September 12th, 2016 by Matt Holden
In the history of sim-racing, nothing has been more associated with chassis setup than the springs. At the same time, nothing has been more misunderstood than the simple coil spring. There are countless different kinds of springs, from coil, to leaf, to torsion springs, and everything in between, and racing series regulate them heavily. When I worked at US Legend Cars as a technical inspector, there was no part of the car more strictly defined than the springs themselves. They had to be a specific length, made of a specific material, possess a constant rate, and maintain a constant diameter (No barrel springs!).
There is even an entire industry dedicated to spring design, and creating the most consistent rates and performance possible with today’s technology. Luckily, we don’t have to worry about things like the spring losing rate over time, or bending, or anything like that in sim-racing, but it’s still important to know how these components work, what they’re used for, and how they can affect your performance on the virtual race track.
To understand spring rate, we must understand Hooke’s Law, which governs the physics behind almost every spring in existence. Hooke’s law simply states that the distance a spring is compressed (its Deflection, x) is directly related to the force the spring is exerting, F. The equation is completed by adding in a spring constant, or “rate”, represented by k. So for all springs, F = kx. It’s that simple.
A spring’s rate is a representation of how much force it will exert for a given amount of compression, or deflection. The rate is expressed in a force-per-distance unit, most often as “pound-per-inch” or “Newton-per-(milli)meter”. So if we have a 500 lb/in spring, it will exert 500 pounds for each inch it is compressed. If we compress it one inch, it exerts 500 pounds. Two inches, and we’ll see 1000 pounds, 1500 pounds for three inches, and so forth. In basic terms, the rate will be an indication of how stiff a spring is, or how resistant it will be when forces are applied to it. A 50 lb/in spring will compress fairly easily under a 25 lb load, while a 1000 lb/in spring will not compress enough for us to be able to notice.
Springs with a constant rate through compression are known as “linear” rate springs. Most real-world springs do not have exact rates and may change slightly through travel, but it’s not enough to cause the spring to not be classified as linear. If the rate of the spring changes through travel, this is known as a “progressive” spring. In many racing series, a progressive spring is illegal, however the vast majority of composite and rubber bump stops are progressive by nature. Below is a graph that would look similar to the output of a spring rating machine:
This graph shows the force vs. travel characteristics of a given spring. I intentionally altered how much force was added per inch of travel to produce a trace that wasn’t perfectly linear, but close enough to be applicable to what we’re talking about. The rate of the spring is the value of the chart’s rate-of-change throughout travel, and is determined by the coefficient of x in the trend line equation shown in the graph. So based on what we see here, the spring I made shows a rate of 248 lb/in and would be labeled as a 250 lb/in spring. The springs we have in iRacing are perfectly linear and are the rate we see in the garage. It’s an impossibility in the real world, but it eliminates a few headaches in the name of simplicity.
Springs in Race Cars
If you’ve driven a car, you’ve ridden on springs. That’s a fact, and we don’t have the widespread technology to have anyone in existence who can’t say that. Springs are a key component in car suspension systems, and are shouldered with a lot of responsibility, from dealing with forces applied from bumps, to controlling body pitch and roll, and in higher performance cars, even handling aerodynamic loads at high speeds.
When I was a kid back in the 90s, I remember seeing NASCAR Cup cars heaving up and down and swaying over to the side as the drivers slung them around the big speedways. I specifically remember watching Jimmie Johnson’s #48 and Bobby Labonte’s #18 jump up and down over the backstretch bumps at Charlotte when they were fighting for the win in the 2005 Coca-Cola 600. Today, that sight is a little more rare, and usually only found on local short tracks, but why is that?
When aerodynamics started to become a problem to consider in the mid 2000s, team engineers realized how they could get a car around the track faster by harnessing the air going around the car and generating grip instead of relying solely on the suspension and tires to do the job. The result was faster cars, and a dramatic change in how the cars looked on track. Even in world-class series such as Formula 1 and WEC’s LMP1 class, the cars are extremely rigid, with little to no suspension movement. I always enjoy pointing out how a Formula 1 car’s suspension will seem to be completely solid at Monza where aerodynamics are extremely important, while the suspension arms move a relatively large amount at Monaco where mechanical grip can be a factor. It’s simply to use aerodynamics as efficiently as possible, and the simple fact that if we set a car up like Johnson’s 2005 Coke 600 winner, it may not even make the show, let alone win the race.
I had a professor in college tell me a very useful thing: “Set the springs for speed, the roll stiffness for cornering.” We discussed roll stiffness last week, but we need to cover the main reason springs are used today: SPEED. As I hinted at with the Monza/Monaco example, springs in today’s race cars are used to control aerodynamics and the car’s attitude at speed, not primarily for mechanical grip. If we have a car that will be traveling at high speeds and we need to control the aerodynamics, we need stiffer springs. If we can sacrifice aero for mechanical grip, then we can get away with softer springs. We’ll go more in-depth with this in another article.
