Date: Wed, 29 Sep 1999 11:47:31 EDT
From: PbPied@aol.com
<<Someone is going to have to explain to me why a simple change in tire
diameter could have any effect on an ABS car's ability to function. In the
case where a larger tire were used, the system should "see" it as just
lower speed (reduced wheel RPM). The opposite would be true of a smaller
tire diameter.
That does, of course, assume that the relationship between front and rear
tire sizes remains constant. If a car came with 195/60's and someone put
205/55's all around, why would that cause a problem? Now if someone does
as I did on my Colt and put 225/50's on the front, and 185/60's on the rear,
that might send an ABS computer into a meltdown mode.... (no, the old tech
'89 Mitsubishi Made car does not have that sophistication... or a suspension
worth a hoot either...) >>
In many systems, they only use speed difference, and then as long as the
tires are the same front to rear, or the same ratio as stock it would be
fine, but there are more "intelligent" systems that look at rate of change,
and these can be slightly confused by tire size changes. A given rpm/sec
change would mean a different amount of G force. These systems are designed
to reduce wheel slip on changing surface conditions. Unfortunately, when you
put on super grippy tires they still release the brakes at the max G's the
system was calibrated for. I sure hope these systems are dissapearing from
the world. Then there are the top of the line systems that also add a G
sensor in the car to compare each wheel to. These can achieve the best
results with different surfaces and tire grips, but once again, the speed
calculated from the G sensor would not match the wheel RPM reading with
different tire sizes. This could cause reduced performance, but with good
programming it could self calibrate quite easilly during a no G coast.
What I am getting at is there are many different systems out there and from
the ones I have driven, they can act very different. If different size tires
screw yours up, you may want to try disconnecting the battery to get it to
re-calibrate. If that doies not work, you may be out of luck without stock
size tires. Oh course only diameter will have an effect, so look at the rpm
at 60 mph figure in the tire specs.
Gary M.
My race cars have not had ABS. My minivan does, and they stink. They release
pressure long before my Yok A540's even start to slip.
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