Springs - Part 1

So, the car remains half torn apart as I wait for parts to arrive - a new water pump, suspension bushings, ball joints, and adjustable control arms. I figured while I wait, I might as well read some of the books I've bought, and see if I can learn something useful.

Well, I think I have.

As you may recall, I've been a bit worried about the current springs installed in the Alfa. For starters, under heavy braking, the sump guard scrapes against the pavement. So I suspect the current springs might be too soft and/or too short. (I also suspect the shocks are worn out, but that's another story.) So, I am trying to find out what to do about springs.

(What follows is a newbie's interpretation on coil springs as used on cars. I am not 100% sure this is accurate, but here it goes.)

You see, springs affect how the car rides (the stiffer the spring, the harsher the ride, kinda), how it handles (stiffer springs can improve handling capability, and keep the car from leaning away from curves and the such), and how high the car rides (longer springs make the car ride higher; shorter springs make the car ride lower.) So, getting the proper stiffness and length of a spring is pretty important in determining how happy the owner feels about the way a car rides and handles.

So, I need to determine which spring stiffness and free lengths to specify for any new springs I install on the car.

What do these things mean, you ask? Well, let's start with stiffness:

Spring stiffness is basically the resistance of a spring to be compressed (or stretched) under a give load or force. Spring stiffness is specified by measuring how much force it takes to compress (or extend) a spring by a certain length. This value is known as the spring constant. For example, if it takes 100lbs to compress a spring by 1 inch, this spring would be said to have a spring constant value of 100 lbs/in. To figure out the spring constant, the following equation suffices:

spring constant (k) = force / difference in length

Or:
k = f/lf - lc

Where:
k - spring constant
f - force
lf - free length with no load
lc - compressed length of spring under the force f.

(This assumes a linear spring, where the spring constant for the spring actually remains constant throughout the range of available compression. Note also that the range of compression is limited - as you compress the spring, the individual coils get closer and closer, until they eventually make contact. Once this happens, the spring is not a spring anymore - it's more of a column of metal which does not behave as a spring would.)

Note that suspension springs do most of their work while being compressed. So most of my discussion will only address springs under compression only.


How about length?

Well, length is just that - the length of the spring. However, you can measure length when the spring is unloaded, or when a certain force is applied to it. The unloaded length of a spring is its free length, while length measured under a give force f can be referred to as compressed length.

Once you know the spring constant (k) and the spring's free length (lf) you can compute the length of the spring for any force using the spring equation shown above. For example, assume:

k=500 lb/in
lf= 10 in

Then, under 1000lbs of force, the spring's compressed length will be:

500 lb/in = 1000 lb / (10inch - lc)

Moving things around:

lc = 10 inch - 1000lb/500(lb/inch) = 8 inches.

Also: once you know a given spring rate (k) and a given free length (lf), you can use these values to order springs from vendor and/or manufacturers. (In short, these two values serve as the initial "size" for specifying and obtaining automotive springs.)

Why does all this matter?

Well, using these relationships and measuring a few things in the car, I can determine which spring rates and spring free lengths I need to use for a given car ride height. In other words, given:

a) The ride height I want to have on my car
b) The spring rates I would like to have on my springs
Then I can determine:
c) the free length of the springs.

Once I figure (B) and (C), I can order springs and be all happy.

Now, my goals with respect to springs is to:
a) Keep my ride height as close to stock as possible--within half an inch lower than stock, but not any higher.
b) Install springs that are stiffer than stock - this will help keep the car from bottoming out during heavy braking, and will help the car handle better
c) Make sure the springs are not so stiff - this would make the ride quality harsher.

So, my approach to deciding on springs will be as follows:

1. Figure out the stock spring rates and free lengths.
2. Figure out the compressed length of the springs with the car riding at stock ride height (as described in the owner's manual.)
3. Survey the various spring kits available for my car, as well as comments posted on various BBs and the such, and figure out what folks think
4. Based on (3), decide (guess, really) which spring rates I want to use
5. Once I decide on spring rates, figure out the free length required for my chosen spring rates.,
6. Order the springs from someone.
7. Install the springs, cross my fingers, and hope this all works.

That's enough babble for now. I will babble more regarding items 1-3 on my next post.