Spring rate confusion

[PeteK]

Sorry this sounds like a newbie question, but I'm really struggling with the spring rate required to get my ride and ride height correct.

I've searched and found info on standard spring rates (from the "BMW E21 323i OEM spring rate inquiry" thread numbers listed below) but I can't understand how they can be right. If the rears are 60 lb/in and I assume 40% of the weight is at the back and it's 1176 kg, even ignoring the effect of the mounting point being inboard, there's got to be 500 lbs on each spring just at rest. This should mean 8.3 inches compressed at rest. The spring is only 12.7 inches long, so that can't be right.

Can anyone help me with where my calculations have gone wrong?



BMW E21 323i OEM spring rate inquiry
Just as an example, standard rear 323i ones are 701-703 lbs. However, the lbs figure is lbs/ft, not lbs/inch, so the spring rates are about 60lbs/inch.

So go through that list and you get.

Front
Code Red - 688-705 - 57- 59
Code White - 708-725 - 59 - 61
Code Green - 728-745" 60 - 62

Rear
Code Red - 613-631 - 51 - 52
Code White - 633-648 - 52 - 54
Code Green - 650-666 - 54 - 55

All the bold are in lbs/inch.

[nas80]

depends on what your after and the spec of your car?

[PeteK]

I'm trying to find a method to calculate the change in ride height given different spring rates and lengths. The 323i is basically stock (sort of) but I'm running a set of "coilovers" I've made that will take a variety of springs. i've got everything from standard 318i, 320A and 323i springs to various sets of Eibach pro springs up to 300 lbs. The car is 4000 km away, so I don't have the luxury of being able to go out and try all the different springs.

I tried the spring rate calculator in Top Tech Tips - Useful calculators, but it gave me the same as my calcs. I.e. The actual spring rate is about double listed by BMW.

Cheers

[nas80]

i have driven a standard sprung e21 and it was very very soft - you dont want that.

if you have a 323i then id go for 200 rear and 300 front but it does depend on your damper settings too you know. what dampers are you using?