Over the eight years that I have been putting together this Toolshed Engineer column, I will humble-brag a bit and admit that I have come up with some pretty innovative shade tree mechanic tips and tricks that can help racers out.

In the interest of full disclosure, this Toolshed Engineer column is not one of those innovative tips. This column is a crackhead idea that I don’t recommend anyone replicate. I have multiple smashed fingers and toes to prove this point. Read on at your own risk.

The stupidity began when I was looking at some stock suspension springs in my garage. I had a bunch of springs lying around that I had grabbed from multiple wrecking yards, but really had no idea what their spring rates were. I collected all these springs because I know not all springs are manufactured the same, even if they are supposed to be “technically” the same spring rate.

By grabbing a bunch of “the same springs” and testing their stiffness coefficient, I could use the best “stock” spring in the car because that spring just happened to be manufactured a little better than the rest. The ultimate goal was to put the stiffest stock springs I had found into my new track day car. Beyond that, I wanted to choose the stiffest of the four springs to be placed in specific corners of the car based on corner-weight data. The theory being if the left front was the heaviest part of the car, I would put the stiffest spring in the left front corner to compensate for the extra weight. So far, this all seemed reasonable. Then I started to build my own spring dyno.

This spring from Eibach is labeled so you know exactly the size and spring rate. The numbers printed on the spring “0800.025.1000” indicate this spring is 8 inches in height, has an inner diameter of 2.5 inches and is rated at 1,000 inch-pounds. But not all springs come nicely labeled like this. OEM springs usually have no markings whatsoever.

To determine what a spring rate is, all you need is a spring dyno. This expensive computerized device, with built-in safety features, places a load on top of a spring and measures the distance the spring is compressed. The spring dyno then churns out data about the spring. I am familiar with this process because I worked directly with Jared Reyes at Eibach Springs and had the springs from my Honda Challenge car dyno tested. Not every spring is created equal. There are variables in metal and the bending/heating process to create springs can change a spring’s stiffness. Eibach put my road race springs in their dyno and told me what each spring’s exact rate was. I used that data to place my springs in specific corners of my Honda Challenge racecar.

Eibach’s expensive spring dyno can determine a spring’s rate. This particular spring was manufactured to be a 700 inch-pound spring. Its actual spring rate was 699.66 inch-pounds which is incredibly accurate.

I had done the right thing in the past, sent my springs to the experts and they provided an outstanding service and extremely accurate data. But on this particular day in my garage while staring at a bunch of springs I knew nothing about, I decided I didn’t need that expert service anymore. I didn’t want to ship my springs and wait for the answer. I could do it myself. Hell, I took physics in college. I understand Hooke’s Law. I don’t need a fancy computerized spring dyno. I could create my own shade-tree spring dyno using some free weights I had in the garage and a ruler. The theory was this: If all I wanted to know was which spring was stiffer, then all I needed to do was stack weight on each spring, measure the distance the spring compressed and the spring that compressed the least was the stiffest of the bunch. Conversely, the one that compressed the most was the softest of the bunch. Easy!

A British dude named Robert Hooke figured out way back in 1676 the relationship between the force applied to stretch a spring, the distance the spring is deformed and how those factors determine the spring rate (spring coefficient). He did it by hanging weights from springs to extend the spring as depicted in the above physics book illustration. The concept works the same by applying a force to compress a spring, which was my hair-brained plan to do in the garage.

Like any physics equation, if you have two of three variables, you can solve for the third unknown variable. My unknown variable was the spring rates of the OEM springs in my garage. The known variables I could utilize would be F, the Force applied to the spring, the weight and x, the distance the spring was deformed. From this I would solve for k, spring stiffness. Even if I didn’t really care about what the numerical value of k was, I could determine which spring had more k or less k based on the amount the spring was deformed (x) by applying the same weight (F) to each spring. The end result would be I would know which spring was the stiffest. Yay!

To measure x, the amount the spring would be deformed, I simply taped a large ruler to the garage wall.

I taped a long ruler to the wall of my garage to measure the deformation of the spring. To accurately see how much the spring deformed along the ruler, I taped a stick to each spring to act as a pointer toward the ruler. I placed the spring on the floor near the wall so the pointer was next to the ruler and took a static measurement prior to adding any weight (Force) on top of the spring. Once I had the static measurement noted, then it was time to add weight to the top of the spring. I would use the exact same weight (Force) on top of each spring for consistency during the testing. Once the full amount of weight was stacked on the spring I would look at the pointer and notate the new measurement on the ruler. Subtracting the new measurement from the static measurement would give me x. This isn’t hard stuff.

