HandsomeMike

Yes. I have the lowering wedges on the front spring and longer bolts in rear.

93Rubie

Just throwing this out there but a well setup car is a very good thing but don't neglect the driver mod. THAT is a HUGE thing. Driver mod can beat car mod.

I'm planning on running STU or CAMS in the future. Goals are to create a C4 that is the ultimate handling machine. Will take time and money, I don't have much of either these days.....

HandsomeMike

Rubie - you could not be more correct about the driver mod! That's why I'm doing the driving school at NCM MSP on 8/13. I'm looking at tranny coolers and brake ducts as we speak!

MatthewMiller

This and Tom's post on jacking are both correct. The thing that is fixed on a swing-axle suspension is the angle of the wheel hub to the control arm. That angle is not fixed on a design with twin lateral links. On a C4 the driveshaft is the upper lateral link, but because of the u-joints it isn't in a fixed relationship to the hub like on a swing-axle design.

Roll Center(RC) height actually determines the amount of jacking at any single suspension height. Any time the RC is above ground, there will be some jacking, because the lateral force at the contact patch is reacted on the inclined line between the contact patch and the RC. Technically, jacking just means that the sprung mass of the car moves upward some amount. This happens on either end of any car where the RC is above the ground plane. It's just that on most cars with lowish RC and decent suspension geometry, there is so little jacking effect that we don't notice because it gets visually overwhelmed by the roll we see. Another way to think of it is that in roll, the outside of the car gets lower to a lesser amount than the inside gets higher. The higher the RC, the more the geometry resists roll. But at 100% "anti-roll" geometry, we have introduced a lot of jacking and a non-linear suspension, neither of which are good.

You can also make the RC below ground, and in those cases we get the opposite of jacking, where the sprung mass actually gets lower in cornering. This sounds great, but you quickly run out of suspension travel, and it actually induces "pro roll" where the geometry encourages more roll. Most true race cars tend to shoot for RCs around ground level, although modern F1 cars and LMPs might be all jacked up (pun intended) because of aero design considerations.

The other thing that must be considered is what happens to the RC as the suspension moves. On a swing-axle design like Tom illustrated, the problem begins with a RC that is really high off the ground at rest, but also keeps getting higher as the lateral force rises during cornering. With that combo, you get jacking effects that are clearly visible in cornering. They are scary, and can lead to rollovers. We recently had a Beetle-based dune buggy at a local autocross, and it compounded the jacking problems by running tall rear tires. That thing came close to rolling over several times. They should not have let it run. Ford's old and ridiculous twin-I-beam front suspension is the same thing, just with longer arms. And they can do the same thing: remember that it was the short-coupled Ford SUVs that had all the rollovers in the Firestone tire debacle years ago. All of this can be mitigated with very heavy spring rates. A Z-bar could also be used, which allows the suspension to roll but resists upward/downward movement of the suspension. Or, just don't design anything with swing axles! Anyway, thankfully no Corvette owner has to worry about this stuff.

I think early C4s experienced mostly the effects of a high rear wheel rate in roll, possibly combined with OE shocks that weren't up to the task of damping those high wheel rates. But as Rubie noted, the camber rod pickup points on the frame were higher in those first three years, so the RC was higher. Ergo, there was some increased jacking compared to the later, revised suspensions. It would be interesting to learn what happens to the RC height of those early C4s in roll, because that would be telling. But what I think is that GM intended the early cars to work with the heavy spring rates, and therefore the higher rear RC was not a big problem. But when customers complained about ride quality and they wanted to soften the rear spring rates, they couldn't keep the RC there because then the jacking would have become significant (i.e., the softer springs would allow the sprung mass to rise up unacceptably in cornering, thereby allowing more weight transfer and less grip). So in the revision they lowered the pickup point and thus the RC. Of course, that also lowered the negative camber gain in roll, so it's all a compromise.

HandsomeMike

Here is a video of my best lap at today's HPDI. "I" for introduction. It's a new program at NCM MSP. Lead/follow. It was awesome. Now for feedback about the car. The car was very neutral in corners. The weak link was tires. My Continental Extreme DW's were good at telling me where I was as far as grip limit but could've used more grip. I also need to replace all rubber bushings with poly. By the end of each 8 lap session the power steering was moaning a little and my every once in a blue moon trans pan drip was dripping about four big drips in the paddock. Surprisingly, my brakes were outstanding, for my novice experience level. The Hawk HPS pads did a great job. As I get better I'll want better pads so I can brake later. Water temp never exceeded 210 and oil temp was around 243. So, I need heavier weight power steering fluid, some DOT4 brake fluid in the 600 temp range to be safe. I also plan to do a trans fluid cooler and temp gauge.

93Rubie

Those would be good upgrades. The HPS pads are not really track material either.

Just for fun my 1st time at a road course. Last summer, Nelson Ledges, sorry the audio isn't better. Lap times (with no traffic) where in the low 1:20's which I'm told is plenty fast for a street car.


I took the car out for a auto-x only the 2nd time this year. Kinda of a last minute "screw the Miata, I'm taking the Corvette" thing. I didn't have my RE71R's on the car just the "track day" tires which are smaller in 255/40/17 and come in the Dunlap ZII* variety. Good tires but NOT my RE71R's in 275/35/18 so even with my PAX win, the car could have been about .4 faster on the better tires and if I had actually messed with my front bar. It was still setup for what I did in June at Pitt Race. Which was to win Late Model Class at the Great Lakes Muscle Car Challenge.

Yesterday

HandsomeMike

Wow! That is a narrow track! (Nelson). I've been bitten by the track bug now and will halt all unnecessary upgrades in favor of only things that make the car more reliable. I have the car right where I want it and will focus on seat time and keeping everything running right. I have always enjoyed autox but the track was way more exciting and rewarding personally.

69427

Hey 93Rubie, what was the track surface like at Nelson Ledges? It's been quite a while since I've been there (on a bike track day), and I'm curious what shape the asphalt is/was.

Tom400CFI

Wow is right! That thing looks like a golf cart path....not a race track! Still...looked like a hoot.

Here's my track. Big, wide, smooth...maybe a little boring. Sorry for the washed out exposure.

HandsomeMike

Photos from the NCM Motorsports Park on Saturday.

Nokones

If you are looking for some light-weight autocross wheels I just happen to have some for sale. If you want to run in BSP I have three sets of Forgeline RS2 17X11 & 17X12 1/2 so you can run the 315 & 335 respectively. If you want to run in stock, I have a set of Revolution three-piece wheels 17X9 1/2 with stock SCCA legal back spacing.

Let me know if you're interested. My email address is: nokones@kenmitchell.com

93Rubie

Its total crap as of last year. I left there with two loose exhaust tips and a rearview mirror having fell off.

That being said, is old school and FAST!! One of the fastest average speed road courses in the country. I was easily hitting 120 thru the kink. Seeing just over 100 on the front stretch.

Look at my speedometer in the video, this ain't no sunday drive.

This got posted the other day and it showed on my Facebook newsfeed. I LOL'ed at this. All the radical rock music and noise, a C7 going thru the Kink at under 100MPH!?!?!

Really, stop driving like a putz, my C4 is WAY faster. Why? Driver mod, *****, and good setup.

https://www.youtube.com/watch?v=bao_...youtu.be&t=46s

93Rubie

Yeah, I'd love to do more track stuff but Cost, risk to car, I don't know how to drive 8 or 9/10's, I get bored with anything less than intense 10/10's driving, street car, etc....

Auto-x allows the 10/10's driving with little risk and its a lot cheaper.

Tom400CFI

You can give 'er hell with no consequences.