Coil Spring compression and lift 101

Donstar · March 20, 2016

How does a shock absorber designed to compress a coil spring (Bilstein 5100, Pro Runner SS) cause the front to lift. It seems counter intuitive.

Chris · March 20, 2016

How does a shock absorber designed to compress a coil spring (Bilstein 5100, Pro Runner SS) cause the front to lift. It seems counter intuitive.

How does Posi-Trac on a Plymouth work? No one knows, it just does.

Donstar · March 20, 2016

How does Posi-Trac on a Plymouth work? No one knows, it just does.

I totally understand your point, but I think this is one of those mechanical marvels I could comprehend with a little help.

Chris · March 20, 2016

I totally understand your point, but I think this is one of those mechanical marvels I could comprehend with a little help.

The body length is greater on the new strut. In past when shocks and springs were separate, the spring determined height, now it is the strut assembly that does.

silveradosid · March 20, 2016

by compressing the spring it pushes the bottom of the shock downwards which pushes the front of the truck up

Mack207 · March 21, 2016

The body length is greater on the new strut. In past when shocks and springs were separate, the spring determined height, now it is the strut assembly that does.

Not true. The body length of the 5100's are longer than stock, but the body length does not determine the height. The spring setting determines height. That's how the bottom setting is still 0 lift.

by compressing the spring it pushes the bottom of the shock downwards which pushes the front of the truck up

Somewhat correct. The spring is compressed into a smaller space, which essentially makes it a stiffer spring. Or you can think about it as the spring being further away from its fully extended state, which gives it more potential energy. That's also why the ride becomes stiffer the higher the setting on the 5100's is.

Donstar · March 21, 2016

by compressing the spring it pushes the bottom of the shock downwards which pushes the front of the truck up

by compressing the spring it pushes the bottom of the shock downwards which pushes the front of the truck up

This makes sense but then the spring is not really any more compressed than stock. The spring remains similar in length. A spacer uses the same principle in creating lift by extending the length of the shock. I wonder why some people report a firmer ride with leveling shocks whereas spacers with the equivalent lift make no difference in ride.

Mack207 · March 21, 2016

This makes sense but then the spring is not really any more compressed than stock. The spring remains similar in length. A spacer uses the same principle in creating lift by extending the length of the shock. I wonder why some people report a firmer ride with leveling shocks whereas spacers with the equivalent lift make no difference in ride.

When set on a setting above "0 lift" on any of the leveling shocks, the spring is compressed more than stock. That's where the lift comes from. The ride becomes firmer because of this, and because the angle of the control arms is greater to the frame and shock.

Think of how aftermarket coilovers work. The height can be adjusted with a spanner wrench by adjusting one of the spring perches. Lower the spring perch, the spring is less compressed, the ride height drops, and the ride becomes softer. It's the opposite for raising the spring perch.

Some people with the spacer lifts report their ride became more firm as well. This is again because of how the suspension moves in an arc.

Donstar · March 21, 2016

When set on a setting above "0 lift" on any of the leveling shocks, the spring is compressed more than stock. That's where the lift comes from. The ride becomes firmer because of this, and because the angle of the control arms is greater to the frame and shock.

Think of how aftermarket coilovers work. The height can be adjusted with a spanner wrench by adjusting one of the spring perches. Lower the spring perch, the spring is less compressed, the ride height drops, and the ride becomes softer. It's the opposite for raising the spring perch.

Some people with the spacer lifts report their ride became more firm as well. This is again because of how the suspension moves in an arc.

This is interesting. Comments like "... the spring is compressed more than stock. That is where the lift comes from..." is why I asked the question. To me compressing something inherently makes it smaller (shorter). It makes no sense to me how squeezing a spring into a smaller space can create lift. Chris and Silveradosid suggest that it is the increased length of the assembly that creates the lift. This tells me that the spring is not being compressed. The shock is being extended and the truck's weight is resting on shocks which are in a more extended state than stock. This would support your statement that when you lower the perch, the ride height drops - as the shock is less extended. If this understanding is correct then the function of the spring will remain the same between stock and level but the difference in shock quality may be noticeable. If using spacers, the ride should remain the same except length of shock travel may be noticeable.

Mack207 · March 21, 2016

This is interesting. Comments like "... the spring is compressed more than stock. That is where the lift comes from..." is why I asked the question. To me compressing something inherently makes it smaller (shorter). It makes no sense to me how squeezing a spring into a smaller space can create lift. Chris and Silveradosid suggest that it is the increased length of the assembly that creates the lift. This tells me that the spring is not being compressed. The shock is being extended and the truck's weight is resting on shocks which are in a more extended state than stock. This would support your statement that when you lower the perch, the ride height drops - as the shock is less extended. If this understanding is correct then the function of the spring will remain the same between stock and level but the difference in shock quality may be noticeable. If using spacers, the ride should remain the same except length of shock travel may be noticeable.

