What is the purpose of pre-load on springs?
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  1. #1
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    Default What is the purpose of pre-load on springs?

    On another thread I'm grappling with setting ride heights and stiffening suspensions on an R12.

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    As many know there are monster springs packed into a fairly small space in the front of these cars. Why not go with shorter stiffer springs. I suspect its something to do with allowing longer more progressive travel but its a bit of a dark art for me.

    P
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    1000+ Posts schlitzaugen's Avatar
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    Nah, it's just a way they found of using a smaller elasticity constant (F=-Kx where k is your constant, x is travel and F is the force deflecting the spring by amount x) to give a smoother rather than harsh ride whilst keeping the car fairly high off the ground and allowing for some loading at the same time.

    These springs will compress more under a load that might not cause any deflection of the suspension in a car with stiffer springs (at similar preload and extent). Of course, the more load, the more compression. The huge preload these springs are given means they would take a air bit of load before they compress any further though.
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    Default Preload

    Quote Originally Posted by Exfrogger View Post
    On another thread I'm grappling with setting ride heights and stiffening suspensions on an R12.

    As many know there are monster springs packed into a fairly small space in the front of these cars. Why not go with shorter stiffer springs. I suspect its something to do with allowing longer more progressive travel but its a bit of a dark art for me.

    P
    Hi
    Springs can be a bit hard to get your head around. They can be short and stiff or long and soft. That's the manufacturers choice. As well, the spring does not always act out at the wheel. As in the 12 the spring acts only part way along a lever suspension arm, so the spring has to be stronger, by the length ratios, to apply the required force at the wheel.

    The actual preload of any spring however has to be sufficient to equal the car load on each wheel. So if the spring is soft then it needs to be 'squashed' a lot to get the required force to lift the car to the static height. All car springs have preload and have to be approached with caution. Long soft ones like the French use are very hard to handle.

    If you want some extra height but the same spring rate, you can use a spacer under the spring. It needs to be strong enough to take the weight and shaped to sit in the spring holder seat. I have made some out of plywood in the past to raise a car up a bit, about 30mm, for rough roads in a third world country. Was OK for the years I was there. Probably a cast or fabricated aluminium spacer would be OK.

    jaahn

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    Thanks guys.

    Shiltzie, you may have to speak louder and slower but I think I get the gist, well sort of...

    I have some tools for handling the R12 springs and have done it a few times now but it remains a job I hate. There's a lip in the R12 shocker that carries the spring perch. I had thought of machining up a spacer as one of our options. I might use a little more than ply wood, tho I do like the African solution...

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    1000+ Posts schlitzaugen's Avatar
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    In short, it's to give a soft ride. French chicks like that.

    Maybe this helps:

    https://en.wikipedia.org/wiki/Hooke's_law

    What is right now the distance from ground to top of wheel arch (at wheel center, steering dead straight) on the car you want to modify?
    My 12 wagon sits quite high (55.5cm rear, 63cm front measured as above, tires about 2bar, have some stuff in the back), but I understand the previous owner has replaced rear springs with some sort of Nissan item. Not sure if fronts have been modified in any way.

    I also am of the understanding that front R17 springs will jack up a 12 a fair bit.

    Overseas (Europe), there are some hard rubber spring seats you can add on the bottom spring perch to the rear of wagons that will give you about 2cm (from memory, unsure, might be more) extra clearance but I don't know of anything for the front.

    If you don't need to take the springs out and only want access to driveshaft or lower wishbone, there's another way. Get yourself some serious threaded rod (I think Bunno's carry grade 10 or at least 8.8 in M10) and stick it through the hole at the top of the strut (has a rubber/plastic cap) tower, bolt the bottom to the upper suspension arm with one of the balljoint bolts and wind it up with a couple of nuts atop the shock tower. That'll take care of it in a safe manner and you can mess around with driveshafts, hubs and lower wishbone.
    Last edited by schlitzaugen; 25th June 2013 at 01:58 AM.
    ACHTUNG ALLES LOOKENPEEPERS

    Das computermachine is nicht fur gefingerpoken und mittengrabben. Ist easy schnappen der springenwerk, blowenfusen und poppencorken mit spitssparken. Ist nicht fur gewerken bei das dummkopfen. Das rubbernecken sightseeren keepen hands in das pockets-relaxen und watch das blinkenlights.

