IRS Axle with 3.07 Limited Slip Diff
The IRS conversion was completed using a 1974 XJ6 4.2litre axle with the wishbones and drive shafts shortened 5 inches (2.5" per side). I probably should have tried 3" aside as the widest tire I can fit is 235mm, but this may involve trimming back the alloy hubs to clear the chassis rails. The Jag rear end is about 10 inches wider than the MG, so I widened the body another 5 inches using a Sebring flared arches kit.
Drive shafts shortened 2.5 inches
I retained the sub frame by having it shortened and fabricated replica Jaguar chassis rails, so that standard rubber mounts could be used.
The conversion went well, but if I was to do it again I would dump the sub frame and mount the diff direct on to a fabricated mount. The main problem with keeping the shortened sub frame is that there is no room for the exhaust to pass through, meaning you have to go under, which leaves less ground clearance.
Jaguar chassis rails copied (Not easy to do, my advice is solid mount it)
I have kept the Jaguar 4 coil-over shock setup, replaced with Gaz gold 13 x 9 inch adjustable units with 200lb springs. The standard Jaguar has 7/16 top and 5/8 inch bottom mounts, while the replacement Gaz units have 1/2 inch eyes top and bottom.
These are 13" x 9" units and due to the shortening of the wishbones required the bottom mount to be moved to get the correct travel and bump stop position.
Gaz Coil-Overs
Car was fitted with Avo coilover shocks all round, but while i was changing the ride height upgraded to Gaz shock absorbers
To use the new shock units the sub frame top shock mounts are reamed out from 7/16 to 1/2 inch. The bottom shock spindle mounts relocated and turned down from 5/8 to 1/2 inch.
Bottom spindle is standard Jaguar with modified turned down spindle on top
The original setup used the front leaf-spring hangers as the forward mount. The geometry in this position was not right , so I have now made plates from 4mm steel to drop the forward mount position to give a better geometry.
Mockup made from MDF then copied in steel. Will be welded in place and use the original mount and a spacer for support
The old setup produced an axle movement of around 12mm during travel. Whereas the modified setup produces 5mm of fore/art movement in each direction as it goes over center. This should be easily taken up by the radius arm bushes and rubber mounted axle cage.
The most difficult area to rebuild are the outer hubs, which contains pre-loaded taper rollers on the outer spindle and end-floated taper wheel bearings in the hub, both of which are both adjusted with shims. If you don't have the tools or the patience opt for exchange items!
Alloy hubs stripped for cleaning and new bearings. The left picture shows one method of finding out the required number of shims to remove to achieve the correct pre-load on the outer spindle.
With the disks being inboard and the two 6 volt battery holders removed, rear disk pad changes and maintenance can be made through the access panel under the rear seat. Standard MG handbrake cable has been modified to fit.
Inboard brakes are a pig to change pads on standard Jag, but access on MG will be much better
All the washers and bolts have been zinc plated, while I was at it I had all the bolts and small items for the rest of the car done at the same time. A box of 30 items and several hundred bolts and washers cost £20 to have plated.
Zinc plating is worthwhile protection for any smaller metal items, looks good to
The diff tags give the ratio as a fraction, in this case 43 over 14 = 3.07, and the extra tag identifies it has power-lock differential fitted (PL).
Close up shows the ratio and power-lock tags normally located at the bottom of the inspection cover.
Left is a standard diff, with limited slip item on the right
Tie-bar using left and right thread adjuster
The diff plate has been copied in 5mm alloy plate
I have weighed the whole set-up for a comparison with a standard B rear end, and the weights are as follows.
Differential 108 lbs
Cage and mounts 26 lbs
Calipers and Disks 35 lbs
Lower wishbones 51 lbs
Drive shaft & alloy hub 49 lbs
4 x Gaz shocks 15 lbs
Front tie-bars 7 lbs
Spindles, bolts & washers 9 lbs
Total 300lbs
For comparison here are the weights for a standard back end obtained from members of an MG BBS
Tube
axle
175 lbs
Shocks &
brackets
20 lbs
Springs &
shackles
30 lbs
Total 225 lbs
This gives weight penalty of 75 lbs over the standard MGB set-up, but this is not the whole story. Ride quality and a reduction in un-sprung weight come in the equation. Well over half of the rear suspension total weight is sprung, as opposed to most the standard B rear end being un-sprung .
To get a comparable suspension set-up from the MG live axle, you would need to add a pan-hard rod, anti-tramp and a limited slip diff. These modifications would probably halve the weight difference between the systems.