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Replacing tires on AWD cars
Cars equipped with Haldex-type AWD systems (e.g. Volvo V70, Audi TT)require replacement of all four tires at once.
Supposedly this is because slight differences in tire diameter can cause Haldex clutches to remain engaged and thus overheat.
Anyone know if this applies to AWD systems that do not use the Haldex system such as the Quattro IV on my '98 A4?
As it so happens I did replace all four when one of my Goodyear F-1 Steels got a cut sidewall and I found out that Goodyear no longer makes replacements.
This information appeared in the Technical Correspondence section. I believe it was the June 2002 issue.
Supposedly this is because slight differences in tire diameter can cause Haldex clutches to remain engaged and thus overheat.
Anyone know if this applies to AWD systems that do not use the Haldex system such as the Quattro IV on my '98 A4?
As it so happens I did replace all four when one of my Goodyear F-1 Steels got a cut sidewall and I found out that Goodyear no longer makes replacements.
This information appeared in the Technical Correspondence section. I believe it was the June 2002 issue.
2001 BMW 330ci/E46, 2008 BMW 335i conv/E93
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Comments
2001 BMW 330ci/E46, 2008 BMW 335i conv/E93
I'd guess that even the grip rates of different tires would affect the system, not only the size. Many of these systems are not designed to operate FULL TIME, and different sizes or grip rates might cause that to happen.
If the tires are worn at all, I'd replace the whole set, or try to shop for a used tire with about the same mileage as yours (good luck).
-juice
-mike
Since the clutch pack you're trying not to fry is between the front and rear driveshafts, this means a smaller tire on either axle will not activate the clutches.
IdahoDoug
-mike
ATs get an 80/20 torque split on non-slip surfaces
MTs get a 50/50.
ATs can go to 60/40, mt's range from 40/60->60/40
VTD/VDC equipped ATs are 45/55 split and no FWD fuse.
-mike
Excellent! This is the kind of stuff I've been wondering. Is there a good tech writeup of the Subaru AWD system that applies to my '97 Legacy system? I'm curious about reaction times, how long it stays engaged at the different torque splits after slippage, etc. I've been under a misconception about Subaru systems for years - thought they were a simple viscous coupling system (former Quattro owner bias here...) before a month ago when I bought 2. Can't seem to find any details beyond marketing fluff like "transfers torque for the best grip....blah blah". Thanks in advance.
IdahoDoug
For the MTs it's viscous center diffy, purely mechanical.
Pre-00 on legacies it used driveshaft sensors to move power between front and rear based on variances in speeds.
00->02 on legacies it moved to using speed sensors on teh driveshafts, weight sensors, and "learned" techniques for shifting power around.
That's for the std. AT trannies.
It's late or I'd go on about VTD/VDC etc.
-mike
But Subies beat Audi Quattro in the snow. Torsen diffs don't work when there is zero traction going to one side. It fails to transfer power and can even ruin the diff.
That's why Audi now uses traction control in addition to Quattro.
Without it, any Subaru AWD will be better on ice.
-juice
I'd be curious about things like reaction time, how fast can it go from the dry road torque split to the max opposite split, what's the torque splitter mechanism (series of wet clutches, how activated, etc)?
On the rear LSD, I was not aware my '97 could have one - any idea how to know this? Previous vehicles with LSD had a metal tab on the rear diff and / or a drain plug with the info permanently stamped on it. My manual does not mention anything, and I've just changed mine and a '96 and neither had any info on an available LSD or any sign on the diffs.
Thanks!
IdahoDoug
-mike
http://i-club.com/forums/showthread.php?s=&threadid=196455
-mike
Great stuff - thanks. I'll do my own searches on the net if you don't have this, but do you know if there's any other technical info around? Things like how fast it reacts to slippage, what the mechanism is, etc. As you know, I recently took the rear driveshaft off a '96 Legacy I bought for my mother and have it in FWD mode. Unfortunately a death in the family ended the plan to tow it to MI for the time being (supposed to leave today), but I'm looking for the information for myself regarding my '97. Normally, I buy a factory manual for every car I've ever owned and read it cover to cover but the Subaru manuals I skimmed at the dealer were apallingly vague. So, I'm in the market for a good aftermarket manual and good "techi" Subaru websites. Got any hints?? Thanks!
IdahoDoug
-mike
Thanks - if you run across any further info let me know. Manufacturers are often fond of publishing how fast their systems (ABS, suspension, torque transfer) monitor. It's easy to have this be fast as it's simply a matter of comparing a few streams of data. But the ability to react takes development money and pricey servos. That's what I'm interested in as it determines how fast the system can actually change the vehicle's behavior and unfortunately this information is rarely available. Seems like this is the case here as well.
Anyhow, let me know if you run across more details.
IdahoDoug
-mike
-mike
IdahoDoug