissed off owners in here (or elsewhere) complaining about differential failures. We don't. Instead, we hear about how many cars are surviving to very high mileage with no major problems.

Reply to
Dave Null Sr.

Tyre suppliers myth..good for business. If it was so important it would be emblazoned across the appropriate section in the hand book in large red capitals. I don't see it. My Subaru dealer tells me that they've never seen a transmission problem due to differing sizes. I'm on my 3rd Subaru...got caught once by the rubber merchants, took advice after the event and now ignore the spiel when I need a new pair. No problems in any respect and a massive £ saving. Maybe there's a difference in UK/US specs /warranty etc but obviously no difference in tyre suppliers!.

-- Clive Norris Selectron (UK) Ltd

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Reply to
Clive

You will get all sorts of stories from dealers, as Clive says. I had an 88' Porsche with crap for tires, so got a set of Goodyear AT crap from a "locally owned and operated" dealer in Santa Fe, NM (Amigo Ti= re) instead of listening to a friend who said use $$Michelin Pilots$$.

On the freeway going home a rear tire went flat. I went back to said dealer. "Did you run this tire when it was flat?" What the hell did he expect, when a tire lets go on the freeway at 75 there's bound to be some "run flat" time. Sheeze... He absolutely refused to refund even though I pointed to crap on the bead of the wheel that he didn't clean off; previous owner had used some liquid "fix flat" at one time or other. Also turned out to be the stem insert was the leak, found =

when I put the $$Michelin Pilots$$ on.

So, only recourse is small claims court and argue your case. Great to be an American?

Be careful; get good recomendations from club members, Sub dealer, etc.

Then do a lot of Hail Marys.

Don

Reply to
Don Nickell

One of the nice things about having a full sized spare is that you can rotate the spare in and the most worn tire out when you rotate the tires. (You DO rotate the tires, right?)

This keeps your spare at roughly the same size so you can just put it on and leave it on if you ever get a flat and you can go 25% more miles before you need to buy new tires. (woohoo!)

If the flat can't be repaired you'll end up with a new spare that's less worn than the other tires, but this would also be true if you never rotated your spare in. I haven't had to do it, but my plan in this case is to replace the most worn tire with the new spare and then carry on as usual. By the time I rotate again, 3 of my existing tires may be worn down to the level of the current spare and continually pulling out the most worn and spreading the wear across those 4 will let the newest tire catch up.

-DanD

Reply to
Dan Duncan

Ditto, but my Subaru mechanic refused to "safety" the car unless the four tires were within a 1/4 diameter, because he said it would cause it to skid under acceleration in snow as the power to each wheel would be different.

-- Dominic Richens | snipped-for-privacy@alumni.uottawa.ca "If you're not *outraged*, you're not paying attention!"

Reply to
Dominic Richens

I can't comment on snow handling as it's a rarity in my part of the world...and when it does snow the UK skids to a halt. I don't see how the power split to the wheels would be significantly different tho due to tyre wear. Grip yes but surely not enough to cause a problem? C

Reply to
Clive

You will get all sorts of stories from dealers, as Clive says. I had an 88' Porsche with crap for tires, so got a set of Goodyear AT crap from a "locally owned and operated" dealer in Santa Fe, NM (Amigo Tire) instead of listening to a friend who said use $$Michelin Pilots$$.

On the freeway going home a rear tire went flat. I went back to said dealer. "Did you run this tire when it was flat?" What the hell did he expect, when a tire lets go on the freeway at 75 there's bound to be some "run flat" time. Sheeze... He absolutely refused to refund even though I pointed to crap on the bead of the wheel that he didn't clean off; previous owner had used some liquid "fix flat" at one time or other. Also turned out to be the stem insert was the leak, found when I put the $$Michelin Pilots$$ on.

So, only recourse is small claims court and argue your case. Great to be an American?

Be careful; get good recomendations from club members, Sub dealer, etc.

Then do a lot of Hail Marys.

Don

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Reply to
Edward Hayes

We've got some people posting about 1/4" difference in *diameter*, and some about 1/4" difference in *circumference*. Which is it? It makes a huge difference - by a factor of pi (3.1415926535897932384626433832795...).

Bill Putney (to reply by e-mail, replace the last letter of the alphabet in my address with "x")

Reply to
Bill Putney

I could be wrong, he might have said circumference, but that's such a small difference I have a problem believing it would make a difference, so I said diameter :-)

-- Dominic Richens | snipped-for-privacy@alumni.uottawa.ca "If you're not *outraged*, you're not paying attention!"

Reply to
Dominic Richens

Underinflation should not cause any problems. The circumference of the tire remains the same independent of low air pressure, so the number of wheel revolutions per mile remains the same.

-- Vic Roberts

Reply to
Victor Roberts

I'm not so sure. I'm sure that overinflation will increase the circumference of the tire. So it stands to reason that under- inflation would decrease the circumference, at least as long as the tire body is contracting with the reduction of air pressure.

Rubber is elastic and will grow and shrink with changes of internal pressure.

Reply to
Jim Stewart

While I admit that over inflation MIGHT stretch the rubber and increase the circumference, note that I said *under inflation* - as in too little air pressure - :-) and you decided to use *over inflation* in your example. And it does *not* stand to reason that just because *over inflation* might stretch the tire under some circumstances, that *under inflation* would make the reduce circumference.

-- Vic Roberts

Reply to
Victor Roberts

Very interesting, didn't think I'd start such a storm... :)

Found the following at Continental Tires describing how one of their pressure monitoring system works:

DDS requires no sensors of its own but evaluates the data gathered via the EBS wheel speed sensors. Any loss of pressure changes the radius of the tires, and results in a specific alteration of the speed signal.

URL:

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I must admit that it is confusing: The tire tread, the part in contact with the road, doesn't stretch or contract much so stays the same length, but the radius of the tire/wheel assembly does change, so something, somewhere, has to give...

Nicolas

Reply to
Sympatico

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Reply to
Edward Hayes

Well, we have a difference of opinion. Who's going to jack up their car and do some tests? Might be interesting to know.

Reply to
Jim Stewart

Time for some math (someone double check this)

1 mile is 63360" the circumfrance of a 215/60/16 is approx 82.3" with that, it requires that wheel to turn 770 times to go a mile

if you increase the circumfrance by 1/4 inch (.25) this wheel will require 767.5 turns to go a mile, which is a 2.5 turn delta.

At 60 MPH, the larger wheel will turn 2.5 turns less than the smaller wheel every minute.

Where is this difference obsorbed and will it cause damage? Is it different for Autos Vs Manual?

Reply to
Mike

There are steel wires going around the circumference so they tend to keep the circumference constant. (You can't really talk about the radius of a mounted tire because it's not circular, being flat on one side.) Of course the steel wires will stretch some when the tire's inflated, but it's not like a balloon.

At slow vehicle speeds there should be no measurable change in rpm of an underinflated tire. But--as I understand it: I'm only repeating what I read elsewhere--at higher speeds the tire develops distortions around its circumference including in the contact patch, so that the effective circumference of the underinflated tire will change, and this is what causes a measurable change in rpm.

Reply to
John Varela

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