reidry

I have setup a 1985 Corvette for autocross. Suspension was completely removed from the car. Poly bushings installed throughout including the differential carrier. New ball joints, all new NDH wheel bearings, heavy springs (BMB, BMH - roughly 400 lb/in rear, 600 lb/in front), adjustable QA1 shocks, rear adj. camber rods, camber brace, beam plates, C5 brake upgrade, SS brake lines, 17 x 9.5 sawblades on all 4 with Kumho Ecsta MX 285/40 rubber, 30mm solid front bar, 26mm solid rear bar.

I'm running -1.1 Camber on all 4, castor is maxed, slightly toe out in front, slightly toe in on rear.

I was running front and rear shocks very stiff which was causing me to raise the tire pressures to keep the tire frequency from dominating. I had pressures up to 38 rear, 42 front. Setup like that the tires aren't sticky enough to get a good launch.

I changed the setup and decreased the settings on the front and rear shocks and lowered the rear tire pressures to 32, front down to 36. Car now feels loose. An instructor that drove my car during the "fun runs" after the practice day agreed that it would suddenly without warning become loose on entry. He suggested a smaller rear bar or wider rear wheels/tires.

I ran a 39.68 setup like this, he ran a 39.2 in my car. With his stock Mustang GT (previous body style) on Victoracers he ran a 37.36. FTD was an Elise at 34.83 with a Porsche GT3 right behind at 34.98. Glenn felt my car might make it into the 38's without race tires.

I would prefer not to go asymetric until I can afford a set of dedicated race wheels & tires. I have a 20mm solid on hand, should I put it in or order up a 24? Should I look elsewhere for an adjustment?

Thanks in advance.

Ryan

larryfs

soften up the rear all the way.

Slalom4me

Would you describe it as 'Snap Oversteer'?

Under braking, the weight transfer is raising the body and bump steer
during the suspension movement results in a transition to toe-out. The
fireman at the back of your hook-n-ladder is giving you some opposite
lock.

On my '89 FE1 with stock ride height, I asked my alignment tech to
set the rear toe and then try lifting up the rear to simulate weight
transfer and measure the toe again. Raising the rear by 2" changes
the total toe approx -0.12 degrees - in other words, the static toe
changes in the direction of toe out.

The effect of 4-wheel steering was fun for me on tight, low speed
courses, it was not so much fun on faster tracks where higher speeds,
harder braking really increased the tendency of the rear to step out
during turn in. To compensate, I did more braking in a straight line and
reduced trail braking. Then I requested more rear toe-in.

If the car is steeply raked front to back, another step which will benefit
and may permit less toe-in is to lower the rear of the car so the rake is
near zero degrees. This gives less leverage during weight transfer and
hence less jacking/toe change.

Some threads I feel you will find useful
.

Solofast

If you look in the C4 Suspension Chart

(http://corvetteactioncenter.com/tech/c4/susp_chart.html )

you will see that they only used the 26mm rear bar with softer springs, with the exception of two years, 1990 and 1991, and those spring rates maxed out at 320 lbs/inch (57n/mm). C4's with too much rear roll stiffness can jack and create nasty oversteer. In stock form that was very evident in the two years noted. You will find that the car is a lot less difficutl to drive under power. The reason is that you are generating squat and getting the toe/jacking issue under control. As noted above, you need to soften the rear end, either with a softer spring, or softer bar. Lowering more in the rear will help, but you will start hitting the bump stops and it will start all over again.

If you go back to 24mm rear bar with the 400 lb spring you will be still on the edge of the issue, If you have a 22mm rear bar I'd try that first.

Slalom4me

I agree that softening the suspension or reducing roll stiffness
permits more squat under acceleration and thus more toe than static.
This would help reduce 'power-on' oversteer at corner exit.

But isn't the OP asking for help with 'power-off' oversteer at corner
entry? I'm not sure that rear spring rate plays as much of a role here.
(More front spring would help counter dive but I don't want to go
there.) If the actual rear toe goes negative when the back lifts, the
car will be squirrelly when it starts to rotate.

Not trying to cross swords with you, just looking at the stated condition.

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Solofast

What I was trying to say was that when we encountered this condition, the car didn't oversteer under power, because the squat lowered the back end, reduced the toe effect and, because it was lower, there was less tendency to jack. Remember that jacking is a non-linear effect, the higher the rear is the more it wants to jack, and the more it jacks the higher it gets. So, if you are to the point where it is starting to jack up, things deteriorate in a hurry. If you don't hit that point nothing dramatic happens and you can hustle right along.

What is causing the problem is the TOTAL roll stiffness, that is the roll stiffness effect of the roll center height, AND that of the springs AND the swaybar. As the rear of the car lifts up under braking, you are getting increased rear roll center height effect on roll stiffness. You can't do much about that, other than lowering the car, but you can effect the other two.

If the car jacks, you can (if you are looking for it) feel the back end come up a bit, the car will loose grip (because you are loosing negative camber and the rear end is toeing out too) and it will step out. It is most pronounced when trail braking on corner entry where things can get really squirrley, but you can also see it in slaloms and offsets if you are lifting to help the car turn in early in the transition. As you roll on the power the suspension squats and you are back under control again, because the rear gets lower, the jacking is reduced, so it gets lower and you hook and go.

