A way to deal with uneven tires.

[friendly_jacek]

I just figured a way to deal with uneven tires and thought about sharing the idea.

 

During recent tires rotation I realized that the rear tires had a funny conical shape and short of grossly misaligned suspension the belts probably snapped. Interestingly, there was no major drivability issue, except for increased tire noise and mild vibrations.

 

The tire shop's diagnosis was partial tread separation and they issued me 2 brand new tires same type for a nominal prorated cost as the old ones had only 8000 miles. Because of the 2 mm tread difference translates to roughly half inch difference in the circuference (twice the acceptable number) I considered running the FWD fuse or shaving the new tires. However, I figured out a way to adjust the tire pressure to equalize the rolling diameter. Turned out that a difference between 32 and 36 PSI neutralizes the difference in the tire diameter. Furthermore, the difference in increased load on the front tires was enough to allow for the same rolling diameter with EQUAL pressure (36 PSI) and UNEQUAL tires.

 

I learned a few valuable lessons from this. Not only one should be anal about identical tires in AWD but also about identical pressures between left and right and also about enough pressure difference between front and rear to allow for same rolling diameter.

 

Lesson 2, pay attention to the production code of the tires you buy. The potenzas 950 I got in 2004 were produced at the beginning of 2001! Now they are 5 years old and no wonder there are disintegrating themself.

[1 Lucky Texan]

I just figured a way to deal with uneven tires and thought about sharing the idea.

 

During recent tires rotation I realized that the rear tires had a funny conical shape and short of grossly misaligned suspension the belts probably snapped. Interestingly, there was no major drivability issue, except for increased tire noise and mild vibrations.

 

The tire shop's diagnosis was partial tread separation and they issued me 2 brand new tires same type for a nominal prorated cost as the old ones had only 8000 miles. Because of the 2 mm tread difference translates to roughly half inch difference in the circuference (twice the acceptable number) I considered running the FWD fuse or shaving the new tires. However, I figured out a way to adjust the tire pressure to equalize the rolling diameter. Turned out that a difference between 32 and 36 PSI neutralizes the difference in the tire diameter. Furthermore, the difference in increased load on the front tires was enough to allow for the same rolling diameter with EQUAL pressure (36 PSI) and UNEQUAL tires.

 

I learned a few valuable lessons from this. Not only one should be anal about identical tires in AWD but also about identical pressures between left and right and also about enough pressure difference between front and rear to allow for same rolling diameter.

 

Lesson 2, pay attention to the production code of the tires you buy. The potenzas 950 I got in 2004 were produced at the beginning of 2001! Now they are 5 years old and no wonder there are disintegrating themself.

 

But doesn't this just change the shape of the tire? The ENTIRE 'perimeter' must still travel around (assuming no 'skipping or something).

 

Kinda like a 'half track' in a way. There wouild still be a difference in axle rotation speed I think.

 

I dunno

[2X2KOB]

But doesn't this just change the shape of the tire? The ENTIRE 'perimeter' must still travel around (assuming no 'skipping or something).

 

The relevant dimension is the distance from the center of the wheel to the road - the radius.

[forester2002s]

Boy, is this ever confusing! HELP!

 

We need some input from knowledgeable tire people.......

[otis]

But doesn't this just change the shape of the tire? The ENTIRE 'perimeter' must still travel around (assuming no 'skipping or something).

 

I think the perimeter or circumference changes with air pressure. just to oversimplify think of a balloon as you blow more air into it. the perimeter grows.

 

with that said, I don't know how siginificant 4 psi of tire pressure will make on a tire. I had a flat and replaced the tire with a used tire i thought had the same tread depth. it turns out the rolling circumference was 1/2" larger. so I lowered the pressure some and the circumference differnce is about .4" now. (keep in mind the error is about +/- .2" . I cant find a flat parking lot long enough to make a more precise measurement )

 

after driving like this for over a year, I've given up trying to equalize tire circumference via tire pressure and now I just roll with it .

