553 Kd1080型载货汽车后桥总成设计（有exb图）
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Kd1080型载货汽车后桥总成设计（有exb图）
Kd1080
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后桥
总成
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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 moment to the vehicle.Second,with good reverse efficiency the drivers hands never truly fix the steering wheel.In this case the steering wheel may be turned by the effect of uneven longitudinal tractive forces,increasing the stabilizing effect of the negative scrub radius.Under braking the same is true.Negative scrub radius tends to keep the car traveling straight even when the braking force is not equal on the left and right side front tires(due to differences in the roadway or the brakes)Caster Angle and TrailWith mechanical trail,shown in Figure 19.1,the tire print follows behind the steering asis in side view.Perhaps the simplest example is on an office chair caster-with any distance of travel,the wheel aligns itself behind the pivot.More trail means that the tire side force has a larger moment arm to act on the kingpin axis.This produces more self-centering effect and is the primary source of self-centering moment about the kingpin axis at speed.Some considerations for choosing the caster angle and trail are:1. More trail will give higher steering force.With all cars, less trail will lower the steering force.In some cases,manual steering can be used on heavy sedans(instead of power steering) if the trail is reduced to almost zero.2. Caster angle,like kingpin inclination,causes the wheel to rise and fall with steer.Unlike kingpin inclination,the effect is opposite from side to side.With symmetric geometry (including equal positive caster on left and right wheels),the effect of left steer is to roll the car to the right,causing a diagonal weight shift.In this case,more load will be carride on the LF-RR diagonal, an oversteer effect in a left-hand turn.The diagonal weight shift will be larger if stiffer springing is used because this is a geometric effect.The distance each wheel rises(or falls) is constant but the weight jacking and chassis roll angle are functions of the front and rear roll stiff-ness.This diagonal load change can be measured with the car on scales and alignment(Weaver) plates.Keep in mind that the front wheels are not steered very much in actual racing.except on the very tightest hairpin turns.For example,on a 100-ft.radius(a 40-50mph turn),a 10-ft.wheelbase neutral steer car needs only about 0.1 rad.(5.7)of steer at the front wheels(with a 16:1 steering ratio this is about 90 at the steering wheel).For cars that turn in one direction only,caster stagger(differences in left and right caster) is used to cause the car to pull to one side due to the car seeking the lowest ride height.Caster stagger will also affect the diagonal weight jacking effect mentioned above. If the caster is opposite (positive on one side and negative the same number of degrees on the other side) then the front of the car will only rise and fall with steer,no diagonal weight jacking will occur.3. Caster angle affects steer-camber but,unlike kingpin inclination,the effect is favorable.With positive caster angle the outside wheel will camber in a negative direction (top of the wheel toward the center of the car) while the inside wheel cambers in a positive direction,again leaning into the turn.In skid recovery,”opposite lock”(steer out of the turn)is used and in this case the steer-camber resulting from caster angle is in the “wrong”direction for increased front tire grip.Conveniently ,this condition results from very low lateral force at the rear so large amounts of front grip are not needed.4. As discussed in Chapter 2,tires have pneumatic trail which effectively adds to (and at high slip angles subtracts from )the mechanical trail.This tire effect is nonlinear with lateral force and affects steering torque and driver feel.In particular,the fact that pneumatic trail approaches zero as the tire reaches the limit will result in lowering the self-centering torque and can be a signal to the driver that the tire is near breakaway.The pneumatic trail”