Looking for some quick feedback, colleague has an Audi TT which is 4WD.
Front tyres are illegal and the rears are sitting with about 4mm.
I know in some big 4WDs they talk of replacing all tyres at same time, presumably for diff reasons or whatever, but would she be OK to replace the front two just now and the back ones later?
I suppose at the end of the day, the car is driving fine just now with a 4mm difference between front and back!!
Thoughts?
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This is like the egg-timer question.
Put new tyres on all round. They will presumably wear unequally just like the present set.
When the fronts have worn down to the legal limit, put the old rear tyres on the front. You will then extract an extra 2mm-worth of wear out of them.
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A new tyre has 7 to 8 mm of tread.
At the moment there is approx 2 to 3 mm difference between the fronts and rears.
With 2 new tyres fitted there is going to be approx 3 mm difference between the fronts and rears.
In short, 2 new tyres isn't going to make much difference to the current set up.
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Most permanent four wheel drive vehicles have a central differential. It'll be in action now, and the other way if new tyres go on the front. (Unless you have a Daimler Ferret... No central diff).
Quite a lot of 4wd cars though are front wheel drive, with drive only going to the rear if the front slips.
I'm sure a bit of web research will show what system the Audi has, and if the tyre tread depth is crucial.
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The TT uses a Haldex clutch. It's not permanent 4x4, but FWD with power fed to the rears via the Haldex as required. No centre diff.
I used to drive a Skoda Octavia 20v Turbo 4x4. Same underpinnings in sheep's clothing.
Should be no issue with slightly different rolling diameters.
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Have she read the manual..last two cars have been 4x4 and the manual in both cases gives pretty detailed information.
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With 2 new tyres fitted there is going to be approx 3 mm difference between the fronts and rears.
Shouldn't that be 6mm, if you're considering diameter, or about 20mm in rolling circumference? That about 1 percent, assuming 225/40 tyres and 18" wheels. Enough to matter? Wouldn't think so, especially if it's working today with a similar difference in diameter, but 4WD systems do seem to be sensitive to these things.
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>> Shouldn't that be 6mm, if you're considering diameter
Yes, you're right. I was quoting the radius, not the diameter.
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Any car that can't operate efficiently, effectively and reliably with any combination of legal tyres is not fit for purpose.
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Totally agree zero. I think it's one of the big Volvo cross-over vehicles that can end up with a ruined transmission if you don't replace the tyres together. So get a puncture when tyres are part worn would be a concern - save money and replace only one tyre or replace all of them!
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>> Any car that can't operate efficiently, effectively and reliably with any combination of legal tyres
>> is not fit for purpose.
It would be no consolation if the Audi TT didn't meet your definition of fit for purpose and broke its transmission would it?
In any case, tyres of the same nominal size and different brands can be surprisingly different in size. When I changed the Bridgestones on the CRV for Michelins, the spare wheel cover would hardly go on - they were probably a good inch bigger in circumference.
It's fairly well known that using identical tyres and evening out wear is a good idea with 4 wheel drives, as far as can be generalised.
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>> >> Any car that can't operate efficiently, effectively and reliably with any combination of legal
>> tyres
>> >> is not fit for purpose.
>>
>> It would be no consolation if the Audi TT didn't meet your definition of fit
>> for purpose and broke its transmission would it?
That fits the criteria of not fit for purpose perfectly.
>> It's fairly well known that using identical tyres and evening out wear is a good
>> idea with 4 wheel drives, as far as can be generalised.
So why are they not equipped with two spare tyres? or even four spare tyres?
Seriously the difference between a new tyre and a worn tyre is insufficient to cause the speedo issues, so its not going to screw the transmission now is it. Or it shouldn't.
Last edited by: Zero on Thu 3 May 12 at 23:12
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>>so its not going to screw the transmission now is it. Or it shouldn't.
No it probably isn't. I admit to being a bit OCD about tyre rotating - it does no harm and might do some good.
Daughter and partner had a Haldex failure on their TT. To be fair it was the controller rather than mechanical, but it was a good few hundred quid if I remember rightly.
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I overcome this with two spare wheels for my 4x4.
The fronts wear about twice as fast as the rears. So once a year, I put the two spares on the front. One of the tyres from the front becomes my new spare, and the other goes into my garage.
It may sound wasteful, but I found previously I wasn't using the spare at all, and as I buy all terrain tyres, the tread pattern was often changed each time I bought new tyres - so I ended up throwing the unused spare away (it was/is far too much hassle to do a 5-wheel rotation every 3-6 months). SO where I was previously buying 5 tyres and using 4, I now buy 6 and use all 6.
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I recall from my quattro days that all being replaced together was the preferred option, but that it was OK to replace in pairs on the same axle.
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>> I recall from my quattro days that all being replaced together was the preferred option,
>> but that it was OK to replace in pairs on the same axle.
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That's what Subaru recommend for my permanent 4WD legacy.
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It has a Haldex 4wd system, of which there are several evolutions but AFAIK VAG has always said in its dumbed down explanations of how this works that torque is apportioned according to differences in rotational speed between the wheels. Quite what the threshold or tolerance around this is I have no idea, but if it was mine I'd want all the tyres to be as near the same rolling radius as possible.
Therefore I'd rotate the wheels every 5000 miles or so to wear them out evenly and replace them all at once. If I had your friend's car and planned to keep it, I'd be replacing all four now and getting them rotated in the future.
The Haldex system can probably cope with the situation as it is, but it may not work as well as intended, and if there are any problems they won't necessarily show up immediately and are likely to be expensive to fix.
I believe the dealers tend to rotate tyres on Quattros at services anyway, though I imagine the intervals could be fairly long.
Of course your friend could always consult the book of words that came with the car.
Last edited by: Manatee on Thu 3 May 12 at 18:47
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>>
>> I believe the dealers tend to rotate tyres on Quattros at services anyway, though I
>> imagine the intervals could be fairly long.
Whenever my Subaru is serviced (every 12k so 2.5-3 times a year) the tyres are rotated.
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So what's the point of a differential?
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You need 'em for going round corners :-)
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Kia Sorentos favoured new rubber on the rear due to excessive diff noises if they were put on the front.
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4WD Tyre Replacement - Cliff Pope
