Spring preload to lift?

[gbub]

Ok, another engineer, let me take a crack at it.

First, spring rate is progressive and measured in lbs/in. If your spring rate is 500, it takes 500 lbs to move the spring 1 inch and you need 500 lbs x2 or 1000 lbs to move it 2 inches. (if it wasn't progressive, putting 500 lbs of force on the spring would fully collapse it).

Now, when you tighten your bottom perch by 1 inch (and pre-load your spring) you are putting 500 lbs of force on the perch (that can't go anywhere if the shock is fully extended). When you put the weight of the vehicle on the coilover assembly, it will only compress if you exceed the 500 lbs of force that the compressed (or loaded) spring is exerting to extend. Before preloading the spring, 500 lbs would compress the coilover 1 inch because the spring wasn't already compressed and pushing back, after preload, you need 1000 lbs to compress coilover 1 inch (again, because the spring is already compressed 1 inch against the shock that can't extend any more).

So preloading the spring both raises the ride height, because the spring is pushing to extend the assembly with a preloaded amount of force, but it also makes it require more force to compress the coilover assembly (thus making it feel stiffer).

Everything was good until your last paragraph. If takes 1000 lbs to compress a spring 2 inches to say 8 inch spring height loaded, it also takes 1000 lbs to compress a preloaded spring to that 8 inch spring height. Just because it was preload compressed 1 inch at 500 lbs, then another 1 inch with 1000 lbs force does not mean it becomes stiffer. A spring has the same force at 8 inches spring height no matter how many steps it took to get there. Preload is just one of those steps to get to the full 1000 lbs load.

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[gbub]

Compressing the spring in any way between the top and bottom spring perch (red area) increases preload and thus ride height because you're compressing the spring and making it harder to compress further. This can be done with a spacer (below or above the spring but still between the spring perches) or by moving the bottom spring perch up in this case.

Putting a spacer in the green position does nothing to preload, but it increases the ride height because it is effectively making the coilover physically longer.

Preloading a spring does not make it harder to compress further. It just will not compress the spring until it exceeds that preload. Once the load exceeds that preload the force/spring height ratio stays the same whether it was preloaded or not.

I agree with your last paragraph. Also, putting a spacer under the spring raises the position of the spring resulting in making the coil over longer when loaded with the vehicle weight.

 

[gbub]

Think of the coilover as a black box, forget about it looks like, just understand that you have two points (the top and bottom mounts) and imagine trying to push those two points together. If I push with a force of 500 lbs and it those points move 3 inches, think of that as the default ride height, now if I change how hard it is to press those two points together, now the 500 lbs of force only moves them 2 inches closer, THAT is all preloading is doing. You're just making it harder to push the 2 points together.

Now, putting a spacer ABOVE the top mounting point of the coilover also does change your ride height, because the point at where that top mounting point attaches has moved down. This has nothing to do with preloading, or the force required to compress the suspension, just changes the mounting location. Unlike changing the preload of the spring, that DOES change the geometry of your suspension thus changes the path the suspension takes moving up and down.

Your black box has a problem. In order to change the distance that the 500 lbs force compresses, you would have to change the spring rate. Preload does not change spring rate.

If your preload was 500 lbs to move it the 3 inches and you increased the preload so it would only move 2 inches. Your black box theory might work. However, no one in there right mind would design a suspension with static load the same as the preload.

You need a new black box.

 

[5GENIDN]

I think there is a "pre load" misconception..... Follow me here....

If I put a load (any load) on a compression spring it gets short does it not?

When I put "preload" on my coil overs the spring does not get any shorter.... Now that is interesting....

That is because I have not really put any additional load on it. What does happen is the dampener actually elongates... Try this on a bench.... you can measure it. The spring will be the exact same length but a 1 inch "preload" makes the mounting points 1 inch further apart. The dampener will be extended 1 inch further. Again the spring is still the same length.

EDIT: Okay I am a dumb shit!!!! It is official.... Everyone else already knew it, I just had to prove it to myself.... Thank you all for allowing me to prove this to myself this in a very public manner.

Do not know what I was thinking. I had to go grab one and I am looking at it and YES the spring gets shorter as I preload because the dampener is at full extension.... Hence preload.

 

[gbub]

I think there is a "pre load" misconception..... Follow me here....

If I put a load (any load) on a compression spring it gets short does it not?

When I put "preload" on my coil overs the spring does not get any shorter.... Now that is interesting....

That is because I have not really put any additional load on it. What does happen is the dampener actually elongates... Try this on a bench.... you can measure it. The spring will be the exact same length but a 1 inch "preload" makes the mounting points 1 inch further apart. The dampener will be extended 1 inch further. Again the spring is still the same length.

