Question for Former Suspension Eengineer

[Ruf-Racer]

How does the AH/TC system monitor wheel speeds and what is the front to rear wheel speed delta "window" allowed by the system? I am hoping to find an 18" wheel/tire choice solution that's totally transparent to the Z06 electronics.


Cheers

R

[DJWorm]

There are speed sensors in each wheel hub which feed data to a Universal Data Bus

The critical demension is the tire's OD
F/R "Delta" (% difference) should be <5%

Doesn't matter what the wheel size is...for example any combination of :

275/40x17 F
305/30x18 F or R
315/35x17 R
335/30x18 R

are all OK as they are all 25.5" OD = 0% delta

I recommend for track use:

C5 Z06:
305/30x18 on 10.5x18 with +0.25" offset FRONT
315/35x17 on 11x17 with +0.25" offset REAR

C6 Z06:
305/30x18 on 10.5x18 with +0.25" offset FRONT
335/30X18 on 12x18 with +0.25" offset

[Ruf-Racer]

I understand the wheel sensors and the data bus ok.What I am looking for are the upper and lower limits of the comparator window.

DJWorm wrote:
F/R "Delta" (% difference) should be <5%


I have seen at least one case where the % difference was < 4% but there was AH intrusion. I think the stock wheel/tire combo presents a difference greater than all 18's with Nitto tires. That seems to be my issue. What are the acceptable operating limits..over and/or under? Does that make sense?

Cheers

R

[DJWorm]

There is no "lower" limit ie. equal size F/R tires have a theoretical limit of 0. The upper limit is 5% TOTAL. This includes the overall diameter difference AND slippage.

IE even wiith a 0% tire OD difference you can still get AH/TC intervention if the slippage is high and nets a 5.5% speed difference. Therefore tires with a 4% difference will activate more often with a smaller margin for slippage.

Acceleration, braking and turning increase speed differences.

This can all be negated by turning the TC off as in Comp. Mode or both OFF as mnost experienced drivers do on dry tracks.

The thing that bothers me the most is the dreaded "Ice" Mode of the ABS. This seems to be aggrevated with the "stickier" the tires are ie. lower UTQP ratings.

[allanlaw]

Originally Posted by DJWorm

There is no "lower" limit ie. equal size F/R tires have a theoretical limit of 0. The upper limit is 5% TOTAL. This includes the overall diameter difference AND slippage.

This doesn't sound right. I am sure that the relevant algorithm "knows" that the stock difference between front and rear diameters is about 4% higher in the rear, and treats that as the norm. Thus, if there is 0 difference, it will trip the TC with much less provocation than in the case of running stock diameters. I have observed this with my C6Z06 and different wheel combinations (and it also makes logical sense). Driving along on the freeway with only a 2% spread at 85 mph engaged the TC and it stayed engaged until I turned it off (dry surface, no braking or acceleration, or any other disturbance).

[allanlaw]

Originally Posted by DJWorm

I recommend for track use:
C6 Z06:
305/30x18 on 10.5x18 with +0.25" offset FRONT

Have you actually tried this on a C6Z? Are you sure it will clear the front of the wheel housing. It's mighty tight in there.

[RocketSled]

The computer system expects the front tires to turn faster than the rear tires by about 3% (the ratio of the front 25.35" to the rear 26.13" diameter delta). If you change the diameter of either the front or rear tire, you change this ratio. Common wisdom says the system will tolerate a change no greater than 5%. That means you can go in either direction by that amount. -2% to +8%. However, the system has no "buffer". For each percent change you make you take away that much margin from the system operation. If the car allows 5% slip at the rear before activating TC, and you make a change that decreases the rear tire diameter or increases the front tire diameter by a total of 5%, the TC will cut in as soon as the rear tires start to spin.

[Ruf-Racer]

There is never "an easy way" is there?


Cheers

R