I think you need to let the front end of the "torque arm" move fore/aft a bit, unless all three chassis side pivots lie along the same straight lateral line (think "at the same location in a pure side view").

Strictly speaking, this is a "traction arm" by virtue of its rod end connection to the chassis, making it functionally identical to the Buick GNX and Kirban's GNX copy rather than similar to the 3rd/4th gen F-body (which is a true torque arm). You can deviate slightly from that straight lateral line criterion with a traction arm if your LCA and traction arm end bushings have sufficient total compliance.

Norm

 

Norm,
I very much appreciate your information and my explanation will probably fall way short, but here goes.

The most important factor in a torque arm mount is the lateral location of the rear. This mount must be kept with a low roll center to aviod unpredictible suspension response.

The system is kinematically free in roll as with a 3 link. The suspension is free to roll when heim joints are used as a result no surprises like "snap oversteer".

The number one complaint with a bushing/stud front control arm mounts is the lack of needed adjustatibilty. This is the main reason we chose not only to go with a heim mounted torque arm but also 4 stage heim mounted shocks,uper and lower adj. control arms, and a custom anti-roll assy. I will post pics of the anti roll as soon as I can.

Thanks for the input,
Brent

 

I see what you're getting at - you need to leave some room to play around with ride height. And it does look to be mostlyfree in roll. Assuming plan view parallel LCAs (or nearly so), your axle's roll steer can be made to fall within a very few percent of zero (either vehicle roll understeer or oversteer depending on the specific LCA inclination). That's good for at least two reasons. First, the axle is remaining closer to perpendicular to the chassis centerline as the car rolls, so it's not like there's a little monkey sitting back there with his own steering wheel (and a mind of his own). And second, when the axle doesn't steer much, the front end of the torque/traction arm isn't going to want to move laterally much (which is accommodated either by bushing compliance or lateral bending of the T/T arm). You may well be within 1/8" here on a 40" T/T arm, possibly 1/16".

Actually, it was the simpler case of two wheel bump that I was concerned with. Ignoring lateral loads and the PHB, you only need restrain the axle in three degrees of freedom (fore/aft motion, plan view steer, and pinion angle change). Either a true torque arm arrangement(with LCAs) or a 3-link provides exactly this number of constraintsand will be kinematically free in bump. The traction arm plus LCAs provides a fourth restraint in that it resists pinion angle change in two different ways. Like a true torque arm, its bending stiffness defines one of these (with an IC at the traction arm chassis-side pivot).And since it is pin-ended, its axial stiffnessdevelops a separate resistance in combination with the LCAs, but acts about a different instant center (the side view virtual intersection of a line drawn through the axle center and traction arm pivot with the LCA axis).

The result of trying to force any sort of rotation about two different IC's is that a 'bind' is developed. In this case, the bind occurs in two wheel bump instead of in roll (which is arguably the more widely recognized bind situation). This bind, which is actually an unintended bump stiffness, is a function of the actual stiffnesses of the LCAs, traction arm, bushings/rod ends, various brackets, etc. You'd feel the end resultas a car that rolls somewhat more in any given corner than the two wheel bump stiffness would lead you to expect (or rides more stiffly than expected for any given roll rate).

That's where the idea of making the three chassis side pivots align in side view comes from - when that occurs, your axle motion is around that single pivot axisregardless of which definition of pinion angle control you look at. Call it a "forced co-incidence", or sort of like getting a 3rd hinge to line up with the other two on a door so that it will still swing easily. More or less, that's what the GNX version is quite close to, and that had to pass far tougher scrutiny than I can give.

Suppose you used a bushing & stud arrangement but captured the bushing within a tube that was itself pivoted to allow free ride height change instead of being fixed in position? I think you could run the pivot bolt through a separate block welded to either the top or bottom of the tube depending on space availability.

Norm

 

That is a lot of good information and it took some time to post.
Thanks for the help.

Brent