Coil Spring compression and lift 101

Donstar · March 21, 2016

Great discussion and very much appreciated. As an aside, I find it interesting that I am not seeing any information which justifies installing levelling shocks over spacers. I was under the impression that levelling shocks somehow would improve function as well as looks. Arguably the quality of the shocks may be an improvement from stock however they don't appear to be a better way to create a little front-end lift!

Mack207 · March 21, 2016

Great discussion and very much appreciated. As an aside, I find it interesting that I am not seeing any information which justifies installing levelling shocks over spacers. I was under the impression that levelling shocks somehow would improve function as well as looks. Arguably the quality of the shocks may be an improvement from stock however they don't appear to be a better way to create a little front-end lift!

The valving was my main reason to upgrade to 5100's. They are valved stiffer than stock, and digressively, which gives a better ride over rough terrain for extended periods (in my opinion). 5100's also have a lifetime warranty (which I've had to use before) and are the best ride I've found for under $300. I've used spacers before as well, and there was never anything wrong with them, they just did nothing to improve the performance or ride of the truck. Are they a better way? That's up for debate. My bloody knuckles from wrestling with a spring compressor tell me that spacers are the easier way, but that's no fun.

Jon A · March 21, 2016

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.

It is not at full extension when the weight of the vehicle is on it. When you jack up the vehicle by the frame, the front wheels do not immediately come off the ground, do they? They don't come off the ground until you use up your EXTENSION TRAVEL. This will happen sooner with the 5100's at a high setting because you have less extension travel available from static ride height.

At full extension the spring is compressed more, when the weight of the vehicle is on it the spring is compressed by the same amount. F=kX . That's how physics works.

Ok, maybe saying "the spring becomes stiffer" isn't the correct phrase. The spring acts like it is stiffer as you raise the settings.

It does not get stiffer, nor does it act like it. It's a linear rate spring--the rate does not change until you hit coilbind. The difference in ride quality is as described above; it has nothing to do with the spring getting stiffer. This is one of the most often repeated myths on the internet.

A spacer kit does not add any travel.

It moves your range of travel. This increases available extension travel and decreases available compression travel relative to the stock ride height, keeps them the same relative to the new higher ride height.

Mack207 · March 21, 2016

It is not at full extension when the weight of the vehicle is on it. When you jack up the vehicle by the frame, the front wheels do not immediately come off the ground, do they? They don't come off the ground until you use up your EXTENSION TRAVEL. This will happen sooner with the 5100's at a high setting because you have less extension travel available from static ride height.

I completely agree. There is less extension / droop travel with 5100's on the higher setting. This would be because the spring is pressing against the top coil perch with greater energy. Nothing else can explain the shock being compressed less when all other factors remain the same. The only thing that changes is the lower coil perch setting.

At full extension the spring is compressed more, when the weight of the vehicle is on it the spring is compressed by the same amount. F=kX . That's how physics works.

What same amount? When the weight of the vehicle is on it, the force is not equal to the force put on it when it's fully extended.

It does not get stiffer, nor does it act like it. It's a linear rate spring--the rate does not change until you hit coilbind. The difference in ride quality is as described above; it has nothing to do with the spring getting stiffer. This is one of the most often repeated myths on the internet.

On my previous vehicle, a Titan, a couple members realized that the springs off of 08+ models were different than the 07 and prior. This led to even more confusion when it came to 5100's. If you were to put a the 08+ spring on the bottom setting of the 5100's, you would get a similar lift and ride as putting the pre-08 springs on the middle setting. This is why I say it's like using a stiffer spring. I never said that the rate measureably changed; it only acts like its stiffer. With 08+ springs on the middle setting, I was able to run without a swaybar and have a comparable ride to stock with a swaybar. With the 08+ springs on the bottom setting, it was like riding in a boat it would sway side to side so much. This tells me that the compression becomes more difficult as the setting is raised.

http://www.titantalk.com/forums/titan-suspension/131439-5100s-install-tips.html#/forumsite/20504/topics/131439?page=1

It moves your range of travel. This increases available extension travel and decreases available compression travel relative to the stock ride height, keeps them the same relative to the new higher ride height.

Correct that a spacer moves the range of travel. It does not increase the extension travel. Only a shock longer than stock can increase the extension travel. That's why coil-over manufacturers like Icon have an "extended travel" c/o available. A spacer increases the overall length of the strut assembly, which pushes the entire assembly down. It's the same as taking the frame-mounted coil bucket off and welding it on lower than factory.

