ASR (traction control) overactive with 18" wheel upgrade

[Stephen Wright]

Hi all

I have a 1998 2.5tdi Audi A6 Avant. When I bought the old girl she had 16" alloys. I have fitted 18" Audi Alloys and now the car is impossible to drive with the Traction Control active. If you turn of the traction control it drives as it should. The front wheels are 225/40/18 and the rears are 245/40/18.

Could anyone shed any light on how I can get the traction control to behave?

Many thanks

[Doctle Odd]

Hi Stephen
were any of the abs sensors disturbed when the wheels were changed, can you get the car scanned (VCDS) changing the wheel size shouldn't make a difference to traction/abs

[NickPicks]

You've got different sized tyres front and rear. My quick calculations show you've gone from a factory wheel radius of 316mm to 318.6 on the front, and 326.6 on the rear. Your wheel speeds will therefore be 2.5% different from front to rear (your speed sensors will be saying the fronts are going 2.5% faster than the rears). I'm not sure if this is enough to trip the ASR into thinking you're starting to wheelspin, but it seems that it is. You can calculate this better by finding out the actual rolling radius / rolling circumference of the specific tyres. Companies like Michelin will publish this, but the likes of DiYung tyres probably haven't even heard of it.

I'd say you're best of changing the rear tyres to match the same size as the front.

You can check the wheel speeds using measuring blocks and data logging in VCDS/vagcom.

[Doctle Odd]

You've got different sized tyres front and rear. My quick calculations show you've gone from a factory wheel radius of 316mm to 318.6 on the front, and 326.6 on the rear. Your wheel speeds will therefore be 2.5% different from front to rear (your speed sensors will be saying the fronts are going 2.5% faster than the rears). I'm not sure if this is enough to trip the ASR into thinking you're starting to wheelspin, but it seems that it is. You can calculate this better by finding out the actual rolling radius / rolling circumference of the specific tyres. Companies like Michelin will publish this, but the likes of DiYung tyres probably haven't even heard of it.

I'd say you're best of changing the rear tyres to match the same size as the front.

You can check the wheel speeds using measuring blocks and data logging in VCDS/vagcom.

Well spotted Nick I assumed the circumference would be the same on all the wheels, dragsters excluded. I must read more slowly!

[Peter D]

Yep they all have to be the same size or at least rolling radius. When I reverse down my fairly steep drive my N/S rear tyre drives over a slightly raised drain cover and as the brakes are on the ABS kicks in for a split second. Does it most days. If it does that on a 1cm drain cover it sure will do it with those profiles. Regards Peter

[mrnice]

I've been running 18' rs6 reps with no probs for 4 months, tyres all same size tho and only very slightly larger rolling radius, I'd try same sizes all round as has been mentioned

[NickPicks]

It's not having a different size that causes a problem - it's having different sizes mixed together, although there must be some tolerance in the system, just don't know how much.

[mrnice]

Thats what i meant, same size all round rather than different size front and rear