preload estimation

[chmercer]

Im trying to find out how much the chassis will move if i preload my springs .5 in. all the simple equations i know of are used to find the spring constant with a certain weight placed on the spring, etc. but im having a hard time figuring out how much the spring is actually pushing back. i dont actually have the springs with me but im estimating that the unloaded height is around a foot, and it will have roughly 650 pounds of load (2600/4) and as i already said .5" of deflection. spring constant of 8kilos, ~ 450 lb/in.

does anyone know the math for this? im dumb :/

[Exar-Kun]

"Im trying to find out how much the chassis will move if i preload my springs .5 in"

?

You're precompressing the spring down .5in, you'r car will sit .5" lower..or amybe I'm misunderatanding your question

.. if the spring rate is constant, the spring would theorhetically push back with 225 pounds of force(since the spring will compress 1" for every 450 pounds of force on it, you're compressing it half an inch, thus to do that would take 225lbs of force).

-ChetMybe explain a bit clearer and I can help more?

[chmercer]

ah sorry. what i mean is i would be compressing the spring .5" up towards the body, so it will slightly raise the chassis, trying to figure out a rough estimate of how much.

[McRussellPants]

You're precompressing the spring down .5in, you'r car will sit .5" lower..or amybe I'm misunderatanding your question

Errrr.... what?

When you preload the spring your car will sit higher.

Preloading the spring 1cm should be like taking 80kgs of weight off of the front end. (R]ride height wise)

so I'd bet that with ~80kgs of preload your car will be 1cm higher.

[naed240sx]

^yes true, unless it extends the shock down further by doing so. Here is a proof:

ok, the spring will be pushing with 225 pounds of force, as exar kun said, so it will take 225 pounds of pressure to even begin to compress the spring when the car is dropped back down on its wheels. So... it will have the same effect as making your car 225 pounds lighter(as far as ride height goes). Assuming your car weighs 2700 pounds, and has a 25-25-25-25 weight distribution at each corner, that is 675 pounds at each corner. So, typically, with 0 preload, your car should compress the springs (625/450=1.5) inches. With the preload, you can subtract 225 from the 675 to get 450. 450/450=1, so it will raise your car 0.5 inches.

This is of course assuming that your shock is no more extended with the preload when your car is off the ground. I am not sure if it extends when you preload or not, but I am assuming that it would extend .5 inches, causing your car to sit at the exact same height.

mmmmm Physics

[McRussellPants]

[QUOTE=naed240sxThis is of course assuming that your shock is no more extended with the preload when your car is off the ground. I am not sure if it extends when you preload or not, but I am assuming that it would extend .5 inches, causing your car to sit at the exact same height. [/QUOTE]

It wont... to have preload you have to have something to load the spring against.

[naed240sx]

ahh very true haha. so 0.5 inches it is.

[C-Kwik]

I'm not sure what you mean by preload, but if tyou are using the same springs and change the spring perch height, then the car's height will change the same distance.

If you are referring to shortening the static unloaded spring length by sandwiching the spring tightly between the upper and lower spring perches, then ultimately, the spring will still compress down to the same loaded static spring length. So in this case, the spring will still hold up the car based on the lower spring perch height. The exception would be if you compressed the spring to the point that the loaded spring length was shorter than the weight on the spring would compress the spring normally.

[naed240sx]

He is talking about compressing the spring with the upper and lower perches. It will make a difference in how far the car is able to compress them, because The spring will be exerting a force opposite to what the car is exerting on them. The preload of the spring would cancel out some of the force of the car, causing the ride height to he higher.

[Red coupe]

CA=W/SRCA= compression amountSR= spring rateW=Weightso 650lb ~ 295kg.5" = 12.7mmand assuming and 8kg/mm springThis also means that W=CA*SRSo we can find out how much weight is being placed on the spring for a given amount of compression

so 0 preload295kg/8kg=36.875mm

Now preload 12.7mmso 12.7mm*8kg/mm=101.6kg of extra pressure on the springso then tension pur on the spring by comressing it 12.7 mm pluss the weight of the car = 101.6+295=396.6kg so 396.6kg/8kg=49.575mm~~50mm of compression

So unless I messed somethen up half an inch of preload would be ~ 50mm - 37mm = 13mm lower, also note that I rounded to the nearest mm in my calculations...so looks like .5 will be .5 lower, sounds right to me.

All math aside remember its a linear spring. Each mm of compression reqires the same amount of weight..

Errrr.... what?

When you preload the spring your car will sit higher.

