Lotus Elan

Front Shock absorbers

PostPost by: alanr » Mon Dec 10, 2018 9:45 pm

rgh0 wrote:
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
Rohan



Ha!....Thanks for that explanation!,,,,,Something else to consider. :D
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PostPost by: alanr » Mon Dec 10, 2018 9:48 pm

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


Yes thanks for that. By coincidence I found that site a few minutes ago.
I will have a play around with it!

Alan.
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PostPost by: rgh0 » Tue Dec 11, 2018 11:27 am

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


Yes thanks for that. By coincidence I found that site a few minutes ago.
I will have a play around with it!

Alan.

Hi Alan

You are part way through a strength of materials 101 and stress analysis 102 engineering course in an unstructured way. Conceptually a coil spring is just a wound up straight torsion rod, once you can see that in your mind the variables become clear, and stress versus deflection, versus coils and length etc etc can be understood at a gut level. That gut level understanding is what design is all about. These days people rely on computers ( or web sites) to do their design but without the gut level understanding they often go down the wrong dead end alley.

cheers
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PostPost by: alanr » Tue Dec 11, 2018 12:38 pm

Yes Rohan my poor old brain is aching with all the permutations.

Different spring supply companies I have discovered use different wire size/coils for the same advertised length spring and spring rate.
I have just received a quote on a 13inch spring, 125lbs/inch rate. When I asked about coils and wire size they tell me it has 16.35 coils, 9.7536mm wire size, which means that it will go solid at 7.5inches..Definitely no good!

I will get there eventually!

Alan.
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PostPost by: nmauduit » Tue Dec 11, 2018 1:08 pm

alanr wrote:Yes Rohan my poor old brain is aching with all the permutations.

Different spring supply companies I have discovered use different wire size/coils for the same advertised length spring and spring rate.
I have just received a quote on a 13inch spring, 125lbs/inch rate. When I asked about coils and wire size they tell me it has 16.35 coils, 9.7536mm wire size, which means that it will go solid at 7.5inches..Definitely no good!

I will get there eventually!

Alan.


are you certain you risk coil binding with this one? 9.7536*16.35/25.4 = 6.3" (I don't know from the top of my head how short is the available spring height with the shock fully compressed), that would mean you need 10" deflection, or 1250 pounds on that wheel... if reaching that amount of load I would think that a bump stop would save you a coil bind that would be of marginal energy anyway, as most of the load would already have been taken up by the deflection encoutered to get there. Then you may also be able to source other springs with a different design (larger diameter and less coils)
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PostPost by: alanr » Tue Dec 11, 2018 5:18 pm

Yes, after thinking about it you are right.... :D

However after discussion with Spax they tell me that ideally I need to be using a 12inch spring with their G792 shock absorbers on a +2.

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PostPost by: nmauduit » Tue Dec 11, 2018 5:21 pm

alanr wrote:Yes, after thinking about it you are right.... :D

However after discussion with Spax they tell me that ideally I need to be using a 12inch spring with their G792 shock absorbers on a +2.

Alan.


I would certainly go by their advice if possible, it is likely to give you more flexibility on body height settings (if spring is on the long side you may not be able to lower your car for the odd track day ;) )
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PostPost by: alanr » Tue Dec 11, 2018 5:27 pm

nmauduit wrote:
alanr wrote:Yes, after thinking about it you are right.... :D

However after discussion with Spax they tell me that ideally I need to be using a 12inch spring with their G792 shock absorbers on a +2.

Alan.


I would certainly go by their advice if possible, it is likely to give you more flexibility on body height settings (if spring is on the long side you may not be able to lower your car for the odd track day ;) )


:D :D :D

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PostPost by: draenog » Wed Dec 12, 2018 3:16 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.

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

Thanks,

Alan.


Sorry to be rather late to the discussion, but I've only just got round to reading the thread...

I have the same Spax adjustable dampers as you from Paul Matty, fitted with standard springs (by Paul Matty). These are currently off the car. I also have some original Armstrong shocks that I got off eBay (from John Clegg on this forum). The picture below shows the comparison between the two...

dampers.jpg and

As you can see, the spring platform is actually higher on the Armstrong shock than the Spax adjustable (on the lowest setting).

The Armstrong shocks match exactly the ones I took off the car to fit the Spax (MOT failure due to leakage). I contemplated getting them rebuilt, but they were so corroded I decided they were a lost cause.
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PostPost by: alanr » Wed Dec 12, 2018 8:09 am

Yes ...I found that out a while ago.That is the problem I am trying to overcome and the determination of appropriate alternative springs :D
Standard springs are unsuitable for the adjustable shocks.

