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Rear suspension lay-out info needed . . .

FM Former Member
I need a little help with a different and some background info on the mathematics used in designing the stock rear IRS suspension.

I have the rail/tube frame in the garage and removed the engine trans, wheels, brakes, and diagonals, mostly to get a good look at what's there. Using (hopefully) a Rotary mid-engine will mean removing the torsion tube and going with end plates and a four link/heim joint set-up with adjustable coil-overs. Easy Peasy.

I'm not sure if there will be room for the over-sized aluminum Porsche diagonal/disc break assembly, so I was thinking of using a four link set-up across the rear as well with a center boxed mount and two parallel bars coming from either side and mounting to the bearing carriers, like the Lotus and some Corvettes and such utilize. Again, with a stout rear frame support/cage, Easy Peasy.

The problem that I have is how to adjust and set this up to operate as cleanly and in the same axis' as the stock assembly. I would SEEM like the up and down movement in the torsion arms would shorten and lengthen the wheel base with each movement, no big deal, but the stock diagonals are bolted and anchored in place and are a fixed length, seemingly preventing the shortening from taking place. I'm missing something here for sure, and whatever that is needs to be designed into the side and rear four bar set-up that I'm shooting for or everything is gonna bind and stress. With the heim joints, it's all loud metal on metal, no bushing, no give, no way to absorb any variations in length through the full swing of the side torsion arms.

Anyone ever run into this? Any solution? Anyone know what I'm missing here? Hopefully I've explained the problem in front of me and used the correct terms to describe it. I could add more info if necessary.

Some info would be terrific. It all might not be worth continuing with, but so far you guys have been great with answers to what I've run into.

Thanks ! !

T
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FM Former Member
I need a little help with a different and some background info on the mathematics used in designing the stock rear IRS suspension.

I have the rail/tube frame in the garage and removed the engine trans, wheels, brakes, and diagonals, mostly to get a good look at what's there. Using (hopefully) a Rotary mid-engine will mean removing the torsion tube and going with end plates and a four link/heim joint set-up with adjustable coil-overs. Easy Peasy.

I'm not sure if there will be room for the over-sized aluminum Porsche diagonal/disc break assembly, so I was thinking of using a four link set-up across the rear as well with a center boxed mount and two parallel bars coming from either side and mounting to the bearing carriers, like the Lotus and some Corvettes and such utilize. Again, with a stout rear frame support/cage, Easy Peasy.

The problem that I have is how to adjust and set this up to operate as cleanly and in the same axis' as the stock assembly. I would SEEM like the up and down movement in the torsion arms would shorten and lengthen the wheel base with each movement, no big deal, but the stock diagonals are bolted and anchored in place and are a fixed length, seemingly preventing the shortening from taking place. I'm missing something here for sure, and whatever that is needs to be designed into the side and rear four bar set-up that I'm shooting for or everything is gonna bind and stress. With the heim joints, it's all loud metal on metal, no bushing, no give, no way to absorb any variations in length through the full swing of the side torsion arms.

Anyone ever run into this? Any solution? Anyone know what I'm missing here? Hopefully I've explained the problem in front of me and used the correct terms to describe it. I could add more info if necessary.

Some info would be terrific. It all might not be worth continuing with, but so far you guys have been great with answers to what I've run into.

Thanks ! !

T
J justinh
The stock irs trailing arms (and front arms too) move in an arc as the suspension cycles so the wheelbase does change. Being fixed length doesn't matter. The setup you're thinking about will also change wheelbase as it moves. It won't bind up unless you mount two links, in the same vertical plane, to the same pickup point or put the pickup points really, really close together. From a geometry standpoint, you're not trying to copy the stock configuration, you want to make it better (more camber gain, less toe gain). And why do you care what the stock spring plate does if you are replacing it?