You can only fix what you have the ability to change, and you can't change the rear roll center effect. Braking is going to lift the rear and you are going to get the effect of increased rear roll center height. So that means you want to lower either the rear spring rate, or the rear sway bar rate to get it back under control. You are right that the car will pitch less with the stiffer springs, so that is why I was suggesting that he lower the rear swaybar stiffness rather than soften the springs. That will bring it under control faster with less hassle.

reidry

Thanks for all the help so far ...

I have the car setup with near zero rake, just slightly nose down. Half-shafts are parallel to the ground.

When the back lifts shouldn't that induce more negative camber, not less? Differential moves up with the body, inner u-joint rises & pulls half shaft up, the wheel top should tip in - more negative camber. At least in straight line braking.

When entering a turn and trail braking, the outside wheel is in bump so as the car tries to rise the wheel looses negative camber.

So my understanding is decreasing the rear roll bar stiffness will allow more rear roll giving more negative camber due to suspension rise and more net negative camber when the body rises due to weight transfer.

Does this sum it up?

I think I'll toss the 20mm solid on the car (have this on hand now) for the Sebring event and evaluate the change. I can always buy a 22 or 23.

Thanks,

Ryan

Sidney004

I agree with what everyone is saying but I would do one simple thing first and that is to take a baseline alignment on the vechicle exactly in the condition it is in now. I have also experienced snap oversteer on my C4 just as you described on corner entry and not on a friendly autox course. It turned out to be the rear alignment. Any rear toe out can induce this condition. I would doublecheck your alignment settings first.

Solofast

You have to look at the camber curves to see what is happening. I don't have a copy of them anymore, but there is camber change in both directions, depending on where you are along the curve and which way you are going. Without looking at where you are I wouldn't venture a guess as to what you will get for a given displacement either way.

What decreasing rear roll stiffness does is lower the rear tire slip angle (reduces basic oversteer coefficient) as well as reduces the tendency to jack (which as noted does all kinds of bad things).

Since you have the 20mm bar that is a good place to start. I might suggest that you take along a set of 20mm poly bushings for the rear bar, since if you want to make a small change in overall bar stiffness changing out bushings is a simple and fast way to do it.

Same for the front, if you go too far, you can pretty quickly change the front bar stiffness with a set of softer bushings to get the car balanced again.

Sidney is right too, you need to know where you are, and if you had some toe out in the back that would make the car a wild ride even if it had the right basic spring and swaybar setup. But in my experience I do think you had a bit too much rear stiffness, and if it was aligned even close to right (about an eighth of an inch of toe in) I would definately soften up the rear bar.

Slalom4me

My recollection in my case is that the car began to step out the
moment I initiated the turn and thus before the rear had a chance to
roll very much in the bump direction on the outside.

I would very much like to see the camber curves SoloFast mentions
in case anyone else can still make these available. They would be
helpful to review.

I had been accustomed to modestly understeering solid and independent
RWD cars before experiencing this kind of oversteer in the C4. After
changing my shorts following the first time it happened, I began to
use it to get through a particularly tight corner, just as you describe.
On a faster, less forgiving track I wasn't comfortable with this and
tried other things.

I'm really enjoying this thread and looking forward to learning more.

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Solofast

What I try to get to in a setup is this:

The front end bites and the back end follows wherever you go...

That includes some serious reefing on the wheel in fast offsets and transitions.

If you are feeling the back end rotating too much, or if you can't have confidence in the car you need to be looking at adding some stability margin. I am not saying that a car should be a pushing pig, and you have to get the front end working to get high levels of grip. But make the changes to get more front end to grip and don't try to buy your way out with with looseness. In an autocross or on the track, you have to have the confidence that if you are working the car hard it is gonna stay behind you.

Lots of people, and even some (but very few) fast ones want the car to rotate and will live with a lot of oversteer to get that rotation, but I just don't think that is the fast way around the track.

BrianCunningham

Most of us would kill to be loose going into a corner.

After reading these posts it sounds more like your brake bias is messed up. I've go loose going into a corner, but it was under hard braking after a long straight.

reidry

Brian,

I'm running C5 fronts with the DRM brake bias spring installed, system has been completely flushed, ATL Super Blue Fluid, bled with speed bleeders, pedal feel is very good.

I'm easy on the brakes and the rears aren't locking, I have pushed the car to several extremes including front and rear tire lock - I guess I could put brake pressure gauges and a bias adjuster in ... is that necessary??

Ryan

Ryan

BrianCunningham

Hard to tell w/o being there. You don't have to lock them to get that to happen, it's the weight transfer, the rears won't have any grip if there no weight on them. You may be able to fix it by cranking your shocks back up.

Try this 1st, crank just the front shocks back up, and see what that does.

As a general rule, you want to change one thing at a time, and take careful notes as you go.

reidry

Ok ... I'm in the middle of chaning the clutch on the car so I've got it up on the lift.

