Seanndogg

I heard this will cause some kind of problems using different size wheels like from a corvette. Will it ?

madwolf

Many people in here have Vette wheels and run 18s in the back and 17s in the front. I've never seen anyone having any problems with them.

stik6shift93

I believe the overall diameter is still the same for both wheels, the 17in has a larger sidewall then the 18 so it equals out.

frmula1

well, no, that would only be true if you put a larger tire on the 17's... but i don't see how diameter should **** with the abs anyway...

stik6shift93

It'd mess with the abs because it'd tell the car that the front wheels were spinning at a different speed than the rears.

Josh-'04 GTO

Different diameters=different sensed wheel speeds=computer thinks your in a poor traction situation=undesirable ABS operation.

madwolf

AFAIK, there are no speed sensors on the front wheels....

Computer only cares about the readings from the back to measure speed.

Josh-'04 GTO

Wrong.

It does have front speed sensors.

Besides, if it didn't, how would the computer know if the front wheels were sliding? If the tire diameter is different, then the computer will think the wheels speeds are different. Therefore, every time you hit the brake you would get what's called false cycling.

madwolf

Yes you're right. It does have front wheel sensors, but I am not sure those go to the actual pcm. They might serve just for the ABS because vehicle speed to the pcm and speedo is measured using the rear sensors...

Josh-'04 GTO

The front wheel sensors do send to the PCM. You can get a reading on all the ABS sensors using PID data.

madwolf

Hmmm..... how are they dealing with this problem on the Vettes then?

Josh-'04 GTO

Easy, the tire diameter is taken into account at GM and programmed into the PCM before the Vette is shipped out the door.

You could actually recalibrate the Camaro PCM to do the same thing, BTW.

madwolf

What you're saying doesn't make sense to me. I have been programming computers for a while now and I've always programmed according to the size of the rear wheel. There aren't separate settings for the front and rear wheels.
When I began to do this, that's what everyone said. The speed in the pcm is measured from the rear wheels ONLY.....

madwolf

I also remember someone on the lt1_edit list talking about somehow adapting the rear wheel sensors to work on the front wheels because he had a custom rear to which you could not adapt speed sensors. I remember they were joking about what would happen when the front wheels would be in the air....

Josh-'04 GTO

So why do you have a VSS then? Vehicle speed sensor on the tranny extension housing measures output shaft speed, which is how fast the car SHOULD be going with the programmed tire size. ABS sensors measure ACTUAL wheel speed and is compared to the others. Approx 4 to 5 mph difference triggers ABS operation.