September 2nd, 2016 by Matt Holden
When we watch a car go around a corner, we almost always take a note of how much it rolls over, or lifts the inside of the car, but how much do we really think about what’s happening? In the yester-years of NASCAR, it was probably never given a second thought to how much a car was rolling over in the corners, but here in 2016, it’s become a science. There’s one more section we need to cover before diving into the garage, a very complex but necessary concept that is key to understanding what the car is doing while on-track: Roll Stiffness.
Roll Stiffness is something that gets covered extremely early in any motorsport-related schooling. When I was in college, I had a Formula SAE car on scales performing a roll-stiffness test within the first month of motorsports classes. On the surface, it’s very simple: How much does the car’s body roll for an applied force? I learned this characteristic in “Pounds per degree”, or lb/°. So if the car had an overall stiffness of 250lb/°, it would take 250 pounds of lateral force to roll the car over 1 degree from horizontal.
On that Formula SAE car, which was specifically built for 60-second autocross runs, we could get away with doing a test of the entire car’s roll stiffness. For a racing vehicle that needs to cover 300 miles, we need to break it down further to “balance” the car. While the entire car will have a given roll stiffness, that value is derived from the roll stiffness values of the two suspension systems in the car. For most racing cars today, that’s two independent suspension systems. However in stock cars, we have an independent, double A-arm suspension in the front, and a Solid Axle, truck-arm suspension in the rear. Two extremely different systems that need to be looked at separately.
Roll Centers and Moment Arms
We’ll start with looking at how the suspension rolls in the first place. On every suspension system, we have two points that play into this: The Center of Gravity and the Roll Center (or Moment Center, to some). Center of Gravity was covered earlier, so go brush up on that if you need to. The Roll Center is an imaginary point somewhere in the suspension which can be thought of as the point the suspension rotates around as forces are applied. For an independent suspension system, locating this Roll Center is very complex and usually requires computer software, or an intern, to locate it without burning up a few hours in the day.
Finding the Roll Center’s location involves extending lines through each of the suspension arms, locating their intersection points, then drawing more lines from those points to the center of the tire contact patch. Got it? Below is an image of how the Roll Center would be located in an independent suspension system like what is on most race vehicles. Each of the suspension arms has lines extended from them (blue for right arms, green for left), and each of those pairs intersects at a point, known as the “Instant Center”. We then draw lines from the Instant Centers to the centers of the tire contact patches (red and pink lines). Where these two lines intersect is where the roll center for that suspension system is! For a stock car, an offset roll center like the one shown here is fairly common, while it will typically be on (or close to) the centerline for a road racing vehicle.
Ready for the rear suspension? This is where it gets complicated: The Roll Center in a solid axle suspension, like on NASCAR stock cars, is right around the center point in the track bar. That’s it, seriously. I’ve got another picture below of the rear suspension assembly on an old COT (it’s a show car, but still works for this explanation. I’ve marked the center of the track bar with a green “X”, and that’s literally all there is to it.
We’ve got all that, our brain has melted, now what? To visualize how suspension systems deal with lateral forces, take another look at the rear suspension picture above. You’ll notice that there’s also a white “X” up in the car, the Center of Gravity, and those lines are connected with a pink dotted-line. Those two points (Roll Center, CG) and the pink line create what’s called a “Moment Arm”, named as such because it generates a Moment, or torque, on the suspension. In the article about the Center of Gravity, I mentioned that all forces act on the car at the Center of Gravity, and this comes into play again here with the suspension! The best way to think of the Moment Arm is as a wrench: Place the bolt you’re trying to turn at the Roll Center, and your hand is the Center of Gravity. When you push on the free end of the wrench, you rotate the bolt. The same phenomenon occurs in suspension systems.
When the car goes into a corner, it experiences a lateral force pushing outward from the inside turn, known as Centrifugal Force. For analysis purposes, this force acts on the suspension systems at the CG, and the Roll Center will basically remain fixed in the car, rolling the suspension to the outside of the turn. In race shops, while preparing for a race weekend, a lot of work goes into moving various suspension components to determine the optimum roll center location for each suspension system, and getting this right before a team loads up for a track is often the reason why a car wins or loses on race-day.
