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1、本文摘于Race Car Vehicle Dynamics 作者：William F. Miliken and Douglas L. MilikenSteering systemsIntroduction This chapter begins with a discussion of steering geometrycaster angle ,trail ,kingpin inclination ,and scrub radius .The next section discuss Ackermann geometry followed by steering racks and gear
2、s .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
3、high priorities .19.1 steering geometry The kingpin in a solid front axle is the steering pivot .In modern independent suspensions , introduced by Maurice olley at Cadillac in 1932,the kingpin is replaced by two (or more ) ball joints that define the steering axis .This axis is not vertical or cente
4、red on the tire contact patch for a number of reason .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 )
5、. 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 motorc
6、ycle 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 geometry As mentioned in chapter 17, kingpin inclination ,spindle length ,and scrub are usually a compromise between packaging
7、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 w
8、ill 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 no caster angle .See the following section on caster angle .
9、For a given kingpin inclination ,a longer 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
10、 the effect that raise ad fall have on centering .3. Kingpin inclination affects the steer camber characteristic .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 ). Positiv
11、e camber results for both left and right-hand steer .the amount of this effect is small ,but significant if the track includes tight turns.4. When a wheel is rolling over a bumpy road ,the rolling radius is constantly changing ,resulting in changes of wheel rotation speed . This gives rise to longit
12、udinal 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 spi
13、ndle 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 steer torques proportional to the scru
14、b radius . If the driving or braking force is different on left and right wheels then there will be a net steering torque felt by the driver (assuming that the steering gear has good enough rev erse efficiency ).The only time that this is not true is with zero scrub (centerpoint steering ) because t
15、here is no moment arm for the drive (or brake ) force 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 .
16、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 drive ,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 determined by the s
