please check my logic

so i have to do some suspension work, and would like to level the car while im in there. this is purely for cosmetics.

its a strut equipped mazda. I want to slightly trim some coils to level it out from where it sits higher in the front than the back (its all factory. my last two p5's were the same way)

my thinking is that since the coils are directly on the struts, that the compression of the springs should be pretty linear.

im having a hard time trying to think of how to describe my thoughts....

lets say the coils, with weight on the car and measuring the distance between the top of one round and the top of the next one down (in the middle of the pack)have a measurement of 1.5 inches. in my mind, cutting one full coil would lower 1.5 inches, as the same weight would still be on the spring, causing the same amount of compression as before. does this sound right?

if im right, from there I can do the math and figure out how much to cut for my desired drop in the front.

did that make any sense whatsoever?

It doesn't quite work that way. I'm on my phone and it's a little tedious to type out on this thing. If someone hasn't answered by the time I get to my computer I'll give it a shot then.

I just woke up so bear with me...

The spring is still inboard of the car's contact patch with the A-arm acting as a lever so any reduction in spring height will be multiplied by the ratio of that lever to realize the actual amount that the vehicle is lowered.

Sort of like a rocker arm, the actual lift of the cam lobe may be .330" but by the time it acts on the valve, through a 1.6:1 rocker arm, the valve will be lifted .528". Increase the lobe lift to .340" and the valve lift with the same rocker becomes .544".

The cam is your spring and the rocker is your A-arm, the valve is your ride height, yada, yada...

I'd start at 1/2 a coil, if that's possible.

Not only what the Jed said but... cutting the spring will stiffen it considerably - possibly out of the bounds of the damper's ability to... well... damp it. Especially in rebound.

Think about how drastic the change is in small adjustments in sway bar length and then apply that to the percentage you are shortening the coiled up version.

Then go here... http://www.ajdesigner.com/phpspringrate/spring_rate_equation_wire_diameter.php

Put in the params for both cases.

Racing Beat or Megan springs If its for cosmetic reasons, perhaps an inexpensive set of lowering springs instead? I drove a car on cut springs - it was horrible.

In reply to chrispy:

I've cut the springs on 90% of the cars I've owned. And every single time it was an improvement.

Additionally, no one makes springs that only lower 3/4 of an inch.

Im trying to avoid the whole cut 1/4 coil at a time and all the labor that involves. I was hoping my theory would work, but apparently not.

Thanks guys. Now I need to decide wether I want to put the time in it, or just accept the reverse rake. Unless someone has a way to get pretty close on the first try.

I'd hazard a guess that 1/2 coil will get you very close to what you want.

Measure the distance between the suspension arm pivot point and the strut mount, and the distance between the pivot point and the hub, then calculate (or post them for the hive to calculate). You could also remove the strut and measure the distance the wheel moves throughout the suspensions travel, then measure the distance the strut would move throughout the suspensions travel.

Dusterbd13 wrote: lets say the coils, with weight on the car and measuring the distance between the top of one round and the top of the next one down (in the middle of the pack)have a measurement of 1.5 inches. in my mind, cutting one full coil would lower 1.5 inches, as the same weight would still be on the spring, causing the same amount of compression as before. does this sound right?

That's how it works. Each individual coil is carrying the weight of the vehicle. Cut the spring in half and the spring will compress to the same "internal" height.

The problem is that you will be losing ride height faster than total spring rate goes up. So you will bottom out more. I say "more" because it seems that Mazda figures that bottoming out in normal driving is acceptable and their cars all ride the bump stops.

Most small cars with some age on them have a nose up rake from:

1. Sagging rear springs(install rubber spring boosters or replace springs). I like the boosters, $12 at oreilly they stiffen the springs up a bit(by filling one coil in with soft rubber) and let you fine tune the ride height.
2. You're looking at the car without 180-500lbs of meatbag in the front row weighing it down.

Worse:
OEM's engineer to achieve minimum costs, which usually means minimum materials used.
A spring design must balance the length, the rate, the package envelope, operating temperature, max deflection, cycle rate, and the design lifetime.
This means balancing material, wire diameter, number of coils, free length, and binding length.
If you are choosing for minimum costs, then you will choose a shorter lifetime, which means that the steel in the spring will be operating right at the edge of elastic deformation.
So cutting off bits only raises the stress, and pushes the operating point farther into deformation zone.

Cutting to the chase: the spring will sag sooner, maybe much sooner. It's not a linear relationship, the curves start getting steep very quickly at the edges.

Lookup an online spring design calculator sometime, and punch in the dimensions of your favorite chassis coil spring.
For a real treat, punch in the dimensions of a valvespring. (!!!)

Carter

Heh. Valvesprings. I remember when people thought it was crucial to have a minimum of .060" between each coil when at max lift. Now, people want to run as close to coil bind as possible when at max lift, after someone realized that slamming the coils together does a dandy job of eliminating any harmonic surging in the spring.

Knurled wrote:

Dusterbd13 wrote: lets say the coils, with weight on the car and measuring the distance between the top of one round and the top of the next one down (in the middle of the pack)have a measurement of 1.5 inches. in my mind, cutting one full coil would lower 1.5 inches, as the same weight would still be on the spring, causing the same amount of compression as before. does this sound right?

That's how it works. Each individual coil is carrying the weight of the vehicle. Cut the spring in half and the spring will compress to the same "internal" height. The problem is that you will be losing ride height faster than total spring rate goes up. So you will bottom out more. I say "more" because it seems that Mazda figures that bottoming out in normal driving is acceptable and their cars all ride the bump stops.

It's not exactly linear like that. Because you are increasing the rate when you remove a coil.

So is it linear enough that it would get me +/- 1/4 inch or so on the first try?

nicksta43 wrote:

Knurled wrote:

Dusterbd13 wrote: lets say the coils, with weight on the car and measuring the distance between the top of one round and the top of the next one down (in the middle of the pack)have a measurement of 1.5 inches. in my mind, cutting one full coil would lower 1.5 inches, as the same weight would still be on the spring, causing the same amount of compression as before. does this sound right?

That's how it works. Each individual coil is carrying the weight of the vehicle. Cut the spring in half and the spring will compress to the same "internal" height. The problem is that you will be losing ride height faster than total spring rate goes up. So you will bottom out more. I say "more" because it seems that Mazda figures that bottoming out in normal driving is acceptable and their cars all ride the bump stops.

It's not exactly linear like that. Because you are increasing the rate when you remove a coil.

To recap.

Each individual coil is carrying the weight of the car. So if there is 500lb on that corner, and the spring compresses five inches between free height and ride height, then the spring rate is 100lb-in. If there are five working coils, each coil compresses one inch to carry that 500lb. Each coil has a rate of 500lb-in. 500lb-in divided by five working coils is 100lb-in total spring rate.

Cut one coil out, you now have four working coils. 500lb-in per coil divided by four working coils is 125lb-in total spring rate, which makes sense because the spring will now only compress four inches.