Lotus Elan

front spring length

PostPost by: wwpete » Fri Jan 29, 2016 3:31 am

I am setting up a +2 for endurance racing. I am putting 160lb springs at the front but I cannot use adjustable spring perches. My question is , what length spring should I use in standard shocks to get the car to sit at factory ride height?
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PostPost by: TroonSprint » Fri Jan 29, 2016 10:52 am

This is a complex question as so many factors are involved, not just the free length of the spring, but also the wire diameter, the outside diameter of the spring, and the number of operational coils. There is a useful calculator here http://www.reliablespring.co.uk/calculator.htm .

I would put the figures in for the standard spring and see what the Force at Compressed Length comes out as. You need to scroll down the screen a tad to find it. For the two-seater Elan it is around 520 lb at the front - I guess the Plus 2 would be a higher figure. The problem is that tiny inaccuracies in your measurements make a huge difference. For example, the wire diameter is raised to the fourth power so has a huge effect on the result.

Anyway, you would then need to play with the figures you input to try and get the same force figure with a calculated rate of 160lb. There is probably a better way, but I'm no mathematician! As you may have worked out, I have spent ages playing with theoretical spring sizes to try and find the best for my car.

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PostPost by: rgh0 » Fri Jan 29, 2016 11:23 am

If your designing the spring in detail then you can use the equations reference and as observed it is very sensitive to wire diameter.

However if you are just trying to select the free length of a 160 lb/in spring compared to the free length of a 110 Lb/in standard spring it is much easier.

On a Plus 2 according to the manual the standard front spring compresses from 14.19 free length to 8.6 inches at normal ride height. This is a load of 615 lbs. A 160 lb/in spring needs to compress 3.84 inches to achieve the same load. To achieve the same length at normal ride height the 160 lb/in spring needs to be 8.6 plus 3.84 inch to give a free length = 12.44 inches.

Some later cars in some markets used a longer free length of 14.59 inch to give a longer fitted length of 9 inch for a higher ride height.

If you want to lower the car a little then you can use a shorter free length to achieve a shorter fitted length and lower the car. I lower my Elan for competition about 5 to 10 mm on the suspension and the rest (about 30 mm) through lower profile tyres.

If the regulations don't allow adjustable spring perches just glue them up once you have set the ride height. I bet the regulations don't define what non adjustable means in detail :)

cheers
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PostPost by: wwpete » Fri Jan 29, 2016 3:27 pm

Thanks. That is what I was looking for. Pounds per inch is pounds per inch. Just needed a length for the loaded car to work out the conversion.
Bill

Guys are right , this site is a wealth of information.
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PostPost by: rgh0 » Fri Jan 29, 2016 11:08 pm

In selecting the actual 160 lb/in front spring your going to use you need to also check the coil bind length to make sure your Ok at full bump, this is often a problem with a plus 2 front spring. You also need to check full droop and ensure the spring does not become loose on its seats, this is not normally a problem with a Plus 2.

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