Adventures in Alignment
Part 1: Basics
Driving enthusiasts and gear heads find many topics to debate. The evidence relied upon is often purely anecdotal. Near the top of this list of topics has to be alignment. The pronouncements we often hear are based upon “belief” rather than fact or the certainties of physics. Indeed, I recently saw a message posted on an internet listserve (Rennlist, of course) in which the writer proclaimed that removing the rear stabilizer bar from a 911 would have no effect upon handling. Incredible.
In an effort to shed some small light on the subject of alignment, let us start with the some basics. Stated most simply, wheel alignment involves the attempt to establish the optimum positioning of the wheels/tires in relation to the vehicle and the road. What is optimum, however, depends upon how you intend to use the vehicle. The variables, generally speaking, are the settings of toe, caster, and camber.
“Toe” is the angle between a vehicle’s longitudinal axis and a plane through the center of the front tire/wheel. In other words, it is the difference between the leading inside edges and the trailing inside edges of the front tires: the angle can be toed “out” or “in”. (Rear wheels may also be subject to toe setting). The toe setting acts to compensate for the lateral forces which are exerted on the tire/wheel. Toe in or out creates a slip angle when the vehicle is moving in a straight line, creating tension within the front-end components. Zero toe would result in no slip angle being present.
“Caster” is the distance between the contact point of the tire/wheel on the road and the point at which the steering axis intersects the road as viewed from the side. If you like it is the forward or rearward tilt of the steering knuckle from vertical. When I think of Caster in general I envision a caster on a swivel chair, noting that the axle center of the wheel is offset from the point at which the caster is fitted to the chair leg. Tilting the top of the knuckle rearward provides for positive caster.
“Caster offset” is the displacement of the center of the wheel in relationship to the steering axis as measured at the wheel center. “Caster Angle” is defined as the angle between the steering axis and the vertical plane as seen from the side of the vehicle. The settings help the wheel decide which way its going to go, making it center-seeking, that is making it return to a straight ahead position after a turn.
“Camber” is the angle of the inclination of the tire/wheel towards the vehicle’s longitudinal plane as measured in the vehicle’s transverse plane. Stated more simply, it is the amount of lean-in or lean-out of the top of the tire/wheel in relation to the centerline of the wheel/tire as seen from the front of the vehicle.
Think of a Champ car at rest, as seen from the front. Because race cars of this type use a great deal of negative camber, the top of the tire/wheel leans in radically and stays that way until the car is up to serious speed. This is negative camber in extremis. The Camber setting significantly influences lateral control of the car. Thus, it is the subject of much fiddling by autocrossers and track junkies. It also has a major impact on tire wear and should be a consideration for everyone.
Not too long ago I replaced the stock tie rods on my SC with the turbo tie rods. This necessitated a trip to the shop for an alignment. Armed with the factory specifications, and a little knowledge (dangerous stuff) of what I wanted to have done, I took the SC to Layland Motors to have the work done. My experience with various settings, how the alignment was done, and the final results are the subject of Part 2.
© W. S. Cline/RoseLaneGarage 2000