C4 rear coilover conversion - with pics
 

I have just done a rear coilover conversion - so will share some pics, results and suggestions to come out of it all for anyone that is interested in this subject.

First up, this conversion is more track intended (hillclimb) than street, but having said that my 85 is street reg and does see the occasional street excursion so I can give feedback on that.

I have changed 3 things in the rear end (breaking the golden rule yes I know !) , coilovers, banski arms and anti squat angle.

I will show the completed setup first - then work backwards.
For info I have initially selected 9" x 350 lb springs on QA1 double adjustable shocks.


https://cimg6.ibsrv.net/gimg/www.cor...90dacff140.jpg

I will do some more pics and explanation from the start in next post.

First thing is selection of the shocks, the rear shocks on the c4 are about 10" compressed and 14" extended.
The QA1 #DD401 Proma Star shocks fit the bill, and these are a double adjustable.
there are other brands that make a similar sized shock also, and in single adjustable if that suits the budget better. they also are a bearing end, so will fit the bolt hole arrangement top and bottom whilst allowing for the misalignment that will result from squeezing them into the c4 rear end. They come with a pair of 1/2" bearings.

I have gone with the 9" x 350 lb springs, going on recommendations from other people who have done the conversion.


https://cimg9.ibsrv.net/gimg/www.cor...36ac848a6b.jpg

Looks nice!

Hi
Just make sure to reinforce the bottom mount, several members have had the aluminum lower bolt and aluminum support break. :ack:

I have run a piece of steel from that shock bolt to brake caliper hub, and another from the nut there also.
There was a post from someone who had theirs break and did that mod, for support to spread the load. That was a month or so ago ..

Yes, dont worry that subject will be covered as I go along :thumbs:

Following!

Test fitting the coilover - let the problems begin !
I started by fitting the coilover onto the stud to see where the clearance issues actually are, the people who have been there before will tell you its the axle and the lower trailing arm link that cause the problems.

First problem is the stud that the shock mounts to - it is a 16mm where it goes thru the hub ear, but is a 5/8" where it mounts the shock (it is .4mm difference). you will remember that the QA1 shocks have 1/2" bearing mounts. After pondering what to do, I found out that QA1 sell a 5/8" bearing for this shock, so ordered a pair of those.


https://cimg8.ibsrv.net/gimg/www.cor...dc404210ff.jpg

The original stud is not long enough - due to the fact the shock bearings have an extra spacing length in the them, and having to space out the shock away from the lower trailing arm. I have put a 5/8" grade 8 bolt in to overcome these issues.
I have jacked the suspension from full droop to full compression and guess what - it actually misses the axle , something is going right !
The top uses a 1/2" bolt and spacer to move it forward enough to ensure that axle clearance.

Here is the test fit on the right side with the Banski arms in place.


https://cimg8.ibsrv.net/gimg/www.cor...8140a3cedf.jpg

I did consider the Vansteel bracket, but I dont like the way it starts extending the coilover assembly upwards and forward the amount it does - looking at too much leverage and going upwards you start to lose compression stroke on the shocker.


https://cimg6.ibsrv.net/gimg/www.cor...a68793d8ed.jpg

This goes to the subject that Gerard just raised, leverage and force at the hub ear. This point is designed for a shocker only, not a spring. The experts say that the shock actually exerts more force on a point than a spring does, but the addition of the spring will add something. There have been people who have snapped the ear off with a coilover assembley.
Also take into account that every inch you space the assembly out (towards the middle of the car) it creates something like 4 x extra force.
After a discussion with my suspension guy he suggested taking the inside conical bush out of the lower banski arm - this will allow the coilover assemble to move in on the bolt as close as possible to the hub and reduce the force acting on it.
I will show some pics later of how I made some strengthening arms to support it all.

I remain a skeptic of this. As noted, a damper can exert a lot more force than that spring when the rear encounters sharp bumps at high speed. Also, nobody ever worries about the strength of the mounting point when they put really stiff race shocks on their C4s. I have custom-valved Bilsteins that are many stiffer in compression than stock, and there's no issue with breaking that ear off. I suspect that the breakages actually are caused by interference with the coilover and some other part, which suddenly creates an infinite spring rate and then breaks the ear. OTOH, it can't hurt to add bracing!

I think you're referring to motion ratio, where you square the distance because it's leverage and the displacement (distance traveled) that both affect the wheel rate vs the spring rate. That won't be the case here, though, because the spring/damper is effectively at a 1:1 motion ratio with the wheel, because it's attached directly to the upright (which moves exactly as much as the tire/wheel). The leverage you're worried about is an issue with respect to the force placed on the ear, but the motion is not part of that equation.

I felt the same way as you about it, this issue was also discussed on the C4 Modified page, I was told by someone whose opinion I trust that the shocks are about 75% of the force and the springs would be about 25%, and that suspension components, such as the hub are over engineered to a certain percentage that would be greater than the 25% extra loading the addition of a spring would make.

