Coil Spring Debate

[Fargo]

That is good to hear. So, basically the guy I talked to was just a tool, ugh! Are you going to try their springs too? Still don't know what to think of the dual rate springs.

My plan was to get some Nth Degree springs. But it seems they are no longer answering the phone or shipping parts. So now I don't know what I will do. I would love to try the Synergy springs, but I need something that carries the weight of a loaded LJ. The Synergy are actually 5lbs lighter rate than my current BDS. So other than gaining some length I don't see any advantage for me. So I am leaning towards trying out the JKS springs. Or maybe holding off a little while to see what happens with Nth Degree.

EDIT: The rates of my BDS work well empty. So the Synergy should be really nice rates. Unless you load down your Jeep like I load my LJ.

 

[bobthetj03]

My plan was to get some Nth Degree springs. But it seems they are no longer answering the phone or shipping parts. So now I don't know what I will do. I would love to try the Synergy springs, but I need something that carries the weight of a loaded LJ. The Synergy are actually 5lbs lighter rate than my current BDS. So other than gaining some length I don't see any advantage for me. So I am leaning towards trying out the JKS springs. Or maybe holding off a little while to see what happens with Nth Degree.

EDIT: The rates of my BDS work well empty. So the Synergy should be really nice rates. Unless you load down your Jeep like I load my LJ.

What is the rate of the BDS springs? Kind of tough to gauge a linear spring against a dual rate spring from what I've been finding out.

 

[jjvw]

What is the rate of the BDS springs? Kind of tough to gauge a linear spring against a dual rate spring from what I've been finding out.

Front 158lbs/in, 18.5" free
Rear 200lbs/in, 13.75" free

Dual (combined) rates is completely different than linear. I don't understand it very well.

 

[Fargo]

Front 158lbs/in, 18.5" free
Rear 200lbs/in, 13.75" free

jjvw got the BDS rates correct. Since the JKS are 8lbs lighter in front and 10 lighter in the rear, I would expect the JKS to ride very similar to the BDS. Maybe a little softer and smoother. They say the dual rate makes it softer.

Dual (combined) rates is completely different than linear. I don't understand it very well.

Your thinking about this formula for combined rates aren't you?
CR = (R1 * R2) / (R1 + R2)

I understand that is the formula used to find the combined rate in coilovers and systems with 2 separate springs. But I don't think that a single 'dual rate' spring is going to act the same way as 2 single springs that are stacked on top of each other.

My knowledge is limited on this as well, but here is my reasoning. In a system with with 2 different springs, like a coilover, each spring is independant of the other. So each spring has to carry the load independantly. The top spring is not aware of the bottom spring and viseversa. So if you have a 50lbs spring and a 100 lb spring they will each compress independant of each other. However, with a dual rate spring, the spring is all one unit so the compression on the lighter rate is also transfered down to the lower rate.

Think of it this way. If I have a linear rate spring rated at 100lbs, then I cut it in half, is it the same as it was before. The formula says that I now have a combined rate of 50lbs (10,0000/200=50). But if I weld that spring back together again I will now have the same 100lbs spring. Since a dual rate spring is all one spring, I don't think that formula is accurate. I think that formula only works when the springs are acting independently.

I'm inclined to think that in a dual rate spring you will feel the softer rate until it is in full coil bind. Then you will feel the firmer rate. But that is just how I am thinking through it, I don't have any actual experience.

Sorry to take this so far offtrack, I thought we were in the JKS Jspec thread. So now back to control arm bushings.

 

[jjvw]

My understanding of dual (or triple, etc) rate springs is that you experience the combined rate until one section goes solid. It doesn't matter if that spring is one continuous spring or two shorter stacked springs like a coil over.

The part I have trouble conceptualizing is how and why the combined rate is less than the two individual rates.

 

[Fargo]

The part I have trouble conceptualizing is how and why the combined rate is less than the two individual rates.

I think its because they each work independently. I don't know if this is correct or not, but here is how I think about. Imagine we have a 100lb spring sitting upright on the floor. If we set a 100lb weight on top of it, it will compress 1". A simple linear rate.

Now imagine that same 100lb spring, but instead of placing the 100lb weight on it, we put a piece of plywood resting across the top of it and we stack on another 100lb spring upright on top of the plywood divider. So we have a stack of 2 100lb springs separated by a divider. Now we place that same 100lbs on top of the 2 stacked springs. How much weight does the spring on the bottom see? Technically it would see the weight of the divider plus the weight of the other spring, plus the weight of the 100lb weight. (Lets just ignore the weight of the divider and 2nd spring as inconsequential). The spring on the bottom has no idea what the weight that is placed on it consist of, because the divider sits between the spring and the weight. So that bottom spring sees the full 100lbs of weight. So it will compress 1". Now how much weight does the top spring see? Its the same thing. The bottom spring will compress 1" until it is holding all the weight above it, then the top spring will also compress 1" to hold the 100lbs that are placed on it. Each spring functions separate from each other. So each spring is required to hold 100% of the weight. So with each spring compressing 1" we now have a total of 2" of compression when using 2 springs. (Working the numbers backwards you will find that translates to a combined rate of 50lbs for each spring. Just what the formula gives us.)

But now what happens if we removed that divider that is between the 2 springs and we weld them together? Do we still have a combined rate or do we just have one 100lb spring that is twice as long? I would argue we just have one 100lb spring. Placing the weight on this single long spring will only compress the spring 1" for each 100lbs. That is why I say a dual rate spring does not act the same as 2 separate springs.

Related thoughts:
I have also read that you can think of a spring like a torsion bar. But I'm not sure how to put that all together. I have read that, like a torsion bar, the longer the material in the spring the less force it would take to twist/compress it. That makes sense if you think of the spring like a lever and the leverage force of it. A longer lever takes less force to lift (or compress) an object. This can be seen in a coil spring by the angle of each coil. A softer spring will have more coils and a gentle slope to fit a longer lever in the same space. A firmer spring will have fewer coils at a steeper slope to fit a shorter lever in the same space.

I think its also important to note that I said in the same space. I believe a 3' length of rod coiled into a 2' tall spring would have a differant rate than if the same rod was coiled into a shorter 1' spring. That 1' spring would have a much softer rate.

That is about the extent of my spring knowledge in a nutshell. I hope it helps. Unless I'm wrong, then I hope it doesn't confuse you.

Sorry for getting off topic.

 

[bobthetj03]

I think that is why a dual rate coil looks the way it does. How the coil is sloped and how the coils are separated changes the rate of that section of spring. I think the bottom section of spring still receives the weight and is affected, but it isn't till the upper portion has reached its rate threshold, then the lower section fully kicks in with its rate. That's how I'm thinking about it.

 

[jjvw]

I'm pretty sure this is accurate.

A 10" spring with 100lb/in rate will compress 1" per 100lbs of weight.

Cutting that 10" spring in half will create two 5" springs each with a rate of 200lb/in. Each 5" section will compress .5" per 100lbs.The combined rate of these two springs together is 100lbs/in.

Both 5" springs experience the full weight of the Jeep at the same time, just like the full length 10" spring does. It does not matter is these two sections are separate springs or attached to each other. The end result will be the same.

In addition to the mental gymnastics above, I don't understand how to quickly conceptualize the combined rate of the two sections of spring are of different free lengths and/or different rates.

 

[Fargo]

I'm pretty sure this is accurate.

A 10" spring with 100lb/in rate will compress 1" per 100lbs of weight.

Cutting that 10" spring in half will create two 5" springs each with a rate of 200lbs. The combined rate of these two springs together is 100lbs/in.
.

I disagree. Why would the rate change after you cut it in half? Its the same coil, just shorter. I would argue you now have two 5" springs with a rate of 100lbs each. The combined rate is 50lbs if they are stacked and used together.

 

[jjvw]

I disagree. Why would the rate change after you cut it in half? Its the same coil, just shorter. I would argue you now have two 5" springs with a rate of 100lbs each. The combined rate is 50lbs if they are stacked and used together.

A 5" coil with a rate of 100lbs will compress 1" per 100lbs. Stack two of those and that 10" assembly will compress 2" per 100 lbs. We just created a 10" spring with a 50lb/in rate.

 

[Fargo]

A 5" coil with a rate of 100lbs will compress 1" per 100lbs. Stack two of those and that 10" assembly will compress 2" per 100 lbs. We just created a 10" spring with a 50lb/in rate.

Exactly. And this was accomplished by cutting the spring in half and stacking it. So what happens if we weld it back together? Wouldn't we have the same 10" spring with 100lb rate that we started with?

 

[bobthetj03]

The length of the spring won't change its rate. Wire diameter, composition of the steel, coil slope determine rate, yes?

 

[jjvw]

The distance per unit of force described by the rate occurs throughout the entire length of the spring. Chop that length in half and you double to required force to move the spring one inch.

Right? Wrong?

 

[Fargo]

The length of the spring won't change its rate. Wire diameter, composition of the steel, coil slope determine rate, yes?

Yes and no. Look at coil slope. That will be determined by the length of the spring right.

EDIT
Assuming your using the same length of stock.

 

[JMT]

I don't think changing the length of a linear spring (cutting it in half) will change the spring rate. If you have a 10" spring and you cut it into two 5" springs, you now have two 5" springs, each with a rate of 100 lbs/in. The reason is because in linear spring rates, the rate is constant through the coils as long as the coils are equally spaced (which is the definition of a linear spring). Stacking them doesn't change the rate.

 

[Fargo]

The distance per unit of force described by the rate occurs throughout the entire length of the spring. Chop that length in half and you double to required force to move the spring one inch.

How so? In the previous sentence, you said the rate occurs throughout the entire length of the spring. How does chopping it in half increase the rate?

 

[bobthetj03]

So, why does a 933 spring have a lower rate the a 934?

 

[Fargo]

Yes and no. Look at coil slope. That will be determined by the length of the spring right.

I don't think changing the length of a linear spring (cutting it in half) will change the spring rate. If you have a 10" spring and you cut it into two 5" springs, you now have two 5" springs, each with a rate of 100 lbs/in. The reason is because in linear spring rates, the rate is constant through the coils as long as the coils are equally spaced (which is the definition of a linear spring). Stacking them doesn't change the rate.

But stacking them DOES change the rate of the ASSEMBLY of the 2 springs.

 

[jjvw]

I need to put a spreadsheet together and compare. It's hard to do this riding in a truck stuck in traffic. The guy driving is no help either...

 

[JMT]

i.e. the rate of two 5" springs that are each 100 lbs/in when stacked will still be 100 lbs/in, not 200 combined or 50 each. It would function no different than a 10" spring with a 100 lbs/in spring rate. Length is irrelevant, coil angle is relevant.