We’ve explained roll center, suspension geometry correction bushings, the benefits of camber, and what sway bars do to a car’s dynamic whole. We touched briefly on body rigidity when we talked about subframe locking collars, but haven’t truly explained the distinction between suspension stiffness and body shell stiffness. For that, let’s look into the difference between a strut bar and a sway bar.
Practically, stiffness and rigidity mean basically the same thing. They both represent the resistance to bending of an object, and we usually see stiffness being the favored word. But for the purposes of this article, stiffness is the word we’ll use for suspension and rigidity is for the stiffness of the body. In effect, strut bars add body rigidity and sway bars add suspension stiffness in a very general way.
The two metrics have a close relationship. Stiffer suspension causes sharper reflexes and different weight transfer characteristics, while softer suspension can add grip and control. For suspension to work effectively, it needs a rigid platform, like a rigid body structure. Strut bars and sway bars are similar in general function, both being formed tubes of metal that have a certain stiffness yet they serve completely different purposes. Here’s how they vary.
A strut bar is a body stiffening device that ties the strut or suspension towers together. Plenty of cars, such as the 2023 Nissan Z, come with strut bars from the factory, but all cars can benefit from them. Unlike stiffer suspension, a more rigid body only benefits the car. It increases ride quality by reducing uncontrolled motions and also improves handling by making the body less twisty, which helps turn-in and direction changes.
Drivers will feel a rigid body the most over choppy pavement, where the chassis is taking impacts and flexing constantly. Cars are always twisting and deforming as they respond to road surfaces, and that deformation is carefully understood by the engineers of the vehicle. Like anything in engineering, it’s a compromise versus cost.
Think of the body of the car as its own spring, or a guitar string. Impacts and cornering twist it out of shape and it has to spring back into its normal shape. Because the body of the car is functionally undamped, it can cause a resonance on several frequencies. The lowest frequencies manifest themselves as cowl shake or general vibration, which is the actual body of the car ringing like a guitar string until it settles down. High-frequency vibrations generally manifest as cabin noise, which is then tuned out with rubber suspension and subframe bushings.
Just like tightening a guitar string, making the body stiffer naturally increases the frequencies of those vibrations, which puts them in a less perceptible range. It also makes the body more resistant to change and therefore more responsive when it does deform. Enter the strut bar, one of the most popular ways of stiffening the car body in a place that is relatively weak.
Letting go of the guitar string metaphor, now think of a box. If you have a cardboard box laying around, use that. Notice that when the box is fully closed, that is when all six sides of it are rigidly secured to one another, that the box is very durable and hard to deform. It wants to stay in a box shape. Open up the top of the box and that rigidity decreases, which allows the box to shear, or skew away from a box shape into a parallelogram more readily. Remove the bottom, and the box loses all shear strength. Think of this box as the engine bay, where the engine bay is a box with no top.
The strut bar effectively adds a top to the box, preventing unwanted movement and shaking. This principle applies all over the car, as the engine bay and cabin are effectively two boxes working together to be as rigid as possible. Thus, the strut bar attempts to close the box and it can be added to the rear strut towers as well. Things like subframe reinforcements and extra structural bracing sold by companies like Cusco all achieve this same effect.
Strut bars are generally a harmless modification, though there are some anecdotes of folks who threw the Cusco catalog at their cars and had cracked strut towers or strange body deformation. It’s important to note that the body of the car was engineered with a certain stiffness in mind and changing that stiffness without a strategy can shift load to places on the body that were never intended to carry it. Stiffen with some caution, but most cars will be just fine.
We’ve gone in-depth about sway bars before. To be clear, they are very different from strut bars in function. A strut bar is designed to be inflexible, but a sway bar is specifically designed to flex. Instead of aluminum or normal steel like a strut bar, a sway bar is constructed of spring steel, like a suspension spring.
Sway bars, which act directly on the suspension during cornering, have a very specific thickness and construction to achieve a desired spring rate. While a strut bar simply fortifies the platform on which the suspension is built, a sway bar is a crucial member of the suspension itself. It controls side-to-side weight transfer and the difference in ride height between two wheels on the same axle while allowing the suspension to behave normally forward-to-back.
Well, they both do different things and achieve different goals. Sway bars will have a more direct effect on handling characteristics than strut bars will, but doing both doesn’t hurt, nor does just one of the two.
The most important thing is to listen to what you and your car need and modify it accordingly. If the car is wobbly and shaky over bumps with fresh suspension, then you likely have a rigidity issue that a strut bar can fix. If the car leans too much in corners or just doesn’t have the right handling balance of oversteer and understeer, a sway bar will go a long way to fixing that.