Car actively trying to swap ends on track?

[d15b7]

hey there. I own a (totally stock except for the sumitomo tires) 1990 ZR-1. and I hold road racing licences with SCCA and NASA and VRG and others, plus I'm a certified instructor for Porsche and BMW CCA and NASA and others. my ZR-1 predominantly understeers unless you are rolling on the power too quickly and then it transitions to very easily correctible oversteer. the car is a pussycat to drive, especially compared to my two race cars (Lotus Super Seven vintage racer and my fast car, a Honda Civic with a K24 engine swap and a big turbo (around 550 whp, and less than 2400 lbs with me in it) stripped and caged with fully adjustable suspension/coilovers/double adjustable shocks.

so -- I'd most definitely say that there is something very amiss with your car. most likely as others have stated the suspension is really off; have a good race shop do a good 'track ready' 4 wheel alignment. have them check to make sure that none of the bushings/ball joints are bad or binding. your car should be fun to drive on track and not a white knuckle ride!

PS if you want better braking performance out of the stock calipers try getting some Raybestos ST43 pads for it; I know they are expensive but they are fantastic and basically will not fade. ever. my Honda runs 15" wheels which limit me to 11" front rotors and the ST43s are the only pads that will last over a 25 min session stopping from 160+ mph at VIR raceway. I've tried all the others (hawk, porterfield, carbotech and others). the ST43s are simply amazing.

[cbaclawski]

Originally Posted by d15b7

hey there. I own a (totally stock except for the sumitomo tires) 1990 ZR-1. and I hold road racing licences with SCCA and NASA and VRG and others, plus I'm a certified instructor for Porsche and BMW CCA and NASA and others. my ZR-1 predominantly understeers unless you are rolling on the power too quickly and then it transitions to very easily correctible oversteer. the car is a pussycat to drive, especially compared to my two race cars (Lotus Super Seven vintage racer and my fast car, a Honda Civic with a K24 engine swap and a big turbo (around 550 whp, and less than 2400 lbs with me in it) stripped and caged with fully adjustable suspension/coilovers/double adjustable shocks.

so -- I'd most definitely say that there is something very amiss with your car. most likely as others have stated the suspension is really off; have a good race shop do a good 'track ready' 4 wheel alignment. have them check to make sure that none of the bushings/ball joints are bad or binding. your car should be fun to drive on track and not a white knuckle ride!

PS if you want better braking performance out of the stock calipers try getting some Raybestos ST43 pads for it; I know they are expensive but they are fantastic and basically will not fade. ever. my Honda runs 15" wheels which limit me to 11" front rotors and the ST43s are the only pads that will last over a 25 min session stopping from 160+ mph at VIR raceway. I've tried all the others (hawk, porterfield, carbotech and others). the ST43s are simply amazing.

That's great to hear! I've run with all those groups! I don't have any competition licenses, but do have a couple TT licenses and have run A group with BMWCCA (e46 330i prepped to be a D-Modified/Nasa ST/TT-5).

To be honest my experience in this car has really made me insecure about my skill level. It is anything BUT a pussycat to drive. I know I wasn't rolling on the power too quickly, as just breathing on it past maintenance would kick out the back, and ANY lift would have the same effect. I swear it was like balancing a bowling ball on a needle, and slow to boot.


I'll start with the alignment, and have the suspension(stock) gone over to look for binding/looseness. The before/after charts should tell the story... hopefully...


Regarding the pads, I'll definitely give those a shot once the DTC-70's need to be replaced (they aren't what I'd call cheap either...) but it might be a while, I don't get a chance to take this one out that often... (too many track cars, not enough track days! - sounds like you have the same issue, lol)


550hp 2400lb civic eh? ~4lb:hp on a FWD car? I can't even imagine what that must be like! I'm scared just thinking about the boost kicking in...

Thanks again

[MarkSS]

ZR1s are not perfectly balanced, so a couple psi less in the rears compared to fronts may help. If you have a pyrometer, dailing in the tire pressure is easier.

It seems like you may have something worn out though. I have seen the trailing arms wear out, you may want to check that as well.

Another

[RussMcB]

Originally Posted by cbaclawski

... Please do post the settings you have and I will start there.

I forget where I found this on the web. Maybe from "VBP Racing"?

[d15b7]

Originally Posted by cbaclawski

550hp 2400lb civic eh? ~4lb:hp on a FWD car? I can't even imagine what that must be like! I'm scared just thinking about the boost kicking in...

ya it takes a smoooth driving style and a very careful right foot application!

here's a short clip of me in the Honda from last fall; I was at NJMP Thunderbolt instructing for the PCA and decided to take the Civic and burn off some of my old obsolete tires from years ago (I used to run it in Limited FWD class before the big EFR7163 turbo. in Limited I ran Nitto NT01 225-45-15 tires). these were 8 yrs old (!) and I was gonna throw them out but figured 'why not use them one last time?'. I run the car in Unlimited now and use a Hoosier A7 295-35-15 up front (massive MASSIVE difference in available traction!)

so in this short clip I was running at only 15 psi (around 450 whp or so maybe a touch less) with the old old beat tires. have to be reallyyyyy cautious applying the throttle (and often I short shift because it'll just light up the fronts coming off the corners). and I only use 4th 5th 6th once I'm out of the pits. even so this clip is from the Red group (Instructors and Racers) and as you can see my old Civic was reeling them in and eating them up haha.

https://1drv.ms/v/s!AmUHWCpbUJmAq34ysK_Q7oEOmnf-

(the 295-35-15 Hoosier A7s plus turning the boost up equates to about 5-6 secs a lap faster).

