How hard is it to convert my stock rear suspension to a custom 3 link? I know how I will make the mount for the upper arm, but what about the geometry? What angle and lenth should it be? And at what angle should the panhard bar be? Im thinking I will make everything adjustable (angles) by putting an extra hole above and below the sandard position.

Also will I need to run a sway bar with this set up?

Thanks - Ron

How hard is it? It is actually fairly complex to scratch design and construct a new suspension. The geometry requires quite a bit of understanding and very meticulous measuring. The PHB should always be level at static height and as long as possible to mitigate any lateral movement as a result of the PHB arc during bump/droop. Always make attaching links with a few alternate mounting locations, you never know until you have driven it for sure if it’s correct and you need the ability to make change. No rear sta-bar necessary with a 3-link

Dennis is right, it is complex. I have started mine, here are the initial picts:
http://www.chevelles.com/showroom/showgallery.php?cat=500&ppuser=26133
Getting enough adjustments has been a challenge. The rear mount for the 3rd link has no adjustment vertically due to the low floor pan in an El camino (chevelles have a bit more room above the axle). the front mount ended up with just 2 positions. Going higher intefered with the body and didn't do anything for geometry. I couldn't go any lower due to driveshaft/yoke interference. These limitations are because I limited myself to fitting within the existing body/frame. If I were to do it better, I would cut out the floor area and make all the links longer. Then again I would be changing the front suspension more as well. I had to limit myself due to budget. Maybe next time.

How does it help if the links are longer? I know from drag racing, that you want the LCA and UCA angles to meet about at the cam shaft level at the back of the motor. Is this ture for a three link as well? Or is starting level better,and than adjust from there? I will be using the stock LCA mounts. I might add another hole below the exsiting hole on the axcle.(for adjustment) The LCA will also be adjustable.

Longer links reduce pinion angle changes and makes the handling more predictable, as I understand it. My top link is only 13" long with 22" long lowers. Idealy you would want the top link at least 70% as long as the lowers. My setup isn't bad, just not idea either.

I was thinking of adding a hump under the back seat. I would have to modify the lower seat, but it just might work! What about angles? What should I be shooting for?

If you want to figure out what to do, time to do some reading. I have Chassis Engineering by Herb Adams and How to make your race car handle by Fred Puhn (i think). There are other books as well. You might talk with Marcus at www.scandc.com. He has done some 3-links. Have fun.

I'll second Randy's suggestions. If you're not careful about this, you can get in way too far over your head and could even end up hurting yourself. Go the library and look up some books by Doug Milliken - he's written textbooks on this kind of thing and it will really help you get a grasp of all that is involved. I'd also suggest going to www.pro-touring.com and then do a search on "3-link". You'll have enough material to keep you busy for a few weeks. And if you want it to handle, you'll need to forget just about everything you've ever "heard" from drag racers - they're only interested in a strong launch.

Good luck,
Ryan

I'm not trying to jump a thread here so please excuse me. Randy was that the rear frame cross member you tied your upper link too? I say this because if that was the stock metal I don't seeing it holding up over time. That metal is too thin for such a crucial peice. You should of ran 2x3 box and replaced the crossmember and then tied the upper link on to it. That open 3/16 just got weaker from welding it. It's also susceptible to stress flexing which will fatigue crack over time. I'm always on the safety and durability when it comes to fabricating anything. That rear end is always trying to roll under torque. The bottom set up is fine as it ties into much thicker frame metal.

RandyB.
builder, painter and fabricator.

RandyB,
Yes it is the stock crossmember I used. It has been braced with tubing so hopefully it will hold. I know Marcus @ SC&C has done this with G-bodies and hasn't had a failure of the crossmember yet. I will be keeping an eye on it, frequently checking it for any signs of cracking.

