coil preload question

I think the above is still a valid question and did not see an explanation to it (sorry if I missed it). and yes, I am taking my new found knowledge of preload to the next level.

I don't think this is related to preload anymore, as we have clearly established that the spring is compressed to exactly the same amount regardless of 5100 spring seat setting. and as per hooke's law, a spring compressed 3" exerts the same amount of force, regardless if it is all 3" from the weight of the truck, or 2.5" from 5100 preload + 0.5 from truck weight. which BTW, makes this independent of the coil being progressive or linear, since the starting point is exactly the same, the spring is already compressed 3" either way, so why would a coil being progressive behave any different using preload?

I appreciate and thanks to all on the explanation of preload. (although I think it's just six of one, half a dozen of the other situation, as it can be explained in terms of length just as well)

 

Actually it is not 6 of one and a half dozen of the other. Geometrically, the spring is the same length, but the struts connecting points are farther apart than they were before you preloaded the spring with enough energy to lift the mass supported by the strut the distance the spindle has lowered away from the frame against the acceleration of gravity.

The suspension is a closed system, with the spring confined by the strut assembly. The spring is not attached only at one end. You are also stating two different systems, one at static load, and a second when the suspension is riding.

While the suspension is riding the length of the spring oscillates based upon loading by the mass of the vehicle accelerating up and down. If the mass of the truck is the same but the spring is now preloaded, the oscillation will be different for the same spring with two different preloads. There is less travel in a preloaded spring if the vehicle weight is the same.

You continually ignore facts and factors in each of your statements and questions.

This is not 'our' logic as you stated earlier, this is basic physics and earth science.

This would be a lot less painful is you would start from the other end. What result are you trying to determine or achieve? Are you trying to get lift and maintain stock travel distance? Are you trying to get lift and maintain stock ride?

 

thanks fifthwind. appreciate the explanation.

as to objective, I always try to completely understand a mod before undertaking it. I know most just take the mod instructions and do it without understanding why. I guess I'm just more inquisitive. An explanation with numbers and equations (scientific basis) will actually help me understand this easier, that's why I said boone's link helped a lot.

so from reading, I wanted to be clear on the relation of coil, shock and how it affects the lift (static part) and ride (dynamic part). I believe I now understand the static part, so will try to understand your explanation on the dynamic part.

let's establish some given.
weight of truck is the same
we use stock coil, so it K factor is the same.
we set 5100 at 2.5"

you said

The variable here is, how much does the truck weight load the coil (static)?

case 1: the weight of the truck only push down on the coil 1" (less than 2.5" setting of 5100).
In this case, yes I can see that the ride will be stiffer, considering the spring still has "reserve" amount of force to push up another 1.5" of equivalent force. A minor bump with minimal force will not move the coil at all, hence stiff ride. correct?

case 2: the weight of the truck push down on the coil 3" (>2.5" setting of 5100).
In this case, I say there is absolutely no difference in ride stiffness and the proof is in hooke's law. The coil is only preloaded for 2.5" of force, yet the truck weight puts down 3" of force, hence the coil will come down another 0.5". At this point, there is absolutely no difference had the 5100 been set to 0" since the "static" equilibrium point on the coil is compressed 3".

So I guess you are saying case 1 is what actually happens right?
if it is case1, then I see a contradiction since case 1 will never give you a lift of 2.5". Here's why

if after assembling the shock and spring (not yet installed on truck), with 5100 set at 2.5", the coil is compressed by 2.5" (to keep the total coilover length exactly the same as stock as others pointed out). Then you install it on the truck and the coilover does not budge (due to excess reserve forice), hence the total length stays the same. compared to stock where the coil comes down by 1" due to truck weight. This means the effective lift gained is only 1" no? if this is not correct, please use numbers to make the correction.

so the question is, setting to 2.5" really give you a 2.5" gain in lift?
I think it can only be a yes for case 2, in which case I still see no clear explanation (using science as basis, and not subjectively) why the ride will be stiffer.

thanks

 

yes nd4, that is exactly my understanding, in terms of "length". and it perfectly explains the lift gain of 1.75"

so in the end, the spring is compressed the exact same amount. so I just need to understand why there will be a difference in stiffness?

