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Old 09-07-2008, 07:42 PM
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c4 irs experts - need help!

so heres one for you guys

i have a 1990 bmw 3 series that i back halfed with a c4 vette suspension.

mechanically everything seems to have worked out perfectly - after 4 months of hard work

the problem i am having now is the car does not want to settle to a consistant ride height. When i layed everything out, i planned for a ground to fender measurement of 24 inches. I adjusted my coil overs approriately, and took it out for a spin.. next thing i know, the car is sitting at 26 inches.. back in the shop, adjust the coils and take it back out. everything is fine. park the car, pull it out of the garage and the car settles to an even different height.

i've never seen anything like this in my life. I'm currently running 400lbs springs, and swapped out to a 250lbs spring thinking the spring rate was way too high. The car felt worse with the 250s and the problem still existed.

My next thought was that the 'dog bones' or '4 link' bars were binding since they have poly inserts. So i now have the bolts on these loose to allow easy travel. The problem seems to be better, but still exists.

any thoughts on where to go next?

here are some pictures of the current setup. The rear is narrowed about 4" on each side - but that does not seem uncommon

i need help before i go insane over here

car sitting at approx ride height - maybe a little on the high side. I had only planned on having 1.5 inches on each side of ride height - 2" max each way



good view of all the mounting points





an earlier picture before all the components were in place





car sitting way way way too low before i had everything adjusted


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Old 09-07-2008, 08:09 PM
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First, welcome to Hotrodders.
Second, that swap is Awesome.

Poly is biting you in the rear-- pardon the pun.

You are barking up the right tree. Try and loosen ALL the bolts that pass through the poly bushings. Bounce the car, check it, repeat. Maybe use some of Energy's goop they supply. (if not already)

Coils will shrink a little at first, so don't panic on that.
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Old 09-07-2008, 08:13 PM
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thanks c-10!

although at the moment i'm not feeling the swap i so awesome but hopefully will be soon.

i was wondering that too - is there a break in period i should expect on the springs?

i've never had that problem in the past.

I'm tempted to remake the 4 links using heim joints on at least one end to help with the motion

it seems like with the c4 setup - everything is fighting each other
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Old 09-08-2008, 06:58 AM
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How much did you narrow that setup?

I've been told that if you take out more than 5 or 6" per side , it changes the lengths of the arcs that the knuckles swing in so much, that the arms on the side really start to bind up, also, you get a bunch of weird alignment changes as the suspension moves.


But that don't matter now, It's done..

I have seen a few suspensions that acted like that, (mostly ones with bellcranks), and yes it was friction at the bushings etc that caused it. The giveaway is that the suspension will move freely while on the rack, but as soon as you load everything, it gets all sticky and will stay wherever you lift or push it down to.


To test the stiction, by hand, pick up the back of the car, while the suspension is loaded, lift it up about an inch or 2 then just let go...The car should settle back exactly to where it was,... then push down on the car...it should rise exactly to where it was .


I had a customer with a 34 coupe with a custom fabricated lay down shock and bellcrank setup in the back...There was so much stiction that the ride height would stay where ever you left it...2" up or 2" down....You'd drive the car, and hit a big bump and it would stay bottomed out...


A set of needle bearings in the bellcrank fixed a good portion of the problem.

You would get the best results from using the "heim" type rod ends with the bearings on all of your linkage ends,,(airframe control), but those are extremely expensive.


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Old 09-08-2008, 08:17 AM
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thanks for the insight mike

i wish i could remember exactly how much i narrowed the axles. its around 4 to 5 inches.

at this point, i'm starting to wish i went with a straight axle - but its a little late, so i have to make this work

One thing that makes this car difficult is the tire width. It doesn't allow for adjustments to be made without driving the car around again. Pushing on the suspension just makes the tires buldge out, etc.

so at rest, the cars suspension usually returns to the same point. it seems to be only after driving that it tends to drift.

I think i may take off the springs and try to lift one of the hubs to see if there is a point in the travel that everything starts to bind up.

and as you mentioned - heims are an expensive solution, but i may need to go that route. would it be possible to make both ends spherical? or would one end need to be solid to prevent everything from moving around?
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Old 09-08-2008, 08:27 AM
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i have installed lots of c4s and the half shafts are almost paralllal to the ground your installation looks like it would bind because the angle of the half shafts
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Old 09-08-2008, 09:02 AM
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Ok - i'll take that into consideration. I guess that makes sense that the axles should be straight at ride height and then go up and down from there.