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    so the answer is that preload doesnt have a purpose as such, but is simply the weight of the car on the spring?

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    Quote Originally Posted by alexander View Post
    so the answer is that preload doesnt have a purpose as such, but is simply the weight of the car on the spring?
    Hi Alexander
    Well yes.
    the spring has to be in it's working range in the static position. It has to be able to go up and down to follow the road surface. So the spring is squeezed or preloaded to equal the car load at the wheel at the required length. If it is preloaded too much the suspension tops out going over humps because the down travel is not enough.

    An example is the small rear engined Renaults. The front suspension sits high in its travel. They always ride better with some weight in the front because the suspension is then preloaded to a mid travel position. Or cut a bit off the springs. IMHO

    jaahn

    "Maybe this helps: https://en.wikipedia.org/wiki/Hooke's_law"
    Maybee not Read that and prepare a short summary for tomorrow !!!!
    Last edited by jaahn; 25th June 2013 at 10:32 AM.

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    Real cars have hydraulics DoubleChevron's Avatar
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    careful with the renault springs. I've even heard of suspension specialists refusing to touch Fuegos 'cos they have nearly killed themselves in the past trying to remove the springs. It sounds like the full length of some Renault springs is longer than some of the spring compressors used to remove them .... so they go "BANG" in a big way when you try to remove them from the spring compressor

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    That is when you use two sets of spring compressors, the rationale is to unload the initial set fully whilst a second set controls the whole 'shebang'. Sorry for the crap joke.... Brendan.

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    1000+ Posts schlitzaugen's Avatar
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    Quote Originally Posted by alexander View Post
    so the answer is that preload doesnt have a purpose as such, but is simply the weight of the car on the spring?

    Actually it does. It decides the point where the spring will work. If you open the link I put up above, you will find the spring deflection vs force graph. Preload is going to determine at what point on that graph is the spring going to be set initially. Around that point (up or down the curve depending on forces applied) is the domain where the spring is engineered to spend its working life.

    PS. Didn't see jaahn's answer. Pretty much the same thing.
    ACHTUNG ALLES LOOKENPEEPERS

    Das computermachine is nicht fur gefingerpoken und mittengrabben. Ist easy schnappen der springenwerk, blowenfusen und poppencorken mit spitssparken. Ist nicht fur gewerken bei das dummkopfen. Das rubbernecken sightseeren keepen hands in das pockets-relaxen und watch das blinkenlights.

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    1000+ Posts jo proffi's Avatar
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    Quote Originally Posted by alexander View Post
    so the answer is that preload doesnt have a purpose as such, but is simply the weight of the car on the spring?
    I thought what you have described is load.
    My trumpet valves have springs in them and i can adjust the pre load by stretching the spring. There is no weight on the valve when it is not pressed, just like when your car is on a 2 post hoist, but there is pre load in the way the springs are contained.

    Whilst i know close on nothing about the performance characteristics of car springs, adding pre load to my trumpet valve springs makes a massive difference to the feel and function of the valve, and is usefull for overcoming friction in a slightly out of condition horn, or for when the monel valves get the dreaded pox and little blisters in the surface cause the valve to bind (but only in slow sensitive bits where any glitch is terribly exposed)
    Jo

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    Hi,
    My answers re preload were in the context of car springs and in particular R12s. Should we split hairs about force and load.

    Of course preload is an important parameter in setting up a spring for a particular use. Springs are everywhere. You may wish for a valve or the like to be immovable up to a set force. Then preload is used. Car hatch struts are another example.

