StewartFisher - So what you're saying is that pre-load basically has no direct effect on the handling, aside from what the lower CoG / different camber angles will bring? So it is just an easy way to alter the ride height (shorter springs are stiffer, all else being equal, LFS works it's magic in that regard in the setup deartment)?

Quote from Bob Smith :

That makes it look like applying the weight of the vehicle to the springs acts in the same weigh as pre-load, which I'm pretty sure would be incorrect.

Of course those pics in that drawing are all from static case where there is no suspension movement. That picture is a bit misleading though...

Explanation picture of how preload can be added (one solution)

EDITEDIEDIT dammit. In my picture the ride height is unneffected by the preload setting

Quote from Shotglass :

yes since weight should only move the equilibrial point to the right but not up

Weight causes deflection and deflection is caused by a force "pushing the spring shorter".

Imho, in purest form, the preload means just that the spring is tensioned to a certain level where certain static force is always applied. The spring has a length that it has when at rest and at preload that length is either smaller or longer. The spring can't tell whether the force put on it is either preload or weight or pull or whatever. The spring just deflects or lenghtens, caused by the forces applied to it.

Quote from Shotglass :

because the damper at full extension is essentially a metal rod

OK, I think we're getting somewhere. I've attached a quick sketch of the system as I understand it.

Fp is the preload force and xp is the preload compression of the spring (k1). k2 is the spring constant of the metal bar of the damper and x2 is the extension of the damper rod. The extension of spring k1 is (xp-x2). We can apply a force F to the end of the damper rod, which is rigidly attached to the spring k1. Let's assume that the system is in equilibrium, i.e. we have added a preload and it is now sitting on a table. The forces at the end of the damper rod must balance.

F + (k2 * x2) = k1 * x1

but x1 = xp - x2, so:

F + (k2 * x2) = k1 * (xp - x2)

Which leads to:

F = (k1 * xp) - x2 * (k1 + k2)

Since the system is in equilibrium, the force F is zero and the preload force in k1 is exactly balanced by the extension of the damper rod k2.
Thinking about this equation, the spring constant of the whole system at this point is zero. That is, any small force F will cause the system to move. The spring constant then increases as F increases until x2 = 0, at which point the damper begins to move and k2 disappears. We're left with:

F = k1 * xp - (x2 * k1)

which, since x2 = 0 at this point, reduces to

F = k1 * xp

which is Shotglass' 'biasing' preload force. However, as x2 becomes negative, the spring rate of the system returns to k1 and the preload simply becomes a 'force offset'. What's the effect of this force offset? It raises the ride height


On reflection it seems that I had misunderstood Shotglass' point. He is, of course, right about the 'biasing' thing but this doesn't affect the spring rate of the system, only the equilibrium point.

Now I leave it to you fine people to ridicule my force diagram

Quote from StewartFisher :

but this doesn't affect the spring rate of the system

i dont think i ever said that

anyway now we at least agree on what happens with a preloaded sping/damper system lying on your desk

and i think your right the only thing that will happen if you put it on a car is that it will shift the equilibrium and thus change the right height (as long as the preload force is lower than the weight of the car)


but what you should consider is the way in which this is different from changing the ride height via shifting mounting points or changing the length of the pushrods

if you change the pushrods you shift the equilibrium at the wheels if you add preload you change it at the springs
the effect of this is that you change the ratio between how much your wheel can move up and down from equilibrium

in my observation, preload reduces "top". ie, the travel between the normal sag point and full extension, essentially eating up travel that was being wasted anyway. preload only affects the apparent stiffness of the spring if too much is used, and it compresses the spring farther than the weight of the vehicle would naturally cause it to sag. a highly preloaded spring may tend to feel very stiff over smooth terrain, but will come to life over bumpier sections.

for those who still don't believe that preload affects the stiffness, imagine a spring held 1cm shorter by a clamp that is fixed in place. if you want to compress the spring further, any force being applied by the clamp would be relieved as soon as you moved the ends of the spring. therefore, to compress it further, you would be doing all the work, and the preload would be contributing nothing, and so the first 1cm of travel beyond the clamp position would require the same application of force as would have been required to move it 2cm from rest.

