DIY progressive rate springs???

[DarkBuddha]

Well, I did a search and couldn't find anything like what I'm thinking, so I figured I'd post a new thread. Here's what I'm wondering:

First, and correct me if I'm wrong here, but progressive rate springs are progressive because the rate of the spring increases under compression. This is because the coils of the spring aren't linearly spaced (i.e. the number of active coils per inch increases in number from one end of the coil to the other). Basically, spring rate is dependent on the number of active coils assuming all other variable stay the same, right?

I started thinking about this and it occurred to me that it might be possible to "fake" a progressive rate spring, or more accurately, a 2 stage spring using spring clamps (which are the opposite of "spring helpers" you've seen in parts stores). The spring clamps could be positioned to section off the spring into 2 sections with an unequal number of active coils. The larger number of coils would have the softer spring rate and the small number of coils would have a higher spring rate. Make sense???

I've used spring clamps in the past without issue (even over long periods of time under hard driving conditions), so I feel ok about using 'em. The question is: does this seem to be sound logic? I certainly recognize that there wouldn't be any exact way to determine the rate of the 2 sections, but it could be tuned by changing the position of the spring clamps.

So, now to the real question: why bother??? Why not just buy progressive rate springs??? Other than being cheap (spring clamps are like $20 and progressive rate springs from Eibach run like $180), I can't think of really good reason... well other than the "tuning" think I mentioned previously.

Any feedback or thoughts much appreciated...

[79T/Aman]

just use a spring rubber like on the stock cars Mr.gasket has them, $20 you done

[chicane67]

Well....... with coilovers, they are called tender springs. But they are only used on a chassis with more suspension travel than what we typically deal with. It also isnt an 'inside/outside' type of setup either..... but I get what your saying. The tender spring is the tunable element.

You could accomplish the same thing with a multi valve shock and a linear spring.

[DarkBuddha]

just use a spring rubber like on the stock cars Mr.gasket has them, $20 you done
Sorta the same idea, except the spring clamps allow for lowering ride height. Still, the idea is to isolate 2 sections of the spring, one with fewer active coils than the other section, thus 2 stages of spring rate. I guess if the stock car guys are having success with it, it must work, right???

Anyway, I actually ran this setup on my Mach for a couple years back when I didn't know anything about spring rates or active coils or any of that... I just did it to lower the ride height without removing the springs. It worked fine, but now I think I may have been getting the side benefit of a 2 stage spring unknowingly. The front always felt pretty good and definitely stiff, but only harsh over severe railroad tracks or potholes.

I suspect this doesn't quite work the way I think though... For example, if the original spring is 450lbs/in and has 8 active coils, and I clamped the spring (which will eliminate one active coil) into sections of say 4 and 3 active coils, then the resulting 2 stage spring would have rates of 900lbs/in and 1200lbs/in. These are crazy high rates, right? My Mach never felt that severe when I was using the clamps... :hmm:

[DarkBuddha]

You could accomplish the same thing with a multi valve shock and a linear spring.
Is that true??? I think my main issue with that route might be cost. I mean, if I was gonna need to replace my shocks and they were under $200 for a set of 4, then that would be a reasonable compromise. But the multi valve Konis I've seen are super pricey. I'm trying to find cheap ways to do stuff, 'cause I'm poor doctoral student.

Edit: I edited my first reply because it sounded incredibly snide and disrespectful, which was not my intent at all, thus this edited version. It still amazes me how innocent comments can sound downright patronizing when read over the internet.

[Norm Peterson]

. . . the idea is to isolate 2 sections of the spring, one with fewer active coils than the other section, thus 2 stages of spring rate.

<snip>

I suspect this doesn't quite work the way I think though... For example, if the original spring is 450lbs/in and has 8 active coils, and I clamped the spring (which will eliminate one active coil) into sections of say 4 and 3 active coils, then the resulting 2 stage spring would have rates of 900lbs/in and 1200lbs/in. These are crazy high rates, right? My Mach never felt that severe when I was using the clamps... :hmm:Remember that the two "pieces" of the spring are both still at work fulltime, so what you really have is a 7 active coil spring of 514 lb/in, or only a little over 14% stiffer than the 450 that you started with. Or you can do the "springs in series" math with the 900 & 1200 for the same result. No different from clamping one full active coil down against a control arm (or strut seat) as far as rate is concerned.

I had initially thought that you might be trying to only partially compress a coil. I think that could be made to provide some variability, but it would be in the "softening" direction rather than stiffening. And it would likely add another source of rattles.

Norm

[chicane67]

Bummer. I missed the un-edited version.......

...but yeah, the cost of the described dampners would run you about 750.00 ea to do it in a linear spring combination.

A cost effective method would be, to cut a shock body for a Bilstein coil-over kit (cut a snap-ring groove in your existing shock body). The cut and snap ring might cost 15-20 a corner and the coil-over set-up runs about 48 a corner. Then you could run a pair of Eibach springs (independant tender and load coils) and tune with the tender spring for load increase/rate. A pair of springs might cost 70.... thats budget race car stuff, there.

For a two spring progressive set-up (specifically with the use of tenders) the following formula is used to determine the initial spring rate:

Ci = Initial Spring Rate
CM = Spring Rate of Main
CT = Spring Rate of Tender Spring

Ci = (CM x CT) / (CM + CT)

The final spring rate (effective after the tender spring is closed) is simply the rate of the active Main spring.

The following formula is used to determine the transition point between the initial rate and the final rate:

TP = Translation Point
Ci = Initial Rate of Main/Tender Spring Combination
(Fc)t = Block Load of Tender Spring

TP = (Fc) t / (Ci)


Other related spring math:

Motion Ratio

A-arm Suspension
MR = Motion Ratio
d1 = Distance from spring centerline to control arm inner pivot center (in) or (mm)
d2 = Distance from outer ball joint to control are inner pivot center (in) or (mm)

MR = (d1/d2)^2

Beam Axle Suspension
MR = Motion Ratio
d3 = Distance between spring centerlines (in) or (mm)
d4 = Distance between tire centerlines

MR = (d3/d4)

Angle Correction Factor
ACF = Angle Correction Factor
A = Spring angle from vertical
C = Spring Rate (lbs/in) or (N/mm)

AFC = (cosine <a) (c)

Wheel Rate (non beam)

WR = Wheel Rate (lbs/in) or (N/mm)
C = Spring Rate (lbs/in) or (N/mm)
MR = Motion Ratio
ACF = Angle Correction Factor

WR = (c) / [(mr)^2 (afc)]

Suspension Frequency
SF = Suspension Frequency (cpm)
WR = Wheel Rate (lbs/in) or (N/mm)
Sprung Weight Vehicle corner weight less unsprung weight

SF = (187.8) (*sq.root of* (wr/sprung weight))

Calculation of Wheel Rate for a given frequency
WR = Wheel Rate (lbs/in) or (N/mm)
SF = Suspension Frequency (cpm)
Sprung Weight Vehicle corner weight less unsprung weight

WR = (sf/187.8)^2 (sprung weight)

Calculation of Spring Rate needed for a given Wheel Rate
C = Spring Rate (lbs/in) or (N/mm)
WR = Wheel Rate (lbs/in) or (N/mm)
MR = Motion Ratio
ACF = Angle Correction Factor

C = (wr) / [(mr) (acf)]

[DarkBuddha]

:candle: Uh, I think maybe I'll stick to composing music and doing sound synthesis and DSP rather than trying to figure out suspension math. In the world of mechanics, I'm strictly a common sense kinda guy. Still, I'm gonna look at that stuff and see if I can learn a bit of something that will help me make better informed decisions. Thanks for the input.