So..I wanted to see how a 'stiffer' spring felt, so I stuffed a pair of those spring helpers in between a coil on each rear spring, and re-lowered the car to the previous height today.

It -dramatically- resolved much of my off-throttle oversteer.

So..with I think..one coil..not in service any more, what spring rate do I roughly have?

87 S4, dual blue paint marks on the springs.

 

2-blue stripe rear S4 shocks (actually mid 84-95) have the following dimensions and spring rate:

Spring rate in lbf/in, 170-174
Free length: 412 mm (16.22")
Wire diameter: 13 mm (0.511")
# Coils: 9.5
ID = 92 mm (3.622")
OD = 92+26 = 118 mm

So if you cut exactly one coil, you drop to 8.5 coils, length 368.6 mm. Not sure how to calculate the change in spring rate. Some of the formulae require knowing the number of 'free coils' (it's less than 8.5, you subtract out the coil that's touching the spring perches), and some ask for Young's modulus. I have no idea.

 

So, by stiffening the rear, you decreased off throttle oversteer?

 

Don't worry about Young's Modulus - it doesn't change. It's a function of the material. Therefore it is a constant and for comparison purposes cancels out when determining overall change.

 

Doing a quick calculation using the numbers supplied above, removing a coil would increase the spring rate ~14% to ~197 lb/in. Surprised to read increasing the rear stiffness reduced oversteer.

 

It certainly feels like it..its not so much of an interesting moment. I'll admit asses are not the best calibrated devices available..

I also like how the rate Ive ended up with, still takes a highway bump or bridge approach softly, but less of a 'sag' into an out of it.

 

Me too.

 

We have FULL coils, not shaved coils, so every coil counts in the rate. ??

 

I need to get this on a rack and report back to you..6500mi since seeing your rack, it's developed a slight pull right. Need more data.

 

No, subtract 1.5 from the number of coils to determine # free coils. Subtract 2 if spring has ground ends.

 

Assuming flat perches. I have not seen my perches yet, but usually if you have full coils the perches will be shaped to fully contact the lowest/highest full coil on the ends and therefore make them act like ground/flat ends.