英文资料翻译IntroductionThis chapter begins with a discussion of steering geometry-caster angle,trail, kingpin inclination, and scrub radius. The next section discusses Ackemann geometry followed by steering racks and gears. Ride steer（bump steer）and roll steer are closely related to each other, without compliance they would be the same.Finally,wheel alignment is discussed. This chapter is tied to Chapter 17 on Suspension Geometry-when designing a new chassis, steering and suspension geometry considerations are high priorities.19.1Steering GeometryThe kingpin in a solid front axle is the steering pivot.In modern independent suspensions,introduced by Maurice Olly at Cadillac in 1932,the kingpin is replaced by two (or more) ball joints that that define the steering axis.This axis is not vertical or centered on the tire contact patch for a number of reasons.See Figure 19.1 to clarify how kingpin location is measured.In front view,the angle is called kingpin inclination and the offset of the steering axis from the center of the tire print measured along the ground is called scrub (or scrub radius).The distance from the kingpin axis to the wheel center plane,measured horizontally at axle height,is the spindle length.In side view the kingpin angle is called caster angle;if the kingpin axis does not pass through the wheel center then side view kingpin offset is present,as in most motorcycle front ends.The distance measured offset is present,as in most motorcycle front ends.The distance measured on the ground from the steering axis to the center of the tire print is the trail (called caster offset in Ref.1)Kingpin Front View GeometryAs mentioned in Chapter 17,kingpin inclination,spindle length,and scrub are usually a compromise between packaging and performance requirements.Some factors to consider include:1. With a positive spindle length(virtually every car is positive as shown in Figure 19.1)the car will be raised up as the wheels are steered away from center.The more the kingpin inclination is tilted from vertical the more the car will be raised when the front wheels are steered.This effect always raises the car,regardless of which direction the wheel is steered,unless the kingpin inclination is true vertical.The effect is symmetric side to side only if there is nocaster angle.See the following sectiong on Caster Angle.For a given kingpin inclination,a lenger positive spindle length will increase the amount of lift with steer.2. The effect of kingpin inclination and spindle length in raising the front end ,by itself,is to aid centering of the steering at low speed.At high speed any trail will probably swamp out the effect that rise and fall have on centering.3. Kingpin inclination affects the steer-camber characteristics.When a wheel is steered,it will lean out at the top,toward positive camber,if the kingpin is inclined in the normal direction(toward the center of the car at the upper end).Positive camber results for both beft and right-band steer.The amount of this effect is small, but significant if the track includes tight turns.4. When a wheel in rolling over a bumpy road,the rolling radius is constantly changing,resulting in changes of wheel rotation speed.This gives rise to longitudinal forces at the wheel center.The reaction of these forces will introduce kickback into the steering in proportion to the spindle length.If the spindle length is zero then there will be no kick from this source.Design changes made in the last model of the GM”p”car(Fiero) shortened the spindle length and this resulted in less wheel kickback on rough roads when compared to early model”p” cars.5. The scrub radius shown in Figure 19.1 is negative,as used on front-wheel-drive cars (see below).Driving or braking forces(at the ground) introduce is different on left and ringht wheels then there will be a net steering torque felt by the driver (assuming that the steering gear has good enough reverse efficiency).The only time that this is not true is with zero scrub (centerpoint steering) because there is no moment arm for the drive( or brake) forces to generate torque about the kingpin.With very wide tires the tire forces often are not centered in the wheel center plane due to slight changes in camber,road surface irregularities,tire nonuniformity(conicity),or other asymmetric effects.These asymmetries can cause steering kickback regardless of the front view geometry.Packaging requirements often conflict with centerpoint steering and many race cars operate more or less okay on smooth tracks with large amounts of scrub.6. For front driv,a negative scrub radius has two strong stabilizing effects:First,fixed steering wheel_if one drive wheel loses traction,the opposing wheel will toe-out an amount delermined by the steer compliance in the system.This wll tend to steer the car in a straight line,even though the tractive force is not equal side-to-side and the unequal tractive force is applying a yaw mome