meldog21

I am posting, actually forwarding a question I got from a friend of mine about high speed instability. Any input? There must be someone here with high speed aero/set up knowledge.

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Dog,

I noticed a kind of uncomfortable instability early this morning(in the dark) on the way to work and wondered if you had any experience with this, or know someone I should talk to.

First, I know this was street stuff and not at a track but it is a general car set up question so I thought I'd give it a shot.

I was accelerating and near the top of 5th at around 165-170mph on a fairly flat open section of freeway. It was still dark so I had the headlights up. The car felt fine accelerating, but as soon as I lifted (no braking) the back end felt light and wiggled back and forth a little. It was kind of like on my sport bike when I brake hard before a corner at the track.

Car info: I just had a fresh corner weight and alignment last week. F -1.5 camber, -.04 toe(combined), +7 caster, R -1.1, -.10 toe(combined). It used to be -1.7 and 0.0. The slight toe in on the front was done to try and counteract my inside edge wear on the front tires, along with a couple tenths less camber. I have approximately 1/2" rake on the rear.

So my questions are: Is a stock to racey set up C5 Z05 just not that stable near top speed? Could that instability be compounded by having the headlights up? Do you really need a wing and splitter to maintain good stability at speed?

If you need more car info let me know and I'll tell you anything I know.

Thanks,

XXXXXXXXXXX(name removed to maintain anonymity)

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So if you have experience on high speed car set up post up and I will forward the info to him.

Dog

tmak26b

Check motor mounts also, I have the same problem and I found some bad mounts (I havent gotten my car back yet)

AU N EGL

165-170 mph in the early morning darkness on public roads ?

"but as soon as I lifted (no braking) the back end felt light and wiggled back and forth a little"

is this guy an idiot or what?

Tell your friend to slow down and not do foolish things on public roads.

First:
public roads are not designed for those speeds do the crown built in to roads, grooves on some pavements, expansion cracks in pavement.

lifting at those speeds is almost certain death. No wt or power to the rear wheels, and even NASCAR cars get light in the rear end and swing around

The alignment looks fine, but that is not the issue. Poor driving and judgment skill is the real question.

Have your friend go to a High Performance Driving School or racing school, No bikes are not the same, to learn how to drive. That is the REAL CAUSE of the problem.

mel you should know this too. Plus Moderators dont like kill or excess speed on public road threads. ( several years ago two CF members were chest thumpping on the forum. them finally met for a race on public roads. Well they crashed and one died. So no public road excess speed, or kill stories. )

GrantB

My FRC is fine if I lift off at 185. It has 1/2" of rake, Z06 suspension with pfadt poly bushings and sway bars. I believe the bushings helped some of the rear-end wiggle, which may be compounded by higher speeds.



Having the headlights up does increase front-end lift, but reduces rear-end lift. The best (free) aero improvements would be increasing rake and blocking off the front fog light screens which normally let air into the front fascia.

I don't believe rear-end lift is the problem, though rear toe-steer might be. When you lift, the rear tires go from being toed-in to neutral toe. This may (and by 'may', I'm completely guessing here; no data to back this up) cause a sensation of instability as the rears suddenly start following the grooves in the road more.

Sidney004

Somebody() should write that guy a ticket!

John Shiels

or bent his key in half.

In any case stock car should be stable at 175 even after lift off which may possibly be from the pavement under the right road conditions. headlights will only make you lose few mph.

175 in the dark where does he think he is Le Mans?

meldog21

Thanks for the agreement on the crazy road speeds. I already gave similar opinions on that. I just don't have the high speed experience or knowledge to provide good answers about the wiggle.

Dog

CP Thunder

Has he had any adjustments made to the fly by wire tables in the ECU? Slamming the TB shut at that speed is the same as stomping the brake pedal........ if so........huge engine deceleration......rear wheel lockup......not pretty at 175 on ANY ROAD

Independent1

A couple of thoughts.

When your friend lifted off the throttle he transferred weight from the rear of the car to the front. That is going to affect howmuch rear traction he has.

Some type of rear spoiler/aero device would help with this issue. Another possibility is to lower the car to increase air speed under the car, which will result in lower pressure underneath the car and hence more downforce.

yellowc6ohio

i've had the same thing happen but mine did it under acceleration. my problem was the clutchpacks in the diff. not sure if the C5's have the same problem with them as the C6's.

meldog21

My friend told me he found something wrong with the car. It turns out that the front bolt on his rear upper A arm was almost out of it's hole, but still on the car. I'm sure the dynamic toe change had something to do with the instability.

I guess other factors could be involved, but having one end of an upper A arm not attached has got to be part if not all of the problem.

Dog