Re: Tire Diameters and Traction Control

["Calvin Hare" <harem3@xxxxxxxxxxx>]

Bob writes:

>From that [the measurements below], you can see the stock sizes 
>already have a 1 % difference between the RPM/Mile with the fronts 
>rolling FASTER than the rears. If I went with the 235/40s in front, 
>the fronts would now turn 2.3 % faster than the rears.  I thought 
>about how the traction control works and did a little mind 
>experiment. If the stock tires have a difference in rotational
>speed, why doesn't the traction control come on? Simple: the traction
>control comes on when it detects the rear wheels are spinning FASTER
>THAN THE FRONTS!  Remember the system only affects what happens
>to the rear wheels by decreasing engine speed or applying the brakes.
>
>Stock Front
>=======
>Size:		225/45-17
>Dia:		24.97"
>RPM/Mile:	808
>Error:		0 %
>
>New Front
>=======
>Size:		235/40-17
>Dia:		24.40"
>RPM/Mile:	827
>Error:		-2.3 %
>
>Stock Rear
>=======
>Size:		245/40-17
>Dia:		24.72"
>RPM/Mile:	816
>Error:		-1.0 %
>

*****************************************************************

I believe just a slight correction is in order.  The info above 
indicates that the stock front tire (225/45-17) is actually slightly 
*larger* in diameter than the stock rear (245/40-17).  These means in 
stock form, the REAR tire is indeed turning FASTER than the front by a 
theoretical 1%.

I believe the individual that experienced ASC+T problems had mounted the 
235/40's on the rear with the stock 225/45's on the front.  This would 
result in the rears spinning 2.3% faster than the fronts which was 
evidently enough to cause problems.

Your setup probably works because the rotational difference between the 
235/40 and the 245/40 is only 1.3% which is probably just within 
tolerances.  Also, as you proposed, it may indeed make a difference that 
you have the faster spinning tire in front.

Just my humble and certainly unresearched thoughts on the subject...

Calvin Hare
'96 M3 - 245/40's all around - No ASC+T Gremlins


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