Front spring length

[PaulFinch]

Hi, have tried this in the racing forum but thought I may have more luck here.

What front spring lengths do people use for running 100 - 120 mm ride height?

Many thanks.

[PaulFinch]

Ok, perhaps this was a little vague.

I am converting my S4 into a bit of a track toy.
I have avo adjustable dampers with adjustable spring seats and I have a set of 9" free length 150lb front springs.

I want to use a ride height of between 100mm and 120mm measured between the sill and the ground as I understand this gives a couple of degrees negative camber.

My question is will the 9" spring be ok or will a shorter spring be better suited?

Thanks,

Paul

[rgh0]

A bit of engineering mathematics ! ( edited to correct or typo and subsequent error in fitted spring length)

Load on front springs is approximately 514 lbs based on the manual data.
i.e. 75 lb spring rate, free length 16.08 inch fitted to car length 9.22 inch
(15.08 - 9.22) x 75 = 514 lbs

if you check that based on suspension dimensions and car weight you get the same number near enough
front corner weight 380 lbs
spring angle 17 degrees
leverage of load on spring = 9/7
(380 x 9/7) / cos 17 = 511 lb which matches the spring data in the manual

If you have the spring platforms in same location as standard but you want a ride height approximately 40 mm / 1.6 inches lower. Then fitted length of spring is reduced by approximately 1.6 inches x 7/9 / cos 17 to allow for the lever arm of the suspension and spring angle. Thus spring fitted length reduced by 1.3 inch to 7.9 inches.

If 150 lb spring and 514 lb load then fitted length of spring is 3.4 inches less than free length.
Thus free length = 3.4 + 7.9 = 11.3 inches.

A 9 inch free length spring is probably to short depending on the amount of adjustment you have on the spring seats assuming you have adjustable seats but you will end up with the adjustable seat near the top of its range. A 12 inch spring is potentially too long as the standard seat location is close to the bottom of the shocker and 12 inch would not allow much downward movement of the seat if you wanted to make adjustments in that direction. I would probably choose a 10 to 11 inch spring which should enable adequate adjustment and also allow raising the car to more normal ride height if required on the typical adjustable seat shock. You also need to check that the spring will not coil bind at full bump once you select spring free length and spring seat location.

cheers
Rohan