I hate to quote Wiki, but some one did go through trouble to post some pretty good info regarding how a composite leaf spring has anti-sway bar like properties.

http://en.wikipedia.org/wiki/Corvette_leaf_spring

 

Hey, I will accept info from any reliable source.

The explanation is quite believable, I'm am somewhat skeptical as far as tangible results.

I have to go now and eat a plate of Crow..

 

I spent 2 years auto crossing with SCCA. It just got too expensive. When my rear wheel bearings wore out, that was the end for me as well as the way they put me in class with lighter and more powerful rice burners. BTW, how's your rear wheel bearings?

 

I need to get the wheel rate from the vendor
it has dual rate springs so it's not that straight forward.

I cranked the compression up on the shocks
that helped a lot

i'm going to order up bigger swaybars for it this week

If that doesn't fit it I'll start messing with the springs

 

I would find out what your actual spring rate is first, there is a formula for working it out on dual rate springs,
but they always seem to use dual rate springs on off-road race cars that use a lot of suspension travel, I dont think they use dual rate springs on bitumen cars,
"roll couple percentage" is important too, have a read of this,
http://en.wikipedia.org/wiki/Suspens...ple_percentage

 

Personal opinion,

I try to get the springs right first, then fine tune with sway bar, shocks last.

 

I got the spring rates

after going through the series equation I'm showing
342 lb/in front
317 lb/in rear

How does that compare to what everyone else is running?

The fronts are angled the rears are not

 

FYI according to my digital level the shocks at rest sit @ a 60 degree angle

I have a SolidWorks model
I'll plug that in and see what I come up with

 

Compared to Z51/Z07 front leaf springs you have HALF as much spring rate. No wonder it rolls.

If I remember right the front FHB Z51 spring is around 650 lbs/in
The Z07 front spring is around 520 lbs/in.

 

Its a dual rate spring

one on top of the other

600 800 front
550 750 rear

I think what's going on is that the initial spring is bottoming out
and then the rate jumps from
317 rear to 750 rear

 

Those rates cannot be compared equally. The motion ratios for a coil over setup are different than the leaf.

 

I think you are on the right track. I recall a few other people with coil overs only having like a 400lb spring in the back. 750lb might be too much causing the oversteer. The front rates look ok, so I would try a bigger bar and see how it feels. A bigger front bar will also help a little with the oversteer.

 

Actually this isn't true for the C4's rear spring. Unlike the front spring which uses the widely spaced, dual pivot setup, the rear spring has a single center mount. It basically behaves like two independent springs without the anti-roll affect.

You are correct in that if we were talking about the front end or a C5 or later Corvette, replacing the leaf spring with same heave rate coils springs would result in a softer overall roll rate if you don't also change the sway bar.

 

Good point. Wheel rates should be comparable I would think?

 

Brian has not posted any pictures of how his springs are positioned, but assuming they are mounted outboard on the lower a-arm near the lower ball joint, his motion ratios may be not that far from stock.

If his coils are mounted inboard of the stock leaf/a-arm interface by much(?) then motion ratios have to be calculated and used to restore his original wheel rates. A stiffer sway bar will probably be in order...LATER

My first question is why dual rate springs? Softer ride on the street? Trying to keep the spring seated when jacking up the car?

Bottom line dual rate or progressive rate springs are going to make it very difficult to tune that suspension.

I would:

Determine the spring rate used the last time the car either "handled well", or rolled less.

Install coils that duplicate the rate that Brian was previously happy with, maybe even bump it up 10%

Do not change roll bars or shock settings from what they were prior to the spring change.

Drive the car for a while and see what "comes up" I suspect body roll will be similar to that with the transverse leaf(assuming similar or slightly stiffer rates with the coil)

If roll is still a bit much AND you have dive on braking(talking front), I would go stiffer yet, maybe another 10%

That suspension is pretty tolerant to high wheel rates and comfort still isn't that bad. Even though my natural frequencys are a little high, I run 1300 lb springs(730 wheel), one end 850 lb(477 wheel) on the other. The Ride is not bad over potholes and eliminated the roll bar totally on the 1300 lb end

Stay away from "roll bars" and "shock tuning" until after the springs are correct

Most important of all, change only one thing at a time

 

The "spring rates" need to be compared at the wheel. If Brian is quoting spring rates for the coil overs for the SPRING, then waynger is right; a little geometry needs to be used to find the rate at the wheel. The rate at the wheel will be LOWER than the rate of the spring itself, b/c the spring (mounted on the shock) has a mechanical disadvantage...the lower arm is a lever.

I believe the spring rates for the stock spring are measured at the wheel. If that is not true, and it's rated at the spring, then since the stock spring mounts slightly inboard of the lower shock mount, the stock spring has a very slight mechanical disadvantage compared to a coil-on-shock spring. SO...if Brian is quoting spring rates for the spring spring, and 93Rubie is quoting factory wheel rates for the stock springs....then Brian has springs that are way, way too soft.

 

both the stock spring and wheels rates are listed here

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

what I listed was the spring rates

I can tell you the spring rate in the back is REAL stiff

the idea behind the two spring rates had nothing to do with street manners, but rather absorbing the smaller bumps.

 

Brian,

I like your thinking of having the softer spring absorb the small bumps, but if you are in the corner and the soft spring has reached coil bind than its not doing anything. I think the stiff sring in the rear should come down in rate quite a bit, and both fronts should go up. For the front I would make the "soft" spring have the same wheel rate as the 88-91 FE1 springs and the "stiff" side be a little more than the Z51/ZO7 springs of the same era. The rear I would not go much stiffer than the 88-91 Z51 rates. I think the old '84 spring is just way to stiff for the C4 suspension.

 

found out today that one of the rear shocks had totally lost it's charge over the winter

fill it up, acted a lot better!

 

got the new springs on the car over the weekend
be interesting to see how the car performs on Sunday