Apocolipse

Specifically C6Z but applies to all I’m sure.

Currently, I have my rake set at the jacking points 0.5” higher in the rear vs front. At my local track (Mosport - high speed) accelerating down the back straight is sketchy once speeds get up over 100. If I let off the throttle it stabilizes. I am assuming this is because the squat from weight transfer removing that rake and causing the car to lift up.

How much rake is everyone running here to prevent this? Is there a negative to running too much rake? Front vs rear instant center heights off too much causing rear jacking?

I would love to fix the instability in the car at those speeds as you really don’t feel good staying WOT when you feel the car getting lighter. Not very confidence inspiring lol

Scooter70

How stock is your car? Hood vents can really help keep the front end down at high speeds.

Apocolipse

ZR1 splitter, side skirts, and extended rear spoiler - so it is currently rear aero bias which adds to the rake reduction at speed. I can’t run an extended splitter as it would hit street driving. Hood I can see would help but I’m hoping to keep that stock since I only track 1% of the time and my coolant temps are fine. I’m guessing I just need to add a ton more static rake to compensate?

davidfarmer

I usually do 1/2-3/4". I agree you would benefit from vented hood, as your are dumping a lot of air under the car, instead of upward.

It could also be your overall stiffness. Your car makes enough power that you are still accelerating pretty hard at those speeds, generating a little squat in the rear.

Static rake would be the first thing to try...you are probably running tall rear tires, so 3/4" isn't a lot.

Mosport, like VIR, has a back straight that isn't flat. Those little dips and rises exacerbates the issue....often feels worse than it really is.

dregg100

Big wing is good but hood vents no good?

Stock suspension?

AZSP33D

Needs improvement in aero balance, many ways to do that. Changing the rear height/rake can also change rear roll center and anti-squat, so it may be better to increase front downforce and / or decrease rear down force.

TrackAire

It's strange that you start feeling an stability issue at only 100 mph while accelerating....that's not that fast enough to create a ton of aero downforce or lift. Before you spend a lot of money trying stuff, I would reduce or remove the rear spoiler and run some sessions to see if that is in fact the issue. I personally don't think the car is squatting or raising that much due to acceleration at 100 plus mph....unless you're running 900 plus hp, the acceleration curve is not that great as you gain speed.

I would also check non aero things that may be causing this instability, especially your A-arm bushings and alignment. When accelerating to 100 mph and beyond, the amount of energy transferred to the A-arm bushings (especially rear) is huge. If you have worn or damaged bushings you might be feeling the A-arms moving around and actually self steering the car....one direction under heavy acceleration and another direction under deceleration. If only one side is defective, then that may make the acceleration instability worse.
Also make sure to check that your tie rod ends, toe settings, alignment, etc. are sound and correct.

Good luck.

Cap'n Pete

I'd make sure your alignment is checked. At the start of this year, my alignment was off. I had suspension work done at 6th Gear (at Mosport) and they setup my ride height & alignment ....... the car felt way better at speed afterward. Obviously there's that sort of loose feeling as you're getting close to the crest before T8 doing ~140MPH, but unless you've got full aero, the car is going to get floaty!

tommyc6z06

So, I lift off just into the last crest (using it for slight braking), then ease off a bit when the car is unweighted, then lean harder into the brakes as the car settles again. I've also waited to brake till after the crest, but corner entry speed is sacrificed by going too deep.
I'd been previously accustomed to an aero car, which was 20mph slower, but really only required a lift into T8, or very mild braking

Anyhow, I touch 151~152mph...4th. Very little twitchiness, over the two unweighted crests/undulations and easy to drive through.

Initially, I was absolutely terrified going over the undulations and backed out for fear I'd lose it. Very scary over 100mph, like you.
I was chasing similar ghosts...added a small splitter, re-aligned (wasn't bad), DRM shocks (OEs were undrivable)...speculated about windows up/down, then realized I may have forgotten the TCS on...and not put it in Competition mode.

If you feel like, come out on Thursday and I'll be happy to take you for a couple of easy laps

Bill Dearborn

I would go a little further with this recommendation. I would remove the aero and go back to a stock configuration (with the small chin splitter and the little spoiler in the back). My pure stock C6Z rode rock solid up to 158 mph on the back straight at the Glen without any issues and it had no issues running the back straight at VIR. If you still have the issue then look at the suspension.

Bill

Speedboosted

Alignment is likely the issue, and not just the static numbers. I'd suspect a worn rear a arm bushing or tie rod, that deflects outward under load.

If you wanted a small step for aero testing, remove the side skirts. They really only add rear downforce and cause more front lift, but I'm not sure that would make it better or worse for you. To me it sounds like there's something in the driveline that's binding/deflecting with load.

fatbillybob

I agree with above. On a lowly C5Z in SCCA T2 trim we got no aero per rules. At Daytona my fastest track I top out at 155-158mph and I can actually see my hood ripple but the car is rock solid.

Apocolipse

Thank you all so much for your vast replies and experiences, it is extremely appreciated and will be testing out many suggestions next track day.

The control arms are all brand new from a C7 zr1 and the same float happened when I had my stock arms too. Only ones I haven’t changed yet are the rear lower arms - with that said I have made some delrin bushings and will put those in next time I have the car on the hoist to rule out rear lower arm movement.

That being said I have also remeasured the static rake and it’s only 1/8” at the jacking points. This may be the issue even though you are correct that 100 isn’t very fast aero wise, but as mentioned before - mosport at the back is not flat so adding that with acceleration in 3rd and the rear aero bias I’m sure it’s squatting enough to make a reverse rake and pull
the front up.

So looks like it’s time to raise the rear up and adjust the toe for it.

Let’s see how it goes.

the pads must have squished down since corner balancing. Rear toe is 1/8” in and front is 0.

TrackAire

It is not a rake issue......something else is going on.

If it is an issue with the a-arms I would say it would be the rear lowers. Keep working on it, you'll get it figured out.

Apocolipse

I will be changing out the bushings to delrin next time it’s on the hoist which will rule out movement there.

Cap'n Pete

You going out this Thursday for JRP / Speed Therapy?? I'll be there as well .

tommyc6z06

great! See you there!