tire wear and all wheel drive

[Andy K]

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

 

[Dave Null Sr.]

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.

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.

 

[GC]

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

 

[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.

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.

 

[Clive]

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

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
www.espguitars.co.uk
www.espshop.co.uk
www.mightymite.com
www.svetlana-tubes.com
www.emgpickups.co.uk
www.tube-shop.com
www.whirlwindusa.com

 

[Don Nickell]

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!.

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

 

[Dan Duncan]

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.

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

 

[Dominic Richens]

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.

 

[Clive]

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


--
Clive Norris
Selectron (UK) Ltd
www.espguitars.co.uk
www.espshop.co.uk
www.mightymite.com
www.svetlana-tubes.com
www.emgpickups.co.uk
www.tube-shop.com
www.whirlwindusa.com
www.deanmarkley.com

 

[Edward Hayes]

I do agree that tires of equal size should be used on all four corners. Audi
recommends 1/4 in difference max for cars driven for max performance and
appantly Subaru has picked up on that number. I can't find anything written
by or any recommendations from Subaru. I would think that if 1/4 or 1/2 inch
difference in circumference would cause a problem would state so in writing.
Tires marked 215/60/16 from different manufacture vary by about 1/4 inch
circumference. Any tire outlet who tells me that I MUST replace all four for
safety or any other reason will loose me as a purchaser. I make this
statement assuming the three tires have useable tread. Salesmen will go to
great lengths to see you 4 tires instead of one. Just my opinion. Others
disputing this should post data and not guesses or tire salesmen's expert
opinion. Eddie

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!.

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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[Bill Putney]

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.

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")

 

[Dominic Richens]

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 (3.14159265358979323...).

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

 

[Victor Roberts]

When looking for a new car (bought a 2000 Legacy L Wagon) I looked at
Volvo wagons with AWD and read some posts and was told by my mechanic
of cases where the rear tires being smaller/less inflated broke the
rear pinion gear (think that's what it's called, the gear at the end
of the driveshaft driving the rear differential).

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.

 

[Jim Stewart]

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.

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.

 

[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.

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.

 

[Sympatico]

Really? Then how does GM and other's make those "LOW TIRE" warnings work?
I'll tell you how, you guys are WRONG. The monitors watch the wheel speed
signals from the ABS, and 'learns' what each one should be. When one tire
goes out of the parameter by a certain amount, the system kicks on the LOW
TIRE light. Obviously for this to work, the rolling circumference of the
tires MUST change with changes to inflation.

NOTE: Yes, some systems DO actually have pressure sensors mounted inside
the tire, but MOST work as described above.

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:
http://www.conti-online.com/generator/www/de/en/continentalteves/continental
teves/themes/products/tire_pressure_loss_detection/dds_0602_en.html

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

 

[Edward Hayes]

The tire radius is not a constant because the force/load on the tire is not
symmetrical. Circumference stays approx the same because the radius front to
back of the tire is greater than the top to bottom when loaded. At least
that's my understanding of why the circumference stays approx constant.
Eddie

 

[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.

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

 

[Mike]

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.

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?

 

[John Varela]

The tire radius is not a constant because the force/load on the tire is not
symmetrical. Circumference stays approx the same because the radius front to
back of the tire is greater than the top to bottom when loaded. At least
that's my understanding of why the circumference stays approx constant.

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.