Just out of curiosity, what type of background do you have in suspension Jon A? All of what I'm posting is from personal experience from installing 5100's and spacers on GM, Ford, and Nissan vehicles. There was so much confusion on these on the Titan forum that I disassembled one of my 5100's just to take pictures and be able to explain the process.

Jon A · March 22, 2016

What same amount? When the weight of the vehicle is on it, the force is not equal to the force put on it when it's fully extended.

When the weight of the vehicle is on it, before the lift and after the lift, the weight of the vehicle remains the same, therefore the force compressing the spring remains the same, thus the displacement from the spring's free length is the same. There's no way around it.

On my previous vehicle, a Titan, a couple members realized that the springs off of 08+ models were different than the 07 and prior.

I haven't had time to go through the thread and figure out what the cause was but it could have been many things--the springs could have been variable rate, they could have gone into coilbind, etc, there are lots of possibilities.

Just out of curiosity, what type of background do you have in suspension Jon A?

I've got a degree in Mechanical Engineering and have been studying suspension design (as well as most other aspects of vehicle dynamics) for more than 20 years through more thousands of pages of text books, SAE papers, etc, than I can count. Suspension design has been an area of particular interest to me and I've designed, fabricated and raced on many suspension components over the years.

Donstar · March 22, 2016

Jon A and Mack207 - You guys are doing an amazing job of brainstorming. There is a lot of misinformation and contradictory explanations on this topic and you two are providing readers with a much greater understanding. Although you express some differences in understanding and experiences, your discussion will add clarity to anyone reading this thread.- Thank you! There is a comment by Jon A that I don't fully understand and was wondering if it is simply a case of how it was written. "At full extension the spring is compressed more, when the weight of the vehicle is on it the spring is compressed by the same amount."

Jon A · March 22, 2016

There is a comment by Jon A that I don't fully understand and was wondering if it is simply a case of how it was written. "At full extension the spring is compressed more, when the weight of the vehicle is on it the spring is compressed by the same amount."

OK, let's do an example to illustrate. Consider a suspension with a 1:1 motion ratio for simplicity.

You have a coil spring rated at 300 #/" and a free length of 10". You need to compress this spring 2" in order to get it onto the shock. Thus, when you hold the coilover in your hands, the spring is "preloaded" 600 lbs. This is the same condition the suspension sees at full droop.

The sprung mass of each front corner weighs 1500 lbs, so with the weight of the vehicle on it, the spring is compressed to 5" length--5" compression X 300 lbs/" = 1500 lbs. At static ride height, you have 3" extension travel (the first two were used up mounting it to the shock) before the shock reaches it's full length and tries to pull the front wheel off the ground.

Now, raise the bottom spring perch 1". It is now "preloaded" 900 lbs. But with the weight of the vehicle on it, it still compresses the spring to a total of 5" because 5X300 still equals 1500. But since the bottom of the spring is now 1" higher, the top of the spring is 1" higher thus the truck is 1" higher. However, in this case you only have 2" of extension travel before the shock is at full length and "topping out." So over rough terrain, you will hit that limit more often and when you do it will be at higher velocity with more energy.

Does that help?

Mack207 · March 22, 2016

I agree with the above. The length of the 5100 with the weight of the truck on it is a measurable object, and you can clearly see that the shock is compressed less at the higher settings. I think we're explaining the same thing, just looking at it different ways. I see it as one taking out the stock spring and swapping it with a stiffer spring, which would compress less with the weight of the truck on it, which means there would be less extension travel available, and more overall height. You're explaining it as increasing the preload of the spring, which is what 5100's actually do, and because the spring will only compress a certain amount with the weight of the truck on it regardless of setting, you gain height, still at the cost of extension travel.

The one place I think we disagree is on the compression stroke. My butt accelerometer tells me that the compression stroke is somehow affected as well, and you say it's impossible, other than the normal increased compression from the suspension moving in an arc and all the angles changing. If it wasn't affected though, I wouldn't have been able to run without a swaybar on my Titan without it feeling like a boat. I tried it first with the stock springs on the bottom setting, and immediately regretted it. I swapped them out for the 08+ springs on the bottom, and it became better, then found the sweet spot with the 08+ springs on the second setting. I highly doubt Nissan went through the trouble of engineering a variable rate coil for their red-headed stepchild of a truck.

It's hard for a shade-tree mechanic to debate a mechanical engineer though lol.

Edited to correct me saying the same thing twice in one sentence.

Spurshot · March 22, 2016

Mack, Jon has all the points spot on. The is no more or less energy pushing on the spring perch. The static force remains the same if the weight of the truck remains the same.

The other variable affecting ride quality is the A-arm angles. As they depart further from the horizontal plane, the mechanical leverage is reduced, then higher force (at the tire) is required to compress the suspension.

