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

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.

silence New Reader
Jan. 27, 2011 1:32 p.m.

I'm interested in hearing a response to this.

Bobzilla Dork
Jan. 27, 2011 1:34 p.m.

With a ruler?

ditchdigger HalfDork
Jan. 27, 2011 1:37 p.m.

Here you go

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

kingbeann New Reader
Jan. 27, 2011 1:40 p.m.
ditchdigger wrote: Here you go 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.

T.J. SuperDork
Jan. 27, 2011 2:45 p.m.


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.

kb58 Reader
Jan. 27, 2011 3:11 p.m.

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.

Teh E36 M3 HalfDork
Jan. 27, 2011 3:20 p.m.
ditchdigger wrote: Here you go 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.

pitbull113 New Reader
Jan. 28, 2011 3:42 p.m.

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

ditchdigger HalfDork
Jan. 28, 2011 4:35 p.m.

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.

Keith SuperDork
Jan. 28, 2011 4:48 p.m.

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.

Curmudgeon SuperDork
Jan. 28, 2011 6:26 p.m.

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 (no affiliation or financial interest!) and they have a specific 'springs for sale' thread.

You'll need to log in to post.

Also on Grassroots Motorsports

MX-5 Cup Racing: Live on Your Screen This Friday and Saturday

1 day ago in News

Catch all the action of the Global MX-5 Cup race at Road America live this weekend.

How to Run Compressed Air at Home

1 day ago in Articles

A guide for installing an air compressor and air tools in your home garage.

2017 Volkswagen Golf SportWagen TSI 4Motion

1 day ago in New Car Reviews

Say hello to the Volkswagen Golf SportWagen TSI S with 4Motion. It comes with a turbocharged 1.8-liter engine that is ...

You Need This: $1600 Street Racer Ford Festiva

1 day ago in News

It already has a GRM sticker and everything.

Get Your Subscription Combo Deals!

1 day ago in News

Get both our magazines for the price of one!

Your Projects: smokindav’s VW GTI Project

1 day ago in News

Love a good MK5 GTI? You might enjoy this build thread.

Wing It: Simple Talk on the Complex Subject of Aerodynamics

2 days ago in Articles

Whether sleek, boxy or somewhere in between, all cars interact with the air while racing down the track.

2018 Acura TLX V6 A-Spec

2 days ago in New Car Reviews

Acura is introducing something new for the 2018 model year: the Acura TLX V6 A-Spec. It's the highest trim level ...

You Need This: 1989 Suzuki Swift GTi

2 days ago in News

For sale: one of Suzuki's under-appreciated hot hatches.

Buyer's Guide: Chevy Cobalt SS

2 days ago in Articles

All the information you'll need on GM’s entry in the sport compact wars.

Wednesday Night GRM Live: Are You Using the Right Chemicals for the Job?

3 days ago in News

What do all these cans even do!?!?

Project SVT Focus Gets Caught Up on Maintenance

3 days ago in Project Cars

Once again, some off-road action sent our project focus under the knife.


Grassroots Motorsports Magazine

Subscribe Today

Also get your instant access to the digital edition of Grassroots Motorsports Magazine!

Learn More