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Last time I went racing I found out that my diff is twisting up under hard acceleration and that's why its hitting on the floor, I even tried real long bump stops and it still hit, that and the fact I found marks on the tailshaft from the diff flange bolts touching on it led me to believe that was the issue, and when you look at the lower control arm setup on a Gemini its not hard to understand how it can happen.
as you can see the pivot point just infront of the spring is where its twisting up, this is not normally an issue in a Gemini as it has a torque tube on the centre of the diff that will stop that, and also most cars don't see the sort of power and traction at the same time .... so my thoughts to remedy this situation varied, tramp rod style things and everything from welding that pivot solid to replacing them with something else.

While im not in a position to go full 4 link with coil overs etc I decided I needed to find an option that allowed the original spring mount to still work, the solution I have settled on is to graft a commodore lower control arm onto the diff and floor. Initial measurements showed it would work out pretty close, the mounts on the diff have the shock mount the same type and in the right spot, as well as the spring mount being perfect, the only issue is that the commodore arm angle inwards and are about 50mm shorter.

conveniently my brother and I have a VP Lexcen we bought for $200 and planned to get needed bits then turf it, so I recently went and rapped the bits needed from it, and also cut the mounts off a VL diff I have left over in the shed from other stuff.
So first things first I needed a way to make sure the diff stayed in the right spot when I started cutting the old mounts off it etc, so I made up a little bracket to locate everything
so then I cut all the old mounts out of the floor and diff and worked out where to locate the commodore arm and once that was decided I fabricated a new mount to weld to the floor and tacked it all together.
Also the angle of the arms should end up slightly lower at the rear which if anything should mean as the diff goes up it should put downward angle on the diff rather than have it turn up like it was before, but more than likely will be fairly neutral!

it looks like the spring height from floor to lower arm is pretty much the same too, so it should mean my ride height isn't changed, and I have the commodore springs too which are thicker so I will cut them to size to take with me as well so if the rear is too soft I can stiffen it with them, the car has always squatted pretty hard in the rear and I have a feeling that's partly that the springs are too soft for what im doing with it.

Cool, will be interesting to see how it goes!

What locates the diff at the top to stop it from twisting?

commodore upper arms, i fitted them to the car initially when i put the commodore diff in it.

OK.
So will the lower arms be parallel?

hard to say for sure till its sitting on the full weight of the car but they should be pretty close or maybe even angled up at the front mount slightly.

you going to gunnedah drags this march ?

Love your work.

Awesome idea, one of my pet hates from my gemini days was those stupid torque tubes and mounts

Holden202T wrote:hard to say for sure till its sitting on the full weight of the car but they should be pretty close or maybe even angled up at the front mount slightly.

you going to gunnedah drags this march ?

Unfortunately no, I'll be heading back from Victoria & not due back till Saturday. If things change, I'll be there.