Lotus Elan

Front Shock absorbers

PostPost by: cobraboy » Sun Dec 09, 2018 7:28 pm

Couple of alloy spacers 1/2" tall dropped on the shocks first will take care of the shorter spring length.
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PostPost by: alanr » Sun Dec 09, 2018 8:16 pm

cobraboy wrote:Couple of alloy spacers 1/2" tall dropped on the shocks first will take care of the shorter spring length.

Hmmm.....
Surely that would compress the springs even more and make the ride height even higher :?
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PostPost by: cobraboy » Sun Dec 09, 2018 9:50 pm

Sorry
Thought you said the springs were shorter and the shocks were longer.
I guess you will figure it out.
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PostPost by: nmauduit » Mon Dec 10, 2018 8:57 am

alanr wrote:To continue this...
I have now fitted to my car Spax adjustables with 14.59inch,110lb springs which, according to Paul Matty, are the standard springs for my car. However like others have mentioned in this thread I have found that even with the shock absorber platforms at their lowest the wishbones are not horizontal and the car sits about an 1.25inches too high. :(
In hindsight I am thinking that I should have gone with my original gut instinct which was to fit 13inch 125lb springs which I can get direct from a spring company. My calculations sort of say in theory it should do the trick and lower the car by about 1inch whilst maintaining more or less the correct spring preload.

I will have to do something, I can't leave it like it is, it will handle horribly. Any ideas anyone?

Alan.


This is likely clear to all but just in case : regarding the shock, for ride height what matters when departing from a stock setup is the distance at which the spring rests above the the shock axis. If the adaptable shock has an adjustable plateform, it is likely to make the spring rest heigher than the stock spring even at the lowest setting, so a stock spring in this setup will raise the body, because the spring is raised from the bottom.

The total shock length would not play a role in body height, it just need to be large enough to accomodate the full suspension range of operation.

If one go for stronger spring than stock, the free length has to be decreased accordingly or the car will mechanically sit higher. If one wants a precise ride height (better than half an inch) with non stock parts it seems challenging to me to get it right the first time (need to calculate precisely the corner weights and account for the suspension leverage, pick springs of a rather precise free length and desired rate ...), which is why when departing from a stock setup most people go for adjustable plateforms.

ps: most springs will settle from new after some use (typically up to 5%, maybe 10% for lower quality ones), I would drive the car a few hundred miles before readjusting the height (or corner weights) ...
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PostPost by: alanr » Mon Dec 10, 2018 11:06 am

Thank you for input. It has put another light on the situation.
Re - Spring raising from the bottom.
If I understand you correctly you are saying that even if all other lengths of the shock absorber are the same if I refit the original springs the ride height of the car will be higher than before because the bottom platform of the adjustable shock absorber is higher than the original Armstrong unit therefore changing the operational axis of the whole shock/spring assembly.

So it seems that refitting the original springs is also not the answer! :(

Thanks,

Alan.
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PostPost by: nmauduit » Mon Dec 10, 2018 11:25 am

alanr wrote:Thank you for input. It has put another light on the situation.
Re - Spring raising from the bottom.
If I understand you correctly you are saying that even if all other lengths of the shock absorber are the same if I refit the original springs the ride height of the car will be higher than before because the bottom platform of the adjustable shock absorber is higher than the original Armstrong unit therefore changing the operational axis of the whole shock/spring assembly.

Thanks,

Alan.


yes (to simpify you can see your car like a table with 4 wheels at the bottom of each leg, legs being actually springs : if you raise the bottom of one leg the corresponding corner will go up, or if a spring will press up more as the spring will be compressed from its base by the amount raised - then for a car there is the suspension leverage to account for...)

the first thing I would do would be to assess the bottom plateform height difference between stock shock and the adjustable one, then compound that by the lever of the suspension and compare that to the extra body height encountered. If all the extra height comes from that difference, I would drive the car a few hundred miles to see where it lands body height wise. If it's only a portion of it, it could be that the spring manufacturer has integrated a margin for the initial sag when put to use, and after sagging the extra height coming from bottom plafetorm level difference will remain. You can then devise if it is acceptable or not.

But taking stronger springs to solve a height problem is not the way I would go : stronger springs will make the car stiffer. If one wants *only* to alter the height then it's same rate but shorter springs that are needed.

There are also tricks but maybe not on a new setup (heating the spring to collaspe a portion of a coil etc..)
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PostPost by: alanr » Mon Dec 10, 2018 11:45 am

Yes I realise that increasing the rate will make the car stiffer which is not what to do. I would like to get it as close to original ride and stiffness as possible.
It seems that the original spec of my car was 14.19inch length at 110lbs, 15.6 coils (measured by me) and not 14.59 17.5 coils supplied by Paul Matty and shown in the Lotus workshop manual that it should have fitted.

The problem is I can't get 110lbs in a shorter spring. The nearest I can find from spring suppliers is 12 or 13inch at either 100 or 125lbs but with 17.5 coils. I don't quite know how the number of coils will affect things.

