[Shawnqa800720]

Hi all,

I recall some threads a while back that were trying to figure out if the move from the 20" wheel setup to and 18" setup would effect the traction and stability control elements of the car when on track.

I am looking to move to a track purpose 18" set, and the tires I plan to run only come in a low profile that is the same as the stock 20" which means I am losing 1" on the tire radius. I know that will definitely impact speedometer, but I am not too concerned about that. I'd suspect traction control would be impacted more than stability control, but it why I'm checking with the experts. Can anyone confirm if this question has be definitively answered?

I do not ever plan on becoming a pro-racer and leave my nannies on when I track to ensure I rarely need to worry about going off-track. I do turn off traction control, but leave stability control in tact. I am outdriving the nannies at this point as they kick on all the time. I know I could be a bit faster with them off now, but at my age, I prefer the safety to a few 10ths of a second.

[carguy55]

I haven't seen any issues issues and I've ran 19" 305 square for years and now run 18" 315 square. Beside the speedometer error it also results in a little lower gear ratio. At this level with stickier tires you should be running with little to no nannies on or you can have negative effects like over heating the rear brakes.

[WhiteMale]

Its not the 18 or 20 it's overall diameter of your tire that is where things get quirky, you have to stay within an inch +/- I think it is. Too much of change in the diameter trips out the Stabilitrak and TCS.

[Shawnqa800720]

Originally Posted by carguy55

I haven't seen any issues issues and I've ran 19" 305 square for years and now run 18" 315 square. Beside the speedometer error it also results in a little lower gear ratio. At this level with stickier tires you should be running with little to no nannies on or you can have negative effects like over heating the rear brakes.

I am running the 1SS, so I only have the options of on/off for the nannies. I turn off traction control, but leave the stability control on. It does heat up the rears quite a bit, but the safety margin is more important to me overall.

Have you ever run full stability control with the 18" setup and did it feel fine or was it tripping more than it would normally?

[Shawnqa800720]

Originally Posted by WhiteMale

Its not the 18 or 20 it's overall diameter of your tire that is where things get quirky, you have to stay within an inch +/- I think it is. Too much of change in the diameter trips out the Stabilitrak and TCS.

Yeah - this is the part I am concerned with. if it truly +/- 1" diameter, I will be well past that. I'd be moving from 315 20R30 and 295 20R30 to a 315 18R30 and 295 18R30, so it would be nearly 2" loss in diameter.

Since stability track is more based on yaw, I wouldn't think that would be impacted by wheel diameter unless there is some level of wheel rotation in the algorithm. Traction control would clearly be an issue because wheel revolutions vs. speed as well as wheel slip play into that algorithm. Since I turn off traction control on track I am not concerned about that. I really need to know if the stability control can be an issue with wheel diameter change.

[clyde]

Originally Posted by Shawnqa800720

Yeah - this is the part I am concerned with. if it truly +/- 1" diameter, I will be well past that. I'd be moving from 315 20R30 and 295 20R30 to a 315 18R30 and 295 18R30, so it would be nearly 2" loss in diameter.

Since stability track is more based on yaw, I wouldn't think that would be impacted by wheel diameter unless there is some level of wheel rotation in the algorithm. Traction control would clearly be an issue because wheel revolutions vs. speed as well as wheel slip play into that algorithm. Since I turn off traction control on track I am not concerned about that. I really need to know if the stability control can be an issue with wheel diameter change.

I don't know what will happen on your car, but the key seems to be the difference between front and rear diameters. Shorter front than rear (by as much as an inch) to equal front/rear seem to be fine. Taller front than rear always seem to cause problems.

[ctrlz]

I believe you have to stay within 3% of the stock rotational diameter to avoid TCS/Stabilitrak errors.

[arpad_m]

Originally Posted by ctrlz

I believe you have to stay within 3% of the stock rotational diameter to avoid TCS/Stabilitrak errors.

Isn't that 3% the maximum safe aggregate difference between front and rear, though? Actually, I'm not sure what happens when someone deviates >3% on both ends (so the relative diff vs stock between front and rear is 0 or very small), but does not adjust the rotational parameters in the tune.

[Shawnqa800720]

Originally Posted by arpad_m

Isn't that 3% the maximum safe aggregate difference between front and rear, though? Actually, I'm not sure what happens when someone deviates >3% on both ends (so the relative diff vs stock between front and rear is 0 or very small), but does not adjust the rotational parameters in the tune.

I would hope that >3% is deviation front to rear, but given how much control GM put into the system it would not be surprising that speedometer-to-wheel rotation is in an algorithm somewhere. Conversely I would hope if that algorithm is in there, it is noticing if all wheels are still spinning in sync, which would essentially negate the wheel diameter change as long as the same for all four.

If it is, I am then curious if this would be a simple fix at a dealership to reprogram the tire diameter...

[ctrlz]

Yes, sorry.
I think the 3% is front-to-rear and maybe even side-to-side.
Without GPS data, I'm not sure how the car could sense any problems even if you put 40" tires all around.

[arpad_m]

Heh, having any side to side delta should be considered a crime against the car .

There is a GPS module in the car (well at least certainly in those that cam with the nav option), so in theory it has location information, even though it feels unlikely that it would be used during ABS and Stabilitrak operation.

[Baddawg53]

I run a 315/30/18 square setup. The nannies don't really care until I start to push the car, then it's super unhappy. I run with everything off, it's kinda scary with it all on with the way it'll grab and release the brakes, really unbalances the car.

I bet on the street you'd never notice, but on track you definitely will and it really slows you down.

[Shawnqa800720]

Originally Posted by Baddawg53

I run a 315/30/18 square setup. The nannies don't really care until I start to push the car, then it's super unhappy. I run with everything off, it's kinda scary with it all on with the way it'll grab and release the brakes, really unbalances the car.

I bet on the street you'd never notice, but on track you definitely will and it really slows you down.

And when you say "unhappy", I assume because you are driving the car to the level where the nannies kick in all the time (as they are supposed to) vs. kicking in because the wheel diameter is off and the car is confused...This is what I am trying to understand - will the wheel diameter change (2" smaller) cause the nannies to misbehave all on its own?


I am driving at the level that my nannies are kicking in a lot (Advanced HPDE run groups, within 1sec lap times when traffic is not a factor). Interesting you mention that it can unbalance the car as I have found it more often is able to save me from when I overdrive the car. The difference is more likely just due to the fact that I have never run with them all the way off because I haven't trusted myself . AND, because I have the SS and it does not have in-between options like the 1LE - mine are either on or off.

I have defaulted to on for stability control and off for traction control because it is safer, if slower. Your "unbalanced" statement is making me question my assumption that I am safer with them on at this point in my driving capability. The fact that I've never been more than 2 wheels off track makes me think I am right, but happy to be corrected by those with more experience than me.

[Baddawg53]

Ok, to be more clear, they intervene well below the limit of the car, as soon as I start to get aggressive with it. I'm talking like 15 to 20 seconds slower than my PB lap times.

I also have the SS model, non 1le. When it decides it's unhappy, it gets very aggressive with it's corrections, and the car is a long way away from losing control. I have no desire to go back to stock wheels though, I love how it handles. I lost some time when I switched and had to slow down with no helpers, but the slow down was temporary and has helped me become a better driver.