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Old 12-08-2011, 05:13 PM   #34
MAXUM
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Quote:
Originally Posted by CateP View Post

Well, with an AWD Subaru you have to replace the tire with the exact same brand (something about treads and the transmission). AND you can't replace one, you have to replace at least two to keep things even. I was looking at $400 for those two tires so I ended up getting a whole new set from Trites. (Trips back and forth from NJ had taken a toll anyway) I am at peace with this decision.
Just to let you know this is not entirely accurate. This is where you can really get screwed if you are at the mercy of those who are fixing your car.

Full time AWD systems are pretty complicated, reason being you're applying power to all 4 wheels at the same time on any surface. The big trouble comes when you're on dry pavement and specifically cornering on dry pavement. All 4 wheels cannot spin at the same speed otherwise you would have significant binding in the drive train, cause excessive tire wear and in high speed condition actually could affect the stability of the vehicle. To compensate for this there is the ability of the drive train to "slip" so that this binding doesn't occur. If I tried to explain the various methods that are used to do this I'd write a novel and bore you to death. The important thing about these systems is that the compensation is able to deal with a certain variation in wheel speed difference. This can be affected by the overall circumference of the tires themselves. IE a worn tire will have less circumference than a new tire will. This is critical for all 4 tires, not just one or two. Now how much give is there in the system? Well to tout the OEM line of course the dealers will tell you that you need to replace more than may be necessary. In reality if you're less than 1/2 way through the tread life replacing a single tire is perfectly fine. More than that it 'could' cause excessive wear to the "slip" mechanism. I use the word could with caution here because it's probably not going to. However putting one new tire on with the other three nearly bologna skins is also not a good idea.

With a system like this the following things are important.

All 4 tires should be of the same make. Changing brands as long as you change all 4 is perfectly fine. Using the same brand and tread is important because each tread pattern will have varying gripping power so this must be matched all the way around.

All 4 tires should be of the same size, mixing tire sizes is a no no with AWD, as each size varies in width and circumference.

Using studded tires with these AWD systems is not a good idea and really not necessary.

If a condition exists where you're outside the normal operational envelope of the slip coupling that coupling can be damaged as a result. It should however be the sacrificial lamb in the entire drive train and protect the more expensive trans axle from sustaining any damage. Not to say those couplings aren't expensive to replace, just not as bad as a trans axle.

Bottom line - all 4 tires need to be somewhat uniform in wear, identical in size and tread pattern. If you replace your tires as long as you're doing all 4 you can change brands. It would be advisable to stick with the recommended tire size.

You should NEVER run a doughnut spare on these vehicles... EVER! If it came with one, throw it out and get a full sized matching spare tire.
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