Lotus Elan

Adjustable Rear Spring Seat

PostPost by: vincereynard » Tue Feb 12, 2019 10:00 am

The chap making adjustable spring seat conversions on my struts has come up with a novel suggestion.

1. Make a threaded sleeve (aircraft alloy) that fits over the damper tube.
2. Make it long enough so it rests on the alloy hub.
3. The bottom 2" (or so) of the sleeve is unthreaded.
4. Reinforce the lower spring seat with a threaded ring.
5. Drill 6 equidistant holes at the bottom of sleeve.
6. Adjust the spring height by turning the tube with a suitable C spanner.

549-feb.-12-09.32.jpg and


Reasons for this arrrangement are -
When he took the original spring seats off they were not welded "square" to the tube. The threaded seat would be.
No welding of the sleeve to tube necessary.
Spring seat can turn as it is compressed.

I cannot think of an immediate objection, other than the spring load would be directly into the hub.
Would that matter? Any comments??

After that he can work on producing some proper adjustable rear wishbones!
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PostPost by: nmauduit » Tue Feb 12, 2019 11:26 am

when looking into the actual dimensions and clearances you may want to check your target springs ID (need a bit of clearance for the spires to move up and down without rubbing the alloy sleeve thread), esp. if you want the narrow springs (I imagine you meant "spring seat can*'t* turn as it is compressed" ... which becomes obvious as one tries to adjust his setup)

then since the hub is cast it is likely that the sleeve will not seat on a perfectly square face.
Last edited by nmauduit on Tue Feb 12, 2019 1:49 pm, edited 1 time in total.
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PostPost by: vincereynard » Tue Feb 12, 2019 12:48 pm

nmauduit wrote:then since the hub is cast it is likely that the sleeve will not reat on a perfectly square face.


Good point, I'll have a word with him!
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PostPost by: JonB » Tue Feb 12, 2019 1:41 pm

Is the top of the hub casting strong enough to withstand the point load of a spring sitting on top of it? I doubt it.
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PostPost by: vincereynard » Tue Feb 12, 2019 4:32 pm

My main concern is with the spring pushing up through the damper and hence damper tube, and the bottom end pushing down on the alloy hub, is there a possibility of the tube being pulled out of the hub?
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PostPost by: nmauduit » Tue Feb 12, 2019 5:04 pm

vincereynard wrote:My main concern is with the spring pushing up through the damper and hence damper tube, and the bottom end pushing down on the alloy hub, is there a possibility of the tube being pulled out of the hub?

if the spring is quite short (so stiff) and catches on the bottom plateform, when you get airborne there may be a way for the spring to pull the tube up a bit - that would not be my main concern as minor and transient for most drivers (more the point effort on the cast hub, that should rather be spread at least on the total circumference - on the Chapman setup the load is spread between top and bottom press fit areas, which is a lot larger).
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PostPost by: 661 » Tue Feb 12, 2019 6:41 pm

Sorry, I'm missing why this is any better than the presently available adjustable spring platforms? (Ok you need to remove the old tube)

Proper adjustable rear A frames.....now you are talking. Preferably not rose jointed and will accept bushes and also be adjustable in situ without having to remove the supporting bolts each time.
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PostPost by: mbell » Tue Feb 12, 2019 8:04 pm

I would expect the hub to be deformed by the tube over time as it not a very hard metal.

If I under stand the main advantage it that the tube can be spun to adjust the height with out teh spring having to rotate, so easier adjustment.

Personally I'd look at adding a small lip to top and bottom of strut tube to hold the sleeve in place. Possibly a simple circular clamps. Or machine the top of the hub and add a hard metal "washer" between tube and hub.
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PostPost by: RichardHawkins » Tue Feb 12, 2019 8:08 pm

I think Spyder sell adjustable rear A frames (wishbones). I made my own adjustable turnbuckles, and found they had seized after brazing, and had to do the job again, using a mixture of metals (brass and stainless). Would probably have been more sensible to have purchased.

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PostPost by: nmauduit » Tue Feb 12, 2019 10:54 pm

mbell wrote:I would expect the hub to be deformed by the tube over time as it not a very hard metal.

If I under stand the main advantage it that the tube can be spun to adjust the height with out teh spring having to rotate, so easier adjustment.

this I don't get : either there is tension in the spring, and that will cause friction on what holds it, so adjustment will be difficult, or there is no tension in the spring, and adjustment will be relatively easy, tube or no tube... or did I miss something?
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PostPost by: vincereynard » Tue Feb 12, 2019 11:18 pm

mbell wrote:Personally I'd look at adding a small lip to top and bottom of strut tube to hold the sleeve in place. Possibly a simple circular clamps. Or machine the top of the hub and add a hard metal "washer" between tube and hub.


Spot on. I had raised my concerns with the man and he has suggested a 5mm steel collar welded to the bottom of the damper tube to take the sleeve. Solves both problems.

661 wrote:Proper adjustable rear A frames.....now you are talking. Preferably not rose jointed and will accept bushes and also be adjustable in situ without having to remove the supporting bolts each time.


His next job is to mod a spare pair of arms I have along the lines of the attached. It still would need the bolts taken out to change camber! I cannot see a realistic way around it so I may give the back one a miss and just have the front toe in adjusted done.
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PostPost by: bitsobrits » Wed Feb 13, 2019 1:24 am

I have that type of adjustable rear control arm on two Elans.

True, you have to remove the inner pivot bolts to adjust, but how often do you adjust camber or toe? Going to the heims on the inner end pretty much eliminates any toe change from bushing deflection or "set" over time.

Btw-I've run the heim inner and outer bearings for years on the street with no issues.
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PostPost by: rgh0 » Wed Feb 13, 2019 10:19 am

A nice inversion of the normal design and avoids the issue of how to attached then new threaded tube to the shaft. Sure some detail design issues to deal with but nothing that is not solvable.

I love creative design thinking and this sort of thought processes is what drove development of Lotus back in the 60's

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PostPost by: vincereynard » Wed Feb 13, 2019 2:59 pm

rgh0 wrote: Sure some detail design issues to deal with but nothing that is not solvable.
Rohan


Rohan
Any design issues I haven't considered other than the additional of a 5mm steel ring to rest sleeve on?
---------------------------------

We wanted a (nearly) full length sleeve from the start. The idea was that -
As the spring can be wound fully down it will be virtually uncompressed.
So, even with the strut assembled, it should allow the strut to be raised up to a point where the wishbone is horizontal. (Jack under the 'bone with a spreader - light as no spring compression.)
This would allow the camber and toe in to be set at the, theoretically, ideal ride height with the wheel off and everything easy to get at.

Bonus - The spring could (MAYBE) be fitted without separate compressors.

Making the sleeve out of aircraft spec alloy saves unsprung weight and prevents corrosion. Indeed it would protect the damper tube. It may even act as cooling fins :)

Having the adjustment "holes" at the bottom of the sleeve, rather than the conventional nut, is easier to get at. 6 round hole (drilled around the sleeve bottom) would give a more secure hold for the C spanner. Although it will need a special C spanner with a round end.

Strange the engineer was concerned with unsprung weight as he makes half scale traction engines!

Any comment or criticisms welcome, preferably before we start .
Last edited by vincereynard on Wed Feb 13, 2019 9:53 pm, edited 1 time in total.
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PostPost by: Craven » Wed Feb 13, 2019 4:45 pm

Check the roundness and size of each of the original damper tubes, not sure if you plan a close fit.
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