Lotus Elan

Rear shock length for cv joint driveshafts

PostPost by: Greg Foster » Mon Mar 03, 2014 8:20 pm

I have removed the SPAX rear shocks from my elan and am installing the AVO shortened length shock for use with CV drive shafts (shafts purchased from Kelsport a few years ago). I have had these new shocks for about 8 months and am just getting around to installing them. As you can see, there is quite a length difference... appx 3-1/4" !? Are these the right ones? Seems to be shorter than needed, but what do I know.
Along with that, I have 12" x 2.25" springs and adjustable spring perches. Shall I cut them to 10" or less so they work with the new shocks?
The adjustable spring perches will sit upon the cut off stubs of the original perch, but protrude above the shock housing about 1.25" I am thinking they need to be cut off also, since I don't need that much adjustment and feel they may impinge upon the bump stops.
Consensus?
Greg
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PostPost by: UAB807F » Mon Mar 03, 2014 10:02 pm

If it's the Kelsport kit that I have, then personally I wouldn't cut down the springs because if you do, the effective rate will rise and upset the front/rear balance. Granted it's probably not going to be a great deal, but it will change things.

I took off the old perches with a combination of grinder & torch as they are only brazed on. I then set the new collar so it was just below the top of the strut and welded the collar in place. When I assembled everything I found that by having the perches at the lowest setting I could just drop the springs on, pull out the damper and assemble it all without having to compress the springs at all - like in the second photo.

To get the ride height I just tightened the two perch collars until the ride looked about right, a trip round the block to settle it all and then back for proper setting up.

Brian

Strut before welding.jpg and


Installed01.jpg and
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PostPost by: peterexpart » Mon Mar 03, 2014 10:05 pm

Brian
I was told by Sue Miller that the Rear Shocks needed restricting by 2" when fitting her CV Drive Shafts, I would have thought 3.25" would leave you with no rear suspension !!
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PostPost by: rgh0 » Mon Mar 03, 2014 10:13 pm

Yes that amount of shorter travel is what i would expect. I just replaced a set of Spax in my Elan with Konis from TTR and they were similarly shorter (approx 60 to 70 mm). i was able to take off the droop limiting cables i had previously fitted. You will not have been using the full travel of the Spax as the donuts originally limited the droop to about the same amount as i get with the Konis now with CV's

If you cut the springs you will lower the car and increase the spring rate. There should be no need to change them if they work now the way you want..You will need to compress them more to fit on the rear strut with its more limited fully open travel

You dont want anything coming above the top of the shock tube. I would take of the remains of the old spring support ( it was brazed in place on my S4) and slide the adjustable mount tube lower before brazing or welding it in place.

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PostPost by: toomspj » Mon Mar 03, 2014 11:18 pm

I will echo Rohan's comments. About 3" less travel is right (you can get away with 2" less but then there's a risk of the joints binding). Top of the adjuster sleeve thread seems to work well being just below the top of the lower tube.

I use 12" springs on my road going Sprint. They are 100 lb spring rate and at the normal ride height they are slightly compressed (preloaded) with the damper fully stroked out. You can just fit them without a spring compressor if you back the spring platforms right off. The ride is a bit firmer than on Colin's original incarnation but quite ok provided you don't run the dampers too high.

For the race car I use 200 lb springs and in that case I use 8" springs, but it is not nice to drive on the road!

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PostPost by: Greg Foster » Tue Mar 04, 2014 1:23 am

P1040052.JPG and
OK,
I have included 1 more picture showing the location of the adjustable mount.
I could cut off more of the original spring perch and drop the tube down but about 1.5" extend above the upright tube at this time.
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PostPost by: Greg Foster » Tue Mar 04, 2014 1:48 am

This picture shows the 12" spring, along with the adj. perch placed upon the original perch location. The AVO shock is fully extended and shows the amount the spring would have to be compressed to make it fit .
I had the spring and spring perch attached with the SPAX shock......that was with the cv joints from Kelsport, the ones I ruined because of the standard shock which I used naively. Live and learn. But now I don't want to mess things up and am asking these questions.
Originally, I was able to install the spring with the perch nuts lowered completely when I had the SPAX shock attached. The assembly was about 1" above the bottom position to get the ride height correct. Therefore I really didn't need all that adjustable tube. This extra length allowed the springs to rub against it and squeak.
Would I be best to pick up some shorter springs?
Greg
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Spring perch, spring and top of AVO shock fully extended
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PostPost by: UAB807F » Tue Mar 04, 2014 6:14 am

peterexpart wrote:Brian
I was told by Sue Miller that the Rear Shocks needed restricting by 2" when fitting her CV Drive Shafts, I would have thought 3.25" would leave you with no rear suspension !!


