Thread: Understanding Spring Rates

Wow, what a way to wake my head up this morning. I'm new to spring rates and such stuff too so I'm no help to you but I'm interested in learning more. I'll be checking back on this thread!

I hope you find your answers and achieve what you were looking for.

SLS is an airbag setup isn't it? Is there a reason you can't use airbags on the front?

I'm continually bemused by people who think of only the spring rate when selecting springs or the typical recommendations that others make, along the lines of, "use xxx lbs springs"

The spring rate is important, but should not be used in isolation.

What you have done, with those calculations is make a far better comparison between different springs, but you still don't have the full picture.

Originally Posted by debruiser

SLS is an airbag setup isn't it? Is there a reason you can't use airbags on the front?

Yep. SLS is Self Levelling Suspension. They (when working properly) inflat and deflate depending on the load in the vehicle. They are the ultimate in progressive rate springs. As they compress the spring rate increases.

Mine are still not hooked up properly with the compressor and the computer handling everything like it should. I currently just have a tire valve on the end of an air line that goes to the bags and I pump them up or let them down as need be. I have everything ready to go to get it all hooked up. Just need time and motivation to hit at the same time.

One thing I have noticed is air bags are soft around centre. What I mean is coil spring rates are fixed progressive of linear. A linear spring has X rate, like my coils have a linear rate of 190lbs per inch. That means it takes 190lbs to compress the spring 1 inch. Air bags on the other hand seem very soft at there set height. It doesn't take much to compress the air bag maybe only 50lbs per inch but they are very progressive. The second inch maybe 150lbs so it would take 200lbs (50 for the first inch and 150 for the second) to compress the air spring 2 inches. Then the third inch might be 400lbs per inch.

I notice the soft around centre thing with the air springs because it will nearly lift over an inch from a full tank of fuel to empty and then drop back when filling. If I let it down when the fuel gets low, I need to pump it up when I fill it.

I guess most wouldn't notice that becasue the computer usually does it for you.

Happy Days

Originally Posted by joel0407

START READING HERE IF YOU ONLY WANT THE SPRING RATE STUFF

Here's my problem.

The suspension guy told me the standard LR springs are 150lbs rate springs and the one I currently have are 190lbs. Now he had previously told me the HD Dobinsons are 250lbs but today he tells me 220lbs (I looked in his book and read 220lbs for myself).

Here's the catch. While the HD springs are heavier rate, they are shorter by 30mm.

I thought "That's no problem, they'll compress less". Well maybe not.

Dobisons Disco 2 springs and 20mm longer, Drivers side to passenger side. The standard rate, +45mm are 390mm, 410mm and the HD are 360mm, 380mm.

Now I have measured my STD +45mm springs in my Disco and they are compressed to 270mm. So they are compressed 120mm, 140mm, Passenger, Drivers sides. Or 4.72, 5.51 inches.

So I calculate that if they take 190lbs to compress the spring each inch (linear) then they are holding 897lbs and 1047lbs.

So those weights will compress the 220lbs springs 103mm and 120mm or roughly 20mm less than the lighter springs are compressed. But the problem is they are starting at 30mm shorter so I'll be down 10mm with the heavier springs.

It doesnt sound right that Dosbinsons would sell 2 different springs, both 45mm lift but one to carry more weight and then make them shorter to start with.

Oh well. I've orderd and paid for them so I'll find out soon enough.

Maybe I am working it out wrong. Spring rates are a little new to me so I'm all ears and ideas.

Happy Days.

your on paper math is correct but you might find that the heavy duty springs arent a linear spring they are progressive

also what happens in the real world and what happens on paper doesnt always line up...

dont forget to extend your bump stops to deal with the longer compressed length of your shock or the first time you bottom out you will crack the shock mount or bend the shocky.

Originally Posted by Bush65

I'm continually bemused by people who think of only the spring rate when selecting springs or the typical recommendations that others make, along the lines of, "use xxx lbs springs"

The spring rate is important, but should not be used in isolation.

What you have done, with those calculations is make a far better comparison between different springs, but you still don't have the full picture.

Fill us in mate.

I cant seem to find many specs of springs. All I get is standard and heavy duty. I was just lucky the guy up here has a sort of cheat book. It listed other brands, it wasn't just a Dobinsons book

Happy Days

Originally Posted by Blknight.aus

your on paper math is correct but you might find that the heavy duty springs arent a linear spring they are progressive.

dont forget to extend your bump stops to deal with the longer compressed length of your shock or the first time you bottom out you will crack the shock mount or bend the shocky.

Yep. Bump stops were extended when the shocks were fitted. It's all been working fine until I added the weight of the winch and cradle. I think it's still over standard height by maybe a smidge but my extended bump stops mean I have less upward travel than standard as well as more weight than standard. Those 2 together aren't a good combination for the life of the bump stops.

The HD springs are listed as Linear.

Happy Days

If there's only 10mm in it you can use shim plates to level the car.

The other thing is the rates are nominal, so the springs will vary slightly, best thing will be to see how it works, I've made some shim/spacer plates to level my Rangie, the plan is to order custom springs when I have the final mass on the car. But to give you an idea I have tried two sets of springs with the same P/N and there's 15mm difference in how the car sits...!

Cheers
Will

Originally Posted by debruiser

SLS is an airbag setup isn't it? Is there a reason you can't use airbags on the front?

Mainly because back then only Range Rovers got full air suspension. Model placement and all that.

The classic EAS rangies share enough of the chassis and front end with a D2 that fitment of air springs is almost a bolt in (once you source the parts).

Spring rate and ride height are seperate concerns. Spring rate needs to be matched to the load carried and ride expectations (natural frequency). Ride height can be tweaked outside that with spacers or different length springs of the same rate.

Bump control (compression and rebound) is then handled by the dampers. Which also have to be tuned to work with a particular spring rate. Though there is some lee-way here.

I'll bet the stiffer springs are shorter to stop unloaded people with heavy duty springs looking like an Alabama mud-truck and handling like one too.