Mojave

I have a 96 that I track, and my current suspension setup is as follows: 89 Z51 115.5 front spring, 84 Z51 87.5 rear spring spring, Bilstein Z51 spec shocks, 30mm front bar, 24mm rear bar. The car currently weighs 2920 full of gas and no driver. Primary use is HPDE, with some auto-x thrown in the mix.

I want to switch to coil overs in the front, to allow me to adjust the spring rate and to allow me to corner weight the car. I'm already running adjustable spring bolts in the rear, so no need to swap over just yet. My question lies is what spring rate to start with. I want to slightly increase the front spring rate, as I think the car would benefit from more front spring.

Is there a formula to convert from the leaf spring rates to coil spring rates? Simply going from 115.5 newtons per millimeter to pounds per inch, I get 659.52 lbs/in. A lot of coil over setups seem to run a spring around 450 lbs, but since the shock is mounted a little further out than the spring, the effective spring rate is less due to increased leverage.

This doesn't seem to quite add up. Does anyone know where I went wrong, or do I really need a 700 lbs/in coil spring to actually increase the spring rate?

1963GrayGray

Chad, hopefully someone can answer your question as I would also be interested in the responses. Who are the manufacturers of coilovers for the C4? I find lots of suppliers for C5 and C6, but not for C4,s.

Dave

Mojave

Exotic Muscle and DRM both list coil overs for C4's. I, however, am broke, and my Bilsteins aren't very old, so I'm going to try and put sleeves on my current shocks. AFCO makes sleeves that have been shown to work on Bilsteins, but they are 2 5/8" springs. The rates and free lengths offered are quite limited compared to the 2.5" offering, particularly by Eibach.

I wonder if a 2.5" spring will fit on a 2 5/8" sleeve...it's only .125"...

VenkmanP

Most people aren't trying to reproduce the 89 Z51 spring rate, as you are. Stock rates range from 52 to 116.

According to the most recent lengthy thread on this subject, you would need 935 lb springs to equal the 700 lb leaf spring that you are looking for. Which seems a bit much.

http://forums.corvetteforum.com/show...light=coilover

Mojave

Interesting, it appears I might actually have been correct. This may throw a bit of a kink into my plan unless I can actually fit a 2.5" spring on a 2 5/8" sleeve.

ghoffman

We have a dual rate package that uses 2 springs in series, so that you have a compliant first inch of travel, and a much stiffer final inch. Please see this thread:
http://forums.corvetteforum.com/show...hlight=hardbar

SDF

I have the DRM coil-over set-up on my 96. My spring rates are 400 lbs front and 275 lbs rear.

BrianCunningham

That's what I'm going with.

Mojave

My car is setup for track use only. I really don't need a dual rate springs because ride quality is not important to me; just lap times.

Those spring rates sounds really, really low, based on what was in the other thread. I want my spring rate above the factory 115.5 spring, which is 659 lbs/in. It was suggested the leaf spring rate directly translate to the ball joint rate. If this is in fact the case, then I need something around a 1015 spring just to match the factory spring I have. According to the online calculator I found, the motion ratio is about .64, give or take, for a coil over.

This is my sticking point: does anyone know how to go from leaf spring rates to the ball joint rate? If I can calculate my factory ball joint rate, I can go from there.

0Vector Vette

My car is an SCCA road race car only. I have had coil overs on it for a couple of years now. I started out with a preety soft setup, 450 front, 325 rear. I have been increasing the rates over the years and am now running 700 front, 550 rear and am very happy with the setup. By the way, I too am using Bilstein shocks with sleeves.

Z06_BluByU

how much is a sleeve setup and where is a good place to find such an animal?!?!