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PostPost by: nmauduit » Wed Dec 12, 2018 10:37 am

alanr wrote:Yes ...I found that out a while ago.That is the problem I am trying to overcome and the determination of appropriate alternative springs :D
Standard springs are unsuitable for the adjustable shocks.

Alan


I guess you got a different model of Spax adjustable than the ones on the above photo, as these yellow Spax would indeed work with stock springs since they can accomodate longer springs then the blue Armstrongs...
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PostPost by: rgh0 » Wed Dec 12, 2018 10:46 am

Design of a custom spring is not hard. Getting it made a little harder.

1. The spring load on a Plus 2 spring at normal ride height can be calculated from the factory manual data.

2. You need to measure for you're new shocks what the distance is at normal ride height from the adjustable bottom platform located at the middle of the adjustable range between the spring platform and the spring top cap to determine what the spring length must be at normal ride height and load.

3. Your need to measure the fully compressed length. This is required to ensure you don't get spring coil bind at full bump and to check maximum spring stress at full bump to ensure you don't over stress the spring

4. You need to measure the fully droop length for the spring to determine what the spring must extend to without loosing contact with its seats. This will not be a problem with a road going Plus 2 front spring, Its an issue designing a front race spring which is much stiffer and does not have the same pre- load travel that a softer road spring has

5. You need to iterate around 3 variables 1. number of coils, 2. free length and 3. wire diameter that gets a spring that meets all the above criteria and that someone can actually make.

The biggest challenge is that most of the options including the original Lotus springs results in excessive stress in full bump which is why the Plus 2 front springs sag significantly overtime. Hot wound springs last better in high stress situations but not many people do hot winding these days. The last person doing it locally for me retired about 20 years ago.


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PostPost by: Elseezed » Wed Dec 12, 2018 11:56 am

Final set up. After a few runs out to settle the springs and re-adjustments. Ride height 5 1/2" to front crossmember, wishbones level, AVO's set to 10. The ride is good and copes well with the many speed bumps I'm surrounded by.
imag0495.jpg and
AVO with standard springs.

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PostPost by: alanr » Wed Dec 12, 2018 2:37 pm

rgh0 wrote:Design of a custom spring is not hard. Getting it made a little harder.

1. The spring load on a Plus 2 spring at normal ride height can be calculated from the factory manual data.

2. You need to measure for you're new shocks what the distance is at normal ride height from the adjustable bottom platform located at the middle of the adjustable range between the spring platform and the spring top cap to determine what the spring length must be at normal ride height and load.

3. Your need to measure the fully compressed length. This is required to ensure you don't get spring coil bind at full bump and to check maximum spring stress at full bump to ensure you don't over stress the spring

4. You need to measure the fully droop length for the spring to determine what the spring must extend to without loosing contact with its seats. This will not be a problem with a road going Plus 2 front spring, Its an issue designing a front race spring which is much stiffer and does not have the same pre- load travel that a softer road spring has



5. You need to iterate around 3 variables 1. number of coils, 2. free length and 3. wire diameter that gets a spring that meets all the above criteria and that someone can actually make.

The biggest challenge is that most of the options including the original Lotus springs results in excessive stress in full bump which is why the Plus 2 front springs sag significantly overtime. Hot wound springs last better in high stress situations but not many people do hot winding these days. The last person doing it locally for me retired about 20 years ago.


cheers
Rohan


Wanting them to fit inside the Armstrong dirt covers I wasn't happy with what Faulkners springs could supply due to the wire thickness they could offer. So I now have a company called the Shock Factory are making me a pair of springs for delivery before Xmas.

1.9 inches inside diameter
12 inches free length
125lbs rate
8.53mm wire diameter
14.625 total coils

This will give approx. 4.74 inches solid length which will be more than adequate.
Hopefully this will solve the problem I have.

Alan.
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PostPost by: nmauduit » Wed Dec 12, 2018 4:51 pm

alanr wrote:
So I now have a company called the Shock Factory are making me a pair of springs for delivery before Xmas.

1.9 inches inside diameter
12 inches free length
125lbs rate
8.53mm wire diameter
14.625 total coils


interesting : would you let me know the cost of having a pair made (in PM if you don't care sharing that publicly) and your satisfaction level when you get them eventually ? thank you !

for my setup I need 2" ID front springs (Koni perches), which are a bit scarce, and had a hard time to source the length/rate I wanted...
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