Tire Diameters and Traction Control

[SMILLER@xxxxxxxxxxxxxx]

Bob Lenarcik is unable to post to the Digest and asked me to forward
this.  I've added my comments below.
_________________________________

From:  lenarcik@domain.elided ("Bob Lenarcik")

When the MXX3s wore out on my 97 M3/2 (12k miles only), I decided to
go with Yoko AVS-S1s. I also did NOT want 225/45-17s on the front, but
I was worried about the traction control issue when I read about
someone having similar concerns in the Roundel. I did some math with
the help of a program a friend wrote (- it calculates all sorts of numbers
for doing +1 or +2 tire upgrades. If anyone wants a copy, I can email it to
you.). I wanted to increase the front tire width but I couldn't afford to get
new wheels to mount 245s in front. I also didn't want to put 245s on a
7.5" rim. I compromised and wanted to get the stock 95 size: 235/40-17.

Here's what I found:

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 %

>From that, 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.

If I increased the RPM/Mile differential, it would have NO EFFECT since
the fronts are still spinning faster than the rears. If I changed sizes so
that there is a built in differential where the rears spin faster than the
fronts, the traction control system would be more likely to activate. If
anything, the traction control probably comes on a little later than it
would otherwise.

I went ahead and got the 235/40s on the front and sure enough, unless I
brake the rears loose, the ASC+T never comes on. The ASC+T works
fine when I do break the rears loose under wet or really fun looking
on-ramps. I usually turn the ASC+T OFF unless the roads are wet or it's
raining.

BTW, the car just clocked 33k miles and the tires (on since 12k) have
over 1/2 of the tread remaining. For the money ($180/ea) they are
definitely a better choice than the MXX3s.

- - Bob

> I thought someone else would have already addressed this by now,
>but I haven't seen it, so here's my .02 on the subject.
>
> If the '97 M3 in question has traction control, it is important that the
>front and rear tires be of the same, or nearly the same diameter.  If the
>fronts and rears differ by more than (something like) 1%, the traction
>control computer can read the rotational speed differences as a loss of
>traction and will then try to correct by reducing the power, or whatever
>else the traction control does on those cars.  I'm pretty sure you don't
>want this.
_________________________________

My comment on Bob's post, since he was responding to my post:

Bob provides real world experience with specific tire sizes, which is
better than my *guess* that the allowable difference between front and
rear diameters is about 1%.  There was a post here on the Digest a few
months back from someone who changed tire sizes on a late E36 M3 and
had a problem with the traction control cutting in unexpectedly.  I can't
remember what sizes that person used, and I can't remember if the front
tires had a larger, or smaller diameter than the rears.  In other words,
YMMV.

I also have a spreadsheet for calculating tire diameters, which I
forwarded to the person who had the original question (too lazy to go
back and find out who that was).  It is important to remember that such
calculations are theoretical only, and actual tire dimensions vary by
brand and model.  If you're considering changing tire sizes on any car
with traction control, you should find out the manufacturer's stated
dimensions in advance of your purchase, and you should also find out
what rim width the tire was mounted on for those dimensions.  Diameter
and width can vary with rim width, too.

Bob and I sure hope this information helps someone.

Scott Miller
Golden Gate Chapter
BMW CCA #44977

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