Ramo7769

I have a C5 prepped for STU. 285 Dunlop Direzzas square on Z06 rear wheels with VB&P springs and swaybars that made the car close to neutral on a ~50 mph skidpad. I'm very pleased with my setup at low speed.

However, I ran an autocross event today and had trouble with braking and and low speed turn in. It was pretty low speed. The cornering speed I had issue with were tight, around 30 mph or maybe even slower. The car pushes unbelievably at that speed. And I'd like to reiterate that the car is beautifully neutral at 50 mph.

Here are my front alignment specs:

Front: Toe: 0, Camber: -2.2deg, Caster: 6.7deg

My thoughts (other than Corvettes hate turning at low speed) are that I could maybe use less camber/caster (less dynamic camber at high steering angles) and some negative toe. But, I'm open to ideas.

Here are my runs but they are from the roof, so it's hard to tell when it's understeering. There is a 90deg left that destroys me on every run. The car isn't responding right, and I never was patient enough to get through it quickly and without hitting cones on exit. Embedding doesn't seem to work at the moment, so here's the link. https://www.youtube.com/watch?v=mLkY...ature=youtu.be

Bill Dearborn

Are you trail braking? I haven't autocrossed much in the last few years but I remember with both of my C5s I had to be done with my braking before turn in and use trailing throttle oversteer to get around a corner quicker. The sequence was brake, lift off the brake, turn in, let the backend come around some and then roll onto the gas to plant the rear and accelerate out of the corner. Loading the front tires with braking and turning violated the friction circle ending in massive understeer that would continue all the way around the corner unless the car was slowed enough for the front tires to regain traction.

Bill

Ramo7769

Video link posted on OP.

Bill, yes, I've tried trail braking, but also tried braking earlier and still seem to plow like I have no front grip at all. In the higher speed corners in this autocross, I'm able to effectively trail brake and it tucks the nose in as I intend. I'm happy with the dynamics at speed.

I was also wondering if my 90 degree left hander was a bad surface too. But, I tried some hard cornering on the way home at 25-30 mph and I just have no grip at that speed and steering angle, regardless of surface. Plow city. But, again, at 50 mph, it rotates beautifully.

mountainbiker2

There's your problem. Those low speed corners take a lot of patients. I know when I'm driving slow corners, I believe everyone else is faster then me, so I push it harder. Then you lose front grip and get more frustrated.

Steve

acrace

Some quick thoughts . . .

1) I'm pretty familiar with that lot, having run many events on it (and having chaired/set up a bunch of courses there). The surface is pretty slippery at the start of the event, but rubbers in nicely and you get decent grip. But if you go offline, you lose that grip in a hurry.

2) It seems to me that the car doesn't want to turn in at low speed. Maybe it's too high corner entry speed. As posted by mountainbiker2, patience and discipline is a virtue.

3) Assuming that you have proper corner entry speed, I'd work on getting the car turned. That's a different solution than a steady state problem. I'd give it some toe out in the front to help with turn-in response.

4) For low speed steady state grip, I actually set my cars up with as much caster as possible to get as much negative camber at high steer angles (as you mentioned).

So I'd break down the low speed under steer problem down into varying segments of the corner. Does it turn in? Once you're at steady state, does it still push? Anything bad during corner exit?

Al Chan

Ramo7769

Thanks, Al. Definitely have seen your name around a lot. I'll have to introduce myself at the next event. Toledo this weekend for me.

Anyway, I have the issue at initial turn-in and at steady state. On exit, I can at least combat the issue with my right foot. But my tail hit a cone or 2 using that method on run #3 which was my best raw time today at 33.8.

I played around on some more asphalt after the event and steady state, limit handling, at 25-30 mph, the car plows all day long.

Toe out seems like a good remedy that would act like ackermann, allowing less slip for the front tires at low speed, higher steering angles. I just have to be careful to maintain a balance for autocross and track. Like I said, I know I can't have my cake and eat it too. I just want to make sure I am making the best compromises I can be. Thanks for the tips.

acrace

I aligned my cars to zero front toe and then would turn it three "flats" per side (toe link) toe out for lower speed stuff. Easy enough to tweak quickly before an event.

Perhaps we'll meet one day. I've pretty much taken the last couple of years off from competition, but still check out various forums on occasion to stay up to date.