[Zephyr]

This is kind of scary. If a few PSI is able to balance the circumfrance of two tires with different tread heights, then surely the same PSI difference could cause two tires with equal tread life to be out of balance. If it is of any significance, the diff gets wrecked.

 

I know that the entire Subaru owning population as a whole cannot be as diligent as a few on this board in respect to tire pressure, so I would imagine that differentials would be falling apart all over the place.

 

What is the difference in 2WD cars? They have a front or rear differential that must be just as susceptible to damage as Subaru's AWD systems.

[friendly_jacek]

This is kind of scary. If a few PSI is able to balance the circumfrance of two tires with different tread heights, then surely the same PSI difference could cause two tires with equal tread life to be out of balance. If it is of any significance, the diff gets wrecked.

 

I know that the entire Subaru owning population as a whole cannot be as diligent as a few on this board in respect to tire pressure, so I would imagine that differentials would be falling apart all over the place.

 

What is the difference in 2WD cars? They have a front or rear differential that must be just as susceptible to damage as Subaru's AWD systems.

 

The center diffs ARE failing frequently. At least I can see numerous posts about torque binds. I would venture to say this is #1 problem on this board, followed by HG problems.

 

 

The diffs in 2WD are different as they do not have slip clutches with limited lifespan.

[tcspeer]

Again, I am not telling anyone to do what I do, but my car has 304,000 miles of postal del. on it. Its a 1997 Subaru Legacy Wagon, it went to work on the Rt. in 1996 ten years now, and I have never had problems replacing tires as needed. I had lots of miles on it before I found out you or not suppose to run them that way. The money I have saved by buying one tire instead of four would easy pay to fix what ever problem most of you worry about. If this was true and as important as most of you think, I would be the first to leave Subaru and buy me a car I could use. My advice is to keep good tires you may not be as lucky as I have, but if you need to put a tire on that does not have the same tread depth to get home, do it and dont sweat it.

[cookie]

Is your car automatic TC?

[Juan]

Again, I am not telling anyone to do what I do, but my car has 304,000 miles of postal del. on it. Its a 1997 Subaru Legacy Wagon, it went to work on the Rt. in 1996 ten years now, and I have never had problems replacing tires as needed. I had lots of miles on it before I found out you or not suppose to run them that way. The money I have saved by buying one tire instead of four would easy pay to fix what ever problem most of you worry about. If this was true and as important as most of you think, I would be the first to leave Subaru and buy me a car I could use. My advice is to keep good tires you may not be as lucky as I have, but if you need to put a tire on that does not have the same tread depth to get home, do it and dont sweat it.

 

I must agree with TC. I think the whole tires-must-match thing is a needless worry to MOST all of us. Bought my L wagon new in '97 and drove it for eight years. I never even heard about keeping tires within some tolorance til I bought my '05 and read about it on this board.I checked the tire pressure maybe twice a year, rotated the the tires once in 50,000 miles, bought new tires when ONE of the four could not be repaired as it was worn out (ran on the spare for two days as I dropped the tire off on my way to work and couldn't make it back till the next day). I even ran LINK CHAINS on the rears to work one day in a fierce ice storm at 30 MPH for twentysome miles AND back home again at 45+ MPH on clean pavement (I didn't want to take the time in the cold to remove them in the parking lot). Never had any problems -none.

 

Subaru is just covering their..., themselves, when they discuss tire variances. just like they tell you not to stick your head out the sunroof and close it on your neck while you're driving. I bet (and I did) that if you keep things sorta close you'll be just fine.

 

Juan

[tcspeer]

Cookie, yes it is automatic.

Is your car automatic TC?

[tcspeer]

Thanks Juan, I needed help on this issue, most postal carriers do like me they replace only the tire that fails and this problem of torque bind is not even discussed in our craft, at least I have not heard it, and we work our cars much harder than most people that use their cars for pleasure.

