Hi Robin,
Right then, you are right about the leverage effect, this is usually called mechanical advantage; a ratio. As I mentioned earlier the most important factors to get right are the WR (wheel rate) and frequency. If we can find common ground you can check my calculations; suggest the Marlin front is as good a place as any.
For mine the levers d1 and d2, just as in your diagram, are:
d1 = 210mm
d2 = 380mm
This gives a ratio of 0.55. These may be the same on your E30 car?
Important input from the expert (Dave Cooper) is to check this by measuring the spring length carefully and stroking the wheel a few inches, it should give the same ratio.
And squared I make it 0.31, then with a 300lb spring rate (SR), WR is:
300 x 0.31 = 93
So if you apply 400lbs to the corner it will compress 4.3 inches at the wheel and 4.3 x 0.55 at the spring.
Ok, dont drift off, there is an extra complcation due to the spring being inclined, so for a more accurate calculation the cos of the angle should be added, for mine 0.9:
WR = SR x (d1/d2)^2 x cos (spring inclination) = 80 approx.
The rear spring by the way is close to upright so dont worry about it there.
Now then, as mentioned earlier the other important factor is the suspension frequency, this is where the personal preference comes in. On most road going cars, it is in the range 1.0 to 1.5 cps. On track cars, its more like 2.0+
The Marlin front with 300lbs SR, making the following assumptions:
F/R ratio is 45:55
Curb weight is 800kg (I'll come back to that one)
Driver and fuel 100kg
Then front corner weight is about 450lbs
My weighed unsprung weight is 95lbs
And WR is 80lbs, from above.
So the weight on each of the front spring is about 350lbs
The frequency is (WR/Sprung weight)^0.5 x 188 (in cpm)
(The 188 is a constant due to gravity) Now divide by 60 for cps (hertz).
f = ((80/350)^0.5 x 188)/60 = 1.5
This is quite stiff, compared to the beemer, which by my calculations is nearer 1.0
The weight issue is a big problem, really until I've actually weighed a car, I have little confidence in the excercise; we know that Marlin quote 750kg for a finished car - and that was with a 4 cyl engine. I would guess they are easy to make heavy than light - and if I'm going to err, it had better be on the heavy side.
But, back to the problem, having got the front sown-up; the rear should be pretty similar in terms of WR, but the freq MUST not be identical or the car will behave very strangely when set in motion.
There you are, you did ask !!
Cheers,
Martin
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