MSR

you need to accurately measure the motion ratio. the wheel rate per leaf spring rate isn't nearly as high as you think it is. when i had a VBP 1125 front spring, i measured the wheel rate to be around 400lbs/in. it was difficult to be really accurate due to friction in the bushings and shocks, but that's pretty close. i can't remember what it was supposed to be theoretically based on my motion ratio measurements, but it was all in the ballpark enough to corroborate my method.

the shock acts on the control arm closer to the ball joint than the leaf spring does, so it should translate to more wheel rate per spring rate when it's a coilover. i never measured the shock angle to calculate that effect, but it's not severe.

all that said, given your car's weight, i think you're better off going with a softer rear spring. also, since you can adjust either rear corner, you can already corner weight it. the only thing front adjustment buys you over where you are now is ride height adjustability (within the confines of maintaining corner weights).

-michael

Mojave

If you use the wheel rate and leaf spring rate listen on the ever popular Hib Halverson chart and wheel rate=motion ratio ^2 * spring rate, you get a motion ratio of around .55 (the values aren't entirely consistent).

It was suggested my other thread in the C4 tech section, http://forums.corvetteforum.com/show....php?t=1866608, that the motion ratio for coil overs is .8 (12/15). So, working backwards from the stock Z51 spring (659.5 lbs/in)

wheel rate = .55^2 *659.5

wheel rate = 199.448, rounded up to 200 lbs/in. This is right in line with what you got, Michael, with an 1125 spring. 1125 is approx double the factory Z51 spring, and 200 is half of 400, which is what you measured.

200 = .8^2 * spring rate

spring rate = 312.5 pounds/in.

If I want to up my wheel rate to 400 lbs/in, the new coil over spring rate is double, so 625 lbs/in.

BrianCunningham

BTW the dual rate springs aren't just for the street, the soft coil gives you more grip where the stiffer rate isn't needed.

Mojave

Hmmm, interesting idea. I have never heard of using dual rate springs in a road race application (mostly off-road where lots of travel is needed). I know some people run a progressive spring, but a lot more people don't. I realize shocks are mostly responsible for transitional feel, but it seems like the car would start to take a set, and then the spring rate would change, causing some interesting effects.

0Vector Vette

I got the sleeve setup from Bilstein. I have had it for several years but I think it was around $200 for the kit. It requires the shock to have a snap-ring groove machined into it. Bilstein can do that for you as well.

briannutter

I'm using the qa1 double adjustables with 450 rate springs on the street. I really regret picking a spring this low..it feels softer than my stock fe1 '94 spring. Go AT LEAST 600.

Mojave

One thing I forgot to take into account with this post is the angle of the shocks. Some quick and dirty measuring with a tape measure of my shocks at my current ride height shows the angle to be between 25 and 30 degrees. Looking at this site, http://www.stockcarproducts.com/pstech8.htm, it appears as though the spring will be only 80% of it's rate due to the angle.


So, the wheel rate for a Z51 spring is right about 200 lbs/in. Before, I calculated a spring rate of 312.5 lbs/in, but that neglected the shock angle. Factoring that in gives:

Real spring rate=312.5 / .8

rate = 390.625

If I want to up my wheel rate to 400 lbs/in, as the VB&P 1125 leaf spring does:

(wheel rate) 400 = .8^2 * spring rate * .8 (shock angle factor)

Spring rate = 781.25, or double the 390 found above.

Mojave

I'm thinking somewhere between 600 and 700 currently.

The Hib Halverson chart says a 94 FE1 car has a wheel rate of 136.4 lbs/in. Using the formula I have thus far, a

wheel rate = 450 * .8^3 (.8 ^2 for the motion ratio of coil overs, plus an extra .8 for shock angle)

wheel rate = 230

So, it looks like it is a fair amount stiffer. Perhaps your shocks are set softer?

ZR1 MK

Ive tried many setups over the years. I have coilovers in 375/275 with stock shocks for street and 500/300 with revalved shocks for track. I recommend minimum of 500/325 for hardcore driving. Ive been in a car with 600 fronts and that was real harsh. Car characteristics and feel will vary greatly depending on shocks, bars and bushings.
If I had to do it over, I would use the Z07 115 spring in the front and do coilovers.