Whats the deal with the quattro and changing tyres then

[Nutkin]

Audi service say my front tyres are worn.

How do i change these ive seen people say axel at a time and all 4 at once and all the rest.

i could spread to axel but all 4 thats gonna be pricey.

 

[PetrolDave]

The Audi recommendation is that the rolling radius of all 4 tyres must be within a small tolerance (something like 3/32" IIRC).

This is to prevent windup in the centre differential which would be caused by tyres with different rolling radii.

So if yout front tyres are worn but your rear tyres are pretty new then changing only the front 2 will be OK, but if the rear 2 are say half worn then you really need to replace all 4 to prevent damaging your centre differential (it's a Torsen type on the A4).

 

[Nutkin]

So it's worth getting some part worms for now

 

[adamss24]

Dont worry that much mate, they are ok if you keep the same size tyre...trust me, they work ok ! its a bit of overkill to replace all 4 tyres at once but it helps traction as they should grip in a similar maneer. I have ran me a4 with 18" alloys with shity tyres on the back and good ones on the front for over 30k with no damage to the centre diff and now i have a space saver 17" on me back for a few weeks while i'm sorting the alloy wheel. I Just take it easy in tight bends as the grip from the space saver is considerably less than the 8.5 " wide tyre and the car is un-nerving !

 

[paul20v]

Audi service say my front tyres are worn.

How do i change these ive seen people say axel at a time and all 4 at once and all the rest.

i could spread to axel but all 4 thats gonna be pricey.

Just change the fronts , none of the tyres wear even it always either front or rear dependant on what you changed last
if you think about it as well you only wear 6.4mm off your tyres in there whole life if this has an effect on rolling radius you would be hard pressed to measure the difference,
audi only scare monger as there wanting to sell tyres as they also say in the owners manual to get your side light bulb changed at a dealer and we all do that dont we

 

[mikeh126]

Both adamss24 and paul20v are correct - the varying degrees of tread wear does not matter.
From the horses mouth (i.e. my A4 Avant owners manual):
" On vehicles with four wheel drive, all four tyres must always be fitted with tyres of the same type, make and tread pattern, as otherwise the driveline can be damaged by continuous differences in the wheel speeds. . . . . . . . . .The use of tyres with varying degrees of wear will, however, not affect the four wheel drive transmission."
So, as advised, - just change the two front tyres and sleep easy!!!!

 

[Mike B]

I think it is more important to keep correct tyre pressures.My quattro came with less than 18psi all round and its obvious that tyey were that low for a long time because they have not needed any more air in the last yesr since then so its was neglect not leaks .I often hear of owners who never ever check pressures or even look to see if the tyre wall looks bulgy which was the case with an MR2 owner this week who had 16 in one rear and 36 in the other with 170bhp powering them

 

[quattrojames]

I agree, my theory has always been that the discrepancy between the front and rear wheels is more when you turn on full lock in a car park than it ever would be having differently worn tyres. I'd always try and avoid brand new tyres with legal limit tyres though.

 

[Nutkin]

well got the car back and measured tread depth, when they said i had 2mm left what they were actually saying was i have 2mm left till the legal limit and even then i measured 4mm accross the fronts, ****** audi this is why i didnt use them for the major service everytime they find something to try and charge me extra for.

At least i don't fall for it but it makes me wonder how many do!

said my rear breaks were 80% worn too which is a bit of an exagerations, but im gonna change them myself soon as caliper winding is easier when its not completely shot.

 

[PetrolDave]

I agree, my theory has always been that the discrepancy between the front and rear wheels is more when you turn on full lock in a car park than it ever would be having differently worn tyres.

You make full lock turns far less than you drive in a straight line.

if you think about it as well you only wear 6.4mm off your tyres in there whole life if this has an effect on rolling radius you would be hard pressed to measure the difference

All the tyres are wearing together, at slightly different rates, so the differences in rolling radius are small.

If causing damage to the centre diff isn't an issue why do Audi Breakdown (RAC) nor recover quattros using a "spectacle"?

 

[Tooks]

You make full lock turns far less than you drive in a straight line.


All the tyres are wearing together, at slightly different rates, so the differences in rolling radius are small.

If causing damage to the centre diff isn't an issue why do Audi Breakdown (RAC) nor recover quattros using a "spectacle"?

Maybe because in that scenario, two wheels would be stopped completely, not like when all four wheels are turning like when driving?

I'd go with the advice in the manual, if it was really a diff killer to run different tread depths on a quattro, then I'm sure Audi would be clear about it either in the manual or at the dealership come service time.

The centre differential is just that, a differential, it's whole purpose in life is to cope with differences between wheel speeds isn't it?

 

[PetrolDave]

The centre differential is just that, a differential, it's whole purpose in life is to cope with differences between wheel speeds isn't it?

If it was an "open" differential then what you say is true, but it's not an "open" differential.

It's a Torsen differential http://en.wikipedia.org/wiki/Torsen which is "open" under approximately equal speed conditions but moves to being a "locked" differential when needed.

 

[Tooks]

Yes, I know it's a Torsen diff, but that doesn't mean it locks up at the slightest difference in wheel rotation speeds front to rear caused by different tread depths.

As your own Wiki link shows, the diff can have a preset Torque Bias Ratio (TBR), that defines under what conditions the diff will 'lock up'. This latitude is presumably why Audi say it's ok to run tyres with different tread depths without any adverse affects.

Later quattro's use even more sophisticated versions of the Torsen, which enable more dynamic torque transfer without the diff locking.

I'm perfectly happy for people to do what they want with regard to tyre setups on their cars, but I don't like the way a lot of folks are scared into the expense of replacing all four tyres. It's wasteful, and as Audi themselves say, unnecessary. In the OP's situation, do you think the garage would have missed the opportunity to recommend he change all four tyres, rather than just two, if it were thought necessary?

 

[PetrolDave]

OK you have it your way.

I guess working for a car design consultancy means I don't know anything...

 

[Tooks]

Don't take it personally chap, I meant no offence.

I don't work for a car design consultancy, I'm only going on my personally gained knowledge from my dad running quattro's ever since the UR, my own experience, and of course what Audi themselves say.

Yes, tyres are cheaper than diffs, no question, but I still don't want to change them if I don't have to.

It's just a car forum, and it's a place to air different views. As I said, people should do whatever they feel comfortable doing.

 

[paul20v]

If causing damage to the centre diff isn't an issue why do Audi Breakdown (RAC) nor recover quattros using a "spectacle"?

Because if your using a spec lift 2 wheels are locked when the others are turning at whatever speed the car is being towed at !
this comment has no comparison to a slight difference in rolling radius from a worn to new tyre .

 

[paul20v]

OK you have it your way.

I guess working for a car design consultancy means I don't know anything...

It also doesnt mean you know everything ,i could also say ive been working on and running quattros of all types for 21years so i know nothing too ,
also we have been down this route with other threads you do what you want and others do what they want .

 

[Nutkin]

Hey im a designer for Jaguar cars and i havnt a clue. lol

 

[Tura01]

Audi service say my front tyres are worn.

How do i change these ive seen people say axel at a time and all 4 at once and all the rest.

i could spread to axel but all 4 thats gonna be pricey

Hello gents

my Audi A3 Quattro manual clearly states all tyres must be same type, size and pattern.

see photo attached from the official Manual.