Live rear axle tech

Jeff Young

GT40s Supporter
Ok, Ron suggested I post here as it appears there is a whole lot of track car knowledge. This particular area of inquiry revolves around that wonderful suspension design known as the "live rear axle."

I've got one on the ITS TR8 I race here with the SCCA in the US. Right now, it has basically a stock rear suspension, basically. It has lower parallel trailing arms with spherical bearings, a stock spring location (which I have to keep) about 2/3 of the down the arms towards the axle, triangulated uppers that angle inward towards the pumpkin of the differential (no sphericals there), a panhard rod, and some DA Konis revalved for my spring rate (300 lbs, fronts are 600).

I presently have a Quaife LSD and we run Hoosier's DOT road race radial, which is basically a racing slick.

Rules say bushings are free (meaning I can run sphericals, urethane, rubber, or air if I want), stock suspension mounting points must remain, but ANY traction bar can be used with traction bar basically defined as anything used to locate or stablize the rear axle.

Car handles decent, corners flat. It is really good in fast sweepers (think uphill esses of VIR). In slower stuff, and quick transitions, car will turn in nicely (no push/understeer), but has serious oversteer issues powering off.

After some discussions elsewhere I've come to the conclusion that I think the upper links are binding badly under roll, causing infinite spring rate, loss of traction and nasty wheelspin.

Under our rules, I can add a third link to the top of the diff and running to the chassis, and then use foam bushings on the triangulated upper links so that effectively they do nothing.

Some here in the states prefer that approach -- the "trilink" -- with a panhard or, better yet, a watts linkage to locate the axle laterally. Others say use the four link w/o a panhard.

Thoughts? Suggestions? Hypothecation? Speechification?
 

Trevor Booth

Lifetime Supporter
Jeff,
in the event that your upper traingular links have the pivot bolts running at 90º to the arm of each link you will get binding if the bushes are not soft to allow for this, however, if they are soft to prevent binding it will cause axle location issues. If the bolts run 90º to the centreline of the car you will not get binding.

A 4 link system with the links as far outboard is preferred as it minimises twist in the axle housing. A tri link connected at the pumpkin will allow axle housing twist.

If you can make it such that the tri link effectively does nothing (with sponge bushes as you suggest) and can fit a 4 link with watts linkage you will have the best system in terms of axle location.
 

Jeff Young

GT40s Supporter
Thanks for the thoughts.

The upper links are triangulated, meaning they meet the axle at a 45 deg angle -- and I agree I think they are binding with the urethane bushings I am using.

I can't relocate the upper links and make them parallel -- have to keep the stock suspension points. BUT -- I can ADD a traction bar, i.e. a third link. So, what cars with similar suspensions in my class do (the 1st Gen RX7 specifically) is run soft foam bushings in the upper links and then use the tri link + a watts/panhard.

As I understand your thinking, in order of best to worst, the best design is:

1. Parallel upper and lower 4 link (not legal in my class);

2. An added third link with foam bushings in the triangulated uppers with a panhard/watts (legal)

3. Stock lower parralels and upper triangulated (legal)

4. Stock lower parrallels and upper triangulated with a panhard that causes MORE binding (legal and what I have).
 
Do the rules state what material the upper factory tri link arms must be made of-- like can you make some plastic replicas of them to allow a three or four link to work.

Also for any 4 link to work successfully without bind it must have either one compliant bush replacing one rod end or an axle bracket that can flex sideways slightly to allow for roll & vertical movement...
 
Jeff, any way you can tape a video recording device to the side of the car and then be able to watch for the camber changes and wheel movement from loaded to unloaded? A lot of (very good) suspension engineering is essentially done in the realm of guesstimating, without actually verifying the what is going on. Understandably, that's not easy to do, but a video of the wheel movement/motion throughout short and medium and long bends can be very illuminating.

Here's a good example: http://www.youtube.com/watch?v=KDVZ1jgWjWA&feature=PlayList&p=A0D1CE4DEDAEB384&playnext_from=PL&playnext=1&index=42
 

Trevor Booth

Lifetime Supporter
Jeff, if you can add a traction bar I.E. third link why cant you add TWO traction bars IE a four link -:)). If you cant do that, option 2 would be the way with a watts rather than a panhard.
 

Trevor Booth

Lifetime Supporter
Jac Mac

Also for any 4 link to work successfully without bind it must have either one compliant bush replacing one rod end or an axle bracket that can flex sideways slightly to allow for roll & vertical movement...

I guess aussies must have found the secret without one compliant bush
 

Ron Earp

Admin
Jeff, if you can add a traction bar I.E. third link why cant you add TWO traction bars IE a four link -.
I wish I could answer that for you but the rules we race by in Improved Touring have all sorts of anomalies. This example is one of the most offensive - you can fab up a tri-link and put on a watts linkage, but no no no no no, you can't make a minor adjustment to the upper links (like suggestion (1) below). [/Bitch Off] We chose to race in the class.

We have not put the car up and run the rear suspension through the range of motion. That is something I'd like to do and I bet we'll learn a lot from it. Also, there are some odd things going on back there with the existing panhard rod. It is curved, maybe two of them are, can't remember.

Do we have an pictures of the existing setup?
 

Jeff Young

GT40s Supporter
Actually I can add two bars. No limit. I just have to keep the stock uppers (and JacMac they have to stay stock except for the bushings). So maybe I run a four link with a watts to control lateral movement, and foam bushings in the upper stock triangulated arms?
 
I remember reading how Corvette racers in the 70's needed to keep the stock trailing arms. However, they got around that by building an almost proper formula car suspension by adding links( additions are allowed as long as the original suspension is still "there") and removing the bushings from the trailing arms altogether since they were no longer needed for locating anything!
I'll see if I can find pics...........
 

Jeff Young

GT40s Supporter
That's essentially what we are looking at doing.

I'm thinking maybe the parallel four link (if I have room for it) with a watts is the ticket. Put foam bushings in the stock uppers and go with it.
 
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