Okay, attached is a little picture. As the wheel upright moves up (or the body moves down) the pushrod causes the bellcrank to rotate and attempt to squash the spring. Raising the spring platform will increase the ride height, and move where the bellcrank is at static ride height (which could then be reduced by changing the length of the pushrod). The preload is set before the damper/spring is fitted to the car, by screwing the platform until it just touches the spring (with the damper fully extended) then turning it a further distance and compressing the spring.

Black is the car body and the wheel upright (upright on the right, body on the left)
Red is the damper body
Blue is the spring
Yellow is the spring platform, that can be raised or lowered on the damper body and sets the preload (i.e. compresses the spring when the damper is fully extended)
Light green is the bellcrank that gives the rising wheel rate
Dark green in the pushrod.

I am of the opinion that Mr Fisher is correct.

Evilgeek - In your example you are pressing the spring away from the spring platform, and in that scenario yes the new load has to take up the old preload. But in this real life racing situation we are compressing the damper unit - the spring platform is always applying it's preload function. Therefore the compress the spring in your example further by moving on end of the clamp, you can apply any load. 1 gram or 100kg will move the spring.

As people have said above, if you apply a load of 100 it compresses x. If you apply 200 it compresses 2x. There is no way in God's World of Physics that applying 100 compresses it x and the next 100 doesn't compress it at all. There is also no way the second load compresses it 0.8x (i.e. the spring get's stiffer), assuming we are not at the point of going coil bound.

Quote from tristancliffe :

Black is the car body and the wheel upright (upright on the right, body on the left)
Red is the damper body
Blue is the spring
Yellow is the spring platform, that can be raised or lowered on the damper body and sets the preload (i.e. compresses the spring when the damper is fully extended)
Light green is the bellcrank that gives the rising wheel rate
Dark green in the pushrod.

Either you are or I am colourblind.

It's a dark yellow. Light yellow would have been too pale.

Ah yes, this is one of the topics that got me a bit disillusioned about real life racing. Not many people actually know why they are doing the things they do.

EDIT: the following isn't entirely true. See further down in this thread for more details.

What I came up with when thinking about preload is that an easier way to think of it is simply as a travel limiter instead of some kind of magic extra force. Applying preload is simply the exact opposite to a bump stop. A bump stop limits how far a spring can travel in bump direction and adding preload limits how far a spring can travel in the droop direction.

In a deflection over force diagram a normal spring does this

a bumpstopped spring does this


and a preloaded and bumpstopped spring does this


So what you're doing is limiting the droop. A popular setting is zero droop i.e. if the force at the wheel decreases to anything lower than the static forces from the weight of the car, the suspension arm will not extend further as a reaction.

What is this good for? I have no answers myself but here's a discussion on exactly that issue: http://fsae.com/eve/ubb.x/a/tpc/f/125607348/m/18910968321

Quote from tristancliffe :

Motorbikes often talk about preload - presumably this is a ride height change, as you do not have rising rate forks/swingarms? Or do people in the motorcycle world think it might do something else.

From what I know of offroad bicycles and suspension adjustment the preload is just there to adjust the sag the bike gets from the rider weight - that's all. As expected it won't change the spring rate - all it does is "make" the spring exert an initial force on the damper and not allow it to compress as much when someone sits on the bike. If you have some extreme pre-load adjustment then it's usually a sign you need a different spring there to start with.

This article here also puts it in plain english together with some graphs for the graph-advocates:

http://www.sportrider.com/tech/146_9510_tech/

Yes, that's what I thought too. On the bike (and to an extent on a car as well) there is an 'optimum' ride height that everything works best at. As rider weight is such a large proportion of a bikes weight, it's fairly critical to get it right. Small tweaks can then be achieved by using the preload to change the static ride height.

