TurbineSurgine

I stumbled across these race shocks in the Bilstein catalog, I couldn't find much information on the forums about them. I know the AK1030 has 360/135 valving and the AK1040 has 410/145 valving. Has anyone use these shocks and what kind of spring rates are the designed for?

This all came about because I wanted use a vbp Extreme front spring, but the problem is my car is a 85 and it would require converting LCA. So I thought maybe I should do front coilovers to get the spring rate I want, these Bilsteins are a 36mm body that would allow a 1 7/8 spring for clearance. I was also thinking that if i kept leaf spring and run a softer coil spring it would be less stress on the shock tower. Any help is greatly appreciated, I know there are some smart guys around here that have been around the block.

BrianCunningham

More info here
http://www.allshocks.com/CAR/CHEVROLET/1984/Corvette

I wouldn't mix coilovers & leafs
Run one or the other.

What do you plan on doing with the car?
Autocross or road race?

TurbineSurgine

This is a mostly a autocross (ssm), but i will also hpde this car. Problem with the 1 7/8 coil springs is i haven't found any springs over 500#. The bilstein catalog list the shocks as ssgt which I presume to be showroom stock grand touring from the old scca days.

BrianCunningham

Early C4 are limited by the shock mounts with regard to coilovers

For SSM I'd get some decent double adjustables

Vette Brakes will make a custom spring for you.

TurbineSurgine

You are correct, the shock towers don't give much room. Most coilovers use 2.5 inch springs, these Bilsteins use a small 36mm body that allows for a 1 7/8 coil spring. Other members have made coilovers this way with sport shocks, and from what I can tell its how the exotic muscle coilovers are made. Bilstein provides machine work service to make a snap ring groove for the coilover sleeve to sit on for 10$. The coilover sleeve and hat kit from Bilstein is 50$ and hypercoil springs are about 70$.

Considering the cost, I am going to give it a go. This setup will allow me to try several spring rates at a affordable cost.

BrianCunningham

Nice looking car!

TurbineSurgine

Thank you

Crepitus

Solofast

Those were the old Corvette Challenge shocks. They were so stiff that they literally beat a car I had to death. After using them for half a season things like switches and plastic parts were literally falling off the car and it creaked and groaned like an old British roadster. The rear hatch would open as I was driving down the freeway. I got a new door jamb switch and when I went to pull the old one out it came out and fell into pieces in my hand....

It got to the point where if I was traveling more than a half an hour to an event I'd change the shocks out as I changed tires, and I'm not one to do any more wrenching than I have to but those shocks were so stiff you couldn't drive the car for more than a half an hour on a smooth freeway before you were begging for mercy.

The idea behind these shocks was that the cars they were putting them on were limited in spring rates, so they were overdoing it with compression damping in the shocks to make up for it.

Bad idea for anything other than a pure track car, and then you really aren't getting an optimum setup, just compensating for soft springs to some extent.

The C4 platform has basic suspension geometry limitations that can't be overcome with springs. You can't just go in and stiffen the heck out of it without really messing up the handling. If you go to much stiffer springs on the front the C4 is just going to push. If you go too high in the back it's going to jack, and you can get past that point with leaf springs that are readily available, so there is no real reason to go to coilovers. The best that you can do is get the right relative spring rates, front to rear, and the leaf spring sets that you can buy or are available from the factory as options offer enough variation and are cheap.

If you are wanting to go up in spring rates you really need to rework the rear geometry to lower the rear roll center so that you can use stiffer springs and/or bars in the back. Otherwise you will just end up with a car that jacks in the back like an old VW, and it will handle horribly.

The C4 was a good package in it's day, but that day is long gone. Stiffening up the springs just emphasizes how flexible the chassis is. Just drive a C4 Z07 car with the roof out and you will see what I mean. It feels like you are sitting on the rear axle and the cowl is twisting when you go around corners.

You can get about to the limits of the chassis with available leaf springs and can tune the balance with sway bars. Because of the limitations to roll stiffness in the rear of the car you really can't do all that much with it unless you rework the rear roll center height. Since you have to keep the original suspension mounting points on the frame, you can't change the location of the lower control arm pivot point on the chassis, which would be the easiest and most simple way to do that. Since the upper control arm is the driveshaft, you can't change that either. You could put on a set of different rear spindles, but so far as I know you would have to make them up from billet stock yourself, which is not a cheap proposition.

