Lotus Elan

Elan Spring Rates for Koni Dampers

PostPost by: Mazzini » Tue Oct 30, 2018 10:33 pm

Does anybody have any experience with Koni front dampers for an Elan? The standard spring seems too long. The springs I have came from PMS and seem close to the workshop manual spec -

Number of coils ? 19.6
Wire diameter - .324 inches or 8.23mm
Rate ? 75lb/in
Free length ? 16.08 or 40.84mm
Fitted length ? 9.22 or 23.4mm

These are non adjustable Koni's for an Elan, not a Plus 2.

Any ideas?
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PostPost by: Frogelan » Thu Nov 01, 2018 4:53 am

Rob,

Have you compressed the springs to see if they might then fit ?

I must confess that I spent hours looking through the Forum on the very subject of lengths, coils, diameters and spring rates and often one is given without the others...

For my GTS Elan I have just bought a set of go faster springs (1.9 Fr, 2.25 Rear) from a retailer in Melton Mowbray just to get the car assembled and planned to do some research from that basis.

The retailer in question was very sheepish about supplying the spring rates but I plan to publish them after measurement in due course.

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PostPost by: Mazzini » Thu Nov 01, 2018 8:08 am

Andrew,

Thank you for taking the time to reply.

I have have a similar set up on my `Sprint. The PMS springs don't sit straight on the damper, they rub, but they do seem to work well. I was wondering is there was a formula for how much the spring needs to be compressed by before it's fitted.

Hope all is well with you and that your project is going well.

Cheerio,

Rob
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PostPost by: Frogelan » Thu Nov 01, 2018 8:27 am

Rob,

I have not dug out my "Buckland" but I'm sure there is no formula for this.

A more general question on the 1.9" springs comes to mind: do these front springs need "dust covers" ?

I have seen these advertised by an Australian outfit (Lotus Marques I think) and they seem to have been fitted originally but the emporia in GB do not seem to carry them.

Why is this....they serve the useful function of avoiding the spring buckling laterally under compression ?
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PostPost by: Mazzini » Thu Nov 01, 2018 10:00 am

Thanks for that. I found it and I guess the spring in that case will creak against the tube not the damper :-)
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PostPost by: nmauduit » Thu Nov 01, 2018 10:05 am

Mazzini wrote:Andrew,

Thank you for taking the time to reply.

I have have a similar set up on my `Sprint. The PMS springs don't sit straight on the damper, they rub, but they do seem to work well. I was wondering is there was a formula for how much the spring needs to be compressed by before it's fitted.

Hope all is well with you and that your project is going well.

Cheerio,

Rob


Plain springs are mostly linear in deflection to the first order, that is for a spring ratio k in in/lbf , when you apply a force of f on the spring you get a deflection of f/k inches. E.g. 120 in.lbf you apply 180 lb spring gets compressed by 1.5 inch from resting position, you apply 180 lb more it goes further 1.5 inch to a total of 3 inches.

The suspension geometry has to be taken in account to establish the correspondance between body weight and spring compression, since the suspension harms can act as a lever, but at rest each spring is compressed to balance the load it holds for keeping the car up. So if the lever is say 2 and the front left quarter weights is about 430 lbf, at rest holding its quarter of the car the front left spring will see 430*2 = 860 lbf, so will be compressed by 860/120 = 7.16 inches
http://www.lotuselan.net/wiki/Suspension_101

ps: the linear behavior does not hold when the coils bind, obviously, or even a little before then. And springs tend to sit some from new after use depending on their quality, up to 5% to 10% is possible.
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PostPost by: Mazzini » Thu Nov 01, 2018 11:31 am

nmauduit - thank you for taking the time to explain, it's appreciated.

Looks a bit questionable when sat on the bench. I'll post a photo of the same type of spring and damper fitted to my Sprint.
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PostPost by: nmauduit » Thu Nov 01, 2018 2:01 pm

hard to tell anything just from the photo, but you have to compress the spring some just to get it to fit in the assembly (shock rod fully extended). This results from the fact that the rate is rather small, hence the free length has to be large enough so that when holding the weight of the car the equilibrium point is at the desired location (proper ground clearance and horizontal suspension arms) with a setup that does not have height adjustment.

When using stiffer springs, one can often install them without a compressor, at times even needing "helper" springs to bridge the gap as even free length of the spring is not enough to maintain contact within the assembly.
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PostPost by: Frogelan » Thu Nov 01, 2018 2:59 pm

Rob

Nicolas' replies are very useful. I hope to be able to see my front Koni shocks sometime next week in deepest Suffolk. I'll take a photo with a tape measure so that you can just check that you have the right length as the difference does seem rather surprising.

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PostPost by: Craven » Thu Nov 01, 2018 3:16 pm

I guess I?m missing something here, you state the manual given fitted length as 23.4mm, just extend the shock out to maximum put the top collar in place and measure the distance between the bottom perch and the underside of the top collar this is the spring fitted length. The main variable that causes ride height problems is the distance between the bottom mounting hole and the bottom perch.
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PostPost by: Mazzini » Thu Nov 01, 2018 6:39 pm

Here is one I prepared earlier, it's the same model Koni damper with the same spring. A two inch compression is required to get the spring mounted.
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PostPost by: Mazzini » Thu Nov 01, 2018 6:43 pm

Craven wrote:I guess I?m missing something here, you state the manual given fitted length as 23.4mm, just extend the shock out to maximum put the top collar in place and measure the distance between the bottom perch and the underside of the top collar this is the spring fitted length. The main variable that causes ride height problems is the distance between the bottom mounting hole and the bottom perch.
FWIW


My question is all about the length of the spring, it seems excessive. I wanted to know if anybody has experience of fitting springs on Koni front dampers, they are a bit of a rarity nowadays.
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PostPost by: nmauduit » Thu Nov 01, 2018 7:11 pm

Mazzini wrote:
Craven wrote:I guess I?m missing something here, you state the manual given fitted length as 23.4mm, just extend the shock out to maximum put the top collar in place and measure the distance between the bottom perch and the underside of the top collar this is the spring fitted length. The main variable that causes ride height problems is the distance between the bottom mounting hole and the bottom perch.
FWIW


My question is all about the length of the spring, it seems excessive. I wanted to know if anybody has experience of fitting springs on Koni front dampers, they are a bit of a rarity nowadays.


From the photo your Konis look just like mine (which work well for me - though now fitted with stiffer springs, shorter then), have you checked the numbers ?

I don't recall specifically if/when I last installed stock springs on front Konis, probably a few years ago : I think the proper Konis would be just like other shocks, in term of extended length etc, what seems odd to you is just the free length of stock springs because of the low spring rate. If your springs are close to stock specs, they'll work close to specs on Konis (remember not to set the Konis too stiff with stock springs, you need to open the bottom valve before putting the springs back on - I understand there are different churches on damper settings, but I go with stiffer when spring rates goes up).
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