I have extra mounting positions for the diff, so i could actually drop it down about 1" which would bring the axles straighter without effecting the rest of the geometry / ride height

this was my first attempt at anything like this, and i took alot of educated guesses in the process - i'm glad it worked out as well as it did but i can see there is still some room for improvement
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Old 09-08-2008, 09:59 AM
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My money is on the whole thing binding up because the geometry is out of whack.

I have a 86 vette and the half shafts are basically parallel to the ground at ride height. The camber arms on mine are probably at half the angle your's are (again at ride height). The tie rods for the toe in/out are basically parallel to the ground.

It looks like you will induce lots of camber and lots of toe out as the body moves down. The reverse is true on body lift.

The half shafts are going to spread the track width as the body settles and that is going to cause binding on the 4 control arms that orient the spindle assemblies.

I'm not sure what the angled coilovers contribute to the problem (makes my head hurt thinking about it), but from pictures I have seen for vettes with rear coilovers they are basically vertical. The 'batwing' and the half shafts are what keeps the rear suspension solid in a left/right orientation.

How are you stopping the differential/batwing assembly from trying to rotate around the rear axle centerline? On vettes they have a 'C Beam' that rigidly connects the front of the differential to the back of the transmission.
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Old 09-08-2008, 11:12 AM
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dave - lots of good points, some of which are inherent design constrictions of the c4 layout (half shafts pushing out the track width , binding the 4 links)

in this picture you can see the front differential mounting point (trapazoid shaped plate). you can also see the rear mounting points for the batwing which have two sets of bolts.

All i need to do is drop the differential down to the next set, and make a spacer to go between the front diff mount and the frame

this will bring the half shafts down about 1 1/8" at the diff side and should help to make them almost parrallel to the ground at ride height. keep in mind the car may not be at ride height in some of the above pictures.



i think that will be step one, and i'll see how much farther that gets me.

the coil over setup is not that much different than what i've seen on other vettes / rods. You need to angle them in to clear some of the other suspension components - unless you make a really funky bottom bracket which would create a huge leverage point.

Thanks for everyones help and suggestions.
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Old 09-08-2008, 11:41 AM
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speaking of your coil over mnt they do have to be on angle . if you have them mounted on the bottom shock mnt you are also looking for problems you can use it but you also have to tye it to the dog bone mnt above it to help support the leverage or the bottom shock mnt could fail
wayne
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Old 09-08-2008, 12:28 PM
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yeah the bracket i have made does worry me to some degree, since it is only held on by one 5/8" bolt.

I've seen some other designs that tie into the dog bone mount also, so i think its in my best interest to do this also.

thanks! all good recommendations here
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Old 09-08-2008, 08:10 PM
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so i dropped down my diff tonight to level out the axles. everything seems to be working out better, and unless i'm imagining things - the car seems to drive better.

The true test will be tomorrow to see if it wants to settle in at a different height.

here is where the suspension sits now. It still needs to come up about a 1/4" - but the axles are near parallel



in hindsight, i should have made my camber rod mounting point a little lower to keep it in line with the axle.

however with the coilovers off, the suspension seems to travel nicely when i lift it by hand.

i'm still going to consider replacing the dog bones with heim joints

here is the front spacer for the diff. I had anticipated lower the diff, but for other reasons.. regardless - i'm glad i thought ahead



maybe i should switch my toe rods to the other side of the hub tab

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Old 09-08-2008, 08:20 PM
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why do you want to raise it up again ?? one other thing are your shafts 90 degrees from cntr line?they should follow the angle of the brace
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Old 09-08-2008, 08:28 PM
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well the car sits pretty mean at the moment, so i wanted to raise the coils just a tad. although its level with the front right now and i have plenty of tire clearance so maybe i will leave it and just adjust out the camber



here is a very preliminary picture - but the axles do follow the same angle as the camber bars. not exactly the same - but i can't see how the stock corvette follows the same angle either.

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Old 09-09-2008, 07:38 AM
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Good work. Now modify the mounting point on the differential for the toe rods so they are parallel to the ground. Otherwise you are going to have the toe in changing too much as the suspension travels up/down.
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