It seems that most learned people (or at least people who are willing to think for themselves, which is VASTLY more important than any silly qualification, and often VASTLY more useful than experience too) seem to agree with my understanding of how preload works.

Quote from tristancliffe :

It seems that most learned people (or at least people who are willing to think for themselves, which is VASTLY more important than any silly qualification, and often VASTLY more useful than experience too) seem to agree with my understanding of how preload works.

Well, the way the "preload affects stiffness" crowd thinks is sort of like the way ideas like "the sun revolves around the earth" are rationalized. And you can't really blame them for it because it's clearly what they experience, however wrong their explanation for it is.

In fact, I've had this discussion with people about bicycle dampers several times when I hear them saying "oh I'll just stiffen up the suspension on the rear for this bit" and they start tightening the preload collar - to which I naturally HAVE to ask "what on earth are you doing other than unbalancing your bike, pray tell?"

Don't some mountain bikes use bellcranks on the spring/damper? Thus increasing the preload might not only raise the ride height, but move the bellcrank into a position that will give, effectively, stiffer suspension?

Personally (and I have done no testing of this yet, but it's something I want to play with next time I do go testing) I think the position on the rising rate curve is fairly unimportant, as it's fairly linear (although not totally by any means).

Quote from tristancliffe :

Yes, that's what I thought too. On the bike (and to an extent on a car as well) there is an 'optimum' ride height that everything works best at. As rider weight is such a large proportion of a bikes weight, it's fairly critical to get it right. Small tweaks can then be achieved by using the preload to change the static ride height.

This is how I used my preload on my bike.

Quote from tristancliffe :

It seems that most learned people (or at least people who are willing to think for themselves, which is VASTLY more important than any silly qualification, and often VASTLY more useful than experience too) seem to agree with my understanding of how preload works.

Sounds sound to me =P Preload doesn't change the springrate, but I am curious as to exactly what it does for a car.

Quote from tristancliffe :

Don't some mountain bikes use bellcranks on the spring/damper? Thus increasing the preload might not only raise the ride height, but move the bellcrank into a position that will give, effectively, stiffer suspension?

Although the people I was referring to use "simple" single-pivot rear-suspension bikes I can't see why it would give stiffer suspension given the bellcranks used on bicycles.

The crank ratios aren't that big really, alot of the one's I've seen are 1:1 except on some "hey, let's drop off a cliff" sort of bikes which would have much different mechanical advantage and torsional/lateral stiffness requirements given the cliff factor. And even those come to think of it mostly use dampers that have changeable springs instead of relying on pre-load tweaking (granted, they need more extreme solutions).

I'll ask a mechanic guy I know who has made his own bicycle and suspension system on the specifics, I reserve the right to be wrong.

Another good discussion: http://forums.autosport.com/showthread.php?threadid=95944

Got me confused, may have to rethink my last post.

Quite a funny discussion. That shaun chap seems to understand it more than most (but I only skim read it), whilst rms is totally confused.

However, I think reading other forum's discussions is meaningless - I have no idea who to trust. Whereas here I know which people generally have their heads screwed on and who doesn't (e.g. if Harjun came in here telling me that preload was a flavour of ice-cream I'd know to ignore him, but if Mr Fisher told me it was a pop-corn making technique I'd be more inclined to consider what he says (but not beleiving him outright, as it has to correlate to my own research and understanding).

But this is heading in the right direction, so keep it coming guys!

Quote from tristancliffe :

Evilgeek - In your example you are pressing the spring away from the spring platform, and in that scenario yes the new load has to take up the old preload. But in this real life racing situation we are compressing the damper unit - the spring platform is always applying it's preload function. Therefore the compress the spring in your example further by moving on end of the clamp, you can apply any load. 1 gram or 100kg will move the spring.

As people have said above, if you apply a load of 100 it compresses x. If you apply 200 it compresses 2x. There is no way in God's World of Physics that applying 100 compresses it x and the next 100 doesn't compress it at all. There is also no way the second load compresses it 0.8x (i.e. the spring get's stiffer), assuming we are not at the point of going coil bound.

