 Vorshlag finally got a new test rig that was probably long overdue - We purchased a Longacre digital spring rater. This is a tool that allows you to install a coil spring, load it at several compressed heights, and measure compressed height and loads digitally. While this is nothing like what we could have attached to our Roehrig shock dyno (the Roehrig Spring Rater was a much more expensive add-on than the stand-alone Longacre spring rater), it does the job and gives us what we want - an accumulation of springs rate data for any springs we test.
Why do we test OEM springs? When you're designing a coilover kit, it is always nice to know what the Original Equipment suspension started with for spring rates. On most of the new shocks we've developed so far we had ideas about factory rates were, but as we get into new models like Miatas, Mustangs, Evos, and more, we want to know what the manufacturer started with. Sometimes, as we've seen, it's, "WHAT were they thinking!?" There's no better way to get the data than having your own spring rater, so we spent the money and got one for ourselves.
We can also use this test fixture to check coilover spring rates. We already know that the Hypercoil coilover springs we use are the best springs out there or else we wouldn't use them - we already tested a variety of coilover springs and these tested on top for consistency, accuracy of marked rating, and linearity of rate. We did not purchase the spring rater to test the coilover springs already on our shelves, rather we wanted to test OEM and various "lowering" springs, and even customer supplied springs.
While there's only so much a $1,000 spring rater can give you, it can still tell most racers what they need to know - the basic spring rate of a spring. The ultimate spring test is a destructive test of the spring - that is where the spring is taken from uncompressed length right down to Coil Bind (fully compressed so that the coils of the spring are touching - the extreme limit of travel). The chart below shows this type of spring rate test. These are straight rate coilover springs - and the test shows they aren't "straight" rates at all. (Straight Rate means it has the same load rating for every inch of travel; a Progressive or Variable Rate spring has a variable spring rate over its travel) Two springs aren't linear at all and when any spring starts to compress near Coil Bind the rate starts to shoot up, then goes to infinite.
Unfortunately, there's no such thing as a perfect straight rate spring, only springs that have a straight rate over a certain displacement range (on some this is longer than others). This chart (courtesy of RRT Suspension) shows what a Roehrig Spring Rater can do. The Roehrig test actually compresses the spring to Coil Bind, then releases the spring and records the spring force as it goes to open length. That's the rub - when you take a spring to Coil Bind it can and usually does ruin the spring. Customers don't like their springs ruined in a rate test, so we choose not to test rates in this manner. We should say "possibly" ruined, since some springs are actually designed to go to coil bind (a flat wound Helper spring, for example), while most are not.
Destructive spring rate test done via Roehrig spring rater. Click for larger version.
You'll notice that some springs are closer to being a straight rate than others. The green one in particular holds its rate almost perfectly until 3.5" of displacement then it begins to ramp until it goes to coil bind. The blue spring actually begins ramping at 1.5" and changes from 527 lb/in to 680 lb/in (at 5.25") before going into coil bind. That's 150 lb/in of change in less than 2 inches! Not very good for a supposed "straight rate" spring (which should keep the same load rating per inch over its entire length of usable travel).
   
Some springs we have tested, both variable rate (OEM) and straight rate (coilover). Click thumbnails for larger pictures.
So that's the basics of what we will and will not cover here. If you're looking for graphs and charts that give you everything you'd ever want to know about springs, stop reading now. We recognize the limitations of our test fixture and the data we collected. No hateraid-I'm-an-expert-engineer emails please! :) We wanted to build a table that showed approximate rates of as many OEM springs as we could get our hands on, as well as check the accuracy and linearity of some straight rate coilover springs.
Since we're testing with springs we do want to use again, we do not want to take chances - None of the springs we're testing will go into Coil Bind unless it is our spring, and going in the trash afterwards. You'll notice we don't get a full 4 inches of compression on some of the tests, which is due to limits in compressed travel of some springs. Again this is an imperfect test just to give users a guideline of what the basic spring rate of a given spring tested at. We did not preload the springs by some magical amount or follow some manufacturer's specs of "20%-70% of usable spring travel". Instead we just compressed each spring in consistent 1.000" increments and measured load at each inch (for coilover springs, which have less travel, we will test them at .25" increments). This means that the first inch of displacement on our OEM spring tests should probably be thrown out, as this is mostly preload, in a range that the spring most likely will not see in use once installed on the car. Using the 2nd and 3rd inch readings will give you a better idea of what the rate of that particular spring is.
 
  
Testing spring lengths and weights. Click thumbnails for larger pictures.
So here's the Vorshlag Challenge : Have some OEM springs laying around? Send them to us and we'll rate them for you, FOR FREE! Its another way we help share the tech, by having a lot of people send in a wide variety of OEM springs and then gather lots of test data. Help us build this spring rate table and collect some useful information about modern sports cars. Want detailed charts? We can collect the data at smaller compressed amounts, like every .50" or even .25" of displacement, if you want more data points.
Note: We are placing this information on our website for informational purposes. This is not a definitive list of all OEM spring rates, just what we have measured - We will not re-post information found elsewhere.
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