When it comes to sway bars (also known as anti-roll bars, stabilizer bars), the common misconception is that their sole purpose is to keep the body of the vehicle from rolling to one side during a tight turn. While that is true, sway bars do more for your car’s handling than you might think.
When a vehicle has excessive body roll, the majority of its weight is shifted to the outer tires during a turn, causing less weight to sit on the inner tires. Lateral grip is something you don’t want to sacrifice on a racetrack. A sway bar’s purpose is to keep suspension movements relative throughout turns. This means that when the vehicle makes a right turn, the left side of the vehicle’s suspension will compress similarly to the right side, keeping more weight on the inner tires.
What is a sway bar and what is its purpose?
Simply put, a sway bar is a torsional spring that connects to both the left and right sides of the suspension to reduce body roll, as Oliver Rathlein of Eibach explained, “The sway bar links both sides of the suspension system to help reduce body roll when cornering. When both wheels take a bump equally, the wheels move the same amount without twisting the anti-roll bar. Individual wheel movement or body roll will force the bar to twist as the lever arms are moved, thereby adding the bar’s own spring rate to that of the car’s springs. Although an anti-roll bar’s main function is to reduce body roll in cornering, it also influences overall handling. You can fine-tune Over- or Understeering with them.”
Besides the performance associated with not having as much body roll, sway bars also improve the weight distribution amongst all four tires. “Since you are cornering flatter, less weight is being transferred to the outside tire,” explained Ben Knaus of Hellwig Products. “Because of this, you can go faster around a corner before losing traction, which means faster and safer cornering.”
For the hobbyist racer or track day enthusiast, aftermarket sway bars are a crucial aspect of the car’s handling. Like Rathlein said earlier, you can fine-tune for over or understeering tendencies. To fine-tune, one needs to first find a sway bar compatible with his or her car in the right diameter. Aftermarket sway bars usually come with up to three holes where the end links attach. Those holes are used for adjusting stiffness. Adjustable end links allow the preload of the sway bar to be adjusted to fine-tune even more.
Different Types of Sway Bars
Sway bars come in all shapes and sizes, but at the end of the day, they all have the same purpose. Solid and hollow (tubular) sway bars are both widely used in this day and age, it really comes down to each driver’s personal preference.
“Both hollow and solid bars work exactly the same way—the only real difference is weight. With the added traction from performance tires and heavier cars with more horsepower—the bar diameter has grown in size to control the handling of the vehicle. Making the larger-diameter sway bars hollow allows us to save weight whenever possible. All of our larger-diameter sway bars are hollow,” Rathlein explained.
A tubular sway bar will obviously weigh less than a solid sway bar, but it will also have a lower spring rate if it’s the same diameter as a solid unit.
“Rule of thumb is that a tubular bar will have a similar rate to a solid bar with an 1/8-inch smaller diameter. So a 1-1/8-inch tubular sway bar will feel like a 1-inch solid bar on the car. Obviously that’s a pretty rough estimate but it’s usually a good starting point,” added Knaus.
Some aftermarket sway bar manufacturers produce splined sway bars. This means that the sway bar itself is a straight, either hollow or solid bar that is splined on the ends to accept the arms that connect to the end links. The splined sway bar is seen in road racing a lot, as it is easily adjustable. Other places where splined sway bars are common are drag racing and off road racing.
There is one more type of sway bar that is currently being used by various high-end automakers and that would be the active anti-roll bar system. The active anti-roll bar system takes commands from the suspension’s ECU to proportionally reduce body lean in turns, while also improving rough-ride quality. Unless you have an E65 BMW 7-series, McLaren 650S, Porsche Panamera, or a third-generation Lexus GS430, chances are your car won’t come equipped with such a complex system.
The Effects of Bushing Material
The functionality of bushings varies based on the material they are made of. Automakers equip cars with softer, rubberized bushings that minimize NVH (noise, vibration, and harshness) because they are cost effective and provide a smoother ride.
After a few years the rubberized bushings will start to dry out (if the car is brand new), but if the car is used, there’s a good chance that the bushings have started to go bad, unless the previous owner has recently installed new ones. A bushing that is too soft or worn out will take away from the effectiveness of the sway bar by allowing it to flex vertically, rather than twist horizontally – this phenomenon is known as bushing deflection.
Popular replacements for factory rubberized bushings are polyurethane bushings. Polyurethane is much more durable than rubber, it doesn’t dry out because of its hydrophobic properties, and the sway bar won’t deflect as much as it would if it was mounted with rubber bushings. Knaus explained, “In our testing we find about a 20-percent loss of rate in our polyurethane bushings. So if your sway bar has a rate of 200 lb/in, once installed on the vehicle you will only be using 160 lb/in of the bar. Different materials and even durometers of polyurethane will have different losses and can change the handling significantly. Also, different materials wear differently and have different amounts of friction. Since the bar is constantly rotating in the bushings, you want it either well lubricated or in a material with low friction, both for feel and bushing life.”
What The Hobbyist/Enthusiast Racer Should Know About Sway Bars
There are a few key factors that the hobbyist and enthusiast racers should know about sway bars. First, bigger isn’t always better.
“I constantly see people comparing different companies’ products, buying the largest diameter sway bar,” said Knaus. “There are so many other factors to compare. There is such thing as a sway bar being too stiff, first off. Also, depending on the material, arm lengths, bushings, etc., a larger-diameter bar may actually have a lower rate than a smaller diameter bar.”
In terms of cornering, you always want a higher roll rate in the front of the vehicle than in the rear. Knaus explained, “The reason for this is your front-to-rear roll rate ratio helps determine understeer versus oversteer. If your front is stiffer you will have understeer, while a stiffer rear will give you oversteer. While getting close to neutral steer is a good goal, it’s always safest to bias towards an understeer. In understeer you can just let off the gas a little and get back in control, while an oversteer condition can go bad quickly.”
All performance cars are designed and engineered to have a low center of gravity, which is why automakers mount sway bars at the lowest point possible in the first place. In virtually all modern-day cars, the sway bar is mounted in the lowest point of the vehicle, meaning that’s the best spot for it. Sometimes you may see a sway bar mounted higher in the chassis; tube-chassis racecars are great examples.
“We design our sway bars to fit exactly in the factory locations. This ensures proper clearance throughout the suspension travel as well as to the ground. On a race car, most chassis builders will mount them as low as possible for the same reason: to achieve a lower center of gravity,” Rathlein commented.
Last, but not least, a sway bar doesn’t lift or lower the car under any circumstances or make the ride rougher.
Knaus explains, “The biggest things I get all the time are people thinking the sway bar ‘lifts’ or ‘lowers’ the vehicle and also how it makes the ride ‘rougher’. If installed correctly, a sway bar should do none of those things. Driving straight down the road, the bar is just rotating up and down in the bushings with the suspension and will not lift or lower the vehicle. Also, while driving straight down a road, you shouldn’t be able to feel if you have a sway bar or not (unless it’s really windy) because it is just rotating up and down with no resistance. The only time a sway bar comes into effect is when one side of the suspension is trying to extend or compress more than the other side.”
Upgraded sway bars are a big factor in a vehicle’s handling characteristics and help with more than just body roll. Don’t just settle for the biggest sway bars because they’re the stiffest, most expensive option; they should be thoroughly researched as there are a plethora of manufacturers, sizes, and styles. With a clearer understanding of these components, you can test what works, and what doesn’t work with your car. Happy cornering!