Piston Slap: Financial Traction, AWD Distraction?

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.

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.