I have a dual spring rate / lift height puzzle for you...

[Jim Oaks]

You have an 18.625" coil spring with a dual rate of 385/650. You're applying 1100 pounds to it. Your current coil spring has a compressed rate of 10.50" How much is the new coil going to compress and how much lift is it going to give you over the 10.50" compressed spring?

I've been working on a page about coil springs and I embedded a calculator to calculate spring rate, and one to calculate how much lift a spring will give you. I even have a list of springs with their lengths and spring rates:

Ford Ranger 4x4 Coil Spring Guide - The Ranger Station

Feel free to use the calculator there if you want.

My issue when calculating how much lift you'll get from various springs is factoring in dual rates. I searched the internet and could only find complex lengthy examples that dealt with dual spring coil overs. Not a dual rate coil spring. The spring below is your example. Tell me what height the spring is going to compress down to, the amount of lift it will give based on the information I posted above, and how you arrived at that answer. I have my own idea of what it is, but I want to see what you guys come up with.

I've never had to deal with dual rate springs, so this is new to me. Some of you may have an answer right away.

 

[bobbywalter]

that looks like a typical progressive rate spring to me.


motion ratio can be a killer trying to figure a good rate.


what is the actual application?

 

[rubydist]

You didn't give us enough information to solve the problem. If the current spring has a compressed length of 10.5" with 1100 lbs on it, we either need to know the spring rate of that spring, or the uncompressed length of that spring (so we can calculate the spring rate) in order to know how much different it will be with the new spring.

And, on the new spring, we need to know how much of the spring has the 385 lbs/in rate and how much of it has the 650 rate. We can guess at that from looking at the photo provided, but we don't know that the photo is of the actual spring you are intending. Typically with non-linear springs, they will publish multiple load v. deflection values or they will tell you the first x" of compression is at the lower rate and then it switches.

Once we have those pieces of data, then we can answer your question.

And, for those of you who are new to multi-rate springs, you will note in the photos Jim provided that the coils at the top are closer together, and the coils at the bottom are further apart. As the spring starts to compress, it is at the lower spring rate. After a certain amount of deflection, the top (closer) coils are compressed to binding which effectively shortens the spring causing the spring rate to increase. The photo does seem to show that there are 2 distinct spacings of the coils, hence the 2 different spring rates quoted. The point where the rate changes can be rather jarring with this big of a jump in rate. Most non-linear springs have a range of coils in the middle where the coil spacing is gradually increased, which gives a more gentle transition between the "soft" and the "firm" portion of the compression.

 

[bobbywalter]

the the rate free length of the current compressed spring is the key factor.


the 385/650 runs into the 1100 to the point that 650 is exceeded.


the compound of the motion ratio based on the known rate/compressed/free length of the current spring would get you to the ball park.

the dynamic response otoh will be shitty.

 

[Jim Oaks]

You have all the information you need. Ignore the current spring. That's just there so you can subtract its compressed length to the compressed length of the new spring to determine how much lift you're going to gain.

With the information you have you should be able to calculate how much the new spring will compress. You have its free length (18.625), springs rates (385PPI/650PPI) and you know you're putting 1100 lbs on it.

 

[bobbywalter]

You have all the information you need. Ignore the current spring. That's just there so you can subtract its compressed length to the compressed length of the new spring to determine how much lift you're going to gain.

With the information you have you should be able to calculate how much the new spring will compress. You have its free length (18.625), springs rates (385PPI/650PPI) and you know you're putting 1100 lbs on it.

you need to know long both sections of spring are.

385 at 4 inches is getting crushed 4 inches with 1100 on it....they compress together regardless....


1100...this assumes not including unsprung.


the current spring with 1100 on it at 10.x inches means everything. if its 650 ppi and 17 in free you are not going to gain....you are going to lose height.

 

[bobbywalter]

1100/385= 2.85 in

the transition is not considering motion ratio.

 

[Jim Oaks]

You know the 10.50" spring has 1100 lbs on it. Compressed is the distance between the beam and the coil bucket. Free is 18.625". The only measurement I don't know is the length of coil that's 385 PPI. It looks like it's 25% of the coil.

 

[bobbywalter]

well...at a .75 motion ratio the static load is 1500ish.

which will eat 4 inches. the lever ratio is real.


with...and if the spring is actually accurate in its dual rating you will have to install it and measure it.

with no other changes i can tell you what the current spring is. based on this known.

 

[bobbywalter]

so the way i guess the 4 inches at zero is this.


top rate x bottom rate divided by the top rate plus the bottom rate.


this is like 4 1/2 inches. which all things equal from the 18 1/2 is is a 4 ish inch lift. is that what you are looking for?

 

[bobbywalter]

but the unknown motion ratio could be a killer.....typical...ttb i assume as you mentioned beam is 1.1-1.3 ish....

that is why i dont like progressives with my beams. they are better riding initially with the lower primary rate...but the 385 gets smashed flat when your whooping on it in short order and your stuck with the 650 secondary instead of the 240 ppi primary...


i just run soft singles and air bags.


typical ttb is 450 ish so the 240 is definitely nicer, whereas a single 200 with a bag is tunable for any situation if you are using the truck as a truck.

 

[alwaysFlOoReD]

It looks like it's 25% of the coil.

Looks like about a 1/3 to my scientific eye instrument. Counting coils its .38 (5/13).
Also you have some glitches in your article. The sq and tg pics are swapped according to the descriptions. There are also a few spelling and other mistakes. I'll look again tomorrow and provide a list if you want. Its bedtime for me now.

 

[Uncle Gump]

 

[Curious Hound]

View attachment 123405

= 42

 

[rubydist]

If you assume that the entire 1100 lbs is applied at the 385 rate, then the deflection is 2.85" and the compressed length is 15.77" - BUT you are guessing that it is all at the lower rate since you don't know everything you need to know. So, you get over 5" of lift at the spring. (note that if part of that 1100 lbs is at the higher spring rate, then the compressed length of the spring is even longer resulting in more lift.)