I must agree with TC. I think the whole tires-must-match thing is a needless worry to MOST all of us. Bought my L wagon new in '97 and drove it for eight years. I never even heard about keeping tires within some tolorance til I bought my '05 and read about it on this board.I checked the tire pressure maybe twice a year, rotated the the tires once in 50,000 miles, bought new tires when ONE of the four could not be repaired as it was worn out (ran on the spare for two days as I dropped the tire off on my way to work and couldn't make it back till the next day). I even ran LINK CHAINS on the rears to work one day in a fierce ice storm at 30 MPH for twentysome miles AND back home again at 45+ MPH on clean pavement (I didn't want to take the time in the cold to remove them in the parking lot). Never had any problems -none.

 

Subaru is just covering their..., themselves, when they discuss tire variances. just like they tell you not to stick your head out the sunroof and close it on your neck while you're driving. I bet (and I did) that if you keep things sorta close you'll be just fine.

 

Juan

[1 Lucky Texan]

Could postal vehicles be a special case though? Seems like having 2 wheels over in the gravel/water/leaf encrusted gutter and swinging the car in/out/around the neighborhood at 20mph is gonna relieve a lot more 'bind' than a 30 minute commute at 70mph on a concrete interstate.

 

I dunno

[cookie]

I think the automatics should be more forgiving which is why I asked TC if he had an automatic. The cutches should slip a bit and will be able to do so more consistantly. Automatic transmisssions on busses are constantly slipping thier big allison clutches and although they do wear it takes time. The clutch plate type transfer on the automatics will be more like that.

The five speeds are going to be more affected especially at high speeds. All you have to do is heat the coupling up and the fluid turns partially solid.

Postal vehicles also often run many stops at low speeds. I think this is probably less likely to damage the trabsfer mechanism.

I suspect postal service is more like what our route busses do. Heat is still your enemy but you run into different problems than you do wiht highway use.

[tcspeer]

Most rural routes are not in neighborhoods, those carriers you see there are city carriers, they use a vechile that is provided by the post office, I dont even know who makes them. Most rural routes are along the hiways and county roads. We are able to get to speed limit many times during the day. As far as the gravel and leaves maybe they do help. That car of yours is really sharp looking, if mine looked that good I would be much more picky about things, then I am on mine.

Could postal vehicles be a special case though? Seems like having 2 wheels over in the gravel/water/leaf encrusted gutter and swinging the car in/out/around the neighborhood at 20mph is gonna relieve a lot more 'bind' than a 30 minute commute at 70mph on a concrete interstate.

 

I dunno

[tcspeer]

This is problably true because all of the ones on rts. are automatic. Iam sure the standards could not hold up to this type of use.

I think the automatics should be more forgiving which is why I asked TC if he had an automatic. The cutches should slip a bit and will be able to do so more consistantly. Automatic transmisssions on busses are constantly slipping thier big allison clutches and although they do wear it takes time. The clutch plate type transfer on the automatics will be more like that.

The five speeds are going to be more affected especially at high speeds. All you have to do is heat the coupling up and the fluid turns partially solid.

Postal vehicles also often run many stops at low speeds. I think this is probably less likely to damage the trabsfer mechanism.

I suspect postal service is more like what our route busses do. Heat is still your enemy but you run into different problems than you do wiht highway use.

[cookie]

I learned to drive on the mail route from Greenville Maine to Rockwood. My uncle taught all the family kids there. We might go five miles without a mailbox , then at the end of a road you wouldnhave several, then maybe one every half mile. I think your route might be similar but with more heat and less 20 below zero weather?

The route sure took a toll on the old Chevy wagons my uncle favored. You got a bit of high speed, a lot of stop and start, and wear on everything.

[Juan]

All you have to do is heat the coupling up and the fluid turns partially solid.

Heat is still your enemy QUOTE]

 

It's all very clear to me now. Really. I thought the system would fail because of some mis-match of gears and grinding and such. Had no idea the fluid could actually solidify. Makes sense, though; one drive shaft constantly turning faster than the other while the fluid constantly tries to "clutch" the two together. Stands to reason things would get hot, especially at high speeds.

