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Login to post Forums » Grassroots Motorsports » How do you measure spring rates

Jan. 27, 2011 12:44 p.m. kingbeann New Reader

I just got some Bilstein sport shocks for my MK3 Jetta which I'm hoping to bring to the $2011 Challenge. I've followed some of the other builds and using others experiences as inspiration - therefore I will be attempting to source some junkyard springs from a truck or van to fit the shocks. I'm targeting about a 450 lbs/in spring rate all the way around. I can find some thick springs, cut 'em down and install to see what happens, OR I'm thinking it would be smarter to have some idea of what spring rates I'm using.

In the absence of any specialized tools, is the math behind this as simple as I think? e.g. place 100 pounds on the spring and it compresses one inch, your rate is 100 lbs/in. I'm assuming linear rate springs of course.
Jan. 27, 2011 1:32 p.m. silence New Reader

I'm interested in hearing a response to this.
Jan. 27, 2011 1:34 p.m. Bobzilla Dork

With a ruler?
Jan. 27, 2011 1:37 p.m. ditchdigger HalfDork

Here you go

http://www.racingsuspensionproducts.com/spring%20rate.htm

All it takes is a caliper to measure wire diameter and a ruler to measure OD or ID and the height. The formula to do the math yourself is in Herb Adams "Chassis Engineering" book
Jan. 27, 2011 1:40 p.m. kingbeann New Reader

ditchdigger wrote:

Here you go

http://www.racingsuspensionproducts.com/spring%20rate.htm

All it takes is a caliper to measure wire diameter and a ruler to measure OD or ID and the height. The formula to do the math yourself is in Herb Adams "Chassis Engineering" book

Cool calculator! I have the book but haven't finished it yet, I'll have to do some more reading.
Jan. 27, 2011 2:45 p.m. T.J. SuperDork

HOW TO RATE COIL SPRINGS

Spring Rate = GD^4/8ND^3

G=Torsional Modulus for Steel = 11.25x10^6

D=Wire Diameter in Inches

N=Number of Active Coils

D=Mean Coil Diameter in Inches

8=A Constant for all Coil Springs

Copied from another forum. Didn't 'fact check' this.
Jan. 27, 2011 3:11 p.m. kb58 Reader

Stand on it and measure how much shorter it gets: Spring rate = your weight / amount spring compresses

And don't forget that spring rate is almost never equal to wheel rate.
Jan. 27, 2011 3:20 p.m. Teh E36 M3 HalfDork

ditchdigger wrote:

Here you go

http://www.racingsuspensionproducts.com/spring%20rate.htm

All it takes is a caliper to measure wire diameter and a ruler to measure OD or ID and the height. The formula to do the math yourself is in Herb Adams "Chassis Engineering" book

It's quite a bit more complex than that. You have to know the difference between "active" coils and not. You essentially are figuring out the length of the spring if you were to unroll it, the diameter, and then use the torsional modulus of steel to determine the rate. The direct measure is probably the most accurate for the non-engineer, but if you pay attention to the instructions and learn the terminology, you can figure it out. I think TJ has the right idea.
Jan. 28, 2011 3:42 p.m. pitbull113 New Reader

if you can get your hands on a moog catalog or other spring manufacturer you'd have a better idea of what springs you'll need to grab at the junkyard. might save you time looking under a hundred cars to find the needle in a haystack
Jan. 28, 2011 4:35 p.m. ditchdigger HalfDork

The description in Chassis Engineering is pretty good about measuring active coils vs. dead. Using it I usually come within 3% of the numbers written on eibach springs.

The problem I have is that most springs these days are progressive rate so both a measured weight or the calculator does not work.
Jan. 28, 2011 4:48 p.m. Keith SuperDork

I don't know if I'd say "most" springs are progressive, but they're easy to spot because the winding gets tighter at one end.

For a challenge car, the easy solution might be to just get some race springs from somewhere. They're labeled I'll bet a sprint car swap meet could bear fruit.
Jan. 28, 2011 6:26 p.m. Curmudgeon SuperDork

The easy answer is yes, if you put 100 lbs on a 100 inch pound spring it will compress 1 inch. That could possibly be a quick and easy method of checking springs in a boneyard; take a known weight (i.e. you can't use your girlfriend to stand on the spring because she won't give you an accurate weight! ) and an accurate measuring device. But the chances of getting that accurate aren't real good. So I'd go with the mathematical method instead. FWIW, you can go to many race car forums, such as roadraceautox.com (no affiliation or financial interest!) and they have a specific 'springs for sale' thread.

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