[cbaclawski]

Originally Posted by MarkSS

ZR1s are not perfectly balanced, so a couple psi less in the rears compared to fronts may help. If you have a pyrometer, dailing in the tire pressure is easier.

It seems like you may have something worn out though. I have seen the trailing arms wear out, you may want to check that as well.

Another

Adding that to the list of things to check, thanks!

Originally Posted by RussMcB

I forget where I found this on the web. Maybe from "VBP Racing"?

Perfect, exactly what i was looking for, Thanks!!

Originally Posted by d15b7

ya it takes a smoooth driving style and a very careful right foot application!

here's a short clip of me in the Honda from last fall; I was at NJMP Thunderbolt instructing for the PCA and decided to take the Civic and burn off some of my old obsolete tires from years ago (I used to run it in Limited FWD class before the big EFR7163 turbo. in Limited I ran Nitto NT01 225-45-15 tires). these were 8 yrs old (!) and I was gonna throw them out but figured 'why not use them one last time?'. I run the car in Unlimited now and use a Hoosier A7 295-35-15 up front (massive MASSIVE difference in available traction!)

so in this short clip I was running at only 15 psi (around 450 whp or so maybe a touch less) with the old old beat tires. have to be reallyyyyy cautious applying the throttle (and often I short shift because it'll just light up the fronts coming off the corners). and I only use 4th 5th 6th once I'm out of the pits. even so this clip is from the Red group (Instructors and Racers) and as you can see my old Civic was reeling them in and eating them up haha.

https://1drv.ms/v/s!AmUHWCpbUJmAq34ysK_Q7oEOmnf-

(the 295-35-15 Hoosier A7s plus turning the boost up equates to about 5-6 secs a lap faster).

Awesome video!

It's always fun to get a point by from a modern Porshe in an old "beater" that has no business doing so (stock anyway)...

[lfalzarano]

Found these baseline specs on Corvette Central tech site.
https://tech.corvettecentral.com/201...ignment-specs/

The rear Toe-in is slightly different, than that mentioned above.

It was also recommended to check your steering linkage for wear.

[Vette Guy]

First, please explain "has been outfitted for road course duty".

Installing a 30mm solid front sway bar w/o upgrading the rear.... this would provide the oversteer you experience.

Second, you are on brakes hard into turns... maybe your rear is not settled proior to turn-in. Are you running DT-70s on the rear? May try DT-60s to reduce bias if rear is getting too squirrely. I'm a believer in the stock calipers with the right pads.... especially on Sumis.

Third, bad shocks would also provide an unplanted rear end, possibly exacerbated by a lowered front if bump steer is not corrected.

Russ provided good alignment settings - recommend -1.5 front/rear camber. As said, you can use a pryometer to dial your alignment in, but that's a whole different level doing alignments at the track in a C4.

Coil-overs do not work better than stock springs. They have 2 advantages - Easily adjusted ride-height and quick change springs at the track. Save your money.

A stock C4 is very predictable and easily correctable, particularly with street tires. I think you will really enjoy your track time once you find the "Ah-ha" problem.

Keep us posted!

Regards,
Mark

[cbaclawski]

Originally Posted by Vette Guy

First, please explain "has been outfitted for road course duty".

Installing a 30mm solid front sway bar w/o upgrading the rear.... this would provide the oversteer you experience.

Second, you are on brakes hard into turns... maybe your rear is not settled proior to turn-in. Are you running DT-70s on the rear? May try DT-60s to reduce bias if rear is getting too squirrely. I'm a believer in the stock calipers with the right pads.... especially on Sumis.

Third, bad shocks would also provide an unplanted rear end, possibly exacerbated by a lowered front if bump steer is not corrected.

Russ provided good alignment settings - recommend -1.5 front/rear camber. As said, you can use a pryometer to dial your alignment in, but that's a whole different level doing alignments at the track in a C4.

Coil-overs do not work better than stock springs. They have 2 advantages - Easily adjusted ride-height and quick change springs at the track. Save your money.

A stock C4 is very predictable and easily correctable, particularly with street tires. I think you will really enjoy your track time once you find the "Ah-ha" problem.

Keep us posted!

Regards,
Mark

1: - racetech seats and harnesses, Ron Davis Radiator, new master with drm bias spring, high temp brake fluid, all 4 calipers replaced, dtc-70 pads on all 4 corners, car shipped to Chicago and thoroughly gone through mechanically, including new tune and removal of secondaries. Suspension components were inspected, but are stock, no aftermarket sways.


2- not really hard on the brakes into turns, even where I'd usually trail brake a bit I over slowed the car and was completely off the brakes and definitely settled at turn in. The problem persisted even in long sweepers, long after turn in. Any hint of throttle until almost full track out would bring it out (I was absolutely not mashing the gas...)


3- The shocks were inspected by Marc Haibeck, he didn't think they required anything, but they are original to the car - Front end stock height.


Thanks much!

[Vette Guy]

Puzzling, but we’ll get there! Does the car roll in corners or does it remain fairly stiff? Do you get any sensation of weight transfer as you add power through turns? Or can you not add enough power? By chance, does your shifter jump up in the console?
Wondering if C-Beam is flexing at attachment points, binding in control arms or issue with sway bar mismatch/end link issue.


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