Lotus Elan

Differential Silent bloc and washer question

PostPost by: huggy » Wed Nov 07, 2018 9:39 pm

Following a strangely broke differential bolt I ordered new bolts and big washers.

Image
Image

It could have been the end of the story....but not with an Elan :mrgreen:

The metal part Inside the silent block look too long ...or the hole Inside the big washer is too small
So big washer touch metal part and do not compress silent block properly (This is perhaps why bolt broke?)

Image
Image
Image


Question are:
-Do I need to cut metal part in silent bloc?
- Or do I need a bigger hole on washer
- Or all of this is perfectly normal but it looks very strange to me :roll:

Thanks for your help :D
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PostPost by: rgh0 » Thu Nov 08, 2018 9:06 am

There are variations in the Silentbloc inner tube dimensions that I have observed. It was a standard industrial component and Lotus may have modified it to suit their installation and subsequent suppliers did not realise this hence the variation or maybe there have been changes to the tube by various manufacturers who have made them.

What ever the case the tube needs to be short enough to adequately compress the washer onto the bottom of the rubber while also locking up the washer onto the inner steel tube as the rubber compresses which prevents the bolt breaking from fatigue due to it flexing

cheers
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PostPost by: nmauduit » Thu Nov 08, 2018 11:18 am

huggy wrote:The metal part Inside the silent block look too long ...or the hole Inside the big washer is too small
So big washer touch metal part and do not compress silent block properly (This is perhaps why bolt broke?)

Question are:
-Do I need to cut metal part in silent bloc?
- Or do I need a bigger hole on washer
- Or all of this is perfectly normal but it looks very strange to me :roll:



I'd first enlarge the hole in the washer so that the tube slides through it, then if pressed against the rubber the tube protrudes file it down so that the rubber can be compressed by the washer, so as to obtain the functionnality described above by Rohan .
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PostPost by: miked » Thu Nov 08, 2018 12:35 pm

Guys, if my memory serves me, don't these always sag and created this gap. Always seem quite soft. I am learning something here as i didnt know about trying to compress. I do recall the the crush tube being very thin walled and having to have proper washers (with correct size ID) to butt the crush tube without damaging it or pushing out of shape. As to opening out the washer, are you saying to tightend until the crush tube lands ontop of the diff mounting ear. As i recall a penny washer sits on the ear bedded into the alloy to protect the alloy. How much compression.
I am also interested in the outcome as will a sag occur after some spirited driving. Hence top side washers in the boot.
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PostPost by: rgh0 » Fri Nov 09, 2018 7:54 am

The tube length short be sort enough that when it is clamped top and bottom between the washers the rubber is compressed by the top and bottom washers by a couple of mm. Thus the diff is firmly held in location by the rubber but isolated by the rubber from the chassis. The rubber should not sag overtime as the outer shell of the Silentbloc is tapered so the rubber is compressed with down loads. The flange on the bottom of the Silentbloc with rubber below it prevents movement on up loads.

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PostPost by: alan.barker » Fri Nov 09, 2018 8:17 am

I know it's not the Mounting but have you fitted a Diff Brace. For the Mounting there are some heavy duty Mounts available eg. Tony Thompson :wink:
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PostPost by: alanr » Fri Nov 09, 2018 11:37 am

I don't think the mount in the photo is a new mount is it?
Is it not just a case that the mount in question is old and the rubber part has badly squashed down and parted from the central tube over time?
New mount needed?
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PostPost by: huggy » Mon Nov 12, 2018 11:51 am

Thanks for all answers.

Mounts are no new but not squashed. Metal insert is just too long from new.

I will go for larger ID washers and try to find the right adjustment: Not a lot of choice if I do not want upper part of diff casing to touch spyder chassis or "seating" on inferior part of chassis
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PostPost by: Craven » Mon Nov 12, 2018 12:18 pm

Certainly news to me, I understood the mount acts as any other Metalastic fitting, the centre sleeve is to isolate vibration from the mounting by rubber, Lotocone, suspension bush etc, the large penny washer limit the tip or travel the centre can move.
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PostPost by: Bigbaldybloke » Sat Nov 17, 2018 12:14 am

Can’t rember the details now but I remember that I had to machine a couple of millimetres off the tube when fitting the diff brace to get the diff in the right position. The mounting appears to settle a bit once the diff weight is on it and the big washer at the bottom looks like it’s not had up against the rubber of the bush. There is a separate thread on this that I have found in the past but couldn’t find it now with a quick search.
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