View GarageI've been chasing driveline vibrations for some time now and its starting to get a bit old. My car has a Supra gearbox and R31 Skyline diff and is quite low, which makes it difficult to get good driveshaft uni angles. Originally the diff pinion was several degrees down from horizontal which resulted in quite a large angle at the rear uni but unfortunately almost none at the front, with 3.9:1 diff gears it vibrated quite badly right around 100km/h.
First step was to get adjustable upper trailing arms, which I thought would solve the issue. However with the ride height (and it may also be a legacy of the angles used when the diff mounts were converted, who knows) I still couldn't get a good result, the diff couldn't safely be rolled back any further or the springs would catch and other clearance issues came up. I ended up raising the gearbox 10mm (can't go any further!) and rolling the diff back quite some way which required trimming the flares so the wheels would still clear. There was no good way to get opposing uni angles so I ended up with it in 'broken back' configuration where the uni angles are identical but in the same direction - velocity is still matched but obviously there is a higher load still and this setup is not ideal for high rpm. I minimised the angles to 0.5 degrees either end to help but it still vibrates around 130km/h and such small angles aren't really good for the unis either.
Now I recently had a thought, why not run a two piece driveshaft to gain some flexibility? The Commodore setup is an ideal candidate with the centre constant velocity joint being fixed in height allowing each end to be altered individually. Couldn't find much info on setting it up as it seems to be quite an uncommon driveshaft config but seems like a good option to me. Crunched some numbers and decided it could work with the parts I happen to have in the shed.
My Torana driveshaft is almost the same length as a VK-VL one, but the heights and angles favoured a longer rear section. The rear section is standard/unmodified VN (approx 100mm longer than VK-VL) and the front section is VK 1/2 (injected six with Borg Warner diff) shortened around 90mm. This was handy as its yoke matches the slip yoke I have for the Supra (1" cup Holden V8 so must be a conversion kit item), and the VN 1 1/16" uni was suitable for the rear.
Shortened the front half, checked the bearing and assembled the CV. Note that I phased the front and rear unis exactly - Holden deliberately mismatched these to several different angles for different models which I understand was in an effort to cancel various other vibrations as well. In this installation the two sections of the shaft run straight in the horizontal plane and I don't have any other issues to deal with so I decided to keep it simple.
With slip yoke and flange:
All bolted up with a little over 1 degree at the front uni, and a little under 2 degrees (to allow for movement on acceleration) in the opposite direction at the rear uni:
I can adjust the front angle between approx 0 and 2 degrees by shimming the centre mount, and the rear between approx 0 and 3(+) degrees in the opposite direction by adjusting the arms. This is ample adjustment for good uni operation and I don't expect to need much difference front and rear because there isn't much movement in the car! The diff is now back closer to where it was originally and the spring angles are good as is wheel placement in the guards. The driveshaft fits fine in the tunnel too, and theres a good few degrees at the CV which should hopefully keep it in good shape.
I made a bracket for the centre bearing mount welded to the bottom half of the driveshaft loop (which unbolts) as its only just behind the loop. The driveshaft loop bolts through the floor to plates on the other side so its plenty strong but unfortunately it does seem to twist the floor pan a little if I push on it so I'm going to have to remove it and run an extra brace to the tunnel - the movement is only very small but I don't want any whatsoever.
So everything seems to have come out right by the numbers but I haven't test driven it just yet because I can't be bothered pulling it all out again at the moment! Will report back when I do but very hopeful at this stage, shorter shafts with smaller diameter has to help at the very least. If its a complete failure then I just have to swap the yokes back on to the one piece shaft and reinstall it (and reset the diff angle).
