Bumpsteer is sometimes described as how much the steering direction jerks when a car hits a bump, but that’s just part of the story. A more complete description of bumpsteer is the amount and direction the toe changes during suspension travel. This condition changes the steering angle as the suspension compresses and rebounds. Since the steering tie rods and the upper and lower control arms travel in separate arcs, it is this disparity in movement that causes the front tires to toe-in or toe-out during suspension travel. According to Steve Gorski of Unisteer: “The idea is to get the pivot points of the steering rack to match the pivot points of the chassis as closely as possible. Global West’s Doug Norrdin concurs: “The key to reducing bumpsteer is making the arcs, the tie rods, and control arms travel as similar as possible. Some people think that the bumpsteer will be minimized if the tie rods are parallel to the ground, but that’s not entirely accurate,” he explains. “After setting the ride height, at our shop we dial 2 inches of suspension travel in the upward and downward direction, which is quite a bit. Then the bumpsteer can be adjusted by changing tie rod length and height, or if that’s not enough, by moving the entire steering rack. On many production cars, this might get you 3 inches of travel in which you have little to no bumpsteer, with some bumpsteer starting to creep in at the extreme ends of suspension travel. Any time you lower a car, you have to readjust the bumpsteer.”
Camber and Caster
Camber and caster are very easy to understand, yet both profoundly impact the handling of a car. When viewed from the front of a car, camber refers to the direction in which the tops of the tires are leaning. A tire that leans inward has negative camber, while a tire that leans outward has positive camber. Since the laws of physics force the outside tire to lean outward during cornering—which reduces the contact patch and cornering grip—most modern suspension systems have negative camber built-in. “Negative camber increases the contact patch, and increases turning thrust to help a car change directions. Lower sidewall tires give less thrust, and taller sidewalls allow running more negative camber,” says Danny Nix of Classic Performance Products (CPP).
On the other hand, caster refers to the angle the spindle is set at when viewed from the side of a car. If an imaginary line was drawn from the upper ball joint to the lower ball joint, the angle of that line would reflect the caster. With positive caster the spindle angles rearward, while with negative caster the spindle angles forward. “The distance from the contact patch to steering axis is called the trail, and the larger the trail, the more the car wants to go straight,” Nix explains. “Likewise, the more positive caster you have, the more camber you can gain while turning. Increasing caster increases steering effort and the self-centering effect of the steering wheel. The downside is that it also reduces steering feedback.”