carefulnow

C6 Z06
Continental 285/645 R18 front, 325/710 R19 rear (same as Lawdogg)
-3 camber front, -1.9 rear

Question for anyone who has run much taller rears than OEM.

Just rolling around the neighbourhood to make sure no parts are falling off of obviously wrong after doing an alignment and replacing rear shocks, something I noticed is that ABS is kicking in almost immediately on moderate braking from very low speeds (~45-50) mph.

When driving I just assumed this was just a result of cold slicks (100 degree tire temp per pyrometer) with a lot of camber on a cool (70 degree) day as I can hear chirping of the tires.

This is the first time I've had scrub slicks on so I have no way to judge what is normal when cold.

But then I remembered some folk writing about all sorts of tire height ratio problems... which I had thought were limited to messages like "Engine Drag Control Active" and am now worried that this ABS engagement might have been a symptom of tire height instead? Is there anything I need to do re: computer parameters for this tire height ratio difference?

Lawdogg

probably just cold slicks. Once warm it will be much harder to get the abs to activate.

Bill Dearborn

Front tire diameter is smaller than stock diameter by a few tenths (25.3 vs 25.7( while the rear tire diameter is over and inch larger than the stock diameter (27.95 vs 26.7). That should definitely bring on engine drag control as the EBCM will see the rear wheels are turning to slow in relation to the front wheels. Under off throttle conditions EDC would add throttle to try and bring up the rear wheel speed (it thinks the rear wheels are sliding under engine braking). Under braking I am not exactly sure what will happen but it will definitely try and correct a wheel speed difference that isn't supposed to be happening. It might ABS the rear wheels to do that.

Pull the ABS fuses and see if it happens after that. No way to program around the issue since the EBCM programming isn't accessible to tuners (even if it was they wouldn't know how to adjust it).

Bill

Lawdogg

Never had an active handling or abs issue with this setup.

froggy47

If you would calculate or look up the difference in height rather than just the sidewall specs that would help.

Having said that, IMO ABS (as opposed to traction control or active handling) is still governed by wheel speed indicators at each corner so when any wheel locks up the abs releases it as needed.

You should never be locking up a wheel under moderate braking unless you have one on ice or such regardless of mismatched front/rear stagger. I have run MANY f/r sizes that are outside the norm. No ABS issues.

So does a c6 engage ABS BEFORE wheel lock up? If yes then that's news to me.

Please educate me.

trackboss

My experience with slicks was that they are like driving on ice until they get up to temp. Unlike R compound, which at least have some grip when cold.

carefulnow

Took it out again after tweaking ride height and redoing alignment, definitely just chirps the tires braking easily then abs kicks in after that so must just be tire temp related.

Sorry was just anxious re: tire heights since the rears are admittedly very tall, but in retrospect I can see this should have zero effect on ABS.

froggy47

Could very well be in many parts of this big country. Even different parts of big states. Sometimes I (and others) forget the extreme variations in ambient temps. I guess my slicks come up to temp backing down the driveway. I have been using quite a few Continental slicks recently. No issues. They do (of course) initiate abs sooner than my a7's but definitely not under moderate braking.