The stub coming out of the diff shouldn't be happening if the 6 link is set up properly and the splines sliding in and out isn't that much of a problem IMO, all trannies do the same and the sliding 1/2 shafts on vipers and olds toronados are similar (now that I think of it, using something like that as a 1/2 shaft....that would be nice, you could c-clip the stub to the diff and have the 1/2 shaft take up the slack)

 

Thanks for the great responce. I feel a little bit overwhelmed by all of the responces. I will try to answer all of the posted questions in following posts.

My situation is as follows.

I have owned my 78 for 4 years now. I rebuilt the front suspension the first winter as well as the motor. The second winter I rebuilt the rear suspension. Nothing much has happened since except for a few things like brakes. When I did the rear I didn’t have much cash so the diff and wheel bearings were not changed. The current handling of the car is terrible. The rear end “slides” out in a turn and the car is very darty.

After removing the rear end I have a pretty good idea why this is happening. My side yoke end play is .105” on ones side and .300” on the other. I have not opened up the diff yet but I assume that the c-clip has fallen off of the one side. My plan is to reuse the side yokes that I have with a 6-link setup. I plan to have the diff looked at to determine what may have been damaged. If anyone has any suggestions for someone to use in the Toronto area that would be great.

My plan for the Cad model was to set up a 6-link system that could be easily modified to see the results in how the suspension would react. I now have the model drawn and need some help setting up the parameters of how the system should act. I understand that a 6-link system will not correct the toe problem but I plan on trying to minimize the change by trying different things.

What should the initial camber and toe be set too at ride height?

What should the initial ride height be? (I was planning on lowering the rear end with longer spring bolts)

What would be a reasonable suspension travel to look at for these numbers? (I used a total of 4” of travel)

Mark.

 

Twin_Turbo

I have no problem posting drawings of all the brackets I come up with.
That also goes for the Cad models. I can supply many different formats, so I should be able to accomadate everyone.

My design will be limited by my dual exhaust that passes directly under the origional camber bracket. The connection point the the lower links will have to be outside my exhaust pipes.

Mark.

 

Red73Vert

I think you are talking a bit over my head. I am not familiar with the C4 setup and I am having trouble following what you are saying. Do you have or know of any pictures that will help me out.

Thanks

Mark.

 

Dual camber rods is what guldstrand uses, they do 2 things, control camber and the setting of the front one opposed to the rear also sets toe, the 1/2 shaft on guldstrand's setup is still the upper suspension member.

Red, Guldstrand uses threaded rods on the camber bracket and on the hub (the fork), the length of the threaded rods is 7" (I asked some of the measurements from a guy that ran the guldstrand setup), so weld in 7" rods threaded for 5/8" and you're set to go, the stock hub fork uses a 5/8 pin and can be replaced w/ a 5/8 bolt by either machining a flat section on one side, like stock, or drilling out the fork to remove the flat spot

Here's the dual bar system:


The gray box sections you see is a display stand, not part of the suspension.

As for trying to reduce toe deviation w/ a 6 link, you won't be able to, the only way to reduce it when using the trailing arm w/ the standard mounting system (shimmed bushing to set toe) is to limit suspension travel, a bandaid.

Mark, since you have the model ready I thought you had the location of the lower strut handy, I'm interested in knowing how much down it is from it's original location, I figured 1" myself but need to check it to make absolutely certain. If you already have that figure I could really use it. I saw you had a modified bracket drawn up in the 6 link pic.

 

0.300 play? Wow, that will certainly do it! A C3 with all the suspension in good shape will handle pretty darn well actually. Of course there's always room for improvement.

As for the rear alignment I like to set them to 0.5 dgr negative camber and 1/8" total toe-in (0.062 per side).

 

Twin_Turbo

The bracket you saw is just a start. My plan is to get the top rod in place (which I have not completed yet) which looks to be more difficult and then have the bottom rod match the top. (parallel and equal length)

Once I have completed my design I will post a complete E-drawing with dimensions of what I have come up with.