You need 2 trailing links to control caster, 2 lateral links to control camber, and a toe link. Usually the toe link is combined with the lower lateral link to form an adjustable a-arm but reversed with 1 inboard pivot and 2 pivots on the bearing carrier. Since on the rear you don't need to change caster, the trailing links can be parallel but don't have to be. The lateral link pickup points need to be located just like you would on a double a-arm suspension to give you your desired camber curve. The tricky part is that your suspension is now moving through arcs in two planes instead of one. That means the suspension as it moves will have toe changes and will gain more camber than the lateral link placement would indicate. A longer top trailing link or moving its chassis pickup point could help limit these toe changes.

Unfortunately, I don't know the formulas to calculate the link lengths for this type of suspension.

A De Dion type suspension is often used on the mid-engined 550. It's just a big bent tube that rigidly connects the hubs. It would replace the lateral and toe links described above. But you give up camber change.

Scratchbuilding a mid-engine frame is a big job. Good luck.
FM Former Member
"And why do you care what the stock spring plate does if you are replacing it? "

Mostly because I want to mimic the movement and use the math to reproduce the spring plates in terms of four links instead. I know how it works on paper, and my little model seems to be fine, but I was wondering if anyone had any real work knowledge of anything that I needed to be careful of and figured that understanding the stock set-up would help in recreating it using for rods, eight heim joints, and four drilled plates.

The rear set-up running across the back seems easier, four parallel rods, two to a side, with the camber adjustment in the rod ends.

So far, the chassis has been a snap, starting with a commercially built base and changing as I go to suit my idea(s). Just wanna be sure that the plan for the rear is a good one, and especially one that will work at highway speeds in a normal weight car. I think that I seen variations on those little jumpy/boingy sand buggies, but this project will be a good deal heavier and see normal use, as an everyday driver, so I wanna over plan and over build it.

J justinh
If your two trailing links are parallel and equal length, they will keep the hub carrier vertical (no caster change) when it moves up and down, and only the wheelbase changes. The stock trailing arm, since it is solid and fixed to the hub, wheelbase and caster both change but rear caster change is irrelevant. If the links are the same length as the distance from the stock torsion tube center to hub center, the wheelbase changes should be nearly the same.

You don't want the lateral links to be parallel, you want them to be unequal length non-parallel just like an a-arm setup to get camber gain in bump.

Here's the camber and toe equations for a semi-trailing arm suspension.

http://www.e30m3project.com/e30m3performance/tech_articles/susp-tech/rear_curves/index.htm

If I measured right, it's 41.5 inches springplate to springplate, 17 inches from center of torsion tube to center of axle, trailing arms are 12.5 inches pivot to pivot along the torsion tube and the inner pivot is 3.25 inches back giving a sweep angle of 14.6 degrees. Near as I can tell, the inner and outer trailing arm pivots are at the same height. If you set the chassis at 5 inches of ground clearance, the center of the torsion tube is 9.25 inches above ground. That puts the roll center about 3.5 inches above ground. If you know what height your torsion tube is, I could calculate the roll center height for the stock trailing arm setup.

If I'm off on any of these measurements let me know.
J justinh
Again, you probably shouldn't bother trying to exactly copy the stock geometry with a completely different type of suspension. Mostly because unless you go true multilink with CAD design and more math than I care to know, it's impossible over the full motion range. You're building fresh, no sense in limiting yourself to what the stock suspension is capable of.
FM Former Member
Thanks for the tips and advice! Funny that we just stripped out two BMWs last Friday/Saturday and had those rear sub-frames (mentioned/pictured in the link) out on the floor checking them over. They'd make a nice rear chassis clip for a project. We were mostly after the rear end housings, transmission, and drive shafts, but these cars seem to have any number of neat bits and pieces hanging off of them.

Once I have the parts for the chassis cut and fabricated, I'll check back in for more help for sure. A '69 Camaro caught my eye over the weekend and the 356 is back up for sale for parts or whole. If it doesn't sell, I'll be back into this project using a 356 body, otherwise it might get covered up with a 914 shell before I'm done.

Later-

T
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