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Replacing tires on AWD cars

andys120andys120 Loudon NHPosts: 16,662
edited March 1 in Audi
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.

2000 BMW 528i, 2001 BMW 330CiC

Comments

  • zueslewiszueslewis Posts: 2,353
    the systems are extremely sensitive. Think of it in a positive light. If you have OEM tires, it's a great chance to upgrade. Check tirerack.com.
  • andys120andys120 Loudon NHPosts: 16,662
    Lots of people don't realize Audi has 2 quattro systems at present.

    2000 BMW 528i, 2001 BMW 330CiC

  • zueslewiszueslewis Posts: 2,353
    even the "rougher" systems like those on the Chevrolet Astro and GMC Safari have shelled transfer cases over tire diameter differences as small as 1/2 inch. Common tires (same size) and rotations are very important - even more so on an Audi, Subaru or other AWD car.
  • Mr_ShiftrightMr_Shiftright CaliforniaPosts: 45,116
    Most shops are wary of selling a single tire to an AWD customer, or even a mismatched pair, for liability reasons. You'll even get flak at Wal-Mart about this.

    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.

    MODERATOR --Need help with anything? Click on my name!

  • ateixeiraateixeira Posts: 72,587
    Subaru says no more than 1/4" difference in rolling circumference. That ain't much.

    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
  • swschradswschrad Posts: 2,171
    I was making an argument that a quarter inch is a lot of rubber in tread depth... and then spied the words "rolling circumference." that's different, quite different indeed. that's Subaru saying buy 'em in sets of four, and your never-used spare is an emergency only tire as soon as you have a few thousand miles on the others.
  • paisanpaisan Posts: 21,181
    The spare is usually a donut tire, on ones with a full-size tire, you would be doing a 5-tire rotation anyway, even if it wasn't an AWD vehicle.

    -mike
  • idahodougidahodoug Posts: 537
    To get home on a smaller diameter space saver spare (actually it may not be a smaller diameter) in an AWD Subaru, open the hood and put one of the spare fuses into the "FWD" slot. This puts the vehicle into front wheel drive only to get home and the rears are just coasting.

    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
  • paisanpaisan Posts: 21,181
    That is only on Automatic AWD Subies. The MT ones cannot be disabled.

    -mike
  • idahodougidahodoug Posts: 537
    True - the MT's use a viscous center coupling, right?.
  • paisanpaisan Posts: 21,181
    Center viscous coupling on the MTs.

    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
  • idahodougidahodoug Posts: 537
    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
  • paisanpaisan Posts: 21,181
    Beyond what's above, I do have some more detail that I posted in one of the other threads.

    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
  • ateixeiraateixeira Posts: 72,587
    Doug: but even in the automatics, the rear LSD (if so equipped) is the viscous fluid-filled type. So I'd still flat bed it if the circumference is way off.

    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
  • idahodougidahodoug Posts: 537
    So, no data available on this stuff for my '97? Any websites (personal or otherwise) to look at?

    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
  • paisanpaisan Posts: 21,181
    I'll try to dig up a post I did about the splits and systems.

    -mike
  • cptpltcptplt Posts: 1,075
    I don't think they had LSDs on Legacys till MY 00. My 98 GT certainly does not.
  • idahodougidahodoug Posts: 537
    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
  • paisanpaisan Posts: 21,181
    It uses electronically controlled clutch-packs inside the 4EAT to control it. I've read but can't find reaction time on your years '95-'99 is something along the lines of "compares axle speeds 5x sec" on the newer post '99 ones it's upped to 10x sec and I think on the pre '95 ones it's 1x sec or thereabouts. I definitely notice a difference between the 90/10 split on my '88 XT6 and the 80/20 split on my dad's '97 legacy. It's much harder to distinguish between my uncle's '97 Outback and my cousin's '00 Outback however.

    -mike
  • idahodougidahodoug Posts: 537
    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
  • paisanpaisan Posts: 21,181
    Will give you a heads up although I doubt I'll find any more detailed info. The info I passed on already is beyond what most other subie nutz know or want to know. Most of them are MT drivers so the 4EAT isn't a concern to them :(

    -mike
  • Mr_ShiftrightMr_Shiftright CaliforniaPosts: 45,116
    Is there any data on what percentage of Subarus are ordered MT and what percentage AT? If so, does it deviate from say the average SUV ratio? Just curious, don't kill yourself digging this up, but if anyone knows I'd like to. Thanks.

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  • paisanpaisan Posts: 21,181
    about the WRXs that show something like 14K MTs and 3-4K ATs. But I think that's cause it's a performance oriented car. My guess is that on the Legacy/Outbacks/Foresters is closer to 80/20 AT/MT I know very few OB/Legacy/Forester owners who have an MT.

    -mike
  • idahodougidahodoug Posts: 537
    Most SUVs do not have an available manual transmission, so the figures would be skewed a bit. In many cases it's due to low ordering, in others it's because the truck manual transmissions engineered for the platform the SUVs are built upon are not suitable for the more genteel SUV buyer. They're a bit unrefined.

    IdahoDoug
This discussion has been closed.