Ackerman Geometry

Without getting into too much geometry and mathematics, the Ackermann Effect is created by the inside and outside wheels traveling at different speeds, angles and distances, depending on how sharp the turning radius is.

Ackerman is determined by the geometry of the steering linkage, in relationship to the wheel base, and track width, If not designed properly, it can cause premature suspension and tire wear, and in some cases cause failure in suspension components caused by binding and scrubbing of the tire. This is often overlooked, and not designed properly in aftermarket Mopar suspensions using rack and pinion type steering, because of limited space in which to place the steering linkage. This is an EXTREMELY important function on a front suspension.

Figure 1

Caster Geomtery

Caster is defined as the angle created by the steering pivot ( upper and lower ball joints) point from the front to back of the vehicle. Caster is positive if the line is angled forward, and negative if backward.

Typically, positive caster will make the vehicle more stable at high speeds, and will increase tire lean when cornering, higher caster settings also effect the cars ability return the wheels back to center on its own. This can also increase steering effort as well. Typical caster settings for a street car is between 3-6 degrees positive. Drag racers will sometimes run in upwards of 8-9 degrees positive caster, while positive more caster makes your car much more stable at high speeds, too much caster will create tire shake from the dynamic forces in the suspension. Neutral or Negative Caster provides easier steering effort,but provides little to no stability causing your car to wonder, or feel like the front is on “ski’s”. A good way to visualize the effects of caster is by a motorcycle, imagine if the rake on the front was reversed ( negative caster), the bike would have absolutely no stability  whatsoever.

Figure 2


The most widely discussed and controversial of the three elements is camber. Camber angle is the measure in degrees of the difference between the wheels vertical alignment perpendicular to the surface. If a wheel is perfectly perpendicular to the surface, its camber would be 0 degrees. Camber is described as negative when the top of the tires begin to tilt inward towards the fender wells. Consequently, when the top of the tires begin to tilt away from the vehicle it is considered positive.

The real advantages to negative camber are seen in the handling characteristics. An aggressive driver will enjoy the benefits of increased grip during heavy cornering with negative camber. During straight acceleration however, negative camber will reduce the contact surface between the tires and road surface.Regrettably, negative camber generates what is referred to as camber thrust. When both tires are angled negatively they push against each other, which is fine as long as both tires are in contact with the road surface. When one tire loses grip, the other tire no longer has an opposing force being applied to it and as a result the vehicle is thrust towards the wheel with no traction. Zero camber will result in more even tire wear over time, but may rob performance during cornering. Ultimately, optimal camber will depend upon your driving style and conditions the vehicle is being driven in. In most street applications, camber is set to ¼ to ½ degree negative camber.

Figure 3

Instant Center

The instant center is an imaginary point that allows for a mathematical “shortcut” in calculating these unknowns. The instant center is also called the instantaneous center of zero velocity (IC). It lies on an imaginary axis of zero velocity, about which the body appears to rotate at a given instant. Instant Center effects several different things, including Camber, Bumpsteer, and Roll Center.

Having an instant center inboard of the car plays a crucial part of the camber change during body roll, this when designed properly will effectively keep a constant even traction patch as the suspension articulates.

Figure 4

Roll Center

This is one of the most critical characteristics of a suspension.  The roll center of a vehicle is the imaginary but accurately defined point on the center-line of the car around which the vehicle rolls, at which the cornering forces in the suspension system are transferred to the vehicle body. The location of the geometric roll center is solely dictated by the suspension geometry. The roll center can be high off the ground, low, or even below the ground. Depending on the type of suspension, the roll center can be an actual pivot point or a virtual point in space and they don’t essentially lie along the center line of the vehicle. SAE’s definition indicates, if the roll centers of a car are at the same height as the sprung mass’ Center of Gravity (CG), it will not reveal any body roll during a corner, a line connecting the rear suspension roll Centre with that of the front is called the roll axis. The reason to find the Roll Center of a car is about predicting how the car reacts while cornering. Knowing where the roll center is in the front of the car gives an idea of what the wheels will be doing as the nose of the car dives under braking or leans in a corner. Without roll center information, one cannot estimate how much the camber angle of the front wheels will change during suspension travel or how much body roll will be present while cornering. Roll center is used to determine the stiffness of spring, ride frequency and various suspension parameters and geometries that will help to keep optimal tire contact in both the conditions of ride( a car’s ability to smooth out a bumpy road) and handling ( a car’s ability to safely accelerate , brake and corner.)

Figure 5

Bump Steer

Bump steer or roll steer is the term for the tendency of the wheel of a car to steer itself as it moves through the suspension stroke. It is typically measured in decimals per inch of travel. Bump steer causes a vehicle to turn itself when one wheel hits a bump or falls down into a hole or rut. Excessive bump steer increases tire wear and makes the vehicle more difficult to handle on rough roads. For example, if the front left wheel rolls over a bump it will compress the suspension on that corner and automatically rotate, causing the car to turn itself momentarily without any input from the steering wheel. This is also referred to as “oversteer” or “understeer” depending on which way the tire turns during compression and rebounding of the suspension. Another example, is that when most vehicles become airborne their front wheels will noticeably toe in.

Figure 4 Shows Bump Steer Properly Designed