What you are describing as preload is not what I refer to as preload. It is just raising the ride height. From my understanding of preload is when the shock/dampener is already fully extended. Adding additional preload does shorten the spring because the shock can't extend anymore. If there is no load on the spring, cannot have a preload.

 

[87-Z28]

 

[Brian_B]

You had me until 2). If you have a coil that compress to a height of 12 inches at 500 lbs load, that should not change if you preload to 250 lbs.

Looking at it another way, say it’s spring rate is 1” per 500lbs, with a 1000lb load the spring would naturally settle 2” down, but adding 250lbs of pre load by tightening the collar should raise ride height by 1/2” (250lbs of effective sprung weight just got negated, the only direction the spring can push and yield is up against the body). So you’d end up +1/2” from baseline (no preload)

 

[Jdyount]

What you are describing as preload is not what I refer to as preload. It is just raising the ride height. From my understanding of preload is when the shock/dampener is already fully extended. Adding additional preload does shorten the spring because the shock can't extend anymore. If there is no load on the spring, cannot have a preload.

Ok, it's official, you have NO CLUE what you're talking about. The shock is a damper, you dont preload the shock. The coil is a spring, if you compress the coil you are loading it. If you compress the spring by shortening the distance between the spring perches while at full droop (while at full coilover extension) you are pre-loading the coil spring (meaning there is no load on the coilover assembly and you are applying a PRE-LOAD to the coil. Thats what pre-load is. On the Eibach 2.0 coilovers, you preload by running the threaded lower spring perch up into the spring compressing it.

That is the definition of pre-loading a coilover, that's it, nothing else.

 

[Jdyount]

The spacer on the left is adding preload by reducing the space available for the spring, not by adding weight. Because of this, the weight of the vehicle will compress the spring to the same length with or without the spacer. The preload is cancelled out because the spacer doesn't add any weight of it's own. The lift is achieved because there is more distance between the vehicle mount and the top of the spring.

I have spring perch spacers. The ride is not stiffer. The shocks will top-out more easily, due to the fact that the shock is already more extended. Preload may kick in here as the weight of the Bronco is unloaded hitting a steep driveway.

You are wrong 100%. Taking up space between the spring perches pre-loads the spring, moving it the first 1 inch (or whatever). That WILL make it harder to mover another inch.

SPRING RATE IS PROGRESSIVE! It get harder to compress as you go. Yall don't understand basic physics is the problem. For something to be stationary while force is appied, there has to be an equal and opposite force. When you preload the spring 500 lbs by compressing it, it is pushing against the end of the shock tube because it is fully extended. If you apply 500 lbs of force to the end of the coilover, all you're doing it taking the load off the stop, it won't start to move until you exceed the pre-loaded force. To move another inch 1000 lbs will have to be applies (for a spring with a spring rate of 500), again, because spring load is PROGRESSIVE.

Yall need to take a physics class.

 

[Willub]

Adding preload reduces down/droop travel, which can cause an earlier top out in suspension stroke, which doesn’t feel good. That’s a “harshness” it can create. It’s also horrible for a bypass style shock that relies on a factory set ride zone. Add preload (via spacer or some garbage) and push it out of that ride zone = “harshness”. Like you said, it does not change anything about the spring once vehicle load is applied. It changes the static ride height. That’s it. Unless you crank preload to the point where vehicle weight can’t compress the spring. Then you’re just an idiot.

 

[Brian_B]

I think this thread needs a "Your Mom something something my preload" joke in here somewhere.

 

[gbub]

Ok, it's official, you have NO CLUE what you're talking about. The shock is a damper, you dont preload the shock. The coil is a spring, if you compress the coil you are loading it. If you compress the spring by shortening the distance between the spring perches while at full droop (while at full coilover extension) you are pre-loading the coil spring (meaning there is no load on the coilover assembly and you are applying a PRE-LOAD to the coil. Thats what pre-load is. On the Eibach 2.0 coilovers, you preload by running the threaded lower spring perch up into the spring compressing it.

That is the definition of pre-loading a coilover, that's it, nothing else.

I think the problem is you have no clue what I am talking about and I may not be explaining myself well.

I also have no clue to some of what you are talking about. Preload I am talking about is done with the coilover uninstalled. The Eibach system already has some preload on the spring with the shock fully extended. If you want to have more lift you run the threaded perch up (as you said) to increase the preload. That does compress the spring but it does load the shock also. That spring is pushing against the fully extended shock. Otherwise, the spring would fly into the air.

Once the coilover is installed, everything changes. The shock is no longer loaded in a static condition with the vehicle on the ground because the weight of the vehicle takes over.

 

[5GENIDN]

Please accept my mea culpa... I can be an idiot as many people on here have probably already recognized.... I am rereading what I wrote yesterday and thinking WTH was I thinking.....

YES preload makes the spring shorter.... tightened from the top or the bottom....

I am really starting to worry about intermittent Alzheimer's....

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