I am assuming that I will want the 2 links the same lenght and parrallel to each other. Is this assumption correct?

Mark.

 

ZD75blue

As soon as I have added the diff and crossmember I will post a link to my FTP site where you will be able to download the native Solidworks assembly if you would like.

Mark.

 

My starting point was with equal length rods. If this is not desireable how do you determine how much neg camber you would like at a specified amount of supension travel. I agree with your statement about body roll changing the relationship with the tire and the ground but, how do you correlate roll with supension travel? Would spring rate and size of sway bars not change the relationship?

Mark

 

Twin_Turbo.

Thanks for the picture. After studying it for a while I now understand how that system controls toe change. Would camber change not be very close to the stock setup?

If this is ture, why is toe setting more important than camber change?

Mark

 

Camber change would be similar to stock, but the bracket is different so the angles of the struts are different.

In your pic you have the lower strut parrallel w/ the 1/2 shaft, if so, that's what I'm after so if you have the difference between the stock and new mounting height of the inner strut bolt, that would be what I need.

 

Toe changes cause the rear to try and steer the car when cornering. It changes the direction the tire is pointing! Fine on a floppy sidewall tire... a dangerous thing with short sidewall Z rated rubber!

Whenver your "done" with it in solid works, I'd like to see it! I'm no where near as gifted as you are with it though!

Toed in in the rear
[.............]

/.............\

Toed out at the rear
[.............]

\............./

 

yes toe oversteer is a big problem under heavy cornering.

 

Nice work Flexus!

I noticed that your initial position (approx ride height) shows the inner end of the half shaft somewhat higher than the outer. Is this the setup that everyone else runs? I would think setting up the car starting with a level half shaft would be more "neutral"... ???

 

TT
Thanks for the photo/info!!! I've been looking these details for 4 months!!!! Very nice!! That dual camber strut setup is very similar to what I was visualizing (and having problems explaining to Mark).

Mark
What I was attempting to explain (and others have been doing a better job of ) was that for road course (turning) type handling you want to keep the tire perpendicular to the ground (not the car) through the turns. This will maintain maximum tire patch. To do this you need to gain negative camber through bump at the same rate as the body rolls. As you go through a turn at speed your car will roll as the suspension absorbs some of the corning force - not a bad thing.

A quick and dirty way to calculate the body roll (in degrees), "B", versus suspension travel; is to measure the distance from hub to hub, divide by 2 ; call this "R". Measure your amount of bump/rebound from the rest position; call this "S". Then the Sine B = S/R. So as suspension travels through it's range of motion you can calculate what the body roll will be and compare it to your negative/positive camber gain. To calculate how much body roll/suspension travel you have gets somewhat complicated, but the distance between the roll center (7" above ground for a stock corvette rear suspension, 3" for the front suspension) and the center of gravity (16" above ground) acts as a lever arm. The farther the distance between the the CG and RC at one end of the car, the more potential for body roll you'll have (all else being equal) and the more work the springs/suspension/tires on that end of the car will have to do.

Regarding toe changes, as others have stated the stock IRS has enough toe change through the range of suspension travel to steer the car. If you look at how the trailing arm moves relative to the half shaft you'll see maximum toe in when the halfshaft is parallel to the ground. Essentially (when viewed from the rear) the hub end of trailing arm is traveling through an arc created by the half shaft while the front of the trailing arm remains fixed. The cosine component of the arc is the problem. You really want to eliminate it, or have the front of the trailing arm slide with it.
A crutch (that works fairly well) identified in the VIP article (below) is to set your ride height so the outside u-joint is an inch below the inside u-joint at rest. Then under load, with say 2" of bump travel, your toe will be at the same as it is at rest since the halfshaft will be the same arc degrees from parallel(now up instead of down). Of course the unloaded (inside) tire will significantly toe-ed out (helping the car turn).