We`ve been running an adj. 3 link with PHB on our G-5 test car `87 Cutlass (LS1/6speed) for about 3 years now. We did start out using the stock crossmember to mount the 3rd link. We reinforced it but after about 5,000 hard street miles and lots of hard launches it looked fine but had actually bent about .20" in the center. The remade it with tubular braces triangulating it to the frame rails and tied it into the cage (which was probably way overkill) and it`s been like a rock ever since. We`ve done a LOT of testing with this configuration and contrary to some speculation it actually required considerably MORE rear swaybar rate and higher rate rear springs. With no rear swaybar and 167lb./in. springs it drove like a pig. We lowered the rear RC about 6" initially and the car cornered flat and was very predictable (much more so that before) but had a lot more understeer. Raising the rear RC slightly reduced the understeer a bit. We added quite a bit more rear spring rate (and retained the 7/8" rear bar) which helped quite a bit and finally installed an SC&C adj. rate rear swaybar prototype. Now we can easily go from understeer to oversteer. We have it dialed in now with just a initial little understeer to allow for some controllable throttle induced oversteer. In order to get a decent length upper link we offset the rear of the mount behind the axle centerline. Some say that`s undesirable but it works very well in this application,in the real world and is more stable and predictable to drive than the shorter version we tried. The angle of the upper link depends on your car and the rest of it`s setup. For example the firmer the front end the more antisquat you can get away with in the rear without rear brake lockup because you`ll transfer less weight under braking. I`m oversimplifying for brevity but basically you`ll want the front of the link slightly lower than the rear and give yourself a few extra holes in either direction,on each end for test and tune. Nobody else can tell you what`s going to work best on YOUR CAR,you`ll have start in the general ball park then determine that for yourself. Mark SC&C

I'll second Randy's suggestions. If you're not careful about this, you can get in way too far over your head and could even end up hurting yourself. Go the library and look up some books by Doug Milliken - he's written textbooks on this kind of thing and it will really help you get a grasp of all that is involved. I'd also suggest going to www.pro-touring.com and then do a search on "3-link". You'll have enough material to keep you busy for a few weeks. And if you want it to handle, you'll need to forget just about everything you've ever "heard" from drag racers - they're only interested in a strong launch.

Good luck,
Ryan

OK, done the search on PT.com...... Now I have a stupid question>>>>
What does SVSA mean??????

Thanks - Ron

Side View Swing Arm; the imaginary line the projects through the upper and lower links to where they would meet if extended out. SVSA should never be under 60"ish as it tends to induce brake hop under severe braking.

I would strongly urge that before picking up a torch or cut off wheel to begin any fab work that you have a strong grasp of the basics; SVSA is one of the very basic and root measurements used when designing a rear suspension.

Side View Swing Arm; the imaginary line the projects through the upper and lower links to where they would meet if extended out. SVSA should never be under 60"ish as it tends to induce brake hop under severe braking.

I would strongly urge that before picking up a torch or cut off wheel to begin any fab work that you have a strong grasp of the basics; SVSA is one of the very basic and root measurements used when designing a rear suspension.

Thanks!!! - And yes, I agree! I am learning (thats what this project is about) as much as I can. Im not going to jump into something I know nothing about. Im in the learning process now. Even if I dont do the three link upgrade, at least I will have a better understanding on how it works.

Oh , almost forgot. You did nice work. I just hate to see someone get hurt. My neck is always on the line when it comes to working on customers cars so better to lean to the for sure side of things. A whole lot of lawyers out there if you know what I mean. Too many times customer will say that's OK do it like this, or that.

Good luck
RandyB..

RandyB,
You're right about them lawyers. :angry:
I did think of changing the crossmember, but decided against it, considering the shock/spring mounts among other things. This project was from the start meant to be simple. The 3-link is the most change I have done to the car. That was due to autocrossing my 03 Mustang GT, can't stand the way the C4L rear end behaves. :eek:
Marcus, thanks for the info on the crossmember. How did you brace it? I hope the bracing I have done works.

How much would things be adverserly affected by offseting the upper link 6" to 8" to the drivers side. It would allow for a longer upper link and more adjustability.

Would love to here some thoughts on this. New job is keeping me on the road to much to get in the garage, but I'm trying to plan for when I get some time off.

If I am not mistaken, the basic B-body cars had just such a setup. Of course they did add a second upper link on the other side with higher power engine combinations.

With a single upper arm in the middle all the forces are balanced from side to side. But move it to one side and there will be more force working on one LCA than the other. The degree of this affect would depend on distance off center for the UCA relative to the distance between the LCAs.

You could also just add another UCA on the other side equally spaced. Steeda sells a product configured like this for the converging 4 link Mustang rear suspension.

Yes, the 60s big-cars had a 3-link. Any of them with a 4-speed, or a Big Block got the 4th link, and all Caprices had the 4th link.

