All wheel drive and tires

[BEJinFbk]

350k km with '00 Audi A6 Quattro 2.5 TDI V6 should be more than enough experience for my statement.

Except that your Audi has a Torsen based center diff, not a viscous coupling - We're talking P80 Volvos here.

Also...a friend had a '02 Volvo XC 70 which had around 300k km when he sold it and he JUST took care that ALL 4 tires are the same dimensions.Car was bought brand new and guess what...never had a problem with transmission.

Yes. We agree - Same Dimensions. And tread wear affects dimension.
Also, it's not the trans that fails. The bevel gear and viscous coupling
are the failure points with mismatched tire diameters.

Like i said...thread wear is NOT counting for AWD system as long as all 4 tires are the same size.

You seem to be contradicting yourself. Maybe there's just a language barrier here.

[misha]

350k km with '00 Audi A6 Quattro 2.5 TDI V6 should be more than enough experience for my statement.

Except that your Audi has a Torsen based center diff, not a viscous coupling - We're talking P80 Volvos here.

Could be.I didn't knew that Quattro system from Audi is that much different from Volvo's.I thought that they all work on the same principle including Subaru...more or less.

Also...a friend had a '02 Volvo XC 70 which had around 300k km when he sold it and he JUST took care that ALL 4 tires are the same dimensions.Car was bought brand new and guess what...never had a problem with transmission.

Yes. We agree - Same Dimensions. And tread wear affects dimension.

When i said same dimensions i meant on same dimensions which are written on tire's wall(profile) excluding tread wear.

Like i said...thread wear is NOT counting for AWD system as long as all 4 tires are the same size.

You seem to be contradicting yourself. Maybe there's just a language barrier here.

I'm not.I'm just saying what i wrote.When i say same dimensions i mean on same dimensions which are written on tires walls(profiles).I'm not counting tread wear difference.

However...i guess that doesn't work for Volvos.
Audi's quattro doesn't care about thread wear difference we're talking about.The system is perfect and immortal.

Thanks for clarifying that and for warning about Volvo's AWD for a future reference in case i get one with AWD.

Are ALL of those Volvo's awd systems that sensitive or there are just some years that are affected....or better say infected?

[tom4342]

Thanks for the input gentleman appreciated.

[MilehighV70XC]

Misha--

Could be.I didn't knew that Quattro system from Audi is that much different from Volvo's.I thought that they all work on the same principle including Subaru...more or less.

Yes, same principal, but completely different ways of addressing it. Instead of doing your own research and finding out the correct answer. You are just going off of incorrect assumptions and giving people very misleading and incorrect information that could really damage their car.

When i said same dimensions i meant on same dimensions which are written on tire's wall(profile) excluding tread wear.

Polskamafia said it correctly in the first reply to this post, "Different tire diameters can cause damage to the AWD".
You are using dimension as a very broad term. The "dimensions" matter to a point. But it is the circumference/diameter that matters more then anything. Why do you think the spare tire/doughnut is a different demension but same circumference? A brand new 225/55/16 will have a completely different circumfernce then a worn down 225/55/16. A barely worn 225/55/17 will have the same circumference as a brand new 225/60/16. And theoretically work just fine because the circumference/diameter is the same.

This is only a major issue on 98-00, maybe 2001. After that Volvo changed to a better design around '01 or '02
The Audi Quattro is a whole different animal. I had a A6 and now an '02 allroad. My parents have a 04 XC. Livng in Colorado I have plenty of friends with Subarus, "the car of Colorado",that I have done work on.

[misha]

Misha--
Instead of doing your own research and finding out the correct answer. You are just going off of incorrect assumptions and giving people very misleading and incorrect information that could really damage their car.

I don't need to to "my own research" because i don't have an AWD car.
I gave such advice because i would personally act exactly the same and that would be....i wouldn't pay attention on thread wear difference.
Everything i said,i would do with MY OWN car,because i was convinced that,if one Audi have such a perfect system,why Volvo would be different.
Turned out that i'm wrong because i thought different.
There wasn't any kind of intention to make either damage to any owner....i was just convinced that AWD system is not different than Quattro and i was talking from personal experience with quattro system.

If you guys say that i'm wrong...OK.
MilehighV70XC ...You have experience with both...so i can say that you probably know what you're talking about.

This is only a major issue on 98-00, maybe 2001.

Is '97 850 with AWD excluded?

[matthew1]

Volvo AWD guide, in case anyone's interested.

[misha]

I am...for possible future reference.
Thanks matthew1!

[scot850]

The linked article on MVS on the AWD systems is good and fair. Biggest issue was lack of knowledge and experience at the dealers and owners in the first Gen AWD. With age has come better knowledge and understanding of the system, but we must remember the systems are now almost 20 years old in the case of a 97 850 AWD. So chances are problems can happen and parts can fail depending on how the vehicle has been maintained and driven.

This subject has had me thinking on what actually causes the system to fail? Is the system monitored by the ABS sensors at all, if this is the case I can understand why tire size would possibly be an issue. Volvo do state a maximum wear variation on the tire (diameter).

As all parts of the system are mechanical with no sensors, what causes the system to fail?

Tire wear in FWD cars is not an issue (other than obviously a safety issue), so you would think that would be saying the trans is not the issue. Then you have the angle gear, the prop-shaft, the VC unit and the rear diff.

The prop-shaft I can't see being the issue for tire wear problems. Yes it fails at the front CV joint mainly due to proximity to the Cat and turbo heat, and the rubber at the center support.

The CV could be a weak link but again, it works through the rear diff, so why would wear on a tire be an issue between the 2 rear tires than say on a RWD car? The VC can fail due to extensive lack of use when the clutch packs dry out and tear up, or if the seals leak and they become dry. Also the from bush support if it fails can cause damage to the VC as it takes more vibration. The front bearing is also a failure point causing a whoosh, whoosh noise. One of mine had the bearing fall into pieces on removal!

So where is the issue caused by tire wear variations? Presumably it is if the ABS is used to monitor the AWD at the wheels or the angle gear. So again, the question is what causes the issue with tire diameter variation due to differing wear on the tires?

Using different tire sizes (i.e. diameters like a 60 or 65 on the same diameter or width would be an issue).

I am now more curious on this issue!

Neil.

[misha]

Hmmm...me too.I don't get it also.
Since i don't own Volvo with AWD,i don't have experience with it like with quattro system.

My opinion is that people exagerate about it being that sensitive.Even if tires on one axle are all bold and on the other one brand new but same written dimensions(on tire profile)...we're talking about 10 mm of tread difference.In real life that could be around 5 mm in overall diameter.But still...that's just my opinion and i will find that out with my own AWD Volvo.

[BEJinFbk]

Except for TRACS, ABS has nothing to do with it.

The only real "sensor" in the system is the VC.
When there's a difference in RPM between the
front and rear axles, it locks up and provides
power to the rear axle.

Assuming that this is happening because you're
experiencing poor traction, the car is effectively
driving on a giant clutch. It's only engaged when
things are slippery and that's how it's designed to
work. When traction is regained, you're FWD again.

If the system is engaged on dry pavement because
differing diameters make it engage when it shouldn't,
you have no "give" and things start working against each
other. That's when parts begin to fail.

It's kind of like old school 4x4's and what happens to
the running gear when you drive around on pavement
with the 4-wheel drive engaged and start breaking stuff.