Custom springs, how to spec?

[Cliff Beer]

I need some custom springs made for one of my vehicles in my garage (not GT40). The current rear springs are sagging and look very tired.

To have custom springs made what specs do I need to give the spring maker?

At present I know the following:

1. the dimensions of the old springs - overall diameter, wire diameter, unsprung height, etc.
2. the approximate weight over the rear,
3. the loaded spring length in order to establish proper ride height.
4. the approximate amount of travel +/- proper ride height for which the spring should travel in normal operation.

What else do I need to know? Anyone had custom springs made before? Tips?

Thanks

ps. if this is in the wrong section please move or edit/delete as necessary - thank you.

 

[Dr. Bob Woods]

Cliff,

Remember that I inquired about buying your car a few months back.

You can certainly match the springs you have now with the same design. Eibach might be able to help. eibach.com/america

However, what you need to know is the natural frequency of the suspension which will determine the wheel rate (spring rate measured at the wheel). Then you need to know the motion ratio of the suspension to arrive at the spring rate you need for that end of the car. Most spring manufacturers go by spring rate.

That being said, you need to know the natural frequency and wheel rate in the front because the difference in the two determines handling. Front softer than the rear -> oversteer, front stiffer than the rear -> understeer. This also depends on the anti-roll bar settings and the difference in between the front and rear track with (as well as all of the other suspension settings and characteristics).

If the car handles ok now, then just duplicate the diameter, wire size, etc., or take the springs to a spring tester to determine the spring rate.

-Bob Woods