Audi RS3 tire replacement

OK, my boss needs a little help here and it's beyond my detailed knowledge.

He has a 2020 RS3.  The factory tire setup is 255/30/R19 in front and 235/35/R19 in back, 3-season tires.  The OE front wheels are half an inch wider than the rears.

He bought a set of aftermarket wheels and Michelin Pilot Sport AS4 tires for winter in a square setup (235/35), from a very reputable local independent tire place.  He was never satisfied with those because there was always a shimmy.  The tire shop tried several different brands of tire, ending up with Conti DWS, but could never make the shimmy go away entirely.

The OE summer Pirellis are A) overpriced junk like most Pirelli tires are, and B) the fronts are shot in under 10,000 miles with horrific inside shoulder wear despite no apparent alignment issues.  Teh googles seems to indicate this is a substantial and typical problem with this tire / car.

He's tired of dicking around with 2 sets of tires and wants to go with a single set of high performance all-seasons on the OE rims.

The question is this:  If he mounts a square tire setup (probably in 235/35/R19) on the factory rims, will the slight stretch on the fronts cause problems for the center differential?

If you do the math, the 255/30s have a sidewall height of 76.5 mm compared to the 235/35s sidewall of 82.25 mm, meaning even with the OE setup the overall circumferences are not the same.  So it seems to me that the little bit of stretch in putting the 235s on the wider front wheels will make them closer to that 76.5 mm number.  Plus he put 5,000 miles on the square winter setup without apparent issue to the center diff.

So I'm inclined to tell him it will be OK, but I'd like some review of my thinking.  Thanks in advance.

I have heard in the past that stretching does not really affect the tire diameter like you would think because the radial belts in the circumference of the tire don't stretch, just the sidewalls do. So I would think he'd be fine.

Also, I believe the RS3 is still a Haldex AWD system where most of the time the center diff is decoupled and it's only driving the front tires.

codrus (Forum Supporter) said:

Also, I believe the RS3 is still a Haldex AWD system where most of the time the center diff is decoupled and it's only driving the front tires.

 

This. Something like 95% FWD.

The tire stretch will not materially affect rolling radius.

The Haldex IS going to be applied to some degree or another all the time. Watching pump pressure with a scan tool under various conditions is pretty enlightening.

pointofdeparture said:

I have heard in the past that stretching does not really affect the tire diameter like you would think because the radial belts in the circumference of the tire don't stretch, just the sidewalls do. So I would think he'd be fine.

Well, the question is that with the OE setup, the fronts have a circumference of 1997 mm to the rears' 2033 mm.  That means the fronts are about 1.8% smaller in circumference, stock.  I can only assume that the center diff is just absorbing this delta as a matter of normal operation - I know that it can vector power where needed but I can't imagine a way it can correct for wheel speed differences except by functioning as a differential does.

If he goes to 235/35 all around, that theoretically brings front and back up to the same circumference, particularly if stretch doesn't really make a notable change in circumference.

In reply to Duke :

Haldex units can slip while being applied, it doesn't hurt them any.  Being able to slip under application is part of how they work so smoothly.

If you want to really bake a noodle, the Focus RS's rear unit (GKN, not Haldex) has a different final drive ratio.  When the clutch is applying, it HAS to be slipping!  This is how they can apply greater amounts of torque to the rear than the front even when there is no tirespin present.

Tires slip under load, even if the tires all had the same loaded radius, they would not be rotating the same speed.  In short, it isn't that big a deal.

In reply to Pete. (l33t FS) :

OK, thanks for the details!  It sounds like they're using the center diff to pick up the slack on the staggered OE fitment anyway.

There isn't a center diff, only a clutch, and slip is how it all works.

FWIW this is one of the front tires in question. Fewer than 10,000 miles, inner edge. The one that didn't wear through to the air is about the same. On the one that is still mounted, that worn shoulder is a sharp cutoff at about a 45d bevel. Alignment was checked and is good.

In spec does not mean good for tire wear, just the handling characteristics the chassis engineers wanted to target, and tire wear is just a maintenance item.

or: You have a $70k high performance car, spending $2000 a year on tires is just part of the deal.

Those tires aren't soft or grippy enough to look that bad in less than 10k miles of normal street driving, especially not on the inside shoulder.  If I didn't know better, I'd say they were rubbing on something.

Duke said:

Those tires aren't soft or grippy enough to look that bad in less than 10k miles of normal street driving, especially not on the inside shoulder.  If I didn't know better, I'd say they were rubbing on something.

I agree, that looks like a tire that's been rubbing, I had something similar happen on my B6 S4 about a decade ago.  Tire was nominally the correct size, but was actually 3/4" wider than the OEM full-size spare that was in the trunk.  It rubbed on the spring perch at full lock and did this in a few thousand miles:

Take a look in the wheel well and see if you can find something shiny that shouldn't be.

In reply to codrus (Forum Supporter) :

It is possible, but those are OE tires on a stock car, and it's both sides.  Shame on Audi if they are self-clearancing that drastically.

IMHO: Something is seriously wrong. Take it back to the dealer. Do not stop. Do not collect $200. 

That is what lots of toe out looks like, with one difference.

Work day is done, can actually pull up the specs for an RS3.  Alldata shows 10 minutes (frickin razzin frazzin....  that's .17 degrees) of total toe-in, plus or minus 10'.  Camber is -52' (-.87 degrees) plus or minus '30.

So the absolute worst it can be and still be in spec is a little over a degree negative camber and zero toe-in.

Those tires look like they have spent a lot of time overinflated and underinflated.  Not sure what the Ackerman is like on that chassis, but a lot of city driving with tight turns will destroy the inner or outer edge, depending on which way the geometry is.

For sure, it doesn't look like a toe issue, because the rest of the tread should be feathered to the side if it wore that much that quickly, BUT a toe issue generally presents as only wrecking the edge tread block.  Radial tires can flop over like that.  But maybe negative camber plus underinflation times 10mm high sidewalls equals that kind of wear?

I'd get an alignment check somewhere else.  And please post the numbers.

Alignment info available Monday.

It's a modern car and as far as I'm aware the TPMS system is functional. The owner is not the most organized person but definitely not the type to ignore a warning light.

You can be chronically underinflated and still not set a light.  Not sure about the indirect systems, but direct systems USUALLY trip under 26psi, which is starting to get to where the tires can run hot enough to have carcass problems.  If the spec tire pressure is 38-40psi, you can be low enough to really hurt tread wear but keep the TPMS happy, with any system.

It's all conjecture, of course.  And some tires just wear awfully.

Pete. (l33t FS) said:

You can be chronically underinflated and still not set a light.  Not sure about the indirect systems, but direct systems USUALLY trip under 26psi, which is starting to get to where the tires can run hot enough to have carcass problems.  If the spec tire pressure is 38-40psi, you can be low enough to really hurt tread wear but keep the TPMS happy, with any system.

I believe Audi is all indirect systems these days (they figured out some better algorithms for managing it) and in my S6 it'll go off with as little as 5 psi difference from the spec.