Going to coilovers is seldom done with a C4 because there really isn't much of an advantage to it.

Better solution is to get a set of Koni adjustable shocks and work on the balance and get tires and rims that work on the car. Since your car has the earlier front suspension you should keep that since it has better kingpin angles and works better than the later suspension in lower speed stuff like autocrosses.

The rear suspension really isn't that different between the early and later cars, we changed over to the later setup, but in reality it didn't make all that much difference. The geometry is a smidgen better, but it wasn't all that much better that you had to do it. Lowering in the back helps lower the roll center, but you run out of rope pretty fast with regards to travel and the camber curve starts to suck. We cut the bump stops some and you have more latitude with mods to the upright, so you may be able to lower it a bit more, but then other things will start to hit and you'll have to flare the fenders.

When these cars lift, like under braking if you're lowered to the max, you can really feel the back end getting loose because the roll center is rising up. Under power it squats and then it behaves, but you gotta slow down sometime...

Having campaigned a C4 in BSP for years and lived with an even more restrictive set of rule than SSM, one becomes painfully aware of the basic limitations of the car that, in light of the rules of the club, make it very difficult to get the car to the next level of performance. Our fully prepped to the limit of the rules BSP C4 ran about the same times as a SS C5Z06, and we had to go the 315 Hoosiers all around to get there.

While we had to use stock components all around on the suspension you can use custom billet uprights and an aluminum LS motor, so you should, in theory, be faster than a well done BSP car, but that's really the only difference I can see that going SSM allows you to be faster than BSP. That's spending a lot of jack for not much time improvement.

I really don't want to be negative, and if you have a car you enjoy and want to run local events with it have a good time, but the basic platform has some limitations that changes like coilovers really aren't going to address. If you want to be able to fine tune front spring rates then maybe it might make sense, but you can get to the same place with bars and that's a lot less expensive way to get there.

Even more importantly is the need for some shock adjustability to tune the car. If you get an adjustable front or rear bar and adjustable shocks you'll be way far ahead in terms of tuning capability than being stuck with single position shocks and coilovers.

JMHO and qualified as such.

BrianCunningham

When I had my old springs one I was three wheeling

I changed them for Z07 equivalent springs and it's a lot more behaved.

Though now it oversteers
Need to get a bigger front bar on it.
I've got adjustable front a-arms so I can get A LOT of camber dialed in.

I have the DRM brackets which changes the geometry

I'd like the lover the rear roll center
Can't change the halfshaft, but I could move the lower arm around with a different bracket.

TurbineSurgine

First let me thank you for all your advice, that was quite the response.

Which set did you use, 1030/31 or 1040/41? I'm assuming that the challenge cars had to use stock springs, and that one of these sets where intended for 84 z51 spring rates and the other was for the 85-87 z51.

Currently koni doesn't offer any 84-87 shocks I'm awar of, I looked long and hard before trying ridetech HQ single adjustable. They are very nice quality shocks with a good click rebound adjuster but the compression dampening is too soft. Ridetech did offer to revalve them so that may happen if this coilover setup doesn't work out.

I have experienced what you where saying about hitting the rear bump stops, and I see what you mean by lowering the roll center. I really think that a higher spring rate all around will help the car. A local BSP '90 vett runs vbp x treme springs with penske shocks, thats 2x the bmb spring rate. He's usually FTD faster or close to a xp rx7, both are seasoned drivers.

When I receive the shocks I will try them with my leaf springs first, then with coil springs. See what happens.

Crepitus

I think QA1 makes a decent double adjustable. they aint a Penske but they dont cost nearly as much either.

I would be interested in seeing the back of a C4 in a Chassis geometry program. And what would need to be moved where to help out camber curves, roll centers and moment arms. 3 and 4 link programs are easy but I have not seen one that models a independent rear

BrianCunningham

i built a CAD model

Solofast

The Corvette Challenge cars used the same springs as the Z51 cars of those years. Here is a link to Hib Halverson's suspension table. It's a must have for anyone doing work on a C4.

http://www.corvetteactioncenter.com/...usp_chart.html

I don't know which ones they were, I believe the Corvette Challenge shocks were the higher rate of the two. You might ask Bilstein which was the higher rate and stay away from them.

You are right Koni doesn't offer the right shock for your car. I had thought they did, but forgot that the later car shocks won't fit your car.