Suspension stiffness is directly adjustable by moving the suspension’s Roll Center (RC). If we raise the RC, we shorten the moment arm, and the suspension becomes stiffer in roll. Similarly, if the moment arm lengthens by lowering the RC, the suspension becomes softer in roll. While we have almost no control over the front roll center location in iRacing’s Stock cars (it’s directly adjustable in some cars, such as the V8 Supercars), we have some small control over the moment arm length in the front. On the rear, however, we can directly adjust the RC’s location.
Ideally, we want the suspension systems to work in harmony and not fight one another over a long race. In a sprint race, we can get away with a chassis imbalance since tire life isn’t a big deal, but over a long race it becomes crucial. When one suspension system is much softer than the other in roll, one tire will become overloaded while it’s buddy-tire will become under loaded. For instance, if we have a very soft rear suspension in roll but a very stiff suspension in the front, the right-front tire can become overloaded while the left-front can become under-loaded. Think of a dirt late model and how they used to run on three tires everywhere for a very exaggerated example. We don’t see that anymore though, mainly because teams realized the left-front tire wasn’t simply a balancing ballast and began using it to help the car turn. When we have the suspension systems working in harmony with all four tires doing their job equally, we have a dynamically balanced race car.
There have been numerous methods to assist engineers in designing cars with balanced suspension, the most well known being the Roll Couple Distribution method. This method was used to calculate the load that would transfer from the inside to the outside of the car in a turn, and attempted to mathematically allow engineers to match the front to the rear of the car and wind up with a balanced race car. How it works, I have no clue, because by the time I started learning all of this stuff there was a new method: equal roll angle. This method attempts to match the suspension by taking the known lateral force for a corner, and building the suspension systems so they’ll roll to the same angle when that load is applied. Doing so should produce a balanced car, and it’s been used at least at the lower levels of stock car racing.
Adjusting RC and Moment Arms
We’ll definitely revisit these things later in the series, but there are ways you can adjust the moment arms and roll center heights in the iRacing garage that can help you get a feel for what is going on. First, and the easiest, is overall track bar height. This is extremely simple, and is typically what I do to the car in a “final practice” type session. Keeping the height split the same on both sides, just raise or lower the entire track bar equally. So if you raise the right-side 2″, raise the left-side 2″. This raises the RC height 2 inches and shortens the Moment Arm (not necessarily 2″ though). A shorter moment arm means a stiffer suspension system in roll. A higher rear roll stiffness will make the car looser, or more prone to oversteer. The opposite will happen by lowering the track bar. Raising the bar will cause less weight to shift off of the left-front while turning, and can help your car maintain speed over a longer run, too.
The front suspension is less adjustable, however. If you are running any of the stock car series in iRacing, you can alter the front moment arm slightly by the front end’s height. A higher front end will usually cause the car to be much looser due to a softer roll stiffness. Gale Force Racing actually ran into that exact issue in the 2014 NASCAR Pro Series when we got to Dover. Some of the drivers raised the cars to allow the car to drop from the high-load turns, but it wound up making the car extremely loose. Since then, I’ve suggested that drivers find a ride height setting they like in the front of the car and make sure to keep the heights at those settings in all cases. If you start changing springs, shocks, or whatever else, but don’t keep the heights the same, you run the risk of changing the moment arm length. That’s two changes, and you probably only intended to make one!
Our next article will start covering springs. Springs are directly tied to the suspension’s roll stiffness in the same way the Roll Center and Moment Arm are, but to different effects. In some cases, coil springs don’t handle roll very well and may contribute very little to the roll stiffness. In others, such as anti-roll bars (or sway bars), they contribute a lot to the stiffness. And then you can add in bumpstops, or sway bar asymmetry, and you have a ton of things to think about.
We’ve covered all the basic principles necessary to start looking at what we can do in the garage. Next we’ll look at springs, from 5-inch Big Spring cars to Coil-over Late Models and Modifieds, to the mysterious bumpstop and bump springs, and even to sway bars, also known as “torsion” springs. I’m looking forward to it, and I’m sure all of you are as well!
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