I don't want to rule the stiffness out as subjective so looking for a more scientific explanation.

 

Paralysis through over analysis.

Hopefully your sex life is a bit easier on your brain...LOL..

When you effectively INCREASE the spring rate by raising the spring perch, the coils do not compress as much, thus giving you lift...the strut is not any LONGER, but it has more 'force' exerted because the perch is higher, effectively raising the spring rate and stiffening the ride a bit, the more you raise the perch, the stiffer the ride, and the less it will compress when reinstalled........

Hope you get it all figured out...

 

You are forgetting the operation of the shock.

By preloading the spring the bottom of the shock is now farther away from the top of the spring, creating more volume below the piston of the shock. This larger volume must now pass through the same size orifice when the wheel moves up and down creating more resistance.

If you remove the shocks from the equation, then yes, there will be no difference.

This was mentioned earlier in the thread. This is also why there are various diameter shocks and valving (dampener) options available. Different wheel and tire combinations have different weights, resulting in different accelerations from the same force.

 

ok, I'm glad you brought that up. and again appreciate your patience in explaining this.

This is the specific statement I need clarification on.

By preloading the spring the bottom of the shock is now farther away from the top of the spring, creating more volume below the piston of the shock.

That is exactly my initial understanding, until someone pointed out that the total coilover length remains exactly the same with preloading. Which means the shock piston remains in exactly the same position, thereby causing the spring to be compressed, hence resulting total coilover length is the same. which does not make any sense, since as you pointed out and I totally agree, the shock is extended.

I was thinking it will be longer, exactly as you described, the shock will be extended, providing more volume below the piston, hence the total coilover length will be longer before installing on the truck. and also to me, explains the stiffer ride because the shock piston already moved, and 5100 being progressive, may explain the cause of stiffness as it will provide a stronger resistance (dampening effect) compared to the stock 0" position. or is the progressiveness only comes into play based on the speed of compression/extension of the shock and not the initial piston position?

so by setting 5100 to 2.5", does the shock get extended (before installing on truck)? if so, then the coil is not compressed?

 

based on this picture. if the spring seat is set to 2.5", the part below the spring seat extends 2.5", hence the shock is extended. the part covered that is inside the coil is now longer by 2.5", therefore the coil is not compressed, and the total lenght is now 2.5" longer. Now if that is not right, and the total coilover lenght is still the same as setting at 0", then that means the shock is NOT extended, causing the coil to be compressed by 2.5"

so which is right? compressed or not compressed?

 

1) All springs must be compressed to be mounted on struts, regardless if preloaded or not.

2) Preloading increases the distance from the bottom of the spring to the bottom of the strut, for nearly all coilovers, making the strut longer when installed. When not installed, it is possible that the piston will limit the expansion of the spring in which case the preloaded spring will be shorter than a spring that is not. Once installed the springs should be of equal length unless limited by the length of the piston. There are physical stops to prevent the piston from limiting travel either up or down if the shock is sized correctly. Allowing the piston to either bottom out or top out will destroy the shock in a short period of time as it is not designed to carry the weight of the truck or a bouncing hub, wheel, and tire assembly against the inside of the ends of the cylinder.

3) Look at some pictures of coilovers, you will see that the preloading is achieved by moving the lower retaining position of the spring.

 

x is the length from spring seat to bottom. setting at 2.5" will extend this by 2.5"
z is total coilover height
y is the height with weight of truck.

to add lift, we either increase X or increase Y-X. Since we use the same coil, and use 5100, we change the length of X.

x remains the same, whether coilover is installed in truck or not.
that means the only variable is Z-X. So it does not matter if the spring is compressed or not, if the spring K is the same, and the weight of the truck is the same, then adding the weight of the truck will compress the spring the exact same amount, which is Z-Y.

Therefore if original loaded height is Y, and if X is constant, therefore Y-X is constant.
extending X, to say X+2.5" will make the total coilover height
Y-X+ (X+2.5") = Y+2.5" ,hence a 2.5" lift.

it is independent of any preloading. even for arguments sake there is preloading before install, but once installed, it is all the same plus the extension below the seat.