I think you might be on the money in relation to infinite spring rate causing a shock that would break something - the outcome of my experience with the spring rates used makes me believe that people are using spring rates that are too soft with the coilovers and the rear ends are most likely bottoming out - but I will discuss that a bit later.

But people were popping out of the woodwork with examples of broken hub ears, and it really wasnt that much extra work to make up some braces - as the saying goes, "what value do you put on peace of mind" ?

I did read a comment by someone from Vansteel that they have never had anyone complain to them about a broken ear from using the adaptor bracket.

After lots of searching .....
I have been running coilovers for many years,
i did break one of the front koni coilover shafts.
They are only 1/2" the next ones will be 5/8",
am leaning to QA1 coilovers to replace the fronts.

The rear coilovers had not given any troubles however my C4 is a weekend toy only, i added similar bracing to the rear just in case.
i have the metal support to shock mount and at nut to knuckle where the trailing arms bolt on.

i have seen several posts over the years with broken lower mounts, (even when not hitting a pothole)....
Obviously the lower offset adapter mount to clear the coilover spring ads so much more stress.

Post 24 for broken mount and 34, 35 for bracing ideas.

https://www.corvetteforum.com/forums...pension-2.html

Yea, hitting massive potholes or welding the knuckle like Brian will tear up the shock mount no matter what. I've got 15 years on one set of coilovers on the street and several years and track miles on another car with no issue.

when it comes to breaking things, we all know "that person" don't we, I've got a brother who's managed to break every car he's ever owned !

One of my favourite sayings is 'there are people who could break a ball bearing in a sand box with a rubber mallet !'

onto the left side, remove the original shaft, replace with 5/8" bolt, fit up the coilover , jack it thru its whole range of travel, and guess what - at its narrowest clearance the spring just touches the axle - of course !

so now I need to find at least 10 mm of clearance, the only way is make a bracket to slide the coilover forward just enough to get that clearance.

6 hours later, it has a tab underneath and to one side to lock it into the ear (and not rotate) and multi drilled to move it in and out as well as backwards and forwards.


https://cimg7.ibsrv.net/gimg/www.cor...a47a967994.jpg

thats an interesting pic, thanks for sharing the picture.
you would think they would have painted or plated that stud - suspension components that look like rusted hulks dont inspire confidence !
the other interesting thing is the coilover being upside down means it needs to be much further inboard to clear the trailing arm, just looking at that spring it reckon its got a fair bit of spring rate in it !

the stud supports are made - the funny thing is that the simple stud setup (right side) required the more complex bracket and the more complex bracket (left side) required a simple support bracket.
I used 30 mm angle steel to make them.


https://cimg8.ibsrv.net/gimg/www.cor...8351ab1ede.jpg


https://cimg3.ibsrv.net/gimg/www.cor...1fc6ab4006.jpg

another shot of the right side,


https://cimg7.ibsrv.net/gimg/www.cor...a5a02a987c.jpg

Awesome, Oz. I have the Van Steele coilover kit on my bucket list of conversions to do, but already have the full Banski setup (Camber/Toe Rods + trailing arms). One thing I noticed right away - it's ROUGH. Like...knock your fillings out, rough - especially with a 4.11 ass.

Did you actually drive your car in phases, or put it all in together?

I'm seriously considering going back to the factory dog bones (leaving the Toe/Camber Banski) just because it's THAT uncomfortable. But if the coilover compensates.....

Curious what you think.

It's thick-gauge wire in that Callaway spring, but the coils are spaced awfully close together (i.e., the wire is quite long) compared to your springs. I bet it's actually pretty low in spring rate.

That's strange. I still run a leaf spring in the rear (550lb/in) and have full rod-end link setup. I only recently installed the Banski trailing arms from stock-rubber dog bones, and I can't tell any difference in ride quality at all! And really, they shouldn't make a big difference, because they aren't reacting any vertical suspension travel into the frame. They only react longitudinal forces from acceleration and braking. I wonder if something in your suspension is binding or interfering with free travel? I know I absent-mindedly installed the rear swaybar upside down once, and it mean the frame was sitting on the arms of the bar. Now THAT was a knock-your-fillings-out kind of ride!

Im betting its something else too - what condition are your shocks in, your leaf, your batwing bushings ?
Have you taken the wheel off, then jacked up the hub thru its whole range of motion and watched what is happening with all the components ?

Thanks for your reply

Everything is brand new. I mean, leafs are original, but bushings and bolts...I had the whole rear out and disassembled. I even polished my half shafts and replaced all u-joints.

It could be just the difference in gear (3.45 -> 4.10), but it just feels RUDE. Really really hard, if that makes any sense.

I polished, re-bushed, and cleared coated by original dog bones so they can go right back in, but it sounds like you guys don't think that's where the body stiffness comes from....

I'll do the coilovers and (leaf spring delete) and see what happens.