Shaving new Tire for subaru AWD

[winnietrey]

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 Suby Forester ( with 15K on the clock). Tire is not reparable. So options are 1)Replace all 4 tires, or 2) Dealer says they will get same tire and shave it down so it matches wear on the other 3. They say all will be peachy.

My understanding is Suby AWD, is extremely sensitive, and if tires don't match as to size (as in tread wear), you will blow the Tranny.

Until yesterday I did not know one could shave a tire. So any thoughts would be greatly appreciated.

Mod If I posted in the wrong place I apologize, please move as you see fit

[naturist]

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.

[colliehauler]

If it was mine I would replace all four tires and save/sell the three remaining tires.

[wildtoad]

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.

Thanks I now know more than before.

[Rolin]

We have a Subaru Forester. In this situation we would shave the tire. Depending on how you drive you can get over 40k miles on a set. Shaving will not create a handling, braking, or traction problem.

Love the Subaru, but it is a bummer when you have a none repairable flat.

[carringb]

Doing it at Les Schwab will keep the tire warranty 100% intact.

[Walaby]

Wonder if it would impact the warranty of the new tire.

I actually had never heard about that in a consumer application.

Interesting.

Mike

[Fast_Mopar]

jerem0621 wrote:

Shaving a tire sounds like a decent option as long as the mechanics do it right. If they do not, you can have some big issues which will be way more expensive than a new set of tires.

Thanks!

Jeremiah

This is correct.

It is true that tire shaving has been around for decades for various reasons, including racing applications. However, finding someone who will do it correctly might be a challenge.

[Sam_Spade]

jerem0621 wrote:
Have you checked the date code on the tires to see how old the tires actually are? If they are more than 3-4 years old by the date code I'd be inclined to replace all four of them.

THIS is an excellent point.

[rjstractor]

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.

[Sam_Spade]

winnietrey wrote:

My understanding is Suby AWD, is extremely sensitive, and if tires don't match as to size (as in tread wear), you will blow the Tranny.

Sounds like a "profit center" for the tire dealer to me.

I suggest that you contact the manufacturer directly.....not some "mechanic" who works at a shop....and get the real skinny.

That type of tire should probably show almost NO wear at 15K.

[midnightsadie]

replace all four ,this your life your dealing with ,and maybe some esle,s.there half shot now.

[wildtoad]

Excuse my ignorance but why would you have to replace all four? I can see getting two for the same axle.

[time2roll]

If shaving 50%+ I would replace all.

[Inlet]

This also happened to me on my 2014 Subaru cross trek. The tires had 22,000 miles on them so I just replaced the set. But I did learn there is quite a market on craigslist of people selling used sets of 3 tires, from Subarus ....such as the Yokahamas . If the remaining 3 have got some life....shop for a used tire.

[ScottG]

Shave.