October 21st, 2016 by Matt Holden
I had a professor in college say, “Springs for speed, bars for handling”. It’s a short, but interesting statement, and it’s a concept that’s easy to forget and go about things in the other way when tuning a race car, be it real or virtual. Modern race cars, oval cars especially, have seen huge advances in the technology that goes into something as simple as a sway bar, or anti roll bar. Just like the coil springs, sway bars can be used for both mechanical grip and aerodynamic grip and come in many different forms. However, they all perform exactly the same role regardless of what car (or type of car) they’re installed in.
What is it?
A sway bar, in simplest terms, is a torsion bar spring linking two corners of a car’s suspension to prevent opposing movements. If the wheel on one side of a car goes up and the other doesn’t, or goes down, the bar exerts forces to bring the two back in line. The bar is typically round, frequently steel (special case for the Indy car at the end of the article), and part of the bar is fixed to the chassis in some way. Sometimes the bar is solid, sometimes it’s hollow, but it all depends simply on how the car’s engineers want the bar to behave.
This image, from Speedway Engineering, shows the components typically seen in a modern racing sway bar. While the sway bar in your passenger vehicle is likely one continuous tube bent around various components in the chassis, racing cars usually employ a “three-piece” sway bar, where the bar (left of picture) can be changed without changing the arms (right of picture). The sway bar arms can feature their own adjustments, such as offset or length, and can also be changed independently of the bar itself.
This image shows the suspension arrangement typical of an open-wheel car, or any car utilizing push- or pull-rod suspension. In this specific arrangement, the anti-roll bar is situated in front of the suspension components with one end fixed to the chassis (not shown) and the other end clamped into a t-shaped arm. When the suspension moves and rotates the rockers, the linkages to the arm twist the bar, creating the anti-roll effect.
NASCAR-style Sway Bar
Following the great war over rear sway bars being “improperly” mounted during the 2012 season in an effort to generate extra rear end skew, NASCAR put a stop to rear sway bars in the Cup series when they introduced the Gen 6 cars. The Xfinity “COT” also arrived on the scene without rear sway bars following a vote among the owners in that series. Similarly, many oval series in the US do not allow the use of rear sway in stock car racing series, and some disallow sway bars altogether. This places a heavy emphasis on the front sway bar to do a lot of the work to “flatten” the front of the car for aerodynamic purposes, while placing the responsibility of rear roll on the rear springs themselves.
The front sway bar in a NASCAR-style stock car is mounted in front of the front suspension, just behind the radiator, in a tube welded to the bottom of the front clip (known as the “sway bar tube”). The bar is placed within this tube where it is allowed to rotate freely and steel arms are clamped onto the ends of the bar. Linkages are placed on the end of the sway bar arm which link it to the suspension’s lower control arm.
I took this picture on a visit to Hendrick Motorsports a few years ago. This particular car is one of Jeff Gordon’s test cars from the 2013 season, so there are some extra bits in the picture we wouldn’t normally see, such as the laser sensors on the splitter and the blue suspension travel sensor behind the shock. Still, the sway bar is mounted in a fairly typical fashion, and can be seen just to the right of the brake assembly. The large, dark grey triangular-shaped piece is the sway bar arm, and the silver circle at the front end of the arm is the end of the sway bar.
Sway Bar Diameter
The bars themselves are available in many different diameters, with this being the major factor in a sway bar’s stiffness. Bars are “rated”, similar to how a spring is given a rate, however sway bars (and torsion springs as well) are usually rated by giving them a force at a specific angle of twist. For oval track racing in the US, sway bars are typically rated by twisting the bar 5° from rest, and then given the force they exert at that angle of twist. If we have a bar rated at 1200lbs, then that specific bar would exert 1200 pounds of force at 5° of twist. Bar length is typically mandated by the sanctioning body, and is typically a non-factor in sway bar selection.
Sway Bar Arm Lengths
Whenever a torque (twisting force) is applied, the resulting force is largely dependent on the length of the arm applying the force. Here is the equation for torque:
T = r*F
T = Torque (lb-ft, N-m)
r = Radius, or arm length (inches, meters)
F = Force applied at the end of the arm (pounds, Newtons)
We can see from this that, for a given torque, if the arm length is increased, the force at the end of the arm must decrease to keep from changing the torque. This is what happens when you go and get a bigger wrench, or get a big pipe to put on the end of the wrench, for a bolt that won’t budge. The longer arm allows you to place a weaker force on the end of the arm to get a large torque at the bolt. This same effect occurs in a sway bar, but typically on a smaller scale.