A simple stick taped to a spring would act as a pointer to the ruler taped to the wall. I placed the stick high on the coils and at the same position for each spring. Before any weight (Force) was added to the top of the spring, I noted the measurement. In this case the static measurement was 38 3/4 inches.

In theory, the physics and the math absolutely checks out on my DIY spring dyno. This was going to work … then we started stacking plates of weights on springs. That’s when things got real stupid in my garage. Springs, by their very nature, are created to expand and contract, which is why we use them in suspension systems. On vehicles, we have springs isolated and securely fastened in a car. They aren’t allowed to bounce all over the garage and try to kill you. That is bad for business.

Like most racers, I have lots of spare ballast lying around the garage, which I could use to add a Force to a spring to help me determine which springs were the stiffest. OEM springs are considerably softer than my Eibach 1,000 inch-pound road race springs, which meant I didn’t need 1,000 pounds of ballast to smash the spring 1 inch.

I needed help stacking 150 pounds of free weights on top of a noodling spring, so I enlisted the help of my partner at Double Nickel Nine Motorsports, Keith Kramer. I told Keith, “Keep these weights balanced on this wobbly spring while I stack on more weight.” This simple request turned out to be difficult and dangerous.

This is us stacking weights, one by one, on top of a spring so we could measure how much the spring compressed to take a measurement to find x. Does this looks safe? It wasn’t.

As you probably already guessed, stacking 150 pounds of free weights on top of a wobbly spring was a recipe for disaster, creating smashed thumbs and toes. I did this project four months ago and I still have an ugly black bruise mark under my right foot’s big toenail. But just because a bunch of weight crashed on the ground and a spring shot across the garage floor didn’t mean we stopped doing it. Oh, no. I needed to know which springs were the stiffest. I had comprised a physics-based method to figure this out, ergo nothing was going to stop me from getting the answer I desired. “Keep stacking!”

We didn’t just cheat death once. We did it over and over again with a bunch of springs, just so I could know which OEM ones were the stiffest. We labeled each spring after the test with the value of k (spring coefficient) to keep track of which one was stiff and which one was soft. What we found was that not all springs are the same. There are legitimate differences in spring rates for OEM springs that are supposedly manufactured to be the same rate.

Weights crashed down, springs spit out, and Keith and I cursed and reeled in pain. As we went through the stacks of springs the cursing got louder and our patience wore thin. We both knew this was a stupid thing we were doing. But stupid never stopped a racecar driver from doing any-damn-thing. Racing is about the edge, and I wanted the edge against my competitors by having the best springs in the best locations on my car. Smashed thumbs? Who cares? I was going to get my answer!

Springs are dangerous. This is why the spring dyno at Eibach has this super thick safety glass on the outside of the machine. This is also why people use spring compressors with pins to safely isolate and hold springs in place when they are pulling springs off of struts. Naturally, we used none of these safety devices when making our DIY spring dyno.

As I said at the beginning of this story, this DIY spring dyno is crazy dangerous. But it did work. I was able to figure out which OEM springs compressed the least. F=kx, meaning less x measurements with the same Force applied equaling a greater k, spring coefficient, and thus they were the stiffer springs of the set. I slapped the best four springs at the most opportune locations on my car and took it out for a drive. Could I feel any discernable difference? Nope. But I can sleep at night knowing I did everything I could to get a single extra inch-pound of spring rate on the heavy left corner of my latest track-day car.

In some race classes, or just to keep your modification points low, you might need to use OEM springs. Since not all OEM springs are created equal, I tested a bunch of them with my dangerous DIY spring dyno and put them in the heaviest corners of my Ford Fiesta ST track day toy.

We used science to solve this particular spring question we had, however, we failed to use common sense. Buyer beware before you try this particular Toolshed Engineer idea at home. Thumbs and toes are important.

Image courtesy of Rob Krider

2 COMMENTS

  1. I used a corner balance scale in a cheap hydraulic press. Put the spring on top of the scale, zero the scale, compress string 1” using the press, read the pounds!

Join the Discussion