![]() You can also see the instant center (blue dot), which is at the convergence point of the upper and lower A-arms. Notice that in dive and roll the RC has migrated about 6 inches laterally. This diagram shows the roll center (black dot). Roll center movement is called migration, and it can move vertically and laterally. Since it’s rather difficult to swap out your springs or sway bars while your car is moving, you want to have the roll centers in a favorable location and keep them there if possible. ![]() If you’re thinking that sounds like a bad thing, you are right! It means that the spring rate and roll rate requirements of the car can change dramatically mid turn. In many cars (and traditional muscle cars certainly fall into this category) the roll centers actually move around quite a bit while you drive. They also determine how body roll affects suspension loading. Why do you care about this? Because the height of the roll centers and roll axis partially determine how much spring rate and roll rate the car requires to handle well. Think of this as if the car’s body is put on a rotisserie and the roll axis is a shaft going through the car that it pivots on. A line drawn between these two points is referred to as the roll axis. In traditional front-engine cars with solid rear axles, the rear roll center is usually higher. The roll center is the pivot point of that pendulum.Įvery car has both a front and rear roll center and they are seldom the same height. Think of the leaning car as an upside-down pendulum. There is a certain amount of lean (or roll) caused by centrifugal force. When any car corners, it grips the road with the tires at ground level, but the body and weight of the car are much higher off the road. This one is a little harder to explain because you can’t see it or easily demonstrate it. Only specifically designed suspensions work well with such high numbers, but older cars can still benefit greatly from higher-than-stock positive caster settings. Modern cars tend to run a lot of positive caster, with 7 degrees being common and some cars running as much as 12 degrees. Guess which one you want to shoot for in your performance car? Note that since caster is measured in relation to the ground, changes in the car’s rake or ride height from front to rear alters the caster. Mild or negative caster settings tend to make manual steering cars easier to steer, but more positive caster provides better on-center feel, more steering feedback, and much better straight-line and high-speed stability. If it’s tilted toward the rear of the car, it’s positive caster. If the top of the spindle is tilted toward the front of the car, it’s called negative caster. The tilt of the spindle, more correctly called the steering knuckle or upright, toward the front or rear of the car in relation to the ground is its caster angle. SPC makes these to fit everything from go-karts to monster trucks. This one is fitted with optional adapters to make accurate toe measurements easier and faster. This SPC Performance Fastrax unit has been a staple at race tracks for years, but is just as quick and easy to use at home. One of the best tuning aids you can buy is a good, portable caster/camber gauge. Since tires tend to have the best adhesion when their tread is flat on the road (at lower speeds and slip angles) or at negative camber (at high speeds and extreme slip angles), properly setting and controlling camber is very important.įor our purposes, positive camber (either static or dynamic) is generally not a good thing. Dynamic camber changes when the suspension moves as a result of the suspension geometry and vehicle movement. Static refers to the settings when you align the front end. The two main aspects of camber are static and dynamic. One degree of negative camber means the top of the tire is leaning inward toward the centerline of the car by 1 degree. One degree of positive camber means the top of the tire is leaning outward by 1 degree from vertical. Zero degrees means perpendicular to the ground, or straight up and down. This is the vertical angle of a tire in relation to the ground. The curvature of the arm has no effect on the geometry it is determined solely by the pickup point locations. Here are the pickup points seen from the front, as the arm would be mounted on a car.
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