There are 2 great resources at http://www.corvettefaq.com under the "suspension" section . The first is the 3 part article on "Corvette Chassis", and the other is the VIP (Vette Improvement Program) by John Greenwood. "Chassis" article has a great explaination of "center of gravity", "roll centers", "roll coupling", as well as the theory behind it all. Being a Mech E you should find it interesting - I did!!! I hope this helps!!!

 

How? Well, you start by picking the brains of a lot of other people who have been doing this for a long time

A 'typical' value for roll-camber, in terms of degrees of wheel camber per degree of body roll is around 0.6-0.7 for an independent suspension. That's the camber rate at curb. So it's the slope of the curve going through curb. That's easy enough to do. The difficulty is what do you want the REST of the curve to look like. That's quite difficult to explain and is probably best left to some textbooks and SAE papers.

How do you correlate roll with suspension travel? Pretty basic geometry, really. You know how wide the track is, so you know how much each tire needs to travel vertically in order for the body to roll 1 degree.

Spring rate and size of the sta-bars won't change the relationship between body roll and suspension motions. But they both affect body roll. So in effect, for a particular suspension/vehicle event -say a particular corner- the body will roll less, so the camber will be less. To that end, a parallalogram linkage with an infinitely stiff suspension is just a suspensionless go-kart, so the linkage won't matter. The softer the suspension gets, the more obvious the suspension issues will become (and it doesn't take much to make it "softer" to the point where you'll see differences).

I hope that helps.

 

Not to nitpick, but I assume you're referring to roll-steer in the rear. Roll oversteer definitely CAN be a big issue, but it's really not an end-all in the broad scheme of things. The BIG issue is how much negative roll steer you have. A vehicle might have +1% roll steer (an understeer effect) or -1% roll steer (an oversteer effect). The difference probably isn't huge between the two, and you probably wouldn't be able to pick one out from the other. But generally speaking, it's not a desirable thing, and certainly not something that we normally design for.

But...there are some vehicles that do have roll oversteer rear suspensions. I can't recall if the F-cars are or not. It's primarily something you'll see on a solid-axle vehicle. In general, when the wheel hits a bump, you want the tire to move up and BACK. Not back a lot, but some. It has a big influence on the impact feel.

If you want to do that with a solid axle, you heed to have trailing arms that are higher in front than they are in at the axle attachment point. This configuration creates roll-oversteer. As the body rolls, the tire is pushed back a bit (like it's supposed to over a bump), but the side effect is that the other side generally moves forward. And they do so such that the net steer of the axle is an oversteer. Trying to swing the back around like a fork truck. But we're talking fractions of degrees of steer.

 

I guess the question I have then is, how far should I be looking at changing my supension for the type of driving I will do. Right now I have huge side yoke end play (.105" & .300") which needs to be fixed.
My initial plan was to reuse the yokes and add an additional top link. I just want a car that handles on the street and allows me to exceed the recomended speed on the odd on-ramp. I was also looking at the fact that for the price of new side yokes, I could easily build the 6-link system. It would just me a time investment. But a 5-link style system like Twin Turbo is building doesn't scare me. It's just a matter of how far to go.

Any thoughts? Will a 5-link car handle that much better than a 6-link?

One Question, I have been using the terms 5-link and 6-link to describe the two different suspension system talked about in this thread.
Does anyone know where these names came from?

 

It's actually 5 bar and 6 link and the way they count is different.

6 link only counts camber control devices + 1/2 shafts, so upper 2 rods , lower 2 rods and 2 half shafts = 6

5 bar counts 1 side,
guldstrand: 2 lower control rods (camber + toe), 2 trailing rods & 1 halfshaft = 5
greenwood (& C4, they use about the same system) 1 toe control rod, 1 camber control rod, 2 trailing rods and 1 halfshaft. = 5


The 5 link would handle better under hard cornering because it does not ihave the toe control problem and the 1/2 shaft is pretty capable of positivly controlling camber IF!!! the stub end play is withing specs, any looseness there and you're in trouble, that's where the 6 link shines.