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Old 10-05-2006, 03:30 PM   #1
DaveL82
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Default How to correct off throttle oversteer?

I had my first chance to run my old C3 1980 vette at Motorsports Ranch last Saturday after working on it for a long time. Had a blast but did find that if I got off the throttle entering a corner the rear wanted to oversteer. Had to break earlier than I wanted (before brake points)and then apply power real early to compensate. Had to carry the power through the corner.

Had no excessive outside wear on tires after two runs. Had the DN 5sp pop out of 3rd twice but that's another issue to address.

Current car setup:

ZZ4 motor and Doug Nash 5 speed
Aluminum rad with electric fans to reduce front end weight
fixed FIA headlights and cover to reduce front end weight
Front has tubular control arms with VPB transverse spring set on next to softest setting.
Carrera shocks on front.
Stock 7/8 sway bar on front.
17 x 10 CCW wheels with Kuhmo MX tires

Coil overs in rear with 350lb springs
18 x 12 CCW wheels with Kuhmo MX tires
no swaybar!

Has a rollbar but still an old flexy chassis.
OMP setas and harness
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Old 10-05-2006, 03:37 PM   #2
69autoXr
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Alignment settings?

Stiffen front spring, stiffer jounce front shocks, stiffer rebound on rear shocks. I think you want to slow down the weight transfer when getting off the gas. Although my knowledge of shock tuning is minimal, so I may be way off base. Maybe one of the shock experts will chime in. Few guys in this area still run (or have ever run) C3's, although RAFTRACER has extensive experience in a C3, hopefully he will see this.

Last edited by 69autoXr; 10-05-2006 at 03:39 PM.
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Old 10-05-2006, 03:56 PM   #3
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Doublecheck rear toe settings. Rear toe out = all sorts of weird oversteer behavior.
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Old 10-05-2006, 03:59 PM   #4
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Dream Car Garage did a special on an L-88 racer they built up. One of the things they discussed was the rear suspension geometry. IIRC, they stated that as the rear unloaded under braking, the rear suspension would transition to toe-out, which caused the rear to be very squirrely. They were limited on what they could do mod-wise, so the best they came up with was to compromise the rear alignment.

I guess you could also go to a stiffer rear spring, just to reduce the deflection. Again a compromise.

HTH, and have a good one,
Mike
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Old 10-05-2006, 04:10 PM   #5
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Quote:
Originally Posted by VetteDrmr
Dream Car Garage did a special on an L-88 racer they built up. One of the things they discussed was the rear suspension geometry. IIRC, they stated that as the rear unloaded under braking, the rear suspension would transition to toe-out, which caused the rear to be very squirrely. They were limited on what they could do mod-wise, so the best they came up with was to compromise the rear alignment.

I guess you could also go to a stiffer rear spring, just to reduce the deflection. Again a compromise.

HTH, and have a good one,
Mike
I have that episode on tape and review that section specifically from time to time. What I don't follow completely is why a stiffer rear spring would help; that would unload the rear more quickly (assuming you leave the shocks alone) and toe out the rear more quickly. I would think stiffer front springs would help more. Again I may be way off base.
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Old 10-05-2006, 05:00 PM   #6
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Quote:
Originally Posted by DaveL82
did find that if I got off the throttle entering a corner the rear wanted to oversteer.
Just don't do that
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Old 10-05-2006, 05:03 PM   #7
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No sway bar in the rear, that can't help. Along with the other suggestions you can start with rear tire pressures.
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Old 10-05-2006, 06:37 PM   #8
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Quote:
Originally Posted by robvuk
Just don't do that
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Old 10-05-2006, 07:07 PM   #9
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You are lucky you weren't driving a 911 Porsche. You would be posting from a hospital bed.

Frank Gonzalez
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Old 10-06-2006, 12:02 AM   #10
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On the rear rebound you want to soften it to lessen throttle off oversteer. That allows the rear suspension to droop faster. If you stiffen it then the rear further unloads during deceleration. Rear sway bar adds rear roll stiffness which in most cases will make the car looser. I have looked at some older vette rear supensions and they are a poor design. I would bet that is the main reason. you can simply try and tune it out with shock settings, and alignment. The camber curve looks to be all over the place with those cars.
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Old 10-06-2006, 06:46 AM   #11
greendot
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Two things I would suggest.
More rear toe in.
A brake proportion valve to reduce rear brake pressure.

The "no rear bar" is a good thing.
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Old 10-06-2006, 10:51 AM   #12
Cashmo
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with greendot and trackboss. More rear toe in, brake proportioning valve and less rear rebound.