It does make the spring shorter, but it gives the spring more energy. You're putting a spring designed for a certain length between coil perches into a space where the coil perches are closer together. This gives the spring more kinetic energy because it's further away from its resting, fully extended state, and from its designed load state. The higher you raise the coil perches, the more energy the spring has, the higher the lift, and the stiffer the ride.

If the overall shock length was the determining factor, it would be the same as adding a spacer to the shock. The only difference in ride would be from the shock valving. But people mention the stiffer ride from having the 5100's set on the highest setting, so we know that the shock length cannot be the provider of lift. If it was, then the ride would be the same regardless of height. We also know that it can't be the shock length because the 5100 has 4 adjustable settings, from 0" lift to 1.8" of lift. Raising the setting does not lengthen the overall shock length, it raises the bottom coil perch so it is closer to the top perch, which gives the spring a smaller place to sit in than designed.

Take a spring you can compress with your hand or finger, and compress it just a little bit on a flat surface. When you move your hand/finger, the spring will bounce upward a little. Then take the same spring, same hand/finger and compress the spring halfway. Remove your hand/finger quickly, the spring will bounce much higher. Same principal.

Donstar · March 21, 2016

If the springs remain compressed or confined to a smaller space then how is lift generated? The extra energy created by raising the spring perch must be released somewhere in order to keep the truck in a permanently raised position. The springs are either pushing the truck up or the suspension down.

Mack207 · March 21, 2016

If the springs remain compressed or confined to a smaller space then how is lift generated? The extra energy created by raising the spring perch must be released somewhere in order to keep the truck in a permanently raised position. The springs are either pushing the truck up or the suspension down.

Well, yeah. It depends on how you want to look at it. The spring is lifting the truck by pushing down on the suspension. It's not an either/or situation.

Mack207 · March 21, 2016

I think you need to look at how the coil-over assembly works. You have a shock with a piston that moves, and around it is the spring. The spring rest on the fixed lower spring perch on the stock shocks, or the adjustable lower spring perch on 5100's. The lower spring perch is connected to the body of the shock, which is connected directly to the lower control arm. There is no vertical movement here. The top of the spring pushes against the top spring perch, which is the same for stock and 5100's. The top spring perch is connected to the piston rod of the shock, which in the one moving part of the shock. When the lower spring perch is raised, the spring pushes against the top spring perch with more energy, which keeps the shock extended more when it's under weight. That's where the lift is from.

Jon A · March 21, 2016

The spring is only compressed more at full droop. At normal ride height it is compressed exactly the same amount. And yes, it's exactly the same rate (stiffness). It's like somebody lifts your floor 1 foot--you don't get any taller or shorter (or stiffer), but your head is now located 1 foot higher.

The rougher ride is not from the spring being stiffer. It's not. It is from having less extension travel. You hit the limit of this travel more often and when you hit it, it hits harder because at full droop, your spring does have more preload and hits the limit with more energy.

A spacer level kit using "outside the coilover" spacers adds the same amount of extension travel as it lifts so this issue does not occur. The limit comes when you go more than 2-2.5" your extension travel now becomes limited by the UCA stop and/or balljoint bind and the same rough ride issue begins to occur.

Mack207 · March 21, 2016

The spring is only compressed more at full droop. At normal ride height it is compressed exactly the same amount. And yes, it's exactly the same rate (stiffness). It's like somebody lifts your floor 1 foot--you don't get any taller or shorter (or stiffer), but your head is now located 1 foot higher.

This doesn't explain how you gain height though. If it's compressed more only at full extension, then it's not really doing you any good except making the shock bottom out harder. If it was compressed the same as stock, you would still gain lift with the 5100's set on the lowest setting because the 5100's are longer than the stock shocks.

The rougher ride is not from the spring being stiffer. It's not. It is from having less extension travel. You hit the limit of this travel more often and when you hit it, it hits harder because at full droop, your spring does have more preload and hits the limit with more energy.

Ok, maybe saying "the spring becomes stiffer" isn't the correct phrase. The spring acts like it is stiffer as you raise the settings. With a properly valved shock, like the 5100's this hard bottoming out doesn't happen nearly as bad. You can feel the difference on compression and rebound as you raise the settings. The shock will bottom out harder, but it can't bottom out harder without being harder to press up. Physics doesn't work that way.

A spacer level kit using "outside the coilover" spacers adds the same amount of extension travel as it lifts so this issue does not occur. The limit comes when you go more than 2-2.5" your extension travel now becomes limited by the UCA stop and/or balljoint bind and the same rough ride issue begins to occur.

A spacer kit does not add any travel. It only adds lift by extending the overall length of the shock, which is what I think you're trying to say. The ride will be rougher if you are over 2-2.5" because of what you said and also because of the angles that the suspension is working at when being compressed. The angle is steeper, which means it takes more energy to compress the spring, which translates to a rougher ride.