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Old 07-27-2008, 04:52 PM   #1
BeBop86
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Question How bad is running two different Tire Brands (front & back)

I just picked up a set of wheels with some usable tread on them, but the front and back wheels have different tires on them.

They are the same "size" (e.g. 215/45/17), but they are different brands (e.g. Kuhmo & Dunlop), and when put next to each-other there's about 1/4", maybe 1/2", total height difference.

So, the front two are the same height, and the rear two are the same height, but the front two are shorter than the rear two.

Make sense?

Thanks.
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Old 07-27-2008, 04:56 PM   #2
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um from what i understand this is a bad thing. i wouldn't drive around like that if you like having a working car.
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Old 07-27-2008, 05:00 PM   #3
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Use the same tires all around.
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Old 07-27-2008, 05:02 PM   #4
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Quote:
Originally Posted by Lancer To WRX View Post
Use the same tires all around.
Obviously I'd use the same tires all around if I were the one to have bought the tires, but since that isn't the case...

I'm simply asking if 1/4" would make a significant difference.
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Old 07-27-2008, 05:03 PM   #5
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Open and read your owner's manual...1/4" is the maximum you can run. If you are foolish enough to run that or great though, you will blow up your center diff.
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Old 07-27-2008, 05:43 PM   #6
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That's 1/4" difference in circumference you can run. 1/4" in diameter = just over 3/4" difference in circumference.
Granted, this is all when the tire in only supporting itself and the wheel, the effective diameter when it's on the car will be different.

Edit: I just checked my owner's manual, and it doesn't give any sort information on what kind of difference in diameter or circumference you can safely run.

Last edited by Counterfit; 07-27-2008 at 06:00 PM.
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Old 07-27-2008, 05:57 PM   #7
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Quote:
Originally Posted by Counterfit View Post
That's 1/4" difference in circumference you can run. 1/4" in diameter = just over 3/4" difference in circumference.
Granted, this is all when the tire in only supporting itself and the wheel, the effective diameter when it's on the car will be different.
So, even though it's only 1/4" taller, it really has 3/4" more tread "length" (if you were to cut it and lay it flat)?
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Old 07-27-2008, 07:20 PM   #8
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Quote:
Originally Posted by BeBop86 View Post
So, even though it's only 1/4" taller, it really has 3/4" more tread "length" (if you were to cut it and lay it flat)?
pi times the diameter, or differance in diameter in this case.

0.25 * 3.14 = 0.78 or a tick over 3/4"
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Old 07-28-2008, 10:38 AM   #9
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unless youre racing the car or something than just run the tires until you need to replace them and then get the same size. out here in the country, 'if it holds air then run em'
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Old 07-28-2008, 11:02 AM   #10
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If you value your center diff you need to run tires that have no difference in circumference greater than 1/4"

like knockknockknock states, if your tires have 1/4" diffrence in diameter, you have over 3/4" difference in circumference
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Old 07-28-2008, 12:16 PM   #11
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Quote:
Originally Posted by 17lyrics View Post
unless youre racing the car or something than just run the tires until you need to replace them and then get the same size. out here in the country, 'if it holds air then run em'
wow, yore a ****ing idiot. read first, then post. work on that k?
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Old 07-28-2008, 02:35 PM   #12
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I think he was being sarcastic? I dunno...
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Old 07-28-2008, 03:00 PM   #13
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So is there a definitive answer to the question, any engineers or technicians on here? I'm a computer engineer so mechanics are not my strong point, but I thought the center diff worked in the following way:

If wheel slippage is detected at the front wheels, center diff will send power to rear wheels and vice versa...

If you are running 2 of a kind in the front, and a different 2 of a kind in the rear, how does that translate into wheel slippage and the center diff being affected? I'm not arguing one way or another but I'd really like to know how this works.


Thx.
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Old 07-28-2008, 03:07 PM   #14
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Quote:
Originally Posted by Domnf15 View Post
So is there a definitive answer to the question, any engineers or technicians on here? I'm a computer engineer so mechanics are not my strong point, but I thought the center diff worked in the following way:

If wheel slippage is detected at the front wheels, center diff will send power to rear wheels and vice versa...

If you are running 2 of a kind in the front, and a different 2 of a kind in the rear, how does that translate into wheel slippage and the center diff being affected? I'm not arguing one way or another but I'd really like to know how this works.


Thx.
Put your car up on a lift or jack stands. Spin one wheel. Notice the other three are spinning. Now think "the center diff is transferring power to the other end of the car!". Now think about if it took slightly longer for the rear wheels to rotate then the fronts. The front wheels would have one time for one full rotation of the center diff and the rears would have a different time. The front or rear wheels are going to win, the center diff is going to lose. Essentially it will assume the smaller rotation is slipping because it is taking less time to rotate when in fact it isn't.
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Old 07-28-2008, 03:14 PM   #15
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It really depends on when you use the tires and how you use the tires. When i drive my Bmw around turns at 60 or so i can feel the back end kick out cause i have two different sets of tires as well. (I have racing tires in the front and regular all seasons in the back) There's so much grip in the front from the racing tires that the back end gets lose. So yes it can be unsafe, it all depends on how you drive at least thats how i see it.
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Old 07-28-2008, 03:21 PM   #16
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It will put a little extra wear and tear on the center differential. Chances are that you'll be fine, but Subaru is extremely conservative with rating the center diff on the WRX, and won't warranty anything if you take it in with a cooked diff, and the wheel circumference difference exceeds 1/4".

If you do some quick calculations, a 215/45-17 tire turns roughly 820 revolutions per mile. The 1/4 inch diameter difference that you're seeing would change that by approximately 10 revolutions per mile. That means that at 60 mph, the plates in your center diff are slipping past each other and stirring up the viscous fluid at approximately 10 revolutions per minute. Even though that's 3 times the recommended limit from Subaru, I have a hard time imagining it can generate enough heat to damage the diff. I think they just set an impossible standard to meet so that they'd always have an out if they didn't feel like covering a diff under warranty.
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Old 07-28-2008, 03:25 PM   #17
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Quote:
Originally Posted by SeriousInquiry View Post
It really depends on when you use the tires and how you use the tires. When i drive my Bmw around turns at 60 or so i can feel the back end kick out cause i have two different sets of tires as well. (I have racing tires in the front and regular all seasons in the back) There's so much grip in the front from the racing tires that the back end gets lose. So yes it can be unsafe, it all depends on how you drive at least thats how i see it.
You're not getting the point here dude, it has nothing to do with safety, more to do with simple mechanics.

If the diameter of the tires is significantly different, the wheels will spin at a different speed than the rest, which can overwork the center diff.
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Old 07-28-2008, 03:25 PM   #18
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Quote:
Originally Posted by Domnf15 View Post
So is there a definitive answer to the question, any engineers or technicians on here? I'm a computer engineer so mechanics are not my strong point, but I thought the center diff worked in the following way:

If wheel slippage is detected at the front wheels, center diff will send power to rear wheels and vice versa...

If you are running 2 of a kind in the front, and a different 2 of a kind in the rear, how does that translate into wheel slippage and the center diff being affected? I'm not arguing one way or another but I'd really like to know how this works.


Thx.
The center diff on a manual WRX isn't anywhere near that smart. It's simply a "limited slip differential" as the name implies. In a viscous-type differential, you have a series of parallel plates separated by a fluid that becomes more resistant to shear the more force you apply to it. In simple terms, it allows the plates to slip past each other slowly, but if they try to slip quickly, the fluid acts more like a solid and forces them to want to turn at the same speed. It works a lot like the torque converter in an automatic transmission.

It can't "send power to the wheels with traction" or anything like that. All it can do is try to keep the average speed of the front tires, and the average speed of the rear tires approximately equal.
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Old 07-28-2008, 03:41 PM   #19
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the simple answer is that it can be very bad if it burns up the center diff. it is not worth risking it. just get a matching set of tires for all four corners.
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Old 07-28-2008, 04:06 PM   #20
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Quote:
Originally Posted by Domnf15 View Post
So is there a definitive answer to the question, any engineers or technicians on here?
Is straight from Subaru good enough?

Quote:
Originally Posted by SOA web site
WARNING: All four tires must be the same in terms of manufacturer, brand (tread pattern), construction, degree of wear, speed symbol, load index and size. Mixing tires of different types, sizes or degrees of wear can result in damage to the vehicle's power train.
http://www.subaru.com/owners/carcare...id=TIRED_TIRES

I've also seen the 1/4" circumference information on the Subaru site, as well. Might be in the owner's manual. Possibly even a FAQ here.
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Old 07-28-2008, 04:08 PM   #21
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Quote:
Originally Posted by The Satch View Post
Put your car up on a lift or jack stands. Spin one wheel. Notice the other three are spinning. Now think "the center diff is transferring power to the other end of the car!". Now think about if it took slightly longer for the rear wheels to rotate then the fronts. The front wheels would have one time for one full rotation of the center diff and the rears would have a different time. The front or rear wheels are going to win, the center diff is going to lose. Essentially it will assume the smaller rotation is slipping because it is taking less time to rotate when in fact it isn't.

Hey Morgan, that's a damn good explanation. It's simple once you understand, but actually explaining it so simply isn't easy. Nice job homie
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Old 07-28-2008, 04:12 PM   #22
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Quote:
Originally Posted by SeriousInquiry View Post
It really depends on when you use the tires and how you use the tires. When i drive my Bmw around turns at 60 or so i can feel the back end kick out cause i have two different sets of tires as well. (I have racing tires in the front and regular all seasons in the back) There's so much grip in the front from the racing tires that the back end gets lose. So yes it can be unsafe, it all depends on how you drive at least thats how i see it.
Most likely your BMW is a 2wd car, which can be run with differing tires on front and rear (although it's still not a fantastic idea in many cases, for many reasons). If you have a "x" model, it wouldn't surprise me to know that you have the same constraint regrading replacing all four wheels.

Also, BMW's with racing tires front and rear can break the rear end loose, too
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Old 07-28-2008, 04:34 PM   #23
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Quote:
Originally Posted by SeriousInquiry View Post
It really depends on when you use the tires and how you use the tires. When i drive my Bmw around turns at 60 or so i can feel the back end kick out cause i have two different sets of tires as well. (I have racing tires in the front and regular all seasons in the back) There's so much grip in the front from the racing tires that the back end gets lose. So yes it can be unsafe, it all depends on how you drive at least thats how i see it.
Please try not to hit anyone when you spin off the road into a tree, kthx.
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