Preloading the spring 1cm should be like taking 80kgs of weight off of the front end. (R]ride height wise)

so I'd bet that with ~80kgs of preload your car will be 1cm higher.

How the hell would your car end up higher... don't speak with such conviction if your not positive.
Modified by Red coupe at 1:04 AM 10/13/2005

[Exar-Kun]

Woute for mathematics!

-Chet

[McRussellPants]

Red Coupe What the hell are you talking about?

The way I think of it is that -

80kg of wieght will compress an 8kg/mm spring 10mm. If you compress a spring 10mm it will take more than 80kg to move the spring further.

so basically with 10mm of preload, your car will sit as high as if 160kg had been removed from the front of the car. (two springs)

The comment that preload lowers a car is completely false. I have my coils preloaded about 1cm on each coner and it definatly raises the car. If you dont believe me - Kuah tells people with KTSs to set the ride height ~1cm lower than you want it with no preload and then to set the final height by adding preload.

[Red coupe]

CA=W/SRCA= compression amountSR= spring rateW=Weightso 650lb ~ 295kg.5" = 12.7mmand assuming and 8kg/mm springThis also means that W=CA*SRSo we can find out how much weight is being placed on the spring for a given amount of compression

so 0 preload295kg/8kg=36.875mm

Now preload 12.7mmso 12.7mm*8kg/mm=101.6kg of extra pressure on the springso then tension pur on the spring by comressing it 12.7 mm pluss the weight of the car = 101.6+295=396.6kgso 396.6kg/8kg=49.575mm~~50mm of compression

So unless I messed somethen up half an inch of preload would be ~ 50mm - 37mm = 13mm lower,

If you would like to point out any errors we can discuss. Like I said I just did the math, which supports Exar Kuns statement, as well as my own logic.Again I am no expert, and can only go off of the model I have provided here. If you would like to provide ANY proof, be it mathematical or imperial we can try to see were I went wrong, but hear-say isn't enough to sway my opinion.

how much did this ~1cm raise your car?

[Exar-Kun]

I think the dissagreement is on how you're talkinga bout preloading it. Are you adjust the top perch, both pereches, using a helper spring, what?

In any case, a spring rate, if constant works like this (on a 150lb/in spring)

1" comrpession requires 150lb2" - 300lb3" - 450lb4" - 600lbetc, etc, etc.

Now, if you preload the spring, let say 150lb worth, it will (if you repload it via the top mount and spin it downward) require you to put 150lbs of pressure on it, compressing it 1". So when the car sits on it, you're addition whatever additional pressure to the sping. Chmercer is talkinga bout sing the lower spring perch, not the upper like I initially thought... in this case, the car would still sit as it normally would once the spring is loaded. IE he's moving the lower perch up .5", so the spring is now pushing upwards with that addiitonal force, but once the car sits on it, it should sit .5" higher....

HOpefully that made some sense.-CHet

[Red coupe]

he's moving the lower perch up .5", so the spring is now pushing upwards with that addiitonal force, but once the car sits on it, it should sit .5" higher....-CHet

So even with the preload, the final compressed legnth of the spring is the same, so the car is moved upwards the same amount as the lower pertch? once the spring is comressed passed the initial amount added by the lower pertch adjustment the pre-loading doesnt effect spring rate or amount of compression does it?

[McRussellPants]

If you would like to point out any errors we can discuss. Like I said I just did the math, which supports Exar Kuns statement, as well as my own logic.

how much did this ~1cm raise your car?

I didnt even really read your math since I know for a fact that preloading a spring will not lower a vehicle.

Edit - that came off rude... but I'm not trying to be

I preloaded the spring about 1cm and lowered the ride height at the same time so I cant give an exact measure ment. however there is way less spring compression when I lower the car onto the spring. it used to droop about 3-4in before (spring tight but at free height) ... now it compresses about 1-2in. (10-15mm of preload).

So even with the preload, the final compressed legnth of the spring is the same, so the car is moved upwards the same amount as the lower pertch? once the spring is comressed passed the initial amount added by the lower pertch adjustment the pre-loading doesnt effect spring rate or amount of compression does it?

The height of the spring changes the by amount you preload it... the shock however stays the same length.

Pre load in relation to corner weighting is different, where preload is adjsuted to get the shocks to be supporting about the same weight on each side of the car. and can be done through the bottom or top mounts.