    Cheers jaahn

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    would someone care to state a concise definition of 'preload'?
    in what units is it measured?

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    1000+ Posts Kim Luck's Avatar
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    I think that the Newton would be perfect for little car springs, whilst DekaNewton would be perfect for big fat ones.
    Don't it always seem to go that you don't know what you've got 'til it's gone............

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    Default Pesky preload !

    Quote Originally Posted by alexander View Post
    would someone care to state a concise definition of 'preload'?
    in what units is it measured?
    Hi Alexander,
    To answer backwards.

    The current accepted unit for force is the Newton.
    From Wiki; 1 N is the force of Earth's gravity on a mass of about 102 g = (1⁄9.81 kg).
    On Earth's surface, a mass of 1 kg exerts a force of approximately 9.81 N [down] (or 1.0 kilogram-force; 1 kgf = 9.80665 N by definition).
    The approximation of 1 kgf corresponding to 10 N (1 decanewton or daN) is sometimes used as a rule of thumb in everyday life and in engineering (as Kim said). Kilo Newtons, kN, have been my standard thinking. 1 kN is about the force of a solid man’s weight( ~100kG).


    Preload. in the context of car springs.

    Picture a car on a hoist with no springs. Fit springs which can hang down freely. Attach the suspension and wheels. The wheels will be much lower than normal relative to the body height. Now as the car is lowered to the ground the springs take the weight and compress until they support the car at its normal height. The springs are now preloaded by the weight of the car and are at their working height.

    You can bolt on the shockies. Now if you lift the car up again, the wheels will not droop so far because the shockies limit the spring travel downwards. So with the wheels off the ground the springs are still preloaded by some amount because the shockies do not allow them to fully extend.

    Most cars these days have spring/shockie units which have to be assembled before they are fitted to the car. So the spring has to be compressed or preloaded by some means before you can fit it to the shockie and bolt it up, because the shockie will not extend far enough for the spring to fit otherwise.
    Jaahn
    Last edited by jaahn; 26th June 2013 at 09:35 PM.

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    Perfect. Thanks.
    Next question: is the exact quantity of preload of any practical significance? Obviously it has to be less than the load applied by the car, or the car will sit with the damper fully extended. And i suppose sufficient difference so the unloading caused by the motion of the car when travelling, does not cause the load to fall to the level of preload, or the damper will fully extend.

    Is there more to it?

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    Last edited by alexander; 26th June 2013 at 07:13 PM.

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    1000+ Posts schlitzaugen's Avatar
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    I think the most important point is the one I made earlier about keeping the spring within a certain space on that elasticity curve over the entire range of possibilities in its service life. Kinda like polarising the base of a transistor. There's a whole bunch of voltages that will open it, but if you want to keep it about a certain point on its characteristic curve, then you need to set the base voltage where you want it.
    ACHTUNG ALLES LOOKENPEEPERS

    Das computermachine is nicht fur gefingerpoken und mittengrabben. Ist easy schnappen der springenwerk, blowenfusen und poppencorken mit spitssparken. Ist nicht fur gewerken bei das dummkopfen. Das rubbernecken sightseeren keepen hands in das pockets-relaxen und watch das blinkenlights.

  18. #18
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    Quote Originally Posted by alexander View Post
    Perfect. Thanks.
    Next question: is the exact quantity of preload of any practical significance? Obviously it has to be less than the load applied by the car, or the car will sit with the damper fully extended. And i suppose sufficient difference so the unloading caused by the motion of the car when travelling, does not cause the load to fall to the level of preload, or the damper will fully extend.
    Is there more to it?
    Hi Alexander
    In general you are correct. The exactness of the preload depends on how exact you require the ride height. Manufacturers use spacers to adjust the ride height for different models and options as well as different springs or selected ranges of spring lengths(usually colour coded).

    Most cars had the longer springs fitted to the drivers side as normal.

    Jaahn

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