Have fun . . . C5s in STU was tempting as a project, but in the end I decided to sit on the couch for 2014.


Al Chan

Matt_27

Did you play with shock settings at all? It might be neutral in steady-state but transient conditions in turn-in can give very different results based on what your shocks are doing. Take some front rebound out or add rear rebound and see what that does. Low speed corners with higher steering angle have a longer transient phase than high speed so that could make the difference. I just started acquiring parts for my FRC to go to STU so I will be going through the same process here in a bit...

Ramo7769

It's definitely been a fun, continued learning experience. I'm still on my stock 1999 damping. Waiting for DRM GB...

Matt_27

Are those DRMs adjustable at all though? Why not some Koni yellows? That's certainly your problem if you are still on stock dampers, especially with higher spring rates. The car will be transformed with Konis, and even more so when you get the front to back rebound balanced out. Do ittttt.

Ramo7769

They can be revalved as needed and they're $400. I personally don't expect much value from tuning rebound and not bump.

All that aside though, my issue is steady state. I'll have a low speed issue with triple adjustable Penskes because they won't make the front tires point in the right directions.

Gordy M

I ran that event on Sunday and that 90 degree turn at the far corner was also a little off camber. Your time of 33.8 on street tires was not that far off considering Mike Losert was at 33.3 on street tires. Also, that course was better suited for the Z06 trans as the stock C5 was under the torque curve at several spots.

Ramo7769

Ah, thanks for that info. You might say that Losert is my target. STU should allow me to get through the corners than his car in street class. But, he's very fast and consistent!

LateBreak

So are you getting on the throttle earlier as well? I track an E36 BMW on occasion, and it simply won't turn at low speeds if you're off the throttle, you have to brake early, turn in, and get on the throttle to get it to rotate. At 2:08 in your video, it looks to me like you're coming in too hot and braking a bit, but not using the throttle to get the car turned.

Ramo7769

Yeah, I agree, 2:08 was definitely my worst moment of the day. I never did that corner well though. You're right. Just need to get some patience, brake earlier and get that big, red nose pointed in. I did it a little better on the following run, but still was late on turn in (braked too late) and came out sideways and hit a cone with the rear corner.

I had some braking issues here also, which I didn't mention. I just did a bleed last night and had some air in some of the lines. ABS kept getting me into ice mode. I think I may be better off with the fuse pulled. Ran it last night. Are these cars typically front skid limited?

LateBreak

Mine always locks up the rears first. What kind of pads are you running?

LateBreak

Also, the DRM bilsteins would be a solid investment, regardless of their effect on this particular problem.

edwardo99

With 1999 shocks your transient response on turn-in will be sloooow. This should not cause the car to push, however, at least not by itself.

edwardo99

I watched the vid. Your car is definitely pushy everywhere on that course. From what I can tell, at 2:08 you were very late to turn in. You jerked the wheel left and it plowed almost straight.

So, you have 3 problems as I see it
1. Transient response is poor. New shocks will help a lot. More front compression and more rebound to make weight shift happen faster across the front. You need also need front toe out so the inside tire will quickly develop slip angle and pull the car into the corner even before weight shift has happened. Get used to setting it at each event like Al advised. I carry a jack and use 2 flats for faster auto-x courses. Zero for street and track. Do you have a big front bar? You will need it to be competitive in slaloms. Must have a stiff front end that reacts. Then tune the balance.
2. The car is pushy steady state. Vettes are highly tunable even in Street class in this regard. Get the right parts on the car, then learn to use tire pressures, alignment, and rake to adjust. (And shock settings if you have adjustables)
3. You are over-steering the present car. Must match inputs to the front end response.

My BS class 2000 turns in like crazy and I have it tuned for slow speed push/ high speed- high grip it's a little looser. It is basically impossible for me to turn the steering wheel too fast at low speed.
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Edit: I forgot to ask, have you lowered the car? Drop it down as low as it will go if you want to be competitive in auto-x. Forget about roll centers. Order: 1. drop it 2. corner weight it 3. align it 4. tune the balance

JiminVirginia

Yes. You owe it to yourself to try a radically different approach to the shock settings, including a version of this, just to see how the car reacts.

Jim