Changing Upper Diff Mounts

[AHM]

The Mounts look fine but the diff is siting on the chassis.

When you have the car jacked-up the suspension / drive shafts pull the diff down onto the chassis. So check the clearance with the wheels on the ground.

R2 is describing that the diff must not touch the chassis cross-member above it. So removing washers as described would appear to be contradictory.
R7 is describing that the cross brace, if present, replaces one washer either side. So 2 washers either side or one washer either side with a cross brace.

The picture on page 4 shows the 2 washers under discussion.

You would hear a terrible noise if the diff touched the chassis at any point. You only need clearance it doesn't really matter how much.

[AHM]

The top mounts don't see much of the wheel torque load as the diff pivots on them so not really affected by solid drive shafts installation. They do resist the prop shaft torque load but this is much smaller.

Surely the top mounts see approximately the same wheel torque load as the bottom ones?

[rgh0]

I guess my comments were a broad qualitative statement not a detailed engineering analysis

The diff sees the torque reaction loads from the output drive shafts and input prop shaft. The majority of this load is carried to the chassis by the bottom torque rods. The tops mounts see some load to balance out the rest of the torques and forces but the top mounts are large and handle this relatively small resultant load easily.
cheers
Rohan

[AHM]

I can assure you that there wasn't much detailed analysis. You had me believing it was a statement of fact! I was thinking how the heck does he know that?

[Quart Meg Miles]

The majority of this load is carried to the chassis by the bottom torque rods.
The tops mounts see some load to balance out the rest of the torques and forces but the top mounts are large and handle this relatively small resultant load easily.

The top mounts have to oppose all the wheel reaction being resisted by the torque rods and as they are the same distance from the output shafts the forces are the same. They are also carrying all the prop reaction although that is considerably smaller.

Forty years ago I had both upper bolts shear off on a gently start from cold and the diff rocked a considerable amount during the few journeys I made like that. The car had done over 100,000 miles but I'm still using the replacement bolts after a lot more than that. Now I'm worrying!

[rgh0]

Your right of course. The top mounts have to provide and equal and opposite horizontal reaction to the bottom torque rods to provide the torque that balances the diff output torque and still hold the diff in one place not going back or forward. I guess I did not express myself clearly

The top mounts act in shear for the diff output shaft torque reaction while the diff torque rods act in tension / compression. The shear loads on the mounts and bolts cause little stress in the top mounts and bolts compared to the loads in tension or compression on the torque rod below the diff.

The tension / compression load on the top mounts and bolts comes from the reaction to the prop shaft input torque which is only a quarter of the output torque due to the diff ratio.

cheers
Rohan

[au-yt]

Being an Elan Sprint and only needing one large washer what clearance under the diff should I see after I change the top mounts?, yes the chassis is straight under the diff.

Graeme

[au-yt]

I can answer my own question, .030" measured with the thickness of a steal rule and a verina caliper.
Doesn't seem like much however the biggest difference was made with the lower reaction links which are now nolathane.

Cheers
Graeme

[rgh0]

If you have the angle bracket like the Sprint should have, you can use no washers in addition between the diff and the rubber mount.

Clearance is not much like you observe - around 1mm +/-

cheers
Rohan

[au-yt]

Thanks Rohan
I do have a upper support and there is only one washer.
Interesting the instructions for the Mick Miller CV conversion make the statement to have the diff as low as possible I am sure I have achieved that!!

Cheers
Graeme