Lotus Elan

Elan Plus 2 diff mountings

PostPost by: NYK » Sun May 27, 2018 11:33 am

The manual states that one or two of the large washers should go between the diff and rubber mount and one large washer above the rubber. The one(s) below I understand, they will spread the load over the entire surface area of the rubber. I don’t see why one needs a large washer above as there is no way (?) that the surface of the washer is going to contact the rubber, or have I missed the point?
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PostPost by: Foxie » Sun May 27, 2018 3:43 pm

One reason might be it would prevent the bolt being pulled through in the event of the rubber failing. I would also imagine under large loading an increasing circle of contact might occur.

As from your pic you appear to have it to hand right now, you could set up a test rig and suspend concrete blocks from the bolt and observe the effect :)

The only problem I've had here after 33 years of Lotusing is with the fixing lugs cracking off. When I had the diff out recently to check it I replaced the last set with a pair of TT heavy duty ones.
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PostPost by: MarkDa » Sun May 27, 2018 5:31 pm

The upper washer will provide a rising rate resistance to the torque load from the diff under hard acceleration.
As you've got all the washers and long enough bolts just instal as specified!
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PostPost by: NYK » Sun May 27, 2018 6:54 pm

MarkDa wrote:The upper washer will provide a rising rate resistance to the torque load from the diff under hard acceleration.
As you've got all the washers and long enough bolts just instal as specified!


Yes, I understand that the bush is designed to oppose the torque from the diff under accelearation and yes I understand that as the rubber deforms there will be a rising rate resistance.......but I still do not understand how this large washer will interact with the rubber when acceleration loads are applied. Perhaps you might enlighten me.
Of course I can just assemble as per manual but I always like to know why I’m doing something.
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PostPost by: Quart Meg Miles » Sun May 27, 2018 9:23 pm

I believe that Foxie's rubber failure protection is the prime reason for a big washer. In 49 years I've only changed the mounts once but did have the bolts shear on a gently start in the early 1970s!

The acceleration torque load will be slight as the pinion is only about 1" radius from memory and the mounts are ten times that from the pinion. The engine torque multiplied by 1st gear ratio, say 3:1, at a radius of 1" will not have much turning force on the ring gear but shock loads from dropping the clutch and adding engine inertia to the torque might have some serious impact. After all, many people are breaking the lugs off their diffs so a lot of force is being applied somehow.

Also the G forces on the mass of the whole diff assembly pass directly to the bushes in shear and will cause the rubber to distort asymmetrically, constrained by that big washer perhaps.
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