Spring Pre-Load: Help me understand the physics.
 

Can anyone help me understand the physics of Spring Pre-load as it relates to a linear-rate spring?

I am having trouble figuring out how pre-loading the spring would affect the net spring-stiffness at all in a linear rate spring.

Other than to precisely control travel, why would you ever adjust pre-load in this situation? Anyone know?

Thanks!

wat? linear rate is linear rate throughout the compression.

do you mean loading the suspension to its working position?

What I'm getting at is, other than for compensating for differing lengths of spring and as an auxiliary method to control ride height and suspension travel, what's the purpose of adjustable spring pre-load on a linear-rate spring?

Preload shouldn't control ride height. A good set like FA moves the whole lower shock instead of the perch the spring sits on. That way ride height is separate from preload. 2 completely different turning coils.

No idea but if not loaded in a point where the car gets a wheel airborne that spring will be completely lose and flop around and then make a hell of a noise when you come back down. Preloaded, the spring will stay tight in the coilover. Also I believe preloaded keeps the wheel pushed down to the ground and whenever airborne or just weight is transferred off the wheel you don't just want gravity pulling the wheel out of the wheel well. You want it pushed out so it meets the ground and keeps traction up. Again no idea, just guessing.

If we are talking about a perfectly linear spring then there is no advantage to running any sort of preload. However springs are not perfectly linear and they tend to deviate away from hooks (ideal) law. I dont know enough about suspension to know how this affects a car on the road. But knowing that no spring is ideal I can see it having some effect on the performance of a suspension setup.

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This is what I suspected, I just don't know how big the deviation actually is. I imagine you have to put the exact spring you're talking about on a spring dyno to get an accurate idea of what that curve looks like.

I didn't see anyone ask but what coil over are you running. Usually 1/4 or 3/8 of an inch of pre load set is the typical.

to optimize your suspension working height to maximize usable stroke for the pretermined track.

Thanks for the information Mitco.

I'm not looking for setup advice or anything else, this was a strictly academic question to confirm my own assumptions.

Apart from corner balancing, I've never messed with preload much

Spring preload serves two purposes that I know of:

1. It keeps the spring in place so that it does not shake free/rattle/lose contact with the perch under extension(very bad).
2. It adds a little bit of "softness" to the ride. OEM springs are always pre loaded to this effect.


Preloading a 400l b spring 1/4 inch effectively gives you 100 lbs of preload. This will "eat up" the first 100 lbs of weight applied to the spring and give it a slight progressive effect.

Since you have effectively used and compressed 100 lbs of weight, you should subtract that weight from your unsprung corner weight when calculating ride heights. i.e.


400 lb corner weight(unsprung) on 400 lb spring= spring will compress 1"

400 lb corner weight(unsprung) on 400 lb spring preloaded 1/4" = spring will only compress 3/4" more.

It does make it a little softer. As I put in my first post.

Scenario 1: No Preload
400 lb corner weight on 400 lb spring

Scenario 2: 1/4" preload
400 Lb corner weight - 100Lb preload on a 400 Lb spring.

To determine spring compression in scenario 2, you have to subtract the preload value from the corner weight. The corner is effectively now only 300 lbs instead of 400. The first 100 lbs is bound up in the spring already, so any compression beyond that will behave similarly as if the corner weighed 300 lbs instead of 400-at least in the low speed range. In the high speed travel situation you can pretty much throw the preload out the window.

The net/effective result of this scenario is a little bit of a progressive feel. As mentioned earlier, Just about every OEM suspension out there does this in order to give the car a smoother ride and handle some of the low speed compression better.

Thank you!