I think that’s where you have being confused.
The physics you are referring are correct but
Preload isn’t a force which is being added to the spring constantly as weight does.
It is an initial force which is caused by compressing the spring to a given initial length. When you compress the spring furthermore the preload has no effect at all.
Of course it does not increase the spring rate but it does increase the minimum force you have to apply in order to start compressing it.

Right, this is the bit I'm hoping to solve!

Lets imagine your scenario with the spring on the bench, with magical forces pressing against it.

You are saying that if you squash it with a load by one inch (preload, and lets say it's 100lb), and then add a further 80lb load on it that no further compression takes place? But if the extra load is 120lb it will compress a further 0.2 inches? i.e. the first 100lbs of extra load is 'absorbed' by the preloading load?

(you can substitue inches and lbs for whatever units you like, but I'm working with nice numbers like 1, 10, 100 etc).

You see, I believed that the preload compresses the spring 1 inch (100lbs force). Adding a further 80lbs will then compress it 0.8 inches MORE (1.8 total compression), and that 120lbs of extra load would be a 1.2" compression (2.2" in total). I think the preload doesn't absorb any extra load.

Who can prove me right or kaynd right?

You can prove it by testing it in your own F3000 or
Just find a spring with constant spring rate and play with it (maybe you can find it in a cheap pen.)

You can also analyze the forces theoretically but I feel too lazy to do it right now. (I will try later)

I shall test it, with a valve spring.

Let me get this straight.

1. The first load is the preload. Call it 100lbs causing 1 inch compression
2. The preload absorbs the first part of any subsequent load. So 100lbs preload plus 50lb load does not move the spring further than 1.
3. But if I apply a 150lb load to an unloaded spring, it will compress 1.5 inches.

How does the spring 'know' the 100lb is preload, and use it to absorb further load rather than being further load in the first place?

I don't think your way can work!

Quote from tristancliffe :

You see, I believed that the preload compresses the spring 1 inch (100lbs force). Adding a further 80lbs will then compress it 0.8 inches MORE (1.8 total compression), and that 120lbs of extra load would be a 1.2" compression (2.2" in total). I think the preload doesn't absorb any extra load.

If that was the case then it would be impossible to adjust sag on motorcycles.

What you are imagining is pushing your car down with 100lbs of force, compressing the springs, and "freezing it" magically so therefore it always stays down. So yeah, if you apply an extra 80lbs of force externally it will go down 0.8" more. However the preload force is not external to the system described above. It's a force inside the system which is the spring forcing itself against the collar and the collar forcing itself against the spring. So you effectively have 100lbs to counter when pushing downwards externally.

All of the above can be total, caffeinated hogwash. Which it likely is. But I'll just leave it there because I like my foot in my mouth.

It only comes into effect if the spring is strong enough to extend the spring assembly to its maximum length even if the car is resting on it.

Simple example:
5 inch spring @ 200lbs/in, contained in a spring/shock absorber assembly with a max length of also 5 inch
car weight at wheel: 100lbs
preload: 1 inch (200lbs)

The spring inside the assembly is preloaded with 200lbs (this load is actually held by the shock absorber which resists the assembly from becoming longer than 5 inches). Now you put the car weight on. The car weight is only 100lbs, so it won't move the spring down, because as soon as you press it together, you take all the load off from the shock absorber and instead put it against the car weight. In fact, even if you add another 100lbs nothing would happen. Only after then you finally reach the critical load value where the spring actually starts compressing.

So if your car is always heavy enough to compress the spring, the preload does nothing but alter the ride height. If it is not, it will basically sit on the fully extended spring assembly and feel like it has very hard springs, up to a point where the load is high enough to actually compress them.

Quote from tristancliffe :

I shall test it, with a valve spring.

How does the spring 'know' the 100lb is preload, and use it to absorb further load rather than being further load in the first place?

The spring doesnt know... the "preload" stops pushing the spring if you want to compress it more


Edit
I am useless at drawing especially using the mouse… but I thing that you can understand what I am trying to say.