Problem with changing wheel size.

[stimpy]

Hi there
Im new to this forum and have a question. have searched precious threads for an answer but i am stumped.

I picked up some 18" sline alloys for my A6 quattro sport. The tyre profile was the wrong size to fit under the front wishbones 50 series instead of 40.
So i decided to fit the rear wheels with the 50 series tyres, as there is room to fit them ok. Then drive to the tyre shop to change the fron 2 to 40 series tyres. All sounds straight forward, rear wheels fitted ok with no clearance problems. Set of to the tyre shop, but only got a couple of hundred yards down the road when the abs warning lit up on the DIS.
Confused i took the wheels of again to check i hadn't damaged the abs sensors. All ok, refitted the 18"'s. Again the abs lights up.
Now here is the weird thing, i put the old 16" wheels back on, took it for a test drive and all is fine????

Any thoughts?

 

[jwhat]

Quattro's should keep the same size tyre on all wheels so as not to wreck the diff, so i'd suggest to change all the tyres to the 40 profile before putting the wheels on the car.

 

[scott b]

its throwing the fault up as the wheels are turning at different speeds, this also happens when you put a fwd or rwd car on a rolling road as one axle wont be turning and one will be

but as above, all the tyres should be the same size and within a couple of mil of each other to prevent diff damage

 

[driftanimal]

agreed with all statements, its as said rolling radius of front to rears was different .... you do need to change the tyres to all same rolling radius.