 

Still, you would think that somebody could come up with a fluid that wouldn't solidify at high temp, or, some sort of emergency cooler to prevent solidification.

Or a warning light/buzzer of some kind.

 

Juan

[jamal]

One problem I see with uneven pressures is handling. By changing the pressure relative to the front/back, you can change how the car will behave in a corner. I wouldn't want wierd, inconsistant cornering.

 

Yeah, it sucks when you blow out a tire when they're all fairly worn and you have to buy 4 tires, but otherwise just check the pressures and rotate the tires on a regular basis. It's part of owning a car.

[Setright]

I have to agree with Texan's first comment. The tyre circumference will not change.

 

That's the whole point of radially wound tyres: The tread doesn't change shape.

 

That means that more air will NOT stretch the tyre like a balloon.

[1 Lucky Texan]

Most rural routes are not in neighborhoods, those carriers you see there are city carriers, they use a vechile that is provided by the post office, I dont even know who makes them. Most rural routes are along the hiways and county roads. We are able to get to speed limit many times during the day. As far as the gravel and leaves maybe they do help. That car of yours is really sharp looking, if mine looked that good I would be much more picky about things, then I am on mine.

 

Well, if you knew what all my other cars ended up looking like, you might not be so flattering! hah!

 

I dunno - just trying to keep my head wrapped around this whole tire circumference issue. I WANT to believe it isn't as bad as it sounds. But I also would not want someone to void their warranty or blow up their center diff. It probably is never gonna be a black/white delineation.

[otis]

I have to agree with Texan's first comment. The tyre circumference will not change.

 

That's the whole point of radially wound tyres: The tread doesn't change shape.

 

That means that more air will NOT stretch the tyre like a balloon.

 

did you mean

a) the tire won't change shape regardless of air pressure. or...

the tire wont significantly change shape with a 4 psi difference in air pressure

 

lets just take an extreme example- fill 1 tire at 32 psi and the other at 10 psi. i can take my glasses off and still see a difference in shape and size.

 

but if you meant to say , then I agree with you.

[friendly_jacek]

I have to agree with Texan's first comment. The tyre circumference will not change.

 

That's the whole point of radially wound tyres: The tread doesn't change shape.

 

That means that more air will NOT stretch the tyre like a balloon.

 

Setright, the issue is not believing, we are not talking religion here. Feel free to do a measument yourself. I did and saw a 2mm difference. Yes, it is too small to see it with just eyes. YMMV with different tire type.

 

One more thing, if you are correct, why underinflated tires wear on edges and overinflated in the center of the tread?

[1 Lucky Texan]

Setright, the issue is not believing, we are not talking religion here. Feel free to do a measument yourself. I did and saw a 2mm difference. Yes, it is too small to see it with just eyes. YMMV with different tire type.

 

One more thing, if you are correct, why underinflated tires wear on edges and overinflated in the center of the tread?

 

But if the tire 'just' gets smaller - why wouldn't it wear evenly straight across?

 

I don't even understand what i wrote - lol!

 

Carl

[OB99W]

Being of a practical as well as scientific nature , I decided a little research was in order. Choosing the rear driver's-side wheel of my '99 OB (with Michelin XW4 tires), I put a height reference mark very near the center. I then took measurements from my garage floor to that reference with 24, 28, and 32 psi inflation (recommended pressure and +/- 4 psi. The variation (change in radius) was about 1/16 inch for each 4 pound change, or 1/8 inch over the range of 24-32 psi. That would yield an effective circumference change of 2*pi*r, over 3/4 inch difference between 24 and 32 psi inflation (naturally, 3/8 inch for a 4 psi change). These are static determinations; a rolling tire will behave somewhat differently, but probably not by much.

 

I'm in the rust/salt belt, and my alloy rims leak at the tire bead area (yes, they've been "resealed", to no avail). The above is one reason why I check tire pressure frequently. I don't know to what degree a Subaru AWD drive train suffers due to tire size/inflation mismatches, but I'd prefer not finding out through personal experience.