Springs - Part 3

On my last post, I mentioned the next step would involve measuring the compressed length of the springs at the stock ride height. How does one do this, you ask?

Well, in theory, it's not too hard: one just needs to set up the suspension to the stock ride height, and measure the distance between the top spring perch and the bottom spring perch.

The problem is that when the car is sitting on the ground, it is hard to get under the car. But if you lift the car and rest it on jack stands, the wheels come off the ground and the suspension droops towards the ground.

So, the best way (given my tools/resources) is to lift the car onto the jack stands, and remove the springs. One can then raise the suspension up/down with a hydraulic jack until one gets the proper suspension ride height. Once this is done, you can use a string and a measuring tape to get a read on the perch-to-perch length.

The stock ride height is determined by measuring the distance from the lower A-arm pivot to the ground (measurement A) , and the distance from the lower ball joint to the ground (measurement B). The difference between A and B should be 34mm +/- a few mm. (I'll have to post a link to a diagram illustrating this -- stay tuned.)

So, by raising the suspension up/down with the hydraulic jack, one can zero-in on the proper ride height and measure the compressed spring length.

(I'll detail the removal of the springs on my next post.)

The compressed length came out to 204mm, or 8.0315".

This is very close to the 200mm test load used in specifying the spring rates (see my previous post.) At this stock ride height, the distance b/w the center of the hub to the fender lip came out to 14 5/8" (371mm.) Furthermore, compressing the suspension 1 1/8" (18/16") caused a compression at the spring perch of 7/16", giving a 18:7 (2.57:1) ratio in travel b/w the outside of the hub and the spring perch.

Why is this useful? Well, these numbers can be used to compute the amount of force with which the spring is compressed at the stock ride height. Assuming the following spring rate and free length (from my previous post):

Front: spring rate = 7.797 kg/mm, free length = 313.5mm

With a compressed lenght of 204mm, then the spring is compressed a total 109.5mm with respect to the stock ride height. The amount of force on the spring is computed with the spring equation:

Fs = Spring Rate * (Free Length - Compressed Length)
= 7.8 kg/mm * 109.5mm = 854.1 kg = 1879.02 lb (aprox.)

This can be used to compute the required free length for springs with different spring rates. Recall:

k = f/(Lf - Lc)

Where:
k = spring rate
f = load at spring
Lf = free lenght of spring
Lc = compressed length

Clearing out Lf, we get:

Lf = (f/k) + Lc

For example, take an 800lb spring (and rounding out a few terms):

Lf = (1879lb / 800lb/in) + 8.0315 = 10.38025"

So, in theory, obtaing a spring with a rate of 800 lb/in and with a free length of 10.38" will yield a ride height very close to stock ride height. This is a big piece of the puzzle - I can now pick my rate, and by plugging in numbers into these equations, I can compute the free length required for the spring. Wee!

(A caveat: the springs are installed with rubber isolators on the top and bottom of the spring. This effectively decreases the actual compressed length somewhat. I wll have to measure these and correct the compressed length before doing computations "for the record". I will do this in my next post.)

A new problem arises when one asks: "What if I want to lower the ride height by X inches?" Well, we have a numbers for that.

Recall that the ratio between the hub-to-fender length and the compressed length is 2.57:1. We can then assume that a 25.7 mm (about 1") decreased ride height implies a 10mm decrease in spring compressed length. We further assume that the force on the compressed spring remains the same (big assumption - probably off by a bit.) We can then punch the numbers into our previous example:
(Note: 10mm = 0.3937")

Lf = f/k + (Lc-10mm)
= (1879lb / 800lb/in) + (8.0315" - 0.39") = 9.99"

Pretty cool, eh?

So, in summary, the numbers to remember are:

f = 1879lb = 854.1 kg
Lc = 8.0315in = 204 mm
Motion at hub:Motion at spring = 2.57:1

And the key equation:
Lf = f/k + (Lc - (correction))

The "correction" term refers to any adjustments in ride height we may want to do, as described above.

The bigger moral of the story: algebra and high school-level physics are actually useful in real life!
:-)


Next up: Back in the garage, removing parts from the suspension.