[Badgerbimmer]

Apex will soon be out with Camaro specific 19" wheels that will allow 305s up front on a ZL1 without spacers. I will likely make the switch. However it got me to thinking going with say 305s on the front and 325s on the back the overall diameter of the wheels/tires will go down 1/2" front and back. Presumably the computer in the car is calibrated for the stock overall wheel diameter. How much impact does this seemingly small change create?

[Move_Over]

I run a 305/30/19 square with no issues to the computer. Only thing that would read wrong is the speedometer.

[Glen e]

the diff on your speedo is a couple mph...google a wheel tire calculator....

[Badgerbimmer]

I run 18s on my M3 versus stock 19s. Only difference was speedo. But that was a 2002, old technology.

[Shawnqa800720]

I've been asking a similar set of questions, and would love to see it end up in the "Ask Al" thread. Specifically diameter ration front to rear. I would think that as long as the ratio of front diameter to rear diameter stays close, differences in diameter would only impact the speedo - not a huge deal. What I need to know, and haven't gotten a solid answer yet is, will changes to the F/R diameter ration impact the control systems.

This thread has a bit on this topic, but just discussion mostly.

https://www.camaro6.com/forums/showt...3#post10404173

[Kairles]

I ran 275/35/18 on my V6 1LE for about 7000miles without any issues. Only thing was the speedo was off.

[Roostie]

Originally Posted by Kairles

I ran 275/35/18 on my V6 1LE for about 7000miles without any issues. Only thing was the speedo was off.

I had the same experience with 275/40/18 on my V6 1LE. I'm going to get another set of the same size, or close to it - definitely smaller diameter that the OEM 20" tires.

[Provoste]

I’ve run the equivalent on 285/35 18 square, 305/30 19 square, and 295/30 20 square. All different front to rear OD and overall OD than factory. In all cases, everything works perfectly: PTM, ABS, etc. As Alex said, the only thing that will be off is your speedo.

[Norm Peterson]

Originally Posted by Shawnqa800720

I've been asking a similar set of questions, and would love to see it end up in the "Ask Al" thread. Specifically diameter ration front to rear. I would think that as long as the ratio of front diameter to rear diameter stays close, differences in diameter would only impact the speedo - not a huge deal. What I need to know, and haven't gotten a solid answer yet is, will changes to the F/R diameter ration impact the control systems.

The ABS only cares about how closely the front and rear wheel rpm ratio matches the OE rpm ratio, which comes directly from the wheel sensors and nowhere else (so far).

Speed in absolute terms is irrelevant, since the ABS and everything piggybacked on it has to be able to work regardless of car speed or wheel/tire rpm.


Norm

[Shawnqa800720]

Originally Posted by Norm Peterson

The ABS only cares about how closely the front and rear wheel rpm ratio matches the OE rpm ratio, which comes directly from the wheel sensors and nowhere else (so far).

Speed in absolute terms is irrelevant, since the ABS and everything piggybacked on it has to be able to work regardless of car speed or wheel/tire rpm.


Norm

Thanks Norm!

Do you know if the same holds true for the control systems (stability & traction control)? I would think the stability control is the most likely to be impacted by a difference in front to rear diameter ratio, but I honestly do not know how the system works at the systems level. I'd love to get a Chevy engineers input to put this one to bed.

My concerns are if the calculations / algorithms used in the stability systems are based off of the stock SS diameters, and you change that ratio, minute errors in the control system mean it will not work as intended. That said, the changes may be so minimal that most drivers wouldn't notice. But, if it is impactful, I'd like to know so I can make my decision with data. There are obviously a ton of folks who have made changes to the diameter ratio, and no one seems to be complaining of issues but as I said, the changes may be so small that most folks wouldn't catch it.

Shawn

[Norm Peterson]

Stability and traction control systems are piggybacked on the ABS and use the same sensors. Mostly, they use the ABS HCU in slightly different ways because the HCU is already set up to individually apply and release the wheel cylinders.

I suppose if you put silly-small diameter tires on all four corners AND IF the ABS algorithm has a maximum expected deceleration rate in its calibration (I'm not saying it does), you perhaps could get some odd behavior under hard to extremely hard braking.


Norm

[Stephen12ZL1]

the factory diameter/size doesn't make any difference as Norm stated. The sensors report wheel speed rpm data and the "computer(s)" are checking for a delta between front and rear or any one wheel out of synch. There is a magic number for diameter(circumference) difference as this exact number is a point of contention. Keep things nearly equal in wheel rpm front vs rear and all systems are happy. The number should be less than 4% delta, however, (as stated above) the exact number isn't known.

[TrackClub]

Having run 295/30/20 square i can confirm the car's balance is virtually the same as with stock staggered set up. Many folks seem to run various square set ups here and none reported any issues as far as i can tell. This incl tires in 20, 19 and 18 sizes (per Provoste's post: he has run them all now).

As far as the nannies...I'd start with considering car's rake: with stock staggered set up the rake is x (dont know precise value). When one puts a staggered set up the rake will be reduced. Reducing the rake makes a car more stable. That's likely why the balance seems the same with a square set up even tho there is more tire in the front and less in the rear. I believe less rake simply compensates for this change.

As far as nannies: TC activates based on rear tire slip levels and should continue performing regardless of diameter changes. Stabilitrak activates based on excessive yaw angles. Specifically based on any differences between a car's actual path vs intended path based on a steering lock angle (such as in oversteer).
Hence I dont believe tire sizes play a role here, as intervention is based on excessive yaw angles only (vs anything to do with tires).

An issue may arise if someone put shorter tires up front and taller tires in the rear however (vs stock).
As doing so would increase the rake and make a car less stable and more prone to oversteer especially on entry and mid corner. The issue then could (hypothetically) become: would Stabilitrak be able to intervene successfully given a much looser car vs original design point.

My 2 highly unscientific cents worth of an amateur opinion

[Badgerbimmer]

Originally Posted by TrackClub

Having run 295/30/20 square i can confirm the car's balance is virtually the same as with stock staggered set up. Many folks seem to run various square set ups here and none reported any issues as far as i can tell. This incl tires in 20, 19 and 18 sizes (per Provoste's post: he has run them all now).

As far as the nannies...I'd start with considering car's rake: with stock staggered set up the rake is x (dont know precise value). When one puts a staggered set up the rake will be reduced. Reducing the rake makes a car more stable. That's likely why the balance seems the same with a square set up even tho there is more tire in the front and less in the rear. I believe less rake simply compensates for this change.

As far as nannies: TC activates based on rear tire slip levels and should continue performing regardless of diameter changes. Stabilitrak activates based on excessive yaw angles. Specifically based on any differences between a car's actual path vs intended path based on a steering lock angle (such as in oversteer).

Hence I dont believe tire sizes play a role here, as intervention is based on excessive yaw angles only (vs anything to do with tires).

An issue may arise if someone put shorter tires up front and taller tires in the rear however (vs stock).
As doing so would increase the rake and make a car less stable and more prone to oversteer especially on entry and mid corner. The issue then could (hypothetically) become: would Stabilitrak be able to intervene successfully given a much looser car vs original design point.

My 2 highly unscientific cents worth of an amateur opinion

As one amateur to another this all makes sense to me.