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Old 12 March 2013, 07:39 PM
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Driver Motorists must buy tires in pairs

Comment: Companies installing new pairs of tires insist that the two new
tires should go on the rear of the vehicle regardless of FWD, AWD or RWD.
They suggest that you should purchase four new tires to be safe. Some
articles on the internet refer to the following lawsuit - San Luis Obispo
County Court Case CV078853 - as the reason. There are settlements of
$8.5M & $10M listed as to why tire companies are following this lead.
Michelin, Goodyear and other tire companies are posting articles to back
up the rear vs. front installation. There are even videos (some appear
bias in how the testing is performed) to back up this ideology. Can you
shed any light on this?
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Old 12 March 2013, 07:40 PM
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Read This! How a bogus Wikipedia entry changed nationwide tire company policy

Some will also tell you the policy it arose from a lawsuit in which a tire company was sued after a customer spun out. So two years ago, we went looking to see if that was really true.

Back then, googling either "oversteer" or "understeer" led inexorably to a Wikipedia page with a reference to "San Luis Obispo County Court Case CV078853". Unfortunately, Wikipedia's link next to the court case reference actually led to a general page for a law firm that appears to specialize in vehicular personal injury lawsuits. (Nice advertising, that.) There was no information about any such case.

http://shallowsky.com/blog/misc/wiki...ny-policy.html
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Old 13 March 2013, 12:15 AM
jimmy101_again jimmy101_again is offline
 
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I seem to recall that Click and Clack agree that if only replacing two tires the new tires should go on the rear of the vehicle. Their reasoning was that you want the best traction on the wheels that you have the least control over, and the least sense of what the tries are doing.
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Old 13 March 2013, 03:05 AM
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DrRocket DrRocket is offline
 
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A lot depends on AWD vs FWD, and what the weather is like.

The thing about front wheel drive cars is that in addition to steering the car, the front tires are also providing forward motion for your car. For all practical purposes, the rear tires are just along for the ride. Therefore, when I only buy two tires, they go on the front .

No matter what set of wheels drive your car, you want the two tires (assuming 2wd) providing thrust to be the same diameter. the reason is the differential. It's not good for the differential to have the two tires spinning at different rpms.
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Old 13 March 2013, 03:08 AM
St. Alia St. Alia is offline
 
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So, does anyone know what that means for an AWD vehicle? As in how many to replace, where they go, etc...

We just recently started looking into buying a car. One of our options is an AWD and I've always had FWD.
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Old 13 March 2013, 03:15 AM
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Quote:
Originally Posted by St. Alia View Post
So, does anyone know what that means for an AWD vehicle? As in how many to replace, where they go, etc...
That depends if it's full time AWD such as a Subaru or Audi, vs part time like a 4 wheel drive truck that's mainly used in RWD.

For AWD, you do want the four corners to match as closely as possible, or problems with the transfer unit can result. It's one of the drawbacks to that type of vehicle. Somewhat higher tire costs if something goes wrong with tires that have some mileage on them.
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Old 13 March 2013, 04:22 AM
overyonder overyonder is offline
 
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Quote:
Originally Posted by DrRocket View Post
The thing about front wheel drive cars is that in addition to steering the car, the front tires are also providing forward motion for your car. For all practical purposes, the rear tires are just along for the ride. Therefore, when I only buy two tires, they go on the front .

No matter what set of wheels drive your car, you want the two tires (assuming 2wd) providing thrust to be the same diameter. the reason is the differential. It's not good for the differential to have the two tires spinning at different rpms.
I'm all for putting 2 new tires in the front, if the still-good 2nd tire has little tread left.

That being said... different "sized" tires due to wear imposes minimal wear on the differential. After all, the differential is made JUST for the purpose of having different rotation speed between left and right (albeit small).

... A 1/32" size difference in a 25" tire represents 25 * 32 = 800 rotations of one tire for a 1 rotation *difference*. (If my midnight math is right).

Given the same 25" tire, the differential would spin one extra turn on one side for roughly every 1 mile traveled. (800 rotations * 25" * pi ) = 5233, just short of one mile.

OY
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Old 13 March 2013, 01:44 PM
overyonder overyonder is offline
 
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Quote:
Originally Posted by overyonder View Post
I'm all for putting 2 new tires in the front, if the still-good 2nd tire has SOME/ENOUGH tread left.
Corrected my own statement

Quote:
Originally Posted by overyonder View Post
... A 1/32" size difference in a 25" tire represents 25 * 32 = 800 rotations of one tire for a 1 rotation *difference*. (If my midnight math is right).

Given the same 25" tire, the differential would spin one extra turn on one side for roughly every 1 mile traveled. (800 rotations * 25" * pi ) = 5233, just short of one mile.
Reviewed this math. I wasn't too far off considering it was near midnight. Reworked it now:

Assuming no flex in the sidewall (or similar amount of flex):

1/32 tire diff

25" tire = x rotations per mile
24 31/32" = y rotatations per mile

x = 5280/ (25*pi/12) = 806.72 rotations
y = 5280/ (24 31/32*pi/12) = 807.73 rotations

OY
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Old 13 March 2013, 11:43 PM
jimmy101_again jimmy101_again is offline
 
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Quote:
Originally Posted by DrRocket View Post
A lot depends on AWD vs FWD, and what the weather is like.

The thing about front wheel drive cars is that in addition to steering the car, the front tires are also providing forward motion for your car. For all practical purposes, the rear tires are just along for the ride. Therefore, when I only buy two tires, they go on the front .

No matter what set of wheels drive your car, you want the two tires (assuming 2wd) providing thrust to be the same diameter. the reason is the differential. It's not good for the differential to have the two tires spinning at different rpms.
I believe it is often assumed that "providing forward motion" is the least important factor, in terms of safety. It is much more important to be able to stop and to keep from spinning. Rarely is the inability to move a significant safety hazard. Front tires typically carry more of the car's weight (which suggest best tires should be in front) but I think the more important reason is that you have control of front wheels (steering, brakes and in most cars power) the rear may be "along for the ride" but when rear traction is foobar you are foobar.

As to the differential, yes miss-matched tires work the differential a bit harder but every time you turn you work the differential much harder than what a small difference in tire diameter does.
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Old 19 March 2013, 09:27 PM
kanazawa kanazawa is offline
 
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Tirerack.com gives a pretty involved explanation:

http://www.tirerack.com/tires/tirete....jsp?techid=52

Members of Tire Rack team had the chance to experience this phenomenon at Michelin's Laurens Proving Grounds. Participants were allowed to drive around a large radius, wet curve in vehicles fitted with tires of different tread depths -- one vehicle with new tires on the rear and half-worn tires on the front and the other with the new tires in the front and half-worn tires on the rear.

It didn't take long for this hands-on experience to confirm that the "proving grounds" name for the facility was correct. The ability to sense and control predictable understeer with the new tires on the rear and the helplessness in trying to control the surprising oversteer with the new tires on the front was emphatically proven.

And even though our drivers had the advantage of knowing we were going to be challenged to maintain car control, spinouts became common during our laps in the car with the new tires on the front and the worn tires on the rear. Michelin advises us that almost every driver spins out at least once when participating in this demonstration!
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