Lotus Elan

Heavy Clunk when Setting Off

PostPost by: david.g.chapman » Fri Oct 08, 2010 6:41 pm

This has only just developed.

When I an stationary in my +2 and take off for a fairly fast start, there is a heavy clunk from underneath the car - enough for me to feel the jolt through the seat. It does not happen at any other time, including doing the same thing in reverse. The car is otherwise going very well with no transmission noises to speak of.

If you pull away more gently there is no clunk.

I have tried:

Checking the wheels for tightness - OK
Visual inspection of all the suspension brackets on the chassis - no cracking, slack bolts or missing bolts.
Exhaust checked for tightness - OK
Engine and gearbox levered against mountings - no clunking and reasonably stiff.
Torque rods on diff - OK.
Tried levering diff upwards - right hand diff mounting (as viewed from the back) looks to have too much movement in it - 2mm with not much levering - no clunking though.
CV bolts checked - OK

Running out of things to try.

Could it be something internal, or something not on the list above?

When you accererate, does that right hand diff mounting go into compression?

Any help will be much appreciated.

Dave Chapman
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PostPost by: andyman » Fri Oct 08, 2010 8:02 pm

Check the propshaft universal joints.My first check would be the propshaft to diff bolts.With a bit of luck it could be an easy fix.
Cheers Andy.
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PostPost by: Mr.Gale » Fri Oct 08, 2010 9:38 pm

What I would do is put the car in 3rd or 4th gear then, by hand, rock the car back and forth. If it is a u joint or in the tranny or diff you may be able to hear the clunk.

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PostPost by: david.g.chapman » Sat Oct 09, 2010 8:36 pm

Thanks for your suggestions. I tried the 3/4 gear trick but could not get any knocking. This would work if the knock happened at very slow take-offs. Prop-shaft looks OK.

I have just removed the fuel tank to look at the offside (UK) upper diff mount. The metal plate at the top of the mount is broken, so the mount has virtually no damping effect under compression! It just holds the diff up.

I now believe that under acceleration it is this side that gets compressed, and the mounting bolt makes an attempt to punch through the fuel tank - not good.

I have ordered two from Spyder Engineering (SM is out of stock).

Knowing my luck this is not causing the noise, but I will post again is something else ends up being the cause - and I find it!

Dave.
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PostPost by: elansprint71 » Sat Oct 09, 2010 9:17 pm

I fitted new Spyder top diff mounts to my Sprint during a chassis change. Four months later I chucked out these trashed items and got a pair of heavy duty ones from Tony Thompson. Still fine after five years.

All rubber parts seem to be rubbish these days, not just Rotoflexes.
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PostPost by: david.g.chapman » Sun Oct 10, 2010 6:31 pm

Thanks for the info, Peter.

As it was 5 years ago, things may have improved - I quizzed Spyder on the phone about rubber quaility and they assured me they were the best on the market - but you never can tell with that line of questioning.

I will still give them a good examination on receipt, though. I think Rohan had a way of taking some stress out of the rubber - I will search the posts.

(edited) - Found the post. Rohan suggested using a large thick penny washer top and bottom to spread the load across the centre tube and the surface of the rubber - I will look into it.

Dave.
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PostPost by: kstrutt11 » Sun Oct 10, 2010 8:30 pm

I had exactly the same noise and it turned out to be the same failure, rear tab of rh diff mount sheared off, I made a reinforcing plate to go over the top of the new mount, hopefully this will help long term durability, the rubber was perfect though on both sides (they were in there when I bought the car 13 years ago, it did have large washers top and bottom.

Good news is it Fixed the problem.
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PostPost by: david.g.chapman » Sun Oct 10, 2010 9:04 pm

Kstrutt,

Do you have more details on your reinforcing plate? Where does it fit onto the mounts?

Looking at another of Rohan's posts, he describes the top washer as clearing the area just under the bolt head (or nut if the bolts are the other way up) to spread the load - so it is not a simple washer - maybe a washer with a larger centre hole under the existing washer - or something conical.

I really must get the old mounts out and have a proper look - but I have a stinking cold and don't feel like doing anything at the moment :( . Tomorrow is another day.

Dave.
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PostPost by: kstrutt11 » Tue Oct 12, 2010 9:55 am

I didn't take any pictures but basically it was the same shape as the mounting face on the chassis and goes on top of the mount sandwiching it between the two and preventing local areas of stress around the fixing.

Kevin
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PostPost by: david.g.chapman » Tue Oct 12, 2010 10:34 am

Thanks, Kevin. I've got the idea.

I now have the Spyder mounts, and have attached a picture of my old mount - with a snapped ear - on the right, and the "new" Spyder mount on the left.

The two mounts look identical, down to the coding on the rubber and the general finish. Maybe Spyder have got a hold of some genuine Lotus bits.

As the old mount lasted 25000 + miles with my CV joints, I am happy to fit them with either Kevin's brace, or a conical washer that goes over the rubber cone at the top.

Cheers,

Dave.
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PostPost by: tcsoar » Tue Oct 12, 2010 11:47 am

When I stripped my plus2 down a few years ago I found exactly what you have. Both of the mounts had been replaced by the po and looked in very good condition, so good in fact that I intially thought that the po had only just fitted them and must have done something wrong. Fairly confident that was not the case now as there have been a few threads covering this previously.

Chris.
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PostPost by: alan71 » Tue Oct 12, 2010 2:21 pm

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PostPost by: david.g.chapman » Tue Oct 12, 2010 5:58 pm

That's it Alan!

Now where is that 3mm thick plate ali I had?

Dave.
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PostPost by: kstrutt11 » Wed Oct 13, 2010 8:26 pm

Thats pretty much what I did, but I used 2mm steel as I had it lying around.

Kevin
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PostPost by: david.g.chapman » Fri Oct 15, 2010 11:04 am

Everything now re-assembled. I have tried out the car and the clunk is gone. :D

Many thanks to you all.

Dave.
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