If I'm not mistaken, I believe offsetting the upper link will effect roll-steer... I know there was a discussion about this at pro-touring.com, but I can't remember what was said.

Didn't those cars have the upper link offset at least a foot. Just courous how much the smaller offset will limit the side effects.

Thanks, and I will go check Pro-Touring.

According to BillShope at CC, the right amount of offset will counter the drive shaft torque and allow for level vehicle launch/acceleration with equal tire loading. Might not be too helpful with a cornering vehicle though, but then it could help if it's keeping side-to-side forces equal though. I believe it was mentioned that the Ramchargers used this back in the day. It's offset to the right though, not the drivers side.

Look for posts by BillShope at corner-carvers.com and page 6 of his site here; http://home.earthlink.net/~whshope/index.html

Peter

Thanks for the info Peter!!!

Peter is right. It's meant to counteract torque. I believe Bill Shope has also mentioned that if the link is not in the perfect location, the rear axle can go into all kinds of harmonics (read: wheel hop like you've never seen before). Good luck!

Ryan

Yep,if the upper link is offset it always to the right (pass.) side. Done properly it makes for harder launches but has the side effect of inducing assymetric forces under braking (can make the car pull to one side). A well designed 3 link is not an easy thing to package in a Chevelle. An upper link about 16" long would be a good thing but bear in mind that the 3rd link on a new Shelby GT-500 is about 9". PHB should be as long as humanly possible and perfectly level at ride height. The best overall height varies from one application to another. On G body (which is very similar) we ended up at 12". A lot of folks will tell you that`s way too high (stock is 18"-19") but that`s what the car works best with. Much lower and we couldn`t get enough spring rate or swaybar rate to keep the car neutral (or even close) and still be bearable to drive on Pa. roads. Mark SC&C

...A well designed 3 link is not an easy thing to package in a Chevelle. An upper link about 16" long would be a good thing but bear in mind that the 3rd link on a new Shelby GT-500 is about 9". PHB should be as long as humanly possible and perfectly level at ride height. The best overall height varies from one application to another. On G body (which is very similar) we ended up at 12". A lot of folks will tell you that`s way too high (stock is 18"-19") but that`s what the car works best with. Much lower and we couldn`t get enough spring rate or swaybar rate to keep the car neutral (or even close) and still be bearable to drive on Pa. roads. Mark SC&C
Interesting trivia regarding PHB height. I would infer, or perhaps confirm expectations, that understeer was present at the lower positions?

Correct. We initially had it set at 9" and it understeered like a pig. Hardly any body roll at all and great mechanical grip but the understeer was horrible. I`ve seen speculation that with this setup you could toss the rear swaybar and run soft rear springs,that`s just plain wrong in our experience. We were running 705/167 springs and a 7/8 rear bar and it still plowed. We went to 200lb rear springs and stiffened up the rear shocks, that made it at least drivable but it still felt heavy and pushed in the corners. MUCH more linear and predictable than with the C4L but it lost the nimble,tossable feel it had before. We ended up raising the PHB and doing a fully adj. road race rear swaybar to test and tweak. 200lb rear springs are about as stiff as we were willing to go because ride was starting to suffer. We ended up running about 3X the stock 7/8" bar`s rate to get the car balanced with just a hint of understeer. The higher bar rate let us back off one click on the rear shocks and bring back some of the ride quality. The car`s awesome now. Very nimble,predictable with excellent turn in. Taking on/off ramps at double the posted speed while popping another CD in is no great feat. Love that car. The 3 link/phb is just such a PITA to package in a kit of any kind. We`re still working on some options but I dunno when we`ll get anything to market. Mark SC&C

For those interested... here is a rather technical description of why and what happens here.

It starts with tires and a concept of grip or ability to hold the road before sliding. A tire has only so much grip and it peaks before dropping a bit when the tire is sliding. As you might expect grip increases when we put more weight on the tire and decreases as we take weight off the tire. Now here is the KEY aspect of this grip concept: these grip changes for weight increase/decrease are NOT balanced and the grip does not increase as much when you add weight as it decreases when you remove weight. Thus optimal grip is achieved when all tires have the same weight loading on them.

Of course when you corner centrifugal force causes some weight transfer from the inside to the outside tires. This is unavoidable. Thus we spend a lot of time tuning how much weight transfer happens and use this to balance the grip between the front and rear for cornering.