Regarding variable rate springs, probably most vehicles on the road in the past 50 years has them.

Donstar · March 22, 2016

OK, let's do an example to illustrate. Consider a suspension with a 1:1 motion ratio for simplicity.

You have a coil spring rated at 300 #/" and a free length of 10". You need to compress this spring 2" in order to get it onto the shock. Thus, when you hold the coilover in your hands, the spring is "preloaded" 600 lbs. This is the same condition the suspension sees at full droop.

The sprung mass of each front corner weighs 1500 lbs, so with the weight of the vehicle on it, the spring is compressed to 5" length--5" compression X 300 lbs/" = 1500 lbs. At static ride height, you have 3" extension travel (the first two were used up mounting it to the shock) before the shock reaches it's full length and tries to pull the front wheel off the ground.

Now, raise the bottom spring perch 1". It is now "preloaded" 900 lbs. But with the weight of the vehicle on it, it still compresses the spring to a total of 5" because 5X300 still equals 1500. But since the bottom of the spring is now 1" higher, the top of the spring is 1" higher thus the truck is 1" higher. However, in this case you only have 2" of extension travel before the shock is at full length and "topping out." So over rough terrain, you will hit that limit more often and when you do it will be at higher velocity with more energy.

Does that help?

Thanks. I was hung up on the part about the spring being "compressed more" at full extension. I am not clear as to the context. I believe you explained that when the vehicle is on a hoist (fully extended) the spring is at the same height (stretch, compress) as it would be if the vehicle was on the ground. The weight of the vehicle on the spring is the same on or off the ground. When you use the words "more compressed", are you comparing a height adjusted shock to a stock setting?

Donstar · March 23, 2016

OK, let's do an example to illustrate. Consider a suspension with a 1:1 motion ratio for simplicity.

You have a coil spring rated at 300 #/" and a free length of 10". You need to compress this spring 2" in order to get it onto the shock. Thus, when you hold the coilover in your hands, the spring is "preloaded" 600 lbs. This is the same condition the suspension sees at full droop.

The sprung mass of each front corner weighs 1500 lbs, so with the weight of the vehicle on it, the spring is compressed to 5" length--5" compression X 300 lbs/" = 1500 lbs. At static ride height, you have 3" extension travel (the first two were used up mounting it to the shock) before the shock reaches it's full length and tries to pull the front wheel off the ground.

Now, raise the bottom spring perch 1". It is now "preloaded" 900 lbs. But with the weight of the vehicle on it, it still compresses the spring to a total of 5" because 5X300 still equals 1500. But since the bottom of the spring is now 1" higher, the top of the spring is 1" higher thus the truck is 1" higher. However, in this case you only have 2" of extension travel before the shock is at full length and "topping out." So over rough terrain, you will hit that limit more often and when you do it will be at higher velocity with more energy.

Does that help?

I carefully read over this along with previous posts and understand what you meant by "more compressed". The way it was written could have been interpreted as the action of lifting the vehicle causes the spring to compress more. I was simply checking for understanding.

I believe I am up to speed now and your explanation is clear. A shock with an elevated collar does use up available compression space but otherwise does not improve or harm the overall suspension characteristics.

Using your example, if the truck needs 600lbs of pre-load and the weight of the truck remains the same, why do you use up more preload with a raised perch?

Donstar · March 24, 2016

Mack, Jon has all the points spot on. The is no more or less energy pushing on the spring perch. The static force remains the same if the weight of the truck remains the same.

The other variable affecting ride quality is the A-arm angles. As they depart further from the horizontal plane, the mechanical leverage is reduced, then higher force (at the tire) is required to compress the suspension.

Regarding variable rate springs, probably most vehicles on the road in the past 50 years has them.

Mack and Jon A were both "spot on" in their understanding. They both represent a lot of the ambiguity and contradiction we read in similar threads. The reason I started this thread was to come up with a clearer explanation. Also, not everyone has a clear understanding of the terminology associated with this topic (sprung/un-sprung, static, preload...) and they purchase based on misinformation. I have read many debates over spacers vs levelling shocks. I want to raise the front of my truck by 1.5" to 2" and would pay for Bilsteins or Pro Runners if they were an improvement over spacers or stock. Jon A was explaining how spring travel is reduced with these shocks. Spacers don't reduce travel but shift the range. I also understand that If you shift this range too much, components (UCA/spring) start bashing in to one another. When we limit our conversation to the suspension systems of the 2014+ gm 1500s I know we can get a pretty precise account of the dynamics of this type of modification.