Paul Matty suggestion is change it to a 12inch spring 140lbs which I think will be too stiff.

Alan.
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PostPost by: alanr » Mon Dec 10, 2018 12:21 pm

After measuring the operational length ( wishbone eye hole to top chassis mount plate)it is the same as the original shock absorber as is the distance between the bottom and top plates on both units. The bottom platform though due to the adjuster mechanism is 1inch higher than the original unit so this is what needs to be compensated for ( axis change) it seems in any new spring.

The learning curve continues....

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PostPost by: nmauduit » Mon Dec 10, 2018 1:09 pm

alanr wrote:After measuring the operational length ( wishbone eye hole to top chassis mount plate)it is the same as the original shock absorber as is the distance between the bottom and top plates on both units. The bottom platform though due to the adjuster mechanism is 1inch higher than the original unit so this is what needs to be compensated for ( axis change) it seems in any new spring.

The learning curve continues....

Alan


yes, if it's 1inch and you want the same setup as stock, you just need a spring of the same rate and 1 inch free length shorter (could be luck or rather by design, having exactly 1 inch difference may help you source the spring). If need be, it is not that expensive to get springs to your specs.

from
lotus-suspension-f42/front-spring-length-t33446.html
front suspension leverage is 9/7/cosd(17), so 1inch extra height would mechanically raise the car 1.35"... so a minimal .15" margin far sag : if Faulkner springs the car may go down 1/2" 3/4" tops after some use, probably not 1.5"
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PostPost by: alanr » Mon Dec 10, 2018 1:37 pm

What are your your thoughts on the number of coils for the same length/rate spring, 15.6 original versus 17.5 for a given spring length/rate that everyone now seems to want to sell me. Assuming that it doesn't produce a binding issue I can't see it making a difference but am I correct?

Thanks for the input..appreciated. :D

Alan.
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PostPost by: JonB » Mon Dec 10, 2018 6:27 pm

Alan, I've been reading this thread with interest.

Not sure it's been mentioned before, but you can get your Armstrong shock absorbers reconditioned.

http://www.vintageandclassicshockabsorb ... 4579579683

I do not know how effective this would be, but someone else could have experience. It's not too expensive.. worth a punt?

Cheers
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PostPost by: nmauduit » Mon Dec 10, 2018 7:30 pm

alanr wrote:What are your your thoughts on the number of coils for the same length/rate spring, 15.6 original versus 17.5 for a given spring length/rate that everyone now seems to want to sell me. Assuming that it doesn't produce a binding issue I can't see it making a difference but am I correct?

Thanks for the input..appreciated. :D

Alan.


no coil number does not play a role as long as they do not bind. The manufacturers has a certain choice of wire diameter and strength to make their springs, and they also have to play on coil number to make the various rates they offer since the wire diameter does not come in continuously varying values.

The manufacturer makes his calculation and provides a spring that has the only 2 parameters that count : free length and rate, no matter the number of coils (that is for linear coil springs - i.e. constant rate - like on elans, for variable rate springs it is more complicated).
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PostPost by: alanr » Mon Dec 10, 2018 7:49 pm

Hi Jon,
Thanks for that very interesting information.
I truly wasn't aware of the possibility of reconditioning the old shock absorbers and would have almost certainly gone down that route had I known it was possible!
Unfortunately I now have purchased the Spax adjustables with new springs so financially I think it is now too late to go down that route.
I have what I now have and so I think I now have to solve the spring rate/length issue utilising the Spax adjustables.

The axis change situation brought to my attention earlier today has also thrown a spanner in my thinking, even more so because I have discovered that the angle of inclination is so much greater on the +2 and is 30degrees against 17degrees on the baby Elan.
I put in a call to Spax Technical dept today and they are coming back to me tomorrow with what they recommend springwise.

Alan.
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PostPost by: rgh0 » Mon Dec 10, 2018 8:08 pm

nmauduit wrote:
alanr wrote:What are your your thoughts on the number of coils for the same length/rate spring, 15.6 original versus 17.5 for a given spring length/rate that everyone now seems to want to sell me. Assuming that it doesn't produce a binding issue I can't see it making a difference but am I correct?

Thanks for the input..appreciated. :D

Alan.


no coil number does not play a role as long as they do not bind. The manufacturers has a certain choice of wire diameter and strength to make their springs, and they also have to play on coil number to make the various rates they offer since the wire diameter does not come in continuously varying values.

The manufacturer makes his calculation and provides a spring that has the only 2 parameters that count : free length and rate, no matter the number of coils (that is for linear coil springs - i.e. constant rate - like on elans, for variable rate springs it is more complicated).



To get the same spring rate with fewer coils you need a smaller diameter wire. This results in the spring steel being more highly stressed when deflecting the spring the same amount. Plus 2 front springs in their original design were close to the limits hence they tend to sag overtime. More highly stressed ones will sag even faster. :(

cheers
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PostPost by: Craven » Mon Dec 10, 2018 8:08 pm

Have a play with something like this.
https://www.acxesspring.com/compression ... tions.html
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