http://www.hunter.com/pub/undercar/2573T/index.htm
Steering Geometry and Caster Measurement
Reprinted with permission from the SAE Technical Paper Series
850219
Daniel B. January
Hunter Engineering Company
Bridgeton, Missouri
ABSTRACT
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 is inclined rearward (in the upward direction) and negative when the steering axis is inclined forward.” (1)*
* Numbers in parentheses designate references at end of paper.
Positive caster tends to produce a stable steering system by generating counterbalancing torques about the steering axes as the wheels roll. The torques vary as the steer (toe) angles change. An equilibrium condition exists when the steer angles of the wheels remain constant with no driver-applied torque to the steering wheel. Ideally, the front wheels steer the vehicle in a straight line in this “neutral steer” condition.
Left and right caster must be equal for this condition to occur, other factors being equal. However, the straight line direction of travel is the thrust line of the nonsteerable rear wheels. If the thrust angle of the rear wheels is altered, the neutral steer direction of the front wheels no longer coincides with the thrust line. See Figure 1. The vehicle then rolls in a circle unless the driver steers the front wheels by applying a torque to the steering wheel. The vehicle “pulls” to the side.
Clearly caster must be defined relative to the thrust line. This is easily done:
Caster Angle – The angle, in side elevation PARALLEL TO THE THRUST LINE OF THE NON-STEERABLE WHEELS, between the steering axis and the vertical. It is considered positive when the steering axis is inclined rearward (in the upward direction) and negative when the steering axis is inclined forward.
This improved definition brings caster measurement in accord with the total alignment concept, where individual toe angles are referenced to the thrust line of the rear wheels.
atriot:
