Shaving new Tire for subaru AWD
Thought I would ask this question in this forum, as from what I read here, you guys seem pretty smart on mechanics. So here is the story. Wife picked up a large hole in the right rear on our 2015 S...
Forum Discussion
rjstractor wrote:wildtoad wrote:
Excuse my ignorance but why would you have to replace all four? I can see getting two for the same axle.
Full time AWD, the thinking is that all four tires need to be exactly the same rolling circumference to avoid excessive drivetrain wear.
I will explain. The problem is not merely excessive drivetrain wear, it is much worse than that.
The reason you have to put same-rolling-diameter tires on a given axle is that the differential between the two wheels contains a pinion gear that rotates on a paddle-like mount in order to allow the wheels to rotate at different speeds when, for example you go around a corner. It slaps around the differential oil as it does so, and that creates a little heat. Normally, with same-sized tires, you get significant rotational differences for like 50 feet as you corner, otherwise, nothing. So no provisions need be made to dissipate that little bit of heat. But if your tires are mismatched, that paddle slaps around constantly, the oil will overheat, and bad things will happen.
AWD vehicles necessarily have a third differential between the two axles, and if any one of your tires is not the same size as the other three, differentials get overworked and overheated. I know this through bitter experience.
A number of years ago I had an AWD Chevy van. On a trip, we blew a tire, crawled off the interstate on the spare, and had both tires on that axle replaced. I failed to notice that idiots at that tire store used the next smaller size tires on that axle. We got 37 miles down that interstate when the third differential exploded in flames. It was an exciting time.
In researching the phenomenon, I found many, many folks who had replaced only two tires on an AWD vehicle, often with the same size tires, and had similar results. For the most part, they got like 100 miles after the tire change before a differential took a dump.
My repair cost "only" $1400 when it happened about 20 years ago. YMMV. But one thing is clear: if you have an AWD vehicle, you MUST make sure all 4 wheels are the same rolling diameter. The OP might get away with buying a single tire and having it shaved to the same diameter as the other three. Just never plan on putting only two new tires on an AWD vehicle. You will regret it.