What we found was that if you go to the higher rate spring you have to back off on rear sway bar stiffness. The 84 Z51 had a 87.5 NM spring and that, when coupled with a 23mm rear bar was just on the edge of jacking up at the stock ride height. That is what I found when driving a brand new 84 Z51 when they first came out.

In building the BSP car, first we put that spring on with a 26mm rear bar and the car jacked up like crazy, even when the car was a low as we dared make it. We went back to a 24mm rear bar with poly bushings and it was still too much rear roll stiffness. We put rubber bushings in the rear bar and it jacked, mostly under trailing throttle, but it still wouldn't behave. We tried putting on more front bar and the car was a mess. It pushed right up until it jacked up and then it would go into massive oversteer. We went back to less front bar and put the stock rubber bushing in the rear bar and then put a 1/16 washer between the frame and the bar clamp in the back bar and that got us to something that worked. Bottom line is that the 84Z51 had as much rear suspension stiffness as you could put in and not have jacking in the back with the stock geometry. Any more than that just made a mess of things.

My stock Z07 car 91 came with a 26mm rear bar and a 57.2 NM spring and that jacked also until I softened the rear bushings by using washers between the frame and the rear bar.

That gives you some data points as to what you can do with springs and bar stiffness combinations with the stock geometry, and then you can set you front springs accordingly. The total roll stiffness combination is what bites you here. You can get to a bad place with too much spring, or too much bar, or too much of both. If you look at Hib's chart you'll see that on the ZR1 the lowered the rear spring rate and put in the 26mm rear bar and that worked also.

TurbineSurgine

I'm running a 32mm bar in front and a 20mm bar in the rear. It does jack a little but the car controllable and it actually helps rotate the car which suits me. I think if I took some rake out it would not jack so much, the rear has to be high enough to stay off the bump stops so that leaves me with raising the front. When I installed the front springs I did the wedges along with cutting the end pads off so its as low as possible, screwed myself on that one. I made some shims out of uhmw plastic to go in there but just haven't got around to installing them. I'd say the car is developing in the right direction and is way better than when I bought it. I'm just enjoying the ride getting there and learning what works, when I get bored with it a for sale sign will be on it and it will be on to the next car.

TurbineSurgine

UPDATE: Finally received my shocks, I ordered the ak 1040 / 1041 and had them machine a c clip groove at the bottom. After ordering I noticed the 1040/41's where removed from the bilstein catalog, not really sure why. I was pretty sure they where 36mm small bodied shocks but when they showed up I found they are not. I must have looked at so many and got confused. So now the front shocks won't work as coilovers as I had planned, I could still use them if I modify the shock towers quite a bit.

So for now I bolted them on as regular shocks with the leaf springs. I test drove the car around the neighborhood on some pretty harsh roads and didn't find them to be extreme or unbearable. I got on a empty two lane, gained some speed and did some slalom like wiggles and the car felt great. My next autocross is this Sunday, I'll return to keep this updated in case someone wants to know. It doesn't seem there are much early c4's racing anymore but it might help someone.

Crepitus

Great thread! Keep the updates coming.

rfn026

I'm glad to see another C4 here.


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I run the Hyperco Corvette Challenge springs. Bilstein did the shocks for me. I just gave them the spring rates and the weights and they set the valving. One of the Corvette engineers said I probably have too much rebound but that's fine with me.

I also use an adjustable rear sway bar.

btw - The roof off thing is an urban myth. In the late 80's GM ran a bunch of test with the roof on and off. The car felt rather different but when the data was downloaded there was no difference between having the roof on and off.

Here's the fastest solo C4 ever. He never had the roof on.

Richard Newton

TurbineSurgine

Well I got the car to the Autocross yesterday, It was a very hot day and by the time I got to my runs I was already exhausted and considering leaving. The shocks where great, I could definitely tell the compression was quite a bit more than the ridetechs I had. I think these are a great option for early C4 budget racers or chump car. I really wish the front could have worked as a coilover without much modification. I might see if there is a 46mm body sleeve that uses 2.25 springs, the other option I might go with is see if bilstien will build me a SG shock with the same dampening.

Here is my fastest run from sunday.

TurbineSurgine

Yes I run now without the top so I can sit properly, I still need to get a roll bar and proper race seats to hold me tight. How high of a spring rate will Hypercoil make. I thought vbp was the only one that made high spring rates.