That means the coil is subjected to exactly the same amount of force (the weight of the truck), regardless of 5100 spring seat setting.

now if you only look at the shock part in the picture. The normal length of shock at 0" is Z. If you set it to 2.5", the length is now extended to Z+2.5", hence the description about piston moving up. If the coil gets compressed the same amount (Z-Y), then it is clear the shock has been extended by 2.5" up.

when you hit a giant bump, will the front coil bump stop hit first before the coil actually bottom out and before the shock get into fully compressed position (piston all the way down)? I trust that bilstein will not allow the shock to go full compress before the coil or bump stop is hit first, even at 2.5" setting.


edit, additional comment.
point 1, yes, in the diagram, the coil is compressed and installed in the strut, but not installed on truck yet.
point 2, yes I realize it will take more compression to get the shock spindle through the top hole. but once bolted, the spring will come back to equilibrium position, I see you said it will be a little shorter since shock resists extension. I think you are saying there is some compression in the coil, but the overall length of the strut assembly (Z) is longer. It has to be less than 2.5" if there is some compression in coil.

 

LOL, you have more patience than me....

 

Still not getting it. NOTHING is changing length. All that's happening to get lift out of 5100's is moving the bottom spring seat up to preload the spring.

Look at it like this. If you jack up the front of the truck with the tires off the ground with the shock setting at 0", and you lower the truck back down it will lower to stock height. If the setting on the 5100 is set to 1.75 and you lower the truck back down it will settle at 1.75" higher than stock while the shock length has not changed. All that's changed now is you have 1.75" less of down travel in the shock. The preload is all that's creating the lift since the spring has more stress on it.

 

Awesome thread, props to all taking the time to explain it. I have found it to be a great read.

 

thanks bueller for the reply.

yes, I agree and follow that. you are extending the length of X by 1.75" below the spring seat. and in order to keep the same coilover length, the coil is pre compressed and does not come down as much when truck weight is added.

question here. if you say it now has 1.75" less down travel, this is not due to the shock getting fully compressed right? it is due to the frame hitting the bump stop right?
I guess I need help describing how if I set 5100 to 0" and use a longer coil spring to give exactly the same 1.75" lift will not lose the same 1.75" down travel.

is the distance of the bump stop to the lower control arm closer setting 5100 above 0"?


thanks.

 

You're confusing down travel with suspension compression. Down travel being the tire traveling down away from the frame.

 

ah ok. my mistake.
since the shock is pulled apart (extended) by 1.75", it then has 1.75" inch less to go longer, say when jacking the front up. (as fifthwind described this as the piston going up by 1.75" inside the shock)

will the shock just fully extend and hit the end? or will the coil will prevent the shock from fully extending?

I guess when I think of stiff ride, I was thinking of compression. so if it affects down travel, does it affect up travel? if so how? It should still have the full up travel range right?

 

this makes sense.

 

is the stiffness/smoothness of the ride a property of the shock dampening?
and is shock dampening affected by initial piston position?

it seems the anwer is yes to both?

 

First off, for simplicity's sake, we'll disregard motion ratio. The front wheel in reality moves at ~7:4 ratio with the shock.

Fully extend and hit the end. The shock is the droop limiter.

Top position of the wheel relative to the frame will be the same as stock (hitting the bumpstop). However, because the position of the wheel is 1.75" farther from the frame at static ride height, the wheel will have 1.75" more uptravel.

Ride quality depends on the type of pavement flaw. A pothole puts the suspension in extension. If the suspension reachs full downtravel while going over the pothole, the ride will be harsh.

 

You're getting closer. The location of the strut on the lower control arm is inboard of the hub but outboard of the pivot. The numbers I will use are round and I am unsure of the accuracy but they will do for an explanation. This geometry means that for every inch of shock travel you get 2 inches of wheel travel. If the shock can travel 8 inches this would mean 16 inches of wheel travel, but the bump stop and control arms will limit this to 12 inches so your shock will only use 6inches of the available 8 inches of travel. The coil spring allows for 9 inches of compression, but the stops only allow for 6 inches of compression. To get 3 inches of lift the lower shock body is moved down only 1-1/2 inches while the stops still allow only 6 inches of travel. If you use a longer travel shock the stops will still only allow 6 inches of travel along the strut, or 12 inches of travel at the wheel.