Lotus Elan

Spring Size ???

PostPost by: Famous Frank » Fri Mar 06, 2009 8:19 pm

I'm switching to small diameter rear springs in my 66 S2 SE. Before I spend hundreds of dollars on new springs, I have in the basement some springs that have been around for almost 30 years. For performance street use, I've been thinking of using springs in the vicinity of 95 lb to 110 lb. I can buy those but I'm having trouble trying to understand what are the springs I already have. I'm not great in math and I know there are formulas on the web but even when I try to put in the numbers, the results seem off to me.

The springs in question are 12 inches tall, 3 inches outside diameter, 2 1/2 inches inside diameter, with a wire diameter of 1/4 inch. Additionally, there are 12 coils (that are not touching anything) and 14 coils if you count the half coils at either end that are also flattened. Can anyone tel me what weight springs I have? Thank you all very much, Frank.
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PostPost by: elancoupe » Fri Mar 06, 2009 10:01 pm

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PostPost by: Famous Frank » Sun Mar 08, 2009 1:50 am

Thank you for the reply and the spring calulator but that's what I don't understand. When you put the numbers in, the answer doesn't make sense to me.

Wire Diameter = .25 inch, the Spring OD + 3 inches, Number of
Active Coils = 12, It then says the Coil Spring Rate is 22.01117580

That just doesn't make sense to me. Is that the correct number? Is it for every inch of deflection? If the spring wire is .25 inch then the 12 inch tall spring can only be compressed a max of 9 inches...........Is this a 22x9 = 198 lb spring??? I don't think so as I can compress this spring an inch or so just by squeezing it. Help !!!!
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PostPost by: CBUEB1771 » Sun Mar 08, 2009 4:17 am

Frank,
The gtthunder calculator has no input for material properties so you can't take it seriously. Try this one: http://www.engineersedge.com/calculator ... _k_pop.htm
This calculator suggests the rate of your springs is on the order of 60 lb/in. You should look for a race preparation shop in your area as they will probably have a spring rate measurement instrument and they can determine the rate directly. Why do you want to go to the smaller OD rear springs? Are you planning on installing wider tires? Forgive me if am overlooking any recent posts you may have made on this subject. If you want to retain the original wheel and tire dimensions then I would not muck with the springs. The larger diameter springs tend to offset bending forces on the strut inserts and make them live longer.
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PostPost by: Famous Frank » Sun Mar 08, 2009 1:26 pm

Russ, thank you for the reply. I do like a slightly wider wheel and tire combination. I have owned one or more Lotus since 1973. Three Europas, Two Lotus Cortinas, a couple of Elan Coupes, but I really love the Elan Roadsters, especially the S2s. I also love great handling. I ran SCCA Solo II Events along with some Vintage Racing with a Formula Ford. I would stick love to stick with the orginal Knock-Ons if I could get a pair that was strong enough for my style of driving. I've had the original Knock-Ons crack at the locating holes, bend to the point where I can't go down the road without the sterring wheel wobbling, etc. That being the case, I'll probably buy some Panasports which alleviates all those problems but they are 5 inches wide. I'd like to make good use of the extra half inch and use a tire in the 175/60 x 13 range,......maybe even a 185/60 x 13. Also, I prefer to lower the car approximately one inch.

I'm using a 3:77 rear with a limited slip and billet diff stub axles, cv joint axles, and I'm looking for some fairly priced billet outer stub axles. The engine is close to 150 hp with 145 lbft of torque.

The smaller diameter rear springs allow me to 1) lower the car to my liking and 2) squeeze in that size tire. Koni are my shocks of choice.

Well, sorry if I'm getting long winded. It really comes down to I want to install the smaller ID rear springs for the above reasons, my wife has recently lost her job, mine is always in jeapardy, and I don't want to spend money on another set of springs if I can use the ones in the basement I bought 30 years ago. Thank goodnes for these cars, they help me keep my sanity!

Thanks again for your help. That's a good calulator.
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PostPost by: dlbarnes1 » Mon Mar 09, 2009 4:05 am

The results from the engineersedge calculator are consistent with the gtthunder calculator when the appropriate value for modulus of rigidity is used (11500000 psi).
In order to achieve a spring rate on the order of 100 lb/in with an ID of 2.5 in and 12 active coils, the wire size should be about 3/8 in. Standard gage wire sizes in this range are 0.362 and 0.394. Although, 0.375 is probably common.

Frank, for a wire size of 0.250 in those springs must have been intended for something else other than the rear suspension of an Elan.

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PostPost by: Famous Frank » Tue Mar 10, 2009 5:02 pm

thanks everyone again. Maybe I should put some eyeglasses on when I measure. The wire diameter is very close to 3/8 inch which I believe would equal something close to .375. That should change numbers drastically.

Also, prior to re-measuring, I put the spring on a scale, zeroed out the scale and compressed the spring 1/2 inch. This gave me a reading of 50 to 55 lb depending on the shake of my arm. If I double that, it would give me a spring of approximately 100 to 110 lb.
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