If we know that the bar will exert a given torque at a specific angle of twist, changing the arm length changes the forces acting on the suspension at the end of the arms. If we increase the length of the arms, we reduce the force acting at the suspension, effectively “softening” the sway bar. Conversely, if we shorten the arms, we will increase the force at the suspension, “stiffening” the bar.
Zooming in on the picture of the #24 test car from earlier, we can see that the sway bar arm installed on the car has three length options, with the linkage installed in the middle of the three available settings. The picture at the top of the article has similar holes. These holes allow for an easy way to fine-tune the sway bar’s characteristics without having to remove components of the sway bar.
Sway Bar Asymmetry
Whenever we run the bar with different-length arms, we are using what’s known as “arm asymmetry” for our car. This is typically where, on a left-turning oval car, the right side arm is shorter than the left side arm. This shorter arm applies a heavier force to the right-front suspension relative to the left-front suspension, which causes the body of the car to roll to the left. The simple result of this is that the right-front wheel rate (spring rate felt at the wheel) is increased while the left-front wheel rate remains the same, effectively the same as increasing the rate of the right-front spring. This, in turn, creates a “heave” characteristic in the sway bar itself, meaning it will begin to resist vertical movement in the chassis instead of purely countering body roll.
This can be used to tune steady-state cornering (car is settled in the corner, no more pitch or heave is occurring) characteristics. Increasing asymmetry will help to flatten out the front of the car without adding preload, which can be extremely helpful for an aerodynamically-dependent car. The increased right-front rate will keep the car from rolling left and right in the corner, and provide a more stable aerodynamic platform.
Keep in mind, however, that increasing asymmetry also increases the vertical stiffness of the front suspension, so an increase in arm asymmetry will also cause the front of the car to run higher, especially on the straights, which increases drag. While this can be ignored at a track with short straights, it can spell trouble for a track like Indianapolis where the straights are fairly long.
Sway Bar Preload
The final adjustment on the sway bar is the bar’s preload. By shortening or lengthening one of the linkages on the sway bar, we can place a load on the sway bar artificially, without the car experiencing any lateral forces. Starting the bar with some amount of preload will roll the car to the left so that the lateral forces wind up rolling the car to a “flat” attitude.
A by-product of preload is a change in dynamic crossweight, or crossweight in the car while cornering. An increase in preload (negative values) will increase the crossweight of the car as lateral forces change, specifically on corner entry and exit. While some drivers dislike preload in any situation, other drivers have been known to like the extra mechanical grip offered on corner exit. Keep in mind, however, that larger sway bars will apply the crossweight change faster than a smaller bar, which can lead to very strange results.
Sway Bar Adjustment
Sway bar tuning has become complex, and is no longer a case of just swapping out diameters to get the handling characteristics you’re looking for. The sway bar is often chosen based on the springs used, track characteristics, and aerodynamic needs, so in the same way that a few articles will be dedicated to spring tuning, the same will be done for sway bars.
Indy Car Arm Material
The Dallara Indy Cars in iRacing offer two choices of sway bar arm material: Steel and Titanium (Ti). Since Titanium is a weaker material than the steel used in sway bar parts, and will result in a “softer” acting sway bar than with steel components. This adjustment could be likened to the arm length adjustment found in stock cars, however this specifically concerns the bending of the components instead of the dimensions of the components.
In-car Adjustable Sway Bars
It was remarkably difficult to find a good picture of these, so I want to give credit to Turner Motorsport’s website for the image above.
Many road-racing vehicles feature a driver-adjustable sway bar in the front and rear, operated by levers in the driver cockpit. These adjustments change the acting angle of the sway bar arm itself by rotating it. When the arm is vertical (Full Stiff Setting), the arm itself is very resistant to bending, and will transmit a large amount of the sway bar’s torque forces directly to the suspension. As the arm is rotated to horizontal (Full Soft Setting), the arm is likely to flex as forces are applied to it. This flex will cause some of the sway bar’s forces to be “lost”, resulting in a softer-feeling sway bar.
Images in this article were sourced from:
Speedway Engineering: http://www.1speedway.com
Turner Motorsports: https://www.turnermotorsport.com/p-340455-e9x-m3-turner-motorsport-racing-blade-style-front-sway-bar-kit/?pdk=AQ
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