While I mentioned a few types of springs earlier, we really only have to worry about three types of springs in the sim-racing world: Coil, Leaf, and Torsion.
Coil springs are the most recognized, and are exactly as their name sounds: a coil of wire. In a coil spring, we have a central axis around which the wire is wound, usually at a constant diameter. If the diameter changes on the length of the spring, it will be named either a “Barrel” spring (narrower at either end) or a “Conical” spring (cone-shaped), but they all work in the same way. Applying pressure to the ends of the spring causes the wire to flex up and down, which it would rather not do, and thus exerts a force to counteract the applied force. Pretty simple!
Torsion springs are found on most cars in the form of a “Sway Bar”, or Anti-Roll Bar. Torsion springs are simply bars of metal that are twisted through a lever, and thus exert a twisting force to counteract that force. Some racing vehicles, such as Formula 1 and LMP1 cars, may use torsion springs on the corners as the wheel springs, twisting the spring through a complex system of pushrods and rocker arms. In those systems, one end is fixed to the chassis and the free end is twisted.
Finally, Leaf springs are a rather antiquated type of spring, but we still see it in some racing vehicles such as the iRacing Street Stock. Leaf springs are made by stacking strips of metal and then bending the stacked set, creating a curved shape. Applying a force to the top of the curve results in the stack acting as a spring. These are seen very often on modern pickup trucks and jeeps, but they seem to be getting replaced slowly by coil springs.
Spring Motion Ratios
Something we don’t have to consider with iRacing is what’s known as a spring’s installation ratio, or “Motion Ratio”. Whenever a spring, shock, or sway bar arm is not mounted directly on the centerline of its respective wheel, it doesn’t move at the same rate as the wheel. For a simple example, if we have a 12” suspension arm, and the spring’s center axis is located 6” inboard of the wheel’s centerline, it will probably move at ½ the rate of the wheel. If the wheel moves up 1”, the spring will move 0.5”, and we have a 2:1 motion ratio. This can obviously get dramatically more complex by introducing spring angles, but for a very simple explanation, we’ll keep everything vertical and parallel.
While it’s not something we deal with directly in sim-racing, it’s a common question: “Why does setup A work in this car but not in this one?” For instance, why can’t we take a setup out of the Class B Xfinity car and drop it straight into the Gen 6 Cup car? More likely than not, the two cars have slightly different suspension systems, and the springs are mounted with different motion ratios, resulting in a different behavior on-track. There’s also the case of different aerodynamics and weight distribution, but it’s something to consider. Also, it’s the main reason why you can’t always take the setup from your real-world Super Late Model and put it in the iRacing Super Late Model. If you don’t know how the motion ratios differ, it’s going to be hard, or nearly impossible, to make it work properly.
Multiple Springs per Suspension System
NASCAR uses an extremely old suspension system in its race vehicles, opting for a solid rear axle suspension system instead of the more common independent systems we see on the road today. The solid axle suspension system is interesting because the two corner springs both affect how the other corner works, specifically in the overall vertical stiffness of the rear end. In an independent rear suspension system, the left-rear spring won’t have much to say in how the right-rear suspension travels, but that’s not the case here. Matt Kenseth’s flip at Talladega this year is going to be extremely helpful in a lot of things, this time we’ll look at where the springs are located in the rear suspension (orange arrows):
The springs are located on the far end of the truck arms (painted white). Despite being located on two different components, they both influence the same suspension system. The two rear springs in a stock car are mounted in parallel, and thus combine rates to get an overall rear stiffness. To find this, we simply add the two rates together, so if we have a 250 lb/in left-rear spring and a 1000 lb/in right-rear spring, the rear suspension has a 1250 lb/in compression stiffness.
The front suspension acts in a similar fashion, however much more complex. If we’re using bump stops, we can wind up with three different springs acting on the suspension in the simplest example: Main spring, bump stop, and sway bar. These are also in parallel, so they can add to find the wheel’s spring rate. This is something to consider as well, since the suspension’s spring rate changes the moment you’ve contacted the bumpstop. Here’s an image of a modern stock suspension system with the various spring highlighted:
Real systems get even further complicated by suspension arm flex (which we have in iRacing, but it’s not really a concern), tire spring rate, and the spring rate of anything else that flexes while the suspension is in motion.
The alternative is when springs are mounted in series, or in-line with one another, usually on the same shock. This is common in dirt racing, especially Rally and dirt oval racing where a softer spring will be used next to a stiffer spring, often called a “tender” or “helper” spring. For this situation, the combined rate is much more complex, found by multiplying the two spring rates, then dividing that by the sum of the rates. For these arrangements, the softer spring is usually expected to bind quickly, meaning the suspension is now solely on the stiffer spring rate.
Next week we’ll dive deeper into springs, and how track characteristics can influence what springs need to be installed on the car.
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