On my Z06 there is a diameter difference of about 0.75" between the front tires and the rear tires. I'm going to 18" wheels on all four corners. What is the range of acceptability/tolerance of the Active Handling system as far as tire diameter differential is concerned? Do I need to maintain a wheel-tire ,front to rear, RPM differential?? It would be very uncool to have the system correct nonexistent understeer at 120 mph. Thanks. :D :steering:

joel(Bink) :cheers:

 

:lurk:

 

I don't think you'll find a clear-cut "limit" spec on this one.

I've read of people that have had no problems running the same tire on all 4 corners, some who have, and even some that have had problems with 1/2" larger rear tire.

If you're going to 18's all around, a 265/35/18 or 275/35/18 front and a 295/35/18 rear will keep the overall tire height ratio very close to stock, and you won't have problems of this nature. If you are thinking of a different sized rear tire, post what you plan to run, or research the sizes, and try to keep the 3/4" to 1" height stagger and you should be fine.

:cheers:

 

Hmmmm ....

I think the tolerance is about 11 km/h or 7 mph speed difference between the wheels.

Assume for a sec that GM had decided to put identical tires on all four wheels. Let's say you make a hard right turn. The inside wheels (RF and RR)will be turning more slowly than the outside wheels (LF and LR). So, they had to allow for some difference in wheel speeds, regardless of tire size, when deciding how AH should look for over/understeer conditions . In addition, GM knows they put larger diameter tires on the rears than on the fronts, compounding the problem for them. So it looks like other than using the wheel speed sensors for determining road speed (basically averaging all four inputs) the wheel speeds aren't really crucial to AH calculations

The reason I suspect you can get away with up to a 7 mph difference is looking at DTC codes C1221 through C1224. The EBCM looks for failing wheel speed sensors by looking for the following scenario to occur for at least 2.5 seconds ...

* The suspect wheel speed equals 0
* The other wheel speeds are greater than 5 mph (8 km/h)
* The other wheel speeds are within 7 mph (11 km/h) of each other.

I suspect the 7 mph spread allows for differences in tire size and cornering wheel speed differentials.

Now I'm basing this guess on reading the explanation of the AH system in the vehicle Service Manual. It is quite possible that there may be other issues involved here, but I cannot see any documentation in the diagnostic sections that specifically discusses different tire sizes on the car.

In addition, the description of the AH system seems to indicate that the main inputs used to calculate when AH should be invoked are a comparison of the desired yaw rate (calculated from steering wheel angle, road speed, and lateral acceleration measured in G's) versus the actual yaw rate (Yaw rate sensor Input in degrees per second)

Another place wheel speed is important is in looking at the ABS system, but it seems from the description that each wheel is handled individually, so different tire sizes, at least from what I can see, shouldn't affect this either.

Now, you do the math and figure out what speed changes you'll introduce by changing the circumference of the front tires ... remember E=mc(squared), though how that will help I have no frickin idea !!!! :lolg:

Have fun,

Steve

 

Only other item I could find was this --- in the TIRES AND WHEELS section of the Service Manual:

Replacement wheels must be equivalent to the OEM wheels in the following ways:
* load capacity
* wheel diameter
* rim width
* wheel offset
* mounting configuration

A wheel of the incorrect size or type may affect the following conditions:
* Wheel and hub-bearing life
* brake cooling
* Speedometer/odometer calibration
* vehicle ground clearance
* tire clearance to the body and chassis

Thus sayeth the manual ...... :yesnod:

One other thought ... maybe someone familiar with a scan tool would know if the PCM and/or the EBCM has the wheel sizes coded in them ... one would think the EBCM has some knowledge of the wheel size in order to convert the pulses from the wheel speed sensors to a calculated road speed. Same with the PCM, it knows from the transmission speed sensor how fast the tranny output shaft is turning, but it would have to know the wheel diameter to convert this to road speed. Possibly this can be reprogrammed in the EBCM and/or PCM with the scan tool ??? Anyone know ???

:flag

Steve


[Modified by BlackZ06, 3:34 AM 5/30/2003]

 

Given my understanding of the system, A/C basically examines the delta between yaw sensor input and steering input - or the movement vector of the vehicle is not aligned close enough with the driver's intention as measured by steering input.

Should have nothing to do with tire sizes.

Pete

 

Y2Kvert4me- I think I'll go with Michelin P Sports 315/30/18 and 285/30/18 if the 315 will fit. I believe there is about a 1/2" difference in diameter. What do you think???? Thanks for the help! :yesnod: :thumbs:

BlackZ06 -thanks for all the info. Great post! :yesnod: I need to read my shop manuals :D Now, you do the math and figure out what speed changes you'll introduce by changing the circumference of the front tires ... remember E=mc(squared), though how that will help I have no frickin idea !!!! ....TOO DAMN FUNNY!!!! :lol: :lolg:

pmarone - it does seem to make a difference as there have been several posts in regard to tire size and active handling recognition. Maybe one of the Gurus will chime in.Thanks for the input! :yesnod: :thumbs:
joel (Bink) :cheers:

 

I drive to the drag strip with my DRs' and they are the same diameter as the front tires 25.7". The trip is 80 miles each way and I have not had any problem with the Active Handling System.I do use the predator to adjust for tire size and it does keep the speedo accurate. I don't know if it has any effect on the AHS.

 

I would say most likely you'll be fine with those sizes.
Someone on the forum is running the 315/30 Pzeros with taller front tires than the 285/30's, and claims no problems....but again, that doesn't necissarily mean you won't.
The 285/30 is your best bet for the fronts, you won't find anything shorter.

:cheers:

 

Thanks Man!Hopefully the wheels will be here soon .Yours look Great! :yesnod: :yesnod: :D
joel (Bink) :cheers:

 

I've also heard a good rule of thumb is to maintain at least 1/2 inch diameter spread F/R...

 

binksZ06 I am running 18's on all 4 corners. Just the other day the road that was leading onto the freeway I was taking it very fast and softly applied my brakes. To my surprise AHS kick in and the vette handle superbly. I felt the vibration on the brake pedal. :auto: :cheers:


[Modified by BodaciousBlue, 3:04 PM 6/3/2003]

 

BodaciousBlue - Thanks for the info!


MANNY- But if you choose wider tires on the rear than on the front, on the same diameter rim, such that the rear tire diameter relates to the front´s just as the originals did,
you´ll be fine. This is what I'll try to do. :yesnod: :yesnod: Thanks!
joel (Bink) :cheers:

 

I read in one of the Vette Mags (don't remember which, but wrote the numbers down) that the ratio between front and rear tire overall heights must be 96% or greater otherwise the traction control will engage on corners. Followed this when upsizing my rims/tires and have no problems.

 

Thanks FKING1! :D :lol: :lol: :lol: :lolg: ( FKING1= Too Funny)
joel (Bink) :cheers: :steering: