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Initial Impressions on 18" slicks [Z07/Road Racing]
I received a customers car a little over a month ago, and the plan is to work through the development issues making these cars monsters on track. The first thing we started with was the tires. Since the Z07 ABS/PTM/TC is calibrated to the pointy end of performance, on the cup2R tire, any deviation from this tire Diameter will result in sub par performance, as the systems self learning capabilities simply do not update with any reasonable amount of speed. Put a tire thats too small on the front, and it thinks the car is locking up the rear brakes too soon so the braking performance decreases and under acceleration has a mismatched slip target and will spin the tires more so than with OEM tire RPK values.
Put a tire thats too small on the back, and the Bias shifts significantly, causing yet again another decrease in braking performance. You have to run a tire combo that has the OEM split (Bias) front : rear. This is really important.
There are currently no hoosiers that offer similar RPK. There are some that are CLOSE in RPK Split front to rear, and we will test those eventually.
The fastest tire that fits, is available, and has less than 1% deviation from OEM is believe it or not, the IMSA sized GT3/GTD tires. 30/68 Front tire, 31/71 rear tire. Fit is REALLY good, considering these are almost impossible to fit on previous generation corvettes, and the only issue is MINOR rubbing in the paddock at low speed, full lock. We dont race there, so we dont really care about that issue.
We are one of the first to get the 18s to fit with stock brakes, so here is what is required for this:
Machine rear caliper bridge approximately 12mm. this gives us a radial clearance in the rear of about 2.5mm to the 8TWELVE EVO wheels, YMMV on other makes and models of wheels. Please check this. We will update with what we know when we know it.
remove parking brake
install dummy rotor, tuck parking brake in the fenderwell
Machine front calipers 2mm, this gives us 4mm total radial clearance to the wheel barrell. We tested it with out machining the caliper, and it picks up track rubber and debris pretty bad. didnt hurt the wheel, but we added clearance.
Initial brain dump with the car:
Power down grip was wonderful. It will barely spin in 1st gear on a launch, and it does not spin at all in 2nd gear once the tires have temp in them. Ambient was 100*
we ran the day in Race mode 2, ESC Off to make sure that the Active handling still did its thing at the limit, and boy does it. Its wonderful.
braking is amazing. Zero issue with braking confidence. We are seeing 1.85-1.9G Decel. Girodisc Iron conversion as we had the OEM brakes off for testing off the car.
My initial thought with the lack of brakes some experience was due to pad/rotor mismatch compared to the OEM Carbon brakes. This appears to not be the case, at least in an Amateur setting. We will test back to back Iron/CCB later as we continue to develop this platform.
Balance is great with the Z07 suspension, a track alignment, and lowered.
EDiff functions REALLY well off throttle. The GM team killed it here.
60+ lbs saved from wheels and tires, this is huge, considering rotational, UNSPRUNG. Testing at G2 Motorsports park Ready to roll Machined caliper installed Front radial barrel clearance as is Front OEM tire and wheel REAR OEM tire and wheel 8TWELVE 18x13 REAR and Michelin 31/71 slick 8TWELVE 18x12 FRONT and Michelin 30/68 slick We gave a lot of rides Shes heavy! Stock weight with iron brakes and 1/2 tank Stock tire testing at Hallett. Featherlite 28' trailer, still clears. Tight, but clears. Calm before the storm
More as I know it, enjoy the pics. This car with so little mods, is easily faster than a current model GT3RS, and I do not say that lightly. The Z07 is a track weapon.
For those wondering what RPK is, its Revolutions per Kilometer. Boiled down, its basically the tire diameter. This is important, as the wheel speed sensors report data as measured from the wheel. When we put different tire sizes on, performance decreases. Do the cars still run? Oh yea. They still stop, they act just fine, but they are limited, and not operating at their full potential The ECUs and Chassis controls for these cars is very thorough, and calibrated very well. Thats why this is so important.
Here are some basic calculations off advertised RPK values. One of our engineers did this math, and it works. This is not the holy grail, but its a dang good start. As always, YMMV.
Green is good
Yellow is Marginal, but OK if coupled with another tire thats similar % off
Red is likely to have ABS/TC issues, again, unless coupled with an equally biased tire, however, its still far from perfect because the car believes its at a certain speed ( and there are MULTIPLE tables referenced off of wheel speed for everything from Fan to diff control to other logics and calibrations)
For those wondering what RPK is, its Revolutions per Kilometer. Boiled down, its basically the tire diameter. This is important, as the wheel speed sensors report data as measured from the wheel. When we put different tire sizes on, performance decreases. Do the cars still run? Oh yea. They still stop, they act just fine, but they are limited, and not operating at their full potential The ECUs and Chassis controls for these cars is very thorough, and calibrated very well. Thats why this is so important.
Here are some basic calculations off advertised RPK values. One of our engineers did this math, and it works. This is not the holy grail, but its a dang good start. As always, YMMV.
Green is good
Yellow is Marginal, but OK if coupled with another tire thats similar % off
Red is likely to have ABS/TC issues, again, unless coupled with an equally biased tire, however, its still far from perfect because the car believes its at a certain speed ( and there are MULTIPLE tables referenced off of wheel speed for everything from Fan to diff control to other logics and calibrations)
We have had good success with green, and bad results with red offsets. Yellow is the gray area, but the intent is to bring awareness to the razor thin margin on tire sizes. As we further our knowledge with this, we will update it as necessary.
We have had good success with green, and bad results with red offsets. Yellow is the gray area, but the intent is to bring awareness to the razor thin margin on tire sizes. As we further our knowledge with this, we will update it as necessary.
So if I keep the front to rear diameters within 1% of OEM I'm still in the green?
Great data and information, appreciate you sharing! Couple of questions if you don't mind.. you said that in race 2 active handling was still doing it's thing, are you referring to traction control? I feel traction control in race 2, but I thought the dynamic stability control was completely off?
Also, what pads did you run with the girodiscs that worked well?
Great data and information, appreciate you sharing! Couple of questions if you don't mind.. you said that in race 2 active handling was still doing it's thing, are you referring to traction control? I feel traction control in race 2, but I thought the dynamic stability control was completely off?
Also, what pads did you run with the girodiscs that worked well?
Thanks!
ESC off in Race 2. It has a LITTLE bit of intrusion with throttle only when you are essentially sideways. Its very easy to drive the car at the limit and over the limit in R2. I run ESC off because it was recommended to me. ESC On was too much intervention, regardless of tire.
We ran the Girodisc GP40 pads front and rear. Its perfect for the rear, but rotor conditioning could use some help in the front, but other than visual disc wear, the car behaved well.
For ABS, the car systems only know about the diameter ratio; as long as we keep very similar ratio, it should work about same as OEM? Sure, perhaps other components might have different strategy in different speeds (e.g. shocks), but ABS should be OK?
I've been using my Z06 Z07 with 295/30/18, 345/35/18, and moved over my AP brakes from Z51 to get 18" wheels to fit. Indeed, the braking performance was not good, and its boost was a lot more than Z51's. I switched the pads to Pagid RSL1 endurance pads that have lower initial bite, and the problem was solved for me. I can brake deep into corners without locking them up, which is encouraging. This doesn't mean I fixed ABS (if it's the source of the issue instead of brake booster map), but looks like I'm able to drive without engaging ABS with lower initial bite provided by these pads.
As with you folks, I've also discovered that ESC off generated best results. That said, PTM race is only an engine power (cut) based traction control at Race+, and this car really is a sweetheart on putting power and when you exceed its limits. As such, I just turn everything off (holding it down 5 seconds). Did you try that with different tire combinations as well? Some of the handling issues with different setups you tried might disappear in that mode. In other words, I'm not against anything you said, but setups with non ideal diameter spreads might still do well with everything off? At that point, if you also have coilovers, the only thing remaining out of your control is the ediff (which I hope would behave) : )
In my case, keeping exact diameters is too hard besides a few slicks, but after reading this thread, I'll revisit the idea of having OEM -like ratio again (Toyo R 305/35/18, 345/35/18 and few others have sizes with perfect ratio, including Hoosiers 315/30/18, 345/35/18 I think at 4-5% front to rear difference). My hope is that having close ratio should mitigate most of these issues anyway, and the speed maps alone should not become a big problem by themselves if ratio is good.
For ABS, the car systems only know about the diameter ratio; as long as we keep very similar ratio, it should work about same as OEM? Sure, perhaps other components might have different strategy in different speeds (e.g. shocks), but ABS should be OK?
I think this is the scenario. I will test absolutely atrocious bias splits soon, and report back. Sadly, this car does not have PDR, So we have to go off of Gforce from the AIM solo, and driver feedback (IMSA GS top contender, for these tests)
I've been using my Z06 Z07 with 295/30/18, 345/35/18, and moved over my AP brakes from Z51 to get 18" wheels to fit. Indeed, the braking performance was not good, and its boost was a lot more than Z51's. I switched the pads to Pagid RSL1 endurance pads that have lower initial bite, and the problem was solved for me. I can brake deep into corners without locking them up, which is encouraging. This doesn't mean I fixed ABS (if it's the source of the issue instead of brake booster map), but looks like I'm able to drive without engaging ABS with lower initial bite provided by these pads.
Id be curious to know what your car does with stock calipers. This car was the Essex test car, and the braking performance was very sub par for most of the season. Multiple pads, tires, and track outings and only thing that solved it was the Stock brakes. In another thread, I break down the piston area, which is significantly different than the AP calipers, and the reason that matters is due to the Eboost/ABS/braking model that exists and we cannot edit it.