I believe Danny's theory on C3's (at least for autox) was that due to poor camber curves you were better off making it so stiff the suspension didn't move much. Having been a passenger in Danny's BSP C3 I can tell you it wasn't a comfortable ride.

Jeff
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Old 10-06-2006, 11:23 AM   #13
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I believe Cashmo is right about Danny's theory - make it a go kart.

I had tried that before Danny got fast and was not successful with it. Yes, I'm that old
Obviously it works for HIM in autocross. I can't say that he ever tried to run that on a track.

I've got lots of tried and removed parts for C2/C3 here.
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Old 10-06-2006, 11:36 AM   #14
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When I said "stiffer rebound on rear shocks" I meant slower return. I guess stiffer was the wrong term, softer rebound is more correct?
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Old 10-06-2006, 11:50 AM   #15
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I think what trackboss is saying is to allow the rear tires to "droop" easier when the body tries to lift from weight transfer so that they stay in contact with the ground. That goes hand in hand with a stiff rear spring so that the suspension travel is less for that weight transfer, but then you have to stiffen the front of the car to maintain steady state front/rear balance of the car.

I think just stiffening the rear spring to fight trailing throttle oversteer will give you steady state oversteer.


However, Dave82's "fix" seems to be "slow in, fast out"

"Had to break earlier than I wanted (before brake points)and then apply power real early to compensate. Had to carry the power through the corner."

There's alot of driver input that comes into play. Are you trailbraking and don't realize it?
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Old 10-06-2006, 12:24 PM   #16
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How much rear ride height adjustment is there in a C3?
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Old 10-06-2006, 12:36 PM   #17
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Dave, pick up the phone and call Dick Guldstrand Ent. in Culver City Calif. and tell them that you have a C3 that you want to RACE. Then buy their entire setup (SCCA & SVRA legal), bolt it on exactly as they recommend, and race it (including the bump steer kit). Dick has been doing this for 43 years (on C2s and C3s) and knows more about the suspension setup than anybody. It will cost you a couple thousand dollars one time, and you will never think about it again. I have been racing C2s and C3s for about 40 years, and I'll guarantee it will work.
BTW I have a friend Kerry Bonner, who instructs at MSR. Look him up and ask him for some driving tips there and at TWS.
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Old 10-06-2006, 03:52 PM   #18
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Thanks for everyones input and it gives me a few places to start. As many of you have pointed out the old C3 suspension is not a good example of a performance design. The camber change can be fixed by moving the lower link which is a relatively easy task.

Rear toe change can only be fixed by tossing the trailing arm and going with two trailing links and a toe link. When I did the coil overs I added points for a toe link just need time to construct the two trailing links/mounts.

As for now I'll try some of the changes suggested. Maybe even starting out with a bit more camber to start with. Spring rate change is a tough one since I then run into toe change with suspension compression as discussed.

Still it's fun trying to sort the old beast out and was able to sense the car trying to oversteer and worked around it even though not optimal for track times.
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Old 10-06-2006, 04:29 PM   #19
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If the rear it toes-out when the rear lifts, then this is exactly the opposite of what the C4 does, but the fix is the same. First measure your rear bump steer to confirm that your suspension is doing this (you can buy a bumpsteer fixture from www.Colemanracing.com ... or make one if you're handy) and then shim the toe link up or down to minimize the bumpsteer effect in your working range (you'll have to replace with a sperical-end type rod).

It'd be helpful to know how your car drove at mid-corner. Is it neutral? Run it in a circle in empty parking lot, gradually go faster + faster and tell us which end washes out first. Hopefully it oversteers a bit and you can merely try dropping the rear spring rate 25 or 50# and see if there's any improvement to corner entry.

Why no sway bars? Is this a normal set-up for a C3?

If you have adjustable shocks then you can increase the rear rebound setting a bit to keep the rear-end from rising so quickly when you let off the throttle... thereby keeping the weight on the back-end a little longer, or you can stiffen up the front compression, or a combo of both... this delaying the weight transfer from the rear tires to the front tires in the throttle-off-corner-entry.

Good luck and keep us apprised of your progress.
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Old 10-06-2006, 05:30 PM   #20
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What was the lap times when you were trailbraking?

What was the lap times when you on the gas before the apex?

I had the same problem at MSR (little bend turn) and just got on the brakes sooner and was on the gas before the apex, to transfer weight to rear tires.......... I actually improved my lap times. And it wasn't so scary, cause the back end wasn't slidding around.

You gotta stay on the gas through riochet and little bend, or as you notice. the rear end gets really light and wants to swap ends.

TIA, Robert
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