Why Sway Bars Should Be Your First Suspension Mod
Even if sway bars seem simple, they are a valuable and somewhat complex mod. Here’s what you need to know to get started.
Modifying cars is deeply personal and can be DIY’d by anybody. Some pursue greater cornering performance while others seek a different driving feel. Whatever it is you want to do, you’ll want to know how sway bars can be one of the most important and affordable parts of a complex dynamic puzzle.
Stiffer sway bars make cars more responsive in corners, whereas softer sway bars do the opposite. It's the easiest and simplest way to make a perceivable change to the balance and dynamics of any car. Sway bars are usually affordable and easy to install, making this a must-do mod for anyone looking to tune a car. Even so, it’s important to understand that the relationship between the front and rear sway bar is much more important than the actual stiffness of either sway bar. It’s similar in principle to springs and dampers, except I think there is much less compromise to be had by modifying sway bars first.
Of the many methods of carrying more cornering speed, there are none more important than practice and tires. Before you modify anything, get familiar with your car. Take it to the track. Rip some canyons. Learn about and listen to the vehicle and give it what it needs. There are thousands of poor cars out there screaming for better driving, but I digress. Once you can identify what you do and don’t like, sway bars are the first real modification you should do.
How do sway bars work?
The operating principle of a sway bar (also known as an anti-roll bar) is incredibly simple. It's a formed section of spring steel that connects the left and right sides of the suspension on a given axle. It can be thick, thin, hollow, or solid, and have different lengths and leverages to determine how stiff or soft the sway bar is. Visualize the sway bar as a sort of extra spring that only works when there are different loads on the left and right sides of the suspension, an occurrence known as roll.
By connecting the two sides together, sway bars prevent roll. Hence, being called anti-roll bars. Like anything suspension, it’s simple enough to modify and understand on a basic level. Yet, there is still some dark magic to making everything play nicely with each other, and it can sometimes seem counterintuitive. Most of that strange behavior comes from the fact that a sway bar forces the inner wheel to move with the outer, more loaded wheel in a corner. This is where engineers get into the complex math of weight transfer and suspension frequencies.
Some folks will look at a car that rolls or leans a lot in corners and infer that there is a lot of weight transfer and that a car that leans less in corners transfers less weight. For a confusing myriad of reasons, this is true and untrue. As a base, it’s important to understand that roll stiffness is the car’s resistance to roll. Roll stiffness comes from a combination of roll bars, the main springs, dampers, and track width.
Weight transfer always happens regardless of roll stiffness. What is being controlled with roll stiffness is the rate or frequency at which the suspension reaches maximum roll and a very small percentage of a vehicle’s overall weight transfer. A softer sway bar will transfer slightly less weight to an opposing wheel and a stiffer sway bar will transfer more. More importantly, a softer sway bar will reduce the frequency of the suspension and a stiffer bar will increase it.
If you want to get into the science of suspension frequency, this paper by Nandan Rajeev and Pratheek Sudi from the JSS Academy Of Technical Education in Bangalore is a great comprehensive primer. For our practical purposes here, you can pretty much think of suspension frequency as, basically, suspension stiffness.
Why modify your suspension at all?
For most of us, a higher suspension frequency means things happen quicker in a cornering situation. A lower frequency will make the car lazier and slower to respond to inputs while giving drivers more time for corrections. Often, cars are set up soft from the factory for ride quality and easy handling. Raising that suspension frequency can give the car superior reflexes but only if you approach it intelligently with step-by-step modding.
Most aftermarket sway bars aren’t tuned to work in conjunction with a factory sway bar, so if you're going to change one it is wise to do both front and rear at the same time for maximum effect. There are rare exceptions, like the 034 Motorsport Mk7 GTI rear sway bar, but most are designed to work as a pair. Think of it this way: You wouldn’t install aftermarket coilovers on only one axle of the car, right?
How to choose and tune a sway bar
Sway bar stiffness is normally defined by its thickness, construction, and leverage. Generally, a sway bar with a larger diameter is stiffer and one with a smaller diameter is softer. Some sway bars are hollow for lightness and require much more thickness to be as stiff as a solid sway bar. There is also the length from the sway bar end link to the sway bar mounting point that determines the leverage and effective spring rate of the sway bar. There is also the issue of where it mounts to on the suspension and if it is moving 1:1 with the wheel. Most of the time, it is moving at around a 0.7:1 motion ratio that changes the effective stiffness of the sway bar. Dampers are usually prone to the same issue.
This is the reason why aftermarket adjustable sway bars have two or three holes for the end link instead of one; it's so one sway bar can become several, and roll stiffness becomes an excellent tuning tool. Those holes are drilled in a way that changes the leverage and stiffness of the sway bar. The closest ones to the sway bar mounts on the body are the stiffest settings.
There is no magic bullet for suspension tuning, nor is there any way to know what is the best setting without trial and error or a lot of complicated math. Conventional wisdom says a softer bar equates to more grip and a stiffer bar equals less grip, within reason. There is a setup window for every component that will work best for the whole car, and sway bars can be counterintuitive. For example, on my 2010 Volkswagen GTI, I set the front sway bar at full soft and the rear sway bar at full stiff in the hope to kill understeer by removing grip from the rear and adding it to the front. This setup did not initially work because of my roll center and suspension geometry, so I went one position stiffer on the front sway bar and found more grip.
Going into the details of how and why this happens would take an exhaustingly long engineering journal, much like the ones I waded through to thoroughly understand the shockingly complex mechanics of these glorified spring bars. If you want to dive deep, here is a great discussion from Rodrigo Santos about weight transfer and how it applies to car setup. Or, this video from former Mercedes-AMG F1 aerodynamicist Kyle Forster:
Basically, controlling the rate of roll with sway bars will affect how the weight settles across the tires, which ties into roll center, center of gravity, and suspension kinematics like camber, caster, and toe-steer.
Modifying or tuning all of those aspects is overwhelming, so taking one part of it and making it work is the name of the game for a DIYer. This is the beauty of sway bar tuning. I learned that there is a precise relationship between roll, weight transfer, and roll center on my GTI in a way that it wants a stiffer front sway bar. This is a behavior that exists on MacPherson-strut cars and not so much on other styles of suspension thanks to its limitations with dynamic camber. Under compression, strut cars actually lose negative camber while dual wishbone and multi-link can be designed to gain camber.
Once I learned that relationship, I killed understeer in my GTI in a way that coilovers, bushings, or lightweight parts couldn’t. Turn-in became crisp instead of pudgy, and mid-corner balance was tuneable with easy turns of a wrench. Practically, sway bars will most affect turn-in and mid-corner balance, and they are much easier to tune than adjustable dampers for that end. Dampers are for the minutiae, and sway bars are for the bigger picture, at least in my experience.
When I paired my Whiteline 24-mm front and rear sway bars with stock springs and dampers, the car became the perfect compromise of balance and ride quality. Although it was certainly firmer over broken pavement, it handled large undulations well and felt like it had reassuring communication thanks to a more linear feel than factory.
In my experience, almost no car is perfect from the factory, at least for my taste. I encourage everyone to start tinkering with their cars and figure out their own preferences. Worst comes to worst, you can always go back to stock. It’s lift-off oversteer or bust my friends, and sway bars get you to at least third base.
Have any questions or experiences with different sway bars yourself? Share in the comments!