As with you folks, I've also discovered that ESC off generated best results. That said, PTM race is only an engine power (cut) based traction control at Race+, and this car really is a sweetheart on putting power and when you exceed its limits. As such, I just turn everything off (holding it down 5 seconds). Did you try that with different tire combinations as well? Some of the handling issues with different setups you tried might disappear in that mode. In other words, I'm not against anything you said, but setups with non ideal diameter spreads might still do well with everything off? At that point, if you also have coilovers, the only thing remaining out of your control is the ediff (which I hope would behave) : )
I did not drive the car in anger with everything off, but I will in future testing. The EDIFF is absolutely wonderful, it really does the lords work.
The Big splits in tire diameter bias really show their ugly side in braking. When run with everything off, it certainly would not show its ugly side of the other facets of chassis and stability control.
In my case, keeping exact diameters is too hard besides a few slicks, but after reading this thread, I'll revisit the idea of having OEM -like ratio again (Toyo R 305/35/18, 345/35/18 and few others have sizes with perfect ratio, including Hoosiers 315/30/18, 345/35/18 I think at 4-5% front to rear difference). My hope is that having close ratio should mitigate most of these issues anyway, and the speed maps alone should not become a big problem by themselves if ratio is good.
I completely agree with this! Its very costly to test a lot of these tires, so from the GO, I chose the fastest tire with one of the best spreads in % variance. Im sure that this thread will be a living document as we all learn together and share our experiences over the decades to come. I still have threads from C5s that are relevant, hard to imagine these cars in 25 years!
Yellow is Marginal, but OK if coupled with another tire thats similar % off
Red is likely to have ABS/TC issues, again, unless coupled with an equally biased tire, however, its still far from perfect because the car believes its at a certain speed ( and there are MULTIPLE tables referenced off of wheel speed for everything from Fan to diff control to other logics and calibrations)
In that chart I see 1 line for 19" pirelli slicks that looked like a square 325 setup f/r?
do you have the data for the 275/675-19 + 315/705-19?
I feel like my contact patch is compromised, but this challenge scrub size is easy to get and economical
Really interesting stuff! I am following along for sure. I am not running slicks yet, but will be trying the Vitour P1 on stock size wheels at almost the OEM ratio (285/20, 345/21) and comparing to the Cup2R’s I have from the car purchase. I have some mag wheels for the P1 but the stock tires will be on stock wheels so WAY heavier.
The vitour 285/30/20 and 345/25/21 have worked great on my car they have good life,are predictable at the limit and dont drop off much till the cords,i have been thru a couple of sets,i use cobalt xr1 pads front,xr2 rear rite now and the braking has been good with this tire i do notice that if i dont turn everything off the esc overheats the rear brakes,i have tried alot of brand pads and these i like for my style i dont know how they would be with slicks this thread is great and this info is what i was waiting for to try a slick tire.
The vitour 285/30/20 and 345/25/21 have worked great on my car they have good life,are predictable at the limit and dont drop off much till the cords,i have been thru a couple of sets,i use cobalt xr1 pads front,xr2 rear rite now and the braking has been good with this tire i do notice that if i dont turn everything off the esc overheats the rear brakes,i have tried alot of brand pads and these i like for my style i dont know how they would be with slicks this thread is great and this info is what i was waiting for to try a slick tire.
Good to know. I will be running the same tires on my new mag wheels with stock CCB brakes for now. I have after market CCB to install after I wear out the stock ones first.
Where’s the videos from this past weekend when it outran the 720S on Hoosiers? LT6 sounded like a racecar straight piped and laid down the fastest time for a street car I’ve ever seen at Hallett!
Louis already mentioned the AP brakes did not work right for them, and it only started working right when they switched back to the OEM brakes of the car. I also had a similar experience.
I first tried the AP brakes on my Z51, and they worked well. When I moved them over to Z06, they were way off,and were engaging ABS all the time. C8 cars come with the same brake booster all across the product line, but looks like they do have a different boost map for every different model(!). The AP brakes were developed on Z51, and there's no differentiation of models for these kits. As such, their valving is way off for Z06. I think they later developed larger kits (390 and 378 rear)on a Z06, but then I don't know if they used a Z06 or Z07 with a CCB. These cars have different boost mapping, so it would work best on only one of them : )
This is an issue for C7 cars as well, but it was less of an issue, since alll non-CCB cars used one booster, and CCB cars used another, and you could easily switch the booster to fix it. On C8, it looks to be a software calibration, and it's not available for tuning.
On my car, I'm still using smaller AP brake kit to fit 18" wheels, but I use lower initial bite enduro pads (Pagid RSL1) to fix the ABS engagement and high brake boost issue that Louis also suffered.
Initial Impressions on 18" slicks [Z07/Road Racing]