Now there are two different weight transfer affects of interest. First the obvious body rolling affect which is driven by the roll center (RC) and center of gravity (CG) height. The longer the distance between these two points, the more the body will try and roll. To some minimal extent the CG will move out as the body rolls so less roll is better. But more importantly body roll complicates and often screws up suspension and tire alignment (this is the reason for the "negative camber" front end parts). So we implement means to control it like stiffer springs and anti-roll bars. We can tune where this control is implemented between front and rear. What we cannot change is the total weight transfer, which is why a low CG is so important.

The second weight transfer affect arises from the distance between the RC and the ground/tire contact. As the RC is raised it increases this form of weight transfer, and as it is lowered it reduces it.

This second affect is why the low PHB causes understeer, it reduces this part of the weight transfer in the rear significantly. It also increases the RC to CG length causing more body roll (messing up front tire alignment and thus traction) unless other compensating changes are made. Like the 3X rear sway bar rate.

As an example my Mustang Bullitt had a C4L rear setup when I got it. It was setup with a performance suspension and cornered fairly flat and quite neutral for stock. I would take speed bumps on one side and as the front rolled over that side of the car would rise and fall. As the rear rolled over the car would stay flat. Obviously this car had most of it's roll control in the front end.

I converted the rear end to IRS using parts from the Cobra. Thanks to Ford Engineering it was a fairly simple bolt in. This changes the rear RC significantly, I have not measured but suspect it is much lower. Thus the rear roll stiffness is increased substantially. Now when I do the speed bump test the car lifts as the rear rolls over too.

I would guess the biggest issue with a 3 link kit is the PHB and exhaust system clearance. The PHB frame mount really needs to be triangulated and tied into both frame rails. Which means stuff going across right where the exhaust is coming down and out aft of the axle.

Lateral-Dynamics.com offers 3-links for other cars but not the chevelle, you might want to give them a call and see if they can help you out. Goodluck

The second weight transfer affect arises from the distance between the RC and the ground/tire contact. As the RC is raised it increases this form of weight transfer, and as it is lowered it reduces it.

This second affect is why the low PHB causes understeer, it reduces this part of the weight transfer in the rear significantly. It also increases the RC to CG length causing more body roll (messing up front tire alignment and thus traction) unless other compensating changes are made. Like the 3X rear sway bar rate.I think you have this backwards (or I'm just reading it wrong :confused: ) - High RCH (close to CoG) will reduce body-roll, Low RCH (way below CoG) will increase body-roll. RCH too high (above CoG) will cause jacking, as the lateral forces try to roll the body in the opposite direction.

I think you have this backwards (or I'm just reading it wrong :confused: ) - High RCH (close to CoG) will reduce body-roll, Low RCH (way below CoG) will increase body-roll. RCH too high (above CoG) will cause jacking, as the lateral forces try to roll the body in the opposite direction.
There is an official name for the weight transfer I was trying to describe but I did/do not remember it off the top of my head. So rather than get it wrong, I omitted it in favor of a more vague description :clonk: Thus I have it right, you are reading it wrong, and I described it poorly (or you would not be reading it wrong) :beers:

So let's see if I can clarify it... You are correct in that a high RC will reduce body roll. "Weight jacking" is related to the weight transfer I was really trying to describe in this paragraph. Any distance above ground for the RC will result in a weight jacking type of weight transfer. Of course as the RC is raised weight transfer from the jacking affect is increased. It can be surprising just how much weight transfer occurs due to this particular affect, it is much more than you might think. Which is what my example/experience and Marcus' experience showed. The novel part of Marcus' experience was it also pointed out the weight transfer tuning aspects of PHB. This makes perfect sense, I had just never taken it beyond the RC tuning to the second order impacts. General wisdom says to get the RC as low as possible but here we saw an example where that was not very wise for other reasons. And I had always been critical of home built PHBs on A-bodys for being too high but now I have reason to rethink that :thumbsup:

Part of why this form of weight transfer is not understood, or under-estimated, is that weight jacking is not something we can see or feel either from inside or outside the car. That is until the inside wheel comes off the ground. But even that can be arbitrary since roll stiffness can cause that same affect.

Note that you could make some goofy things happen with roll centers, like build a car that leans *into* turns. Unfortunately this would be very sub-optimal because it would require a very high RC leading to huge weight jacking problems.