Steering Geometry and Caster Measurement

Caster of a steerable vehicle wheel is defined herein to be relative to the thrust line of the non-steerable wheels. This incorporates caster into the “total alignment” concept. A method of measuring caster in accordance with this definition is derived. The restrictions, limitations, and accuracy of the method are investigated, and practical implementation procedures are suggested. The purpose of this paper is to define the caster angle of a steerable vehicle wheel to be referenced to the thrust line of the non-steerable wheels. A further purpose is to derive and characterize an optimal method of measuring caster which can be implemented in a practical manner. Recent years have seen the development of the “total alignment” concept, which relates the toe angles of the steerable vehicle wheels to the thrust line of the nonsteerable wheels. Increased sophistication of vehicle suspension systems has made this more important, while advances in alignment measurement instrumentation have incorporated the concept and assumed the corresponding computational burden. Referencing caster to the thrust angle is part of the total alignment concept, but caster of a steerable wheel is difficult to measure. It is the angle between the vertical and the projection of the invisible steering axis onto a vertical plane containing the thrust line of the vehicle. It is not easy to attach measuring devices to a projection of an invisible axis. Indirect measurement methods are available that are very accurate, if they are implemented properly and certain restrictions are observed. REFERENCING CASTER TO THE THRUST LINE Caster has been defined as follows: “6.2.1 Caster Angle — The angle in side elevation between the steering axis and the vertical. It is considered positive when the steering axis