Ah, but the restriction is on droop, not compression, and there's a lot of movement on those damper shafts. As Rohan said, I'd have been surprised if the full range of the OEM damper shaft was used and equally it wouldn't surprise me to find that the doughnuts ended up being a droop limiter.

Once you've assembled everything the dampers are going to move to some mid-point at static ride height as the car weight goes on the spring and the damper works over a relatively small compression range to the bump stops even on the OEM set-up.

I didn't measure the allowable compression movement before and after, but I'd guess that if I had the same spring rates then the movement would be identical to having the old spring perches in place. Static ride height at the rear is exactly the same for sure because that was my start/end point check.

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PostPost by: rgh0 » Tue Mar 04, 2014 6:58 am

The Elan had 3 inches of bump travel and 3.2 inches of droop travel in the rear suspension strut at standard ride height ( its shown in the workshop manual section D fig 1) I am not sure if the data is the hard limit on the actual original shocker travel or approximate numbers as limited by the Aeon spring in bump and the donuts in droop

The new shocks will not affect bump travel as limited by the Aeon spring or equivalent ( I assume you still have it). I think you will find droop travel will be similar also with the shocker limiting it to what the donut did previously.

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PostPost by: UAB807F » Tue Mar 04, 2014 7:17 am

Ahh, I didn't realise there was 3" each way, I've missed that in the manual. I'm intrigued now, I'll have to see if I can measure how much of the damper shaft is showing at static height. I must admit I didn't give it much thought at the time.

I remember reading about the Aeon rubber stoppers working in tandem with the metal springs as a sort of rising-rate effect, but as mine were knackered I replaced them with the Kelsport ones after talking with Pat Thomas, so I suppose there's some difference. They do work well though, I've stopped grounding the exhaust on our lanes since I did the swop :)

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PostPost by: peterexpart » Tue Mar 04, 2014 9:49 am

Ah, but the restriction is on droop, not compression, and there's a lot of movement on those damper shafts. As Rohan said, I'd have been surprised if the full range of the OEM damper shaft was used and equally it wouldn't surprise me to find that the doughnuts ended up being a droop limiter.

Once you've assembled everything the dampers are going to move to some mid-point at static ride height as the car weight goes on the spring and the damper works over a relatively small compression range to the bump stops even on the OEM set-up.

Brian
Yes, I had missed that it would be droop and not compression, but on rebound would that much restriction not mean that the shocks were mechanically holding the car at a lower static height putting all of the strain on the top shock mountings ?
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PostPost by: rgh0 » Tue Mar 04, 2014 10:03 am

Total travel on AVO shock looks like around 5 to 5.5 inches from the first photo versus around 9 inches for the Spax.. Required standard travel required per the manual is 6.2 inches so it appears a little restriction on droop will result.

There should be around 2 to 2.5 inches of droop travel from standard ride height at which point the shock limits further travel versus the standard 3.2 inches.

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PostPost by: rgh0 » Tue Mar 04, 2014 12:00 pm

As a point of comparison I measured up the original Armstrong shocks that came out of my Elan - travel was 205mm ( 8 inches) with the shoulder on the rod for the top spring cap 10mm above the shock top at full compression and 215 mm above it on full extension.

Does anyone have a set of the TTR Koni' to measure the same dimensions on. I know it was around 60 to 70 mm less than the Spax but I did not measure it precisely when I installed them.

cheers
Rohan
Last edited by rgh0 on Wed Mar 05, 2014 12:21 am, edited 1 time in total.
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PostPost by: Greg Foster » Tue Mar 04, 2014 11:21 pm

Thanks Gents,
I will take your advice and install the adjustable mount in a lower position and use the original springs as they are.
Greg
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PostPost by: Andy8421 » Fri Mar 07, 2014 11:28 am

I have a pair of non adjustable 'new old stock' koni shock absorbers, and access to a decent screw cutting lathe. I am contemplating a DIY reduction in shaft length as I have obtained a couple of CV driveshafts and I want to avoid full droop lockup.
Would anyone know if the centre shaft in the shock is hardened, and if the flat on the shaft is necessary to locate on the lotocone or if it can be dispensed with?

Thanks
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