I got to looking at the rear sway bar getting ready to change it while the exhaust is off. Grabbed the bar, thought "that's odd, feels a little small and why does it wobble around??" Grabbed my trusty calipers ... tada 23mm bar!!

Moving around in those mounts certainly isn't making my handling predictable. I'll have to get some 23mm bushings but for the next event I'm going to swap in my 20mm stocker.

I never even looked twice when that bar arrived, assumed the forum member sold me what I asked for! Live and learn!

Ryan

Slalom4me

I don't know what correspondence you had with the
seller, but in the sale thread he described the rear bar as follows:
According to Hib Halverson's suspension chart, the '84 Z51 rear bar
is a 23 mm. Z51's weren't 26 mm until '90.

.

reidry

It's no problem, my oversight. I won't be going back to the seller. If I need a 26 in the future I'll hunt one up.

Ryan

Slalom4me

No, I have the C5 brakes and DRM bias spring also. Until proven otherwise,
I feel the bias is a red herring for you here. I feel likewise about
camber change. You state you have -1.1 camber all the way around
now - this will help grip and tire life. Short of changing ride height to relocate
the range of camber change being used, there isn't much you can do about
the camber curve that is simple to achieve. As far as lowering putting
you onto the bump stops, my opinion is that a 400# spring is stiff enough
to allow some room for ride height adjustment. There is another variable
on your car that no one has mentioned yet - damper rebound settings.
Soft rebound settings wouldn't be a cause of Snap Oversteer, but could
contribute to it. However, there are already enough factors on the
table, so set rebound adjustment aside for the time being.

Please consider Sidney004's advice - baseline your current alignment
as your first step. Any competent shop will be measuring 'Before' values
- it is common practice in my part of the world and the shops keep printouts
attached to copies of work orders. Good practice when you make the
appointment is to advise the shop that you want a copy of the Before
& After printout to take with you - if they balk, go elsewhere.

Prior to going for the alignment, prepare by reviewing the many posts here
with C4 setting recommendations. Also take a look at recommendations offered
by Vette Brakes and Products. Notice how the Autocross and Track settings
differ - this is in part due to the different speeds and
stability/transitional requirements between the events. In particular,
rear toe-in is increased for higher speed use. Decide which usage best
meets your needs and take these settings with you to the
appointment. As mentioned elsewhere, be sure to have the car in
its track configuration (fuel load, tire pressures, driver weighting)
when you arrive.

About the swaybars. My view is that these are a fine tuning aid to
complement proper spring selection and that sway bars are selected
to establish balance at steady-state conditions at low speed on a 100'+
skid pad. (Front and rear aero is used to establish steady-state balance
at high speed).

Resolving the issue of the 23mm bar in 26mm bushings (or changing to the
20mm bar/bushings) will eliminate the two-stage rear roll control aspect the
loose fit was effectively creating. But I don't believe that the transition in
roll control that occured as the play in the bushings was taken up and the
swaybar began to have effect would/should be severe enough at initial
turn-in to shock load the tires past their grip point - there wouldn't normally
be enough slip angle developed yet. But - what if the tire had already had
begun to develop slip angle? (How? Toe-out.)

Switching to the 20mm is quick, solves the loose bushing problem and
won't alter current settings before you get a chance to baseline the
alignment. But I don't think it is the main solution to the initial
issue.

Sorry to continue to flog the alignment horse - this was just supposed to
be a post to say that gauges and an adjuster should not be necessary.
Incidently, member CentralCoaster did research into bias spring selection
in the course of setting up Wilwoods on his early ('85?) C4 - he could
be a resource if you decide to pursue this angle.

.

Slalom4me

Melrose has this Handling FAQ that may be of interest.

They used to offer alignment specs, but now appear to sell the info
as part of a DIY booklet.

.

Solofast

Some of all of this doesn't quite make sense...

With your rear spring stiffness and a 26mm rear bar with poly bushings I would have thought that you would have had too much rear roll stiffness. But with a 23mm bar if you had rubber bushings in it, I wouldn't have thought that you would have had too much rear roll stiffness. (Just going on experience with most of the sprng/swaybar combinations, and knowing the 84's in stock Z51 trim had a tendency to jack, and so did the 90/91's. The 84's had a 535 pound rear spring and a 23mm rear bar, the 90/91's had a 350 pound rear spring and a 26mm rear bar, but also a slightly lower rear roll center, so that kinda bounds what you can do at close to stock ride height with springs and bars)

Still any bar that is loose in poly bushings is going to be a scary thing. You are going from no bar to a pretty good amount of bar when you run out of slop, and that is a bad thing.

Swaybar bushings make a big difference in the overall working of a sway bar, so you need to be careful because a stiffer bushing can have a big impact in how the car responds.

Try the 20mm bar you have now, and if that is too soft, put some poly bushings in it, and if that doesn't work and you need more stiffness go to your 23mm bar with rubber bushings next. In fact I might be tempted to just put in rubber rear bar bushings in the 23mm bar and see if that does it, since that is the easiest and fastest thing you can do....Over that range you have a lot of tuning variability and should be able to get it tied down.