When it comes down to it, there’s no easy way to explain the engineering behind suspension design. It’s a complex science where tweaking one variable affects a dozen other variables, and the resulting domino effect can be quite frustrating for a weekend warrior trying to shave off a tenth or two on the autocross. To compound the situation, once you start diving into suspension tuning, you’re met with a roadblock of fancy terminology that few people can clearly explain. That’s what this story is all about. Think of it as a glossary of common suspension terms defined by the leading engineers and manufacturers in the aftermarket. We got our field of experts to uncover the secrets behind everything from Ackerman and antisquat, to scrub radius and instant center. Let’s dig in!
As a car negotiates a corner, the inside front wheel travels in a tighter radius than the outside wheel. Ideally, it would be possible for the inside tire to turn at a tighter angle than the outside tire. Ackerman refers to any tricks or tweaks in the suspension geometry to help the inside tire travel in a tighter arc, but this can be difficult to accomplish. One method is to set the front tires to toe-out. While this does help situate the inside tire at a more advantageous angle during cornering, it also points the outside tire in the wrong direction. Alternately, some chassis builders have experimented with intentionally dialing in a small amount of bumpsteer to toe-out the tires during cornering. Ultimately, the benefit of Ackerman geometry is up for debate. Many experts agree that its benefits are questionable in a street car, and some racers swear by it while others don’t. “The theory behind Ackerman geometry is that without it, the inside tire scrubs and chirps, resulting in a loss of cornering speed. On some tracks, you can also set a car up for reverse Ackerman, where the outside tire turns at a sharper angle than the inside tire,” Dale Schwartz of Schwartz Performance explains.
When most street cars accelerate forward, weight transfers rearward and causes the back of the car to squat. In an effort to enhance the forward bite of the rear tires, the suspension can be set up to counteract this squatting force and drive the rearend housing into the ground. Aptly referred to as antisquat, this effect is very well optimized in drag cars, but it can also benefit road course machines as well. “Drag cars can actually have so much antisquat that they raise the back end of the car during acceleration. What’s happening is that as the pinion gear rotates the ring gear and torques the rearend housing, that force is being used to drive the tires into the pavement,” Matt Jones of Art Morrison Enterprises (AME) explains. In cars with a four-link rear suspension, angling the control arms upward to move the instant center height closer to the center of gravity height increases antisquat. “Antisquat is calculated based on the instant center location in relation to the center of gravity. As long as it doesn’t compromise other aspects of suspension geometry, antisquat can help put the power down when exiting a corner.”