Now, if you preload the spring, let say 150lb worth, it will (if you repload it via the top mount and spin it downward) require you to put 150lbs of pressure on it, compressing it 1".-CHet

This is where I don't nessicarily agree with. when you compress with something like a shock (where it will collapse but not expand) where it isnt weight, compressing the spirng an 1inch there will be 150lbs of force pushing on whats compressing the spring... meaning that if you set something that weighs 150lbs on top of it the spring is already pushing back with the same force. I hope this explains why preload will raise a car. I dont know how much sence that makes to someone esle (it does in my head if it doenst I'll try and think of a better way to explain it)

[Exar-Kun]

Found this doing reseach to clarify it myself.

"Well in that case, you are not affecting the spring preload anyway, but merely the ride height. All four springs together can only be preloaded by a fixed amount equal to the unsprung mass times the force ratio of the suspension on the spring perch. The only way to change the preload is to add preload to one or two springs and remove an equal amount from the other(s). The only way that can happen is by raising or lowering the perches of diagonal pairs of springs. So if you raise the 400lb/in spring perches one inch on the LF-RR pair, they each compress 1/2" and gain 200lbs of extra preload, while the RF-LR pair extend 1/2" and lose 200lbs of preload. If you raise all four, no springs are compressed and therefore there is no added preload, but rather the sprung mass is raised 1" at the upper spring perch and the springs remain at the same installed height."

Talking about rasing the lower perches.

More:

Okay, to clarify, this is not correct :

"If you have the unloaded spring already exerting 400lbs on the perch, then putting 800lbs of opposing force on it won't compress it 2". Instead...

Quote:

"Alternatively, there is already 400 lbs of preload, so the spring compresses an additional 1 inch... "

"Bingo! What is getting overlooked in this thread so far is that the spring perches are not what preloads the springs once the coilover is installed in the car. If you were to preload the springs so much with the perches that the car's weight didn't compress the springs at all, your car would have no rebound suspension travel at all. I've never heard of a car being set up like that, although perhaps in some specialized application like tractor-pulling it might be done.

The preload on an installed spring comes from the car's weight compressing the spring. You can't increase the total preload that all four springs have unless you make the car heavier. The only thing you can do is change the way the car's weight (it's unsprung weight, really) is shared among the four springs. And you can only do that across diagonal corners."

Ie, Chmercer, if he moves all the spring perches up .5" it will exert 150lb/in of force on the car, so the car will sit as if it was 150lb lighter than if the spring was unloaded... BUT THE NET EFFECT WILL BE THE SAME.

Ie, in the thread reference above..the guy puts 400lbs of preload on a 400lb/in spring, then sets a 800lb weight on it..it compresses a total of 2" (or an additional 1" from the preload height)..

which would be the SAME as not preloading it and putting the 800lb on it.

I stand a bit corrected about it, since I was thinking of it slightly differnetly, but had a tough time explaining

"So even with the preload, the final compressed legnth of the spring is the same, so the car is moved upwards the same amount as the lower pertch? once the spring is comressed passed the initial amount added by the lower pertch adjustment the pre-loading doesnt effect spring rate or amount of compression does it? "

Correct!... but this assumes he's 'preloading' all 4 of the springs.

YAY!-Chet

[McRussellPants]

I think that is more explaining Preload in relation to corner weighting and coilovers with only single height adjustment, but the points are valid.

Ie, Chmercer, if he moves all the spring perches up .5" it will exert 150lb/in of force on the car, so the car will sit as if it was 150lb lighter than if the spring was unloaded... BUT THE NET EFFECT WILL BE THE SAME.

Ie, in the thread reference above..the guy puts 400lbs of preload on a 400lb/in spring, then sets a 800lb weight on it..it compresses a total of 2" (or an additional 1" from the preload height)..

which would be the SAME as not preloading it and putting the 800lb on it.

Cool... Thats really the point I was trying to make... I just don't really type right with small text and 4hrs of sleep.

[sleepyRPS13]

is that the same as cross weight?http://www.ybapublications.co....n.htm

[C-Kwik]

For a given spring and car weight, the lower spring perch will determine the ride height. Even if you used a shorter shock piston rod, it would compress the spring by the amount the rod is shorter. But if you put enough weight on the spring to overcome the resistance the spring provides at the preloaded compression, then it will just simply push the spring to the same point it would if it had no preload. At this point, the length of the shock rod no longer has any effect on the spring length.

So if you installed a spring that compresses from 8" free length to a 4" length under the car's weight, then preloading the spring 1" would still result in the spring compressing to a 4" length under the car's weight(assuming same spring and weight. Putting Since the spring height will be the same compressed length, the only thing controlling the height then will the lower spring perch's height.