Mocking the Jag IRS in the Rover P6

This weekend was spent mocking up the Jag parts to the Rover. In the comfort of the house I drew up the mounting of the diff and it’s suspension.

GA of design

How to put this plan into action? Well being an engineer I first want to test everything for position; first I got the diff under the car and raised so the output shafts are in the exact position the Rover’s were, helped by the Laser Level.


When mounting a propshaft with two UJs the input and output flanges should be parallel to stop vibration, remember these are UJs not CVs. On measuring the engine rocker covers I found the engine is mounted to point up 5 degree at the nose. I checked this by measuring the gearbox output flange and it too is at 5 degree from the vertical.

gearbox output angleGearbox output

But I need the radius arms’ forward mounts to be in line with the two LCA inner mounts. Ref the CAD drawing. This is because the LCA is mounted to the underside of the diff using needle bearings, with no give at all. Since the radius arms are to be solid mounted to the LCAs and rod end bearings at there forward ends, there will be no rubber or other deformable links so the three chassis mounting points must be in a line (two LCA and one raduis arm).
Now if I was to mount the rear diff at 5 degrees nose up the forward mounts of the raduis arms would be higher than the stiff bar that runs under the Rover from side to side. Or in other words there would be nothing to them to.
I lowered the nose of the diff until the LCA mounts pointed at this cross bar. Here I used the laser, locked non-level, to point through the mounts. I found this position still gave a 2.5 degree nose up position.

Positioning Radius ArmsPosition Raduis Arm on Cross Link

In these photos you can also see I’ve mounted the propshaft and checked that a single piece shaft will clear. The standard P6 shaft ended at the cross member so the new shaft will be a couple of feet longer and will be made bespoke for the car. Here a view looking forwards towards gearbox. The wood is pointing the shaft higher that the gearbox, so the clearance will be slightly better than is shown here.


So the diff is sitting quite smartly under the P6.


Here you can see how the Jag LCA mounts under the diff. It is held between the pressed plates and the casting, gripping the two spacers.
The rear of the diff will be mounted to the beam running away from the camera, picking up in the pressed plate behind the LCA mount and picking up the four bolts on the top of the diff. The front will be held on by a frame, for now the wood, to a cross member in the P6 chassis which runs between the standard spring damper mounts.

Here is a photo of the whole setup mocked up, the P6 bump stop will be removed.

GA of rear susp

It clear to see how the LCA is mounted under the diff in two places. The raduis arm, here the rusty tube, runs to the cross beam under the propshaft. These are the three point that need to be in line.
The diff is not held very stiffly in the fore-aft direction, as all the fore-aft loads are taken by the radius arms, leaving only a torque excerted by the LCA on the diff. The drive shaft to the wheel can only pass a torque back to the diff, so the diff will never see a fore-aft  force. The subframe behind the diff together with a couple of mounts infront of the diff will take all the torque transmitted and reacted by the suspension.

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One Response to Mocking the Jag IRS in the Rover P6

  1. Philip says:

    Many years ago I built a rally P6b, and used the Jag diff with the de-dion rear end. I cut and shut the driveshafts to suit. Worked well.

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