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

[bobbywalter]

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.)

its not the 385....its 250 ish...they move together.

the point of the variable/progressive rate coil is the softer ride of the 250 over the 385 and 650.


i am unclear of what the goal was. because this only applies to a 90 degree s/a as to what would actually happen. not the case with sla or ttb. for sure ball park.

there is no fixed stop with a single multirate coil over like you have with a coilover setup...


it just depends on what you do with the truck. i use my truck as a truck and overload the fawk out of it. the rate of sag you get from the progressives was cutting into my time and tire life so running a single rate with air bags inside worked perfect. super soft for crawling and high rate for hiway cruise....better than a sway bar.

 

[bobbywalter]

put another way.....i used to switch coils and leafs when i got home from wheeling.

i used 3/4 ton ttb truck coils and 8 pack leafs for work duty.


with 5 kids that was getting to be a time suk. then i got cost out of maintaining the 8.8...it was just too weak for what i do and i went to one tons and leafs all around.

dialing that in was an absolute pia. but livable. i would never had believed i would have left leafs on the truck more then a year or so in the front, back in 07....but with the weight of the diesel and 4l80 it works good enough. but took alot of jankin around.

 

[Curious Hound]

i am unclear of what the goal was

I think, in this case, there is no "one goal". Jim's trying to get some info collected into one article that would give someone the tools to design for their partucular goal, whatever that may be.

In my head, narrowing down exactly what happens with spring rates and load on a dual or progressive rate spring, would be nice to see. As we see, there are a lot of variables. Load on the spring, unloaded height of spring, how many coils at the lower rate, how many coils at the higher rate, amount of travel before the lower rate is all used up and you're just sitting on the stiffer coils, and maybe more?

My most recent coil replacement/upgrade was complicated enough just using simple single rate springs. Of course, I should have gotten the truck weighed first. I was just guessing on static loading on the front. When I weighed the truck this past September, I was shocked to find it approaching 6k.

The last factor is "what springs are easily obtainable to use and what is the spring rate, load rating and free height of each. When I was searching, it seemed that getting data was the hardest part. It would be nice if vendors would provide the data on their websites. Instead, what you find is "xx years F150 front coils, $yy.yy price" not very informative.

 

[bobbywalter]

I think, in this case, there is no "one goal". Jim's trying to get some info collected into one article that would give someone the tools to design for their partucular goal, whatever that may be.

In my head, narrowing down exactly what happens with spring rates and load on a dual or progressive rate spring, would be nice to see. As we see, there are a lot of variables. Load on the spring, unloaded height of spring, how many coils at the lower rate, how many coils at the higher rate, amount of travel before the lower rate is all used up and you're just sitting on the stiffer coils, and maybe more?

My most recent coil replacement/upgrade was complicated enough just using simple single rate springs. Of course, I should have gotten the truck weighed first. I was just guessing on static loading on the front. When I weighed the truck this past September, I was shocked to find it approaching 6k.

aye....my naked weight is 6k. tooled and spooled it gets heavy.


if that is the case then the formula i listed above is the starting point for it.

i might have screwed up the math....
upper x lower 385x650 = 250250 divided by 385+650 = 1035

250250 divided by 1035 = 241.78...

so the working ppi is 240 ish untill the 385 coils crush...then its 650. that is really good for taking hits.

shock valving is difficult though to have them stout enough to take the 650 impact if you are really stroking out the suspension.


that is base 90 degree working angle...which is a start.


so 1100 divided by 242 = 4.54 in crush. so basically the result is the 4 in lift.

 

[Jim Oaks]

James Duff 5.5-inch Ranger spring 385PPI/650PPI and 18.625" long

The coil spring specs I gave you are for a James Duff 5.5-Inch lift coil for a 1983-1997 Ford Ranger 4x4.

I've done the whole 385x650 / 385+650 = 241.78 (we'll say 242).

1100/242 = 4.54 of compression.

18.625 - 4.52 = 14.084" compressed spring height which is 3.58" taller than the 10.50" spring it's replacing.

