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C4 rear coilover conversion - with pics

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Old 04-14-2018, 03:35 AM
  #21  
Drew95
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Originally Posted by blackozvet View Post
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 ?
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.
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Old 04-14-2018, 04:02 AM
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Originally Posted by Drew95 View Post
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.
I would do the jacking up and down of the rear end with the wheel off - there will be something somewhere thats causing that harshness !

If you get stuck Ive got a Vizsla thats really good at diagnosing rear end issues,

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Old 04-14-2018, 04:25 PM
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Originally Posted by blackozvet View Post
I would do the jacking up and down of the rear end with the wheel off - there will be something somewhere thats causing that harshness !
I agree 100% - except you should also unbolt the ends of the spring and shocks first, so it has no resistance to movement. Try that before you make any other changes. A switch to coilovers shouldn't make any difference in ride quality (unless you change effective spring rates, of course) either. I'm also not really clear how the change in rear gears would affect ride quality.

So what kind of bushings are in your batwing mounts? What shocks are you running? Assuming nothing is interfering with travel, shocks always make the biggest difference in ride harshness, followed by bushings. Did you change out the rear swaybar?

If you get stuck Ive got a Vizsla thats really good at diagnosing rear end issues,
If the above doesn't turn up any problems, then try this!
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Old 04-15-2018, 04:47 AM
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First drive - the install was finished just in time for my clubs annual show - Corvettes rock the Bay, no time for a wheel align but the camber isnt looking too bad. its a 50 km round trip so a good little test.

this is how she rolls out the garage, next to my mates 454 C3 - the 2 black bitches !





First impressions - the rear end is really smooth, despite the bearing joints on the banski arms and the shocks there is no clunking or noise, its very soft and compliant, and very nice into and out of roundabouts (you can feel the difference with the lower anti-squat too when you accelerate), the only thing Im worried about is that maybe the 350 lb springs are going to be too soft for the track ?

The only pic I have of the car at the show,
(we managed to get over 90 cars - everything from C1 to C7)


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Old 04-15-2018, 01:21 PM
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Originally Posted by blackozvet View Post
If you get stuck Ive got a Vizsla thats really good at diagnosing rear end issues,


Might have to take you up on that offer!
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Old 04-15-2018, 01:26 PM
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Originally Posted by MatthewMiller View Post
So what kind of bushings are in your batwing mounts? What shocks are you running? Assuming nothing is interfering with travel, shocks always make the biggest difference in ride harshness, followed by bushings. Did you change out the rear swaybar?
When the rear was out, the batwing got cleaned up, gears changed, and went back in with a bushing kit I got from Corvettemods. They were prothane/urethane bushings. I didn't change the sway bar and the shocks are the stock bilsteins.

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Old 04-15-2018, 11:17 PM
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Originally Posted by Drew95 View Post
When the rear was out, the batwing got cleaned up, gears changed, and went back in with a bushing kit I got from Corvettemods. They were prothane/urethane bushings.
My guess is that this is the source of most of your ride harshness and noise. All the rear spring loads and roughly half the swaybar loads have to travel through those two batwing bushing. The only major vertical loads that don't are the rear shock loads.
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Old 04-16-2018, 04:12 AM
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Feedback from track time - round 1 of the Wintercup series at Collingrove Hillclimb.
Wheel alignment has been done.

we are just coming out of summer here in Australia, but as soon as you put on a race meet down comes the rain !
pretty good turn out in the 3 different capacity classes for 2wd street reg, my class (over 3500 cc) had 10 entries.

I was right about the 350 lb springs - they were too soft, the rear end was a bit squirmy, and it had the inevitable understeer when I pushed it. I checked the rubber slides and they were all the way up so the shocks were getting almost complete compression before hitting the bump stops. Best time was a 35.4 - this run was with 2 wheels sliding thru the grass in the tight esses when I tried to push it hard. this compares to a 35.2 at the last meeting in similar weather conditions (with the z51 326 lb leaf)

Disappointing after all the work but still quick enough to be the fastest 2wd street reg car.
But I guess it just a matter of doing more homework to get the rear end rates right.

After the meeting went and saw my suspension man and had a chat, he stated that it would be wheel rate related and I need to do some measuring of a few things and he will do the calculations.

One of the first things to measure is where is the spring mounted in comparison to the original leaf ? From there it is about comparing motion ratios.
Here is a pic from the rear showing where the leaf locates (I have dropped the leaf bolt in the hub for reference) and where the new coilover locates, you can see how much inboard it is - another 1.5 inches inboard.
1.5 inches can cause a fairly decent loss of wheel rate - in simple terms wheel rate is the effective spring rate at the hub or wheel taking into account a number of measurements.




the next measurement is the motion ratio - one certain way to check is to jack up the hub and mark and measure the shock travel to see how far the shock/spring is moving in relation to the wheel. the motion ratio on my setup is .75 - for every inch the wheel hub moves up - the shock/spring travels .75 of an inch.

cant get photo to load ?

I put the measurements (arm lengths, corner weights etc) into a suspension calculator and it said the motion ratio is .81 (I know its actually worse than that at .75) and the suggested spring rate is 402 lbs. so I know at a calculated guess that I need north of 400 lbs, maybe 420 lbs ?

Last edited by blackozvet; 04-16-2018 at 04:14 AM. Reason: pic missing
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Old 04-16-2018, 08:01 AM
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I don't think motion ratios apply here. Both the leaf spring and the shock mount are directly connected to the suspension upright, not at some point along a control arm. On a control arm, one end moves as much as the wheel moves and the other end doesn't move at all. So if you'r spring were mounted along that control arm, the motion ratio is equal to how close to the moving end of the arm the spring is mounted. But when the spring is connected directly to the upright, it must move as far as the upright itself. It doesn't matter where on the upright you mount something - the motion ratio is 1:1.

OTOH, since the spring is mounted with an inward angle toward its top, then you are losing a bit of motion ration in that way: vertical displacement of the upright isn't causing a perfectly vertical displacement of the coilover, but instead that displacement is split into a vertical and a horizontal component. Also, I think that this is a slightly falling rate, but I'd have to draw it out to be sure I'm seeing that aspect correctly. Falling-vs-rising rate would also depend on the ride height as well.

Last edited by MatthewMiller; 04-16-2018 at 08:02 AM.
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Old 04-16-2018, 06:33 PM
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Originally Posted by MatthewMiller View Post
I don't think motion ratios apply here. Both the leaf spring and the shock mount are directly connected to the suspension upright, not at some point along a control arm. On a control arm, one end moves as much as the wheel moves and the other end doesn't move at all. So if you'r spring were mounted along that control arm, the motion ratio is equal to how close to the moving end of the arm the spring is mounted. But when the spring is connected directly to the upright, it must move as far as the upright itself. It doesn't matter where on the upright you mount something - the motion ratio is 1:1.
OTOH, since the spring is mounted with an inward angle toward its top, then you are losing a bit of motion ration in that way: vertical displacement of the upright isn't causing a perfectly vertical displacement of the coilover, but instead that displacement is split into a vertical and a horizontal component. Also, I think that this is a slightly falling rate, but I'd have to draw it out to be sure I'm seeing that aspect correctly. Falling-vs-rising rate would also depend on the ride height as well.
It does have a motion ratio of .75 - I have physically measured it,
for every 1 inch the hub moves , the shock moves 3/4"


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Old 04-16-2018, 09:34 PM
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Originally Posted by blackozvet View Post
It does have a motion ratio of .75 - I have physically measured it,
for every 1 inch the hub moves , the shock moves 3/4"
That has to be from the angle of the shock relative to vertical, then - not from a motion ratio in the common meaning of the term. And the problem with that application of the term is that if it's due to the inclination of the shock, then it will vary depending on where in the suspension travel you start. For example, if you start measuring from full droop and move the wheel up 1", you might get 3/4" shaft displacement, but if you measure at regular ride height it might be something else because the geometry changes.

OTOH, if you measure the displacement of the bottom shock bolt (where the coilover mounts to the upright) relative to the wheel movement, you will find that it's 1:1, or so close as to be insignificantly different. It has to be, because the upright is one solid, rigid piece of aluminum. It's true that it isn't moving straight up - it's moving in an arc around the front-view instant center formed by the upper and lower suspension links, on a virtual swing arm (SVSA). But that virtual swing arm is so long that a few inches of inboard mount is minimal. I haven't measured it, but let's suppose that the SVSA is only 4' long at normal ride height. Then in order to have a 0.75:1 motion ratio, your coilover would have to mount 1' inboard from the camber arm pivot bolt. We know that's not the case. Judging from your picture a couple posts back, it's only 2-3" inboard of the camber arm pivot. The SVSA probably also gets considerably shorter at full droop, though. So again, it's important to measure starting from actual static ride height position.
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Old 04-17-2018, 02:15 AM
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Originally Posted by MatthewMiller View Post
That has to be from the angle of the shock relative to vertical, then - not from a motion ratio in the common meaning of the term. And the problem with that application of the term is that if it's due to the inclination of the shock, then it will vary depending on where in the suspension travel you start. For example, if you start measuring from full droop and move the wheel up 1", you might get 3/4" shaft displacement, but if you measure at regular ride height it might be something else because the geometry changes.
OTOH, if you measure the displacement of the bottom shock bolt (where the coilover mounts to the upright) relative to the wheel movement, you will find that it's 1:1, or so close as to be insignificantly different. It has to be, because the upright is one solid, rigid piece of aluminum. It's true that it isn't moving straight up - it's moving in an arc around the front-view instant center formed by the upper and lower suspension links, on a virtual swing arm (SVSA). But that virtual swing arm is so long that a few inches of inboard mount is minimal. I haven't measured it, but let's suppose that the SVSA is only 4' long at normal ride height. Then in order to have a 0.75:1 motion ratio, your coilover would have to mount 1' inboard from the camber arm pivot bolt. We know that's not the case. Judging from your picture a couple posts back, it's only 2-3" inboard of the camber arm pivot. The SVSA probably also gets considerably shorter at full droop, though. So again, it's important to measure starting from actual static ride height position.
Im not sure what all that means

I do know that the actual motion ratio is .75 - I measured it thru all 4 inches of shock travel and it is constant. This is how I was told to measure it as it is a true motion ratio and not theory based.
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Old 05-05-2018, 12:01 AM
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Update - after the understeer I measured up the QA1 springs rated at 350 using the specs checker, they came out to 333 lbs - will get them checked on a spring checker but it doesnt help if your springs are less than what they are supposed to be (however small)

I havent had the calculations done to determine spring and swaybar rates so I put the leaf back in for the meeting last week.

Measured the motion ratio of the leaf - it is a true 1:1 ratio, remembering that the coilover motion ratio is .75

So ran the car with leaf back in the rear, still using the double adjustable shocks.
The understeer is gone - so that would tend to prove that the loss of wheel rate with the coilovers is real.

Next step will be doing the calculations sheet to get the spring and swaybar rates sorted.
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Old 05-05-2018, 10:52 AM
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ihatebarkingdogs - in reply to your question (I got the email notification but its not on the thread ?)

My rear leaf is a z51 NYU @ 326 LBS.
So if the qa1 springs are about 333 lbs minus the motion ratio loss - then its a reasonable amount of spring rate and wheel rate being lost. Particularly when you take into account the roll couple (balance against the front rates) which is running 600 lb springs and a z51 30mm rollbar.
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Old 05-07-2018, 06:37 AM
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Here's a video from last meet - long sit on the start line was due to car ahead of me slowing, but you can hear how it will spin the rear tyres on the bumpy track.
Bit more work required on the compression/ rebound adjustments.


Last edited by blackozvet; 05-07-2018 at 07:11 AM. Reason: reload video
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Old 05-07-2018, 10:20 AM
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Nice video. I'm amazed that these things can go around corners. Mine is only good in a straight line. I think your main problem is that someone mounted your steering wheel on the wrong side. Good luck. I can't help your with your suspension. My rear coilovers are 180lbs.
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Old 05-07-2018, 11:22 AM
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Originally Posted by 383vett View Post
Nice video. I'm amazed that these things can go around corners. Mine is only good in a straight line.
I think the biggest difficulty you're having in that aspect of performance is the fact that your front tires are always off the ground...
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Old 05-07-2018, 06:26 PM
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Originally Posted by 383vett View Post
Nice video. I'm amazed that these things can go around corners. Mine is only good in a straight line. I think your main problem is that someone mounted your steering wheel on the wrong side. Good luck. I can't help your with your suspension. My rear coilovers are 180lbs.
Thanks, it is quite amazing that these old "yank tanks" as American cars are called in Australia are as quick as they are - the steering wheel being on the right does cause a space problem, if you look at your car you will see how much smaller the space is in the right side footwell !

Suspension requirements between going in a straight line and going around corners is a whole lot different !
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Old 05-07-2018, 06:31 PM
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Originally Posted by MatthewMiller View Post
I think the biggest difficulty you're having in that aspect of performance is the fact that your front tires are always off the ground...
Never thought of that!


Originally Posted by blackozvet View Post
Thanks, it is quite amazing that these old "yank tanks" as American cars are called in Australia are as quick as they are - the steering wheel being on the right does cause a space problem, if you look at your car you will see how much smaller the space is in the right side footwell !

Suspension requirements between going in a straight line and going around corners is a whole lot different !
Must be tight between the pedals and your feet.
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Old 05-07-2018, 06:40 PM
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Originally Posted by 383vett View Post
Must be tight between the pedals and your feet.
there is only enough room for a brake pedal and an accelerator pedal,
I drive it like a go-kart !
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