Piston Slap: Financial Traction, AWD Distraction?

Sajeev Mehta
by Sajeev Mehta
piston slap financial traction awd distraction

John writes:

Hey Sajeev,

Since you requested goofy questions for Piston Slap, I’ve got one:

A friend of mine told me that her dad had a Subaru (I don’t know what particular model). He had one of the tires blow out, and even though he knew he should either replace all four tires, or have the new one shaved down to match the other three, he decided to risk it and just use the one new tire as is. Consequently, the all wheel drive system got messed up due to the ever so slight mismatch in tire diameters. Is this for real? I’ve never heard anything like that before.

Sajeev answers:

I didn’t request “goofy” questions, per se…but let’s not split hairs. And this is far from a goofy question.

Because this problem is for real, a good explanation is here. And it’s not an “ever so slight” mismatch with the tires, if the ¼” circumference discussed on UltimateSubaru.org is valid. I regularly dissuade people from buying AWD cars, unless they live in colder climates where municipalities simply can’t regularly plow all their streets. Which is a lot of the country, but not a lot of the population.

AWD systems are heavier, thirstier and cost more to buy. For the long term owner, they cost more to keep functional. And for anyone who loses one tire from a quartet that had a lot of life beforehand, things get real ugly. This is almost as silly as fretting over horsepower figures when wide-open throttle is rarely applied.

I will try to remember this letter the next time someone asks my opinion on an AWD vehicle, when I know that someone doesn’t need it. And that’s not a slam on Subaru or Audi, at least not intentionally. The concept of shaving down a perfectly good tire will certainly get some “financial traction” in people’s minds!

Send your queries to sajeev@thetruthaboutcars.com. Spare no details and ask for a speedy resolution if you’re in a hurry.

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4 of 47 comments
  • Wmba Wmba on Feb 13, 2012

    The most common Subaru AWD system is the Multi-Plate Transfer clutch behind the 4 speed auto (4EAT) and the new CVT. It is always engaged when the car is in gear, and has a 60/40 front-rear balance statically. (90/10 went out for the 1998s. When the tires are all the same make and nominal size and the car is travelling in a straight line, the front and rear wheel speed is the same, and the MPT clutch plates do not slip relative to one another, so no wear happens. When going round curves, the front wheels travel a lesser distance than the rears (in any vehicle), so there will be slippage between the clutch plates to allow for the difference. This is a short-term phenomenon, of course. When one wheel is differently sized, the clutch has to slip all the time, wearing it. The same can be said if the two wheels on an axle are more worn than than the pair on the other axle. So some tolerance has to be incorporated to allow the MPT clutch system to have some longevity. Subaru says 1/4 inch on tire circumference, which amounts to only about 0.3 %. My opinion is that it is just to cover their asses. Also, remember, you can yank a fuse to make the car FWD only if you get a flat and have to use the spacesaver spare, because it's NOT the same circumference by a long shot. At a 75 mph cruise, the typical tire size of 215/45 - 17 rotates at 1025 rpm, so the clutch plates scrape over each other at a mere three (3) rpm, if there is an overall 0.3% difference between front and rear tire circumferences. That's one revolution every 20 seconds, or three times faster than a sweep second hand on an analog clock. If only one tire is replaced, then that axle turns only 0.15% slower than the other with the worn tires, and the differential on the axle with the new big tire takes up the difference between tires. No biggie. Subaru and the other AWD manufacturers know this tiny difference doesn't amount to a hill of beans, but won't come out and say what the real tolerance is. I'd bet it's easily twice as much. The thing to remember is that different makes of tires with the same nominal size can be quite different in actual size and tread width, so get at least the same make and model if you have to replace one. I wouldn't worry until the tread depth difference is at 3/32nds or more. Then shaving is called for. Put the new tire on the front to make it wear more quickly to the other old tires' size. For manual tranny Subies, the center differential will look after the mismatched front and rear wheel axle speeds. The viscous limited slip device connected between the front and rear output shafts of the center diff won't work up much of a sweat with a 5 or 10 rpm relative speed difference. 100 rpm or more, sure, that's what it's there for, to lock up. With the 5 speed auto in older Legacy GTs, 3.0 and 3.6 six cylinder vehicles, and early 4 speed WRX autos, there is a center differential, like the manual tranny cars. There is also a lockup clutch to lock the front and rear driveshafts together when conditions warrant. There is no fuse to pull to put the car into FWD as there is with the much more common MPT setup in an average Subie, because it isn't needed due to the center differential. After some 2 1/2 winters and 9000 miles on my Legacy GT auto, my front right tire expired with a large nail through the upper sidewall. Unrepairable. A new Toyo winter tire was 3/32 deeper in tread, and the Subaru dealer installed it. On the front. I have had no troubles at all. Car tracks straight in the almost two winters since. Didn't expect to, either. A reasonable amount of common sense has to be applied. I have only had AWD cars since 1988, and haven't had to replace so much as a CV boot, so where all this extra maintenance is supposed to happen compared to FWD is beyond me. Having traction all the time is a true luxury, unlike leather seats or real wood inserts on the dash. I am constantly amazed at the poor traction of FWD when I travel in friends or colleagues cars in snowy conditions, in fact it's white knuckle time for me. Sure, if you live in the sunny south, then two-wheel drive is fine, I guess. I prefer my real luxury to be on my side, no matter the weather.

    • See 1 previous
    • Rpn453 Rpn453 on Feb 13, 2012

      "The thing to remember is that different makes of tires with the same nominal size can be quite different in actual size and tread width, so get at least the same make and model if you have to replace one." Sometimes that's not even enough! From a forum I freqent, here's a situation that an actual tire engineer encountered when four new apparently-same tires were installed on some sort of AWD Cadillac: "Last week I got a call from a Cadillac dealer on a 2008 AWD. He had installed a new set of tires and within a week the guy was back with tranny noise. The tech diagnosed this as tires with different diameters - about 0.4". Same size, same make, same model, same speed rating, but different plants. They called to ask about next steps. I arranged to have the tires shipped to me. They arrived today! Not only different plants but the date codes were only 8 weeks apart - AND - the earlier tires were made of rayon with a single nylon cap while the later tires had polyester and 2 cap plies. And I could see the difference without even mounting the tires up. Arrgh! $10K to fix everything!"

  • Trucky McTruckface Trucky McTruckface on Feb 13, 2012

    I owned one AWD vehicle - a fourth-generation 4Runner. It was great, but it also cured me of any notion that AWD is remotely necessary anywhere south of I-70. The few times it could really come in handy don't make up for the added expense.

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