That can't be right.

1100/385 = 2.85
1100/650 = 1.69

2.85+1.69 = 4.54 which is pretty much the same number we got above.

It looks like the upper 25% of the spring is the 385 PPI part. Let's call it 4.625" and the lower 3/4 of the spring 14".

4.625" - 2.85" = 1.77"
14.000" - 1.69 = 12.31"

12.31" + 1.77" = 14.08" compressed spring height which is 3.58" taller than the 10.50" spring it's replacing.

If this is a 5.5-inch spring, we're missing 2-inches somewhere. Somehow last night I came up with a calculation that resulted in that spring providing a 5.75" lift, but I don't recall how I did it.

If I do this calculation with linear spring rates, they work out.

A BDS 6-inch Ranger spring is 400 PPI and 20" long.

1100 / 400 = 2.75" (1100 is a close weight estimate)
20.00" - 2.75" = 17.25"

17.25" - 10.50" (Regular Cab) = 6.75"
17.25" - 11.50" (Super Cab) = 5.75"

A Skyjacker 6-inch Ranger spring 435 PPI and 18.8750″ long

1100/435 = 2.52"
18.8750" - 252" = 16.355"

16.355" - 10.50" (Regular Cab)= 5.85"
16.355" - 11.50" (Super Cab) = 4.85" (which is pretty close to where mine sits now)

 

[Jim Oaks]

I know from doing these calculations that the SKY176 6-inch spring @Curious Hound used should have given him roughly 7-inches of lift. What did it end up providing? I know he had to make some coilbucket adjustments because it was too high.

 

[bobbywalter]

there are formulas.

and what actually happens.


24 in free 140 ppi makes for 5 in of lift static with ttb on long arms 12 in over stock and 4 in drop and vms pivots with 1.7 in canted seats.

18 in 450 ppi no lift normal seats made nearly identical static lift with same setup.....basically 15 in compressed height.

i cant make the math work. but it is what it is.


that all goes to shit driving because the frequency is insane with the 140 ppi, hence the bags.


i am 3600 front 2400 rear at cruise weight. so 1600 corner front...goes to 1200 corner with unsprung. i suspect this is why leafs are fine as there is a good or maybe abnormal amount of unsprung weight.

so i am figuring coilovers would be a smart move for ride and hard impact with high unsprung weight. worked on a psd diesel swap on a fsb @ 2006 or so with e350 coils and it was good but cracked the k member and twisted up the beam and arms a bit and he just went to leafs... when i think about going back to beams that always pops up and stops me...

but a fresh ttb build with a ecoboost 4 whizzer would be worth taking the time to run the calculator.


that said, whatever suspension i ended up with i would just start with a known rate spring and install the set and see what it does. then dial from there because i know what that particular setup does at that point. figuring out all of the factors...and there are alot with ttb is not something i would take the time to do, but with the example of eric, i sure as hell can see why trying to figure it out would be useful to the group as a whole...

i am just not ever gonna be that smart.

 

[Jim Oaks]

I've seen @4x4junkie talk about the 1.5 leverage if the TTB before, but I've never understood what it is, or where it comes from. I've searched the net and not found an explanation.

What I do know is that I've looked at coil springs for a 1985 F150 TTB and 1979 F150 with a live axle and they have pretty close to the same spring rates. I was expecting the TTB to have a much higher rate.

I think this is info worth pursuing. This same basic info can carry over to the fullsize TTBs as well.

As old as TTB is, we could end up in a situation where nobody is making Ranger kits anymore.

 

[Curious Hound]

I've seen @4x4junkie talk about the 1.5 leverage if the TTB before, but I've never understood what it is, or where it comes from.

Not 100% sure. But my thought is that it comes from the way the ttb works. You have 2 different lengths to consider; axle beams pivot to spring perch and axle beams pivot to wheel centerline. The weight of the truck pushes down on the axle beam at the spring perch, length A. Then the weight is transferred to the ground via the wheel, length B from the pivot, which is longer than length A. The lever arms are different, so the forces are different.

To further complicate matters, the passenger side axle beam is a different length than the driver side beam. So if you build both sides identical, you get different results. It's noticeable when you go to do the alignment or start measuring and comparing compressed spring heights from one side to the other. This may even contribute partially to the proverbial "Ranger lean" issue that we normally attribute to fuel tank placement, driver position, etc.

 

[Curious Hound]

To expand on all that, you might be thinking "IFS setups and solid axle setups also have a difference between the pivot to spring length and pivot to wheel length". And that is true. But, in IFS, those lengths are much shorter than ttb. I don't know exactly how that affects it. The length (lever arm) from pivot to spring perch probably close to 1/2 the length from pivot to wheel center. It probably just makes the math easier for the engineers. In a solid axle setup, you have a much longer lever arm. But there's another catch. The axle is not pivoting from a fixed point on the frame. It pivots off of the opposite wheels contact with the ground. It's a whole different beast. In fact, if just one side is being raised by a rock, it actually puts upward force on the frame on that side AND a much lesser lifting force on the frame on the other side. So both sides of the vehicle are being lifted. Whereas with ttb, the side with the rock is lifted and that lifting force, raising the frame actually unloads the opposite side slightly.

 

[rubydist]

its not the 385....its 250 ish...they move together.

the point of the variable/progressive rate coil is the softer ride of the 250 over the 385 and 650.

What you said is only true if you have 2 springs, one with the 385 rate and one with the 650 rate. That is not the case here. Here we have one spring with coils wound so that part of the spring gets bound up at roughly 30% deflection. In the case that we have here, the initial spring rate is 385 because all of the coils are twisting, and the later rate is 650 because after the close coils are bound, a significantly shorter portion of the coils are twisting therefore giving it a higher rate. So the math I did is correct for a single dual rate spring (with the assumptions I provided.)

 

[bobbywalter]

What you said is only true if you have 2 springs, one with the 385 rate and one with the 650 rate. That is not the case here. Here we have one spring with coils wound so that part of the spring gets bound up at roughly 30% deflection. In the case that we have here, the initial spring rate is 385 because all of the coils are twisting, and the later rate is 650 because after the close coils are bound, a significantly shorter portion of the coils are twisting therefore giving it a higher rate. So the math I did is correct for a single dual rate spring (with the assumptions I provided.)

no.

they all move together until bind.

its 242 ppi rate. then 650.

 

[bobbywalter]

if you have 2 400 ppi springs on top of each other....one is 3 inches long....one is a foot...the working ppi is 200 until the short spring binds then its 400.


the difference is merely with this spring it is harder to fix the stop/bind point

 

[alwaysFlOoReD]

I asked ChatGPT;

# Given values
initial_length = 18.625 # inches
soft_rate = 385 # lb/in
stiff_rate = 650 # lb/in
applied_force = 1100 # lbs
compressed_length_old = 10.50 # inches

# Step 1: Find transition force (force at which soft rate bottoms out)
# Assume transition force is approximately when soft rate section fully compresses.
# Transition force = soft_rate * compression until transition.
# To estimate, we'll assume a reasonable transition point.

# Let’s assume transition compression happens around a third of total compression.
# We'll refine this if needed.
transition_force = soft_rate * (initial_length / 3)

# Step 2: Compute compression in soft and stiff rate sections

if applied_force <= transition_force:
# The force is within the soft rate range only
total_compression = applied_force / soft_rate
else:
# First, compress fully at soft rate
soft_compression = transition_force / soft_rate

# Remaining force for stiff rate section
remaining_force = applied_force - transition_force

# Compression in stiff rate section
stiff_compression = remaining_force / stiff_rate

# Total compression
total_compression = soft_compression + stiff_compression

# Step 3: Find new compressed length
new_compressed_length = initial_length - total_compression

# Step 4: Find lift difference compared to old compressed length
lift_difference = new_compressed_length - compressed_length_old

total_compression, new_compressed_length, lift_difference.

 

[bobbywalter]

if only that actually happened.