Effect on coil spring rate from rubber mounts 2

[carbonarc]

We're trying to decide whether or not to add rubber bushings on the top of the front coil springs on a vintage Alfa Romeo race car. The rubber bushing is stock and has some practial advantages, not related to the spring rate.

I've read and been told that adding a rubber mount to the top of racing coil springs will severly reduce the overall spring rate. But I wonder how it can have much of an effect when considering a rubber mount that is about 3/4" thick and likely compresses less than 1/4". In my application the coil springs compress about 3" during full suspension travel. The spring rate is 1250#/inch but I have no information on the rate of the rubber. Can someone clear this up?

Curt Griggs

 

[GregLocock]

Ok, the rule for adding springs in series is that the total rate = k1*k2/(k1+k2) - in practice the softer spring dominates, and the total will always be softer than the single elements.

In your case the steel spring will be significantly softer than the spring mount, by about a factor of 12 if your measurements are right (they seem reasonable).

So in your case the net effect will be a drop in spring rate by 8%. This will be offset by the extra length of the toal assembly, so your ride height will /increase/, which may be the cause of your informant's comment.

Oh, remember that compliance is the inverse of rate, a source of endless confusion.





Cheers

Greg Locock

 

[mloew]

carbonarc,

If the system is two springs in series:

K_total = ((k_{road spring})^-1 + (k_mount)^-1)^-1

You must also note that those mounts are highly non-linear.

Best regards,

Matthew Ian Loew
"Luck is the residue of design."
Branch Rickey

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[carbonarc]

Thanks to Greg and Matthew for you prompt and professional replys. This is my first post to the group, but I've been monitoring some of the threads for a while. I must say I'm impressed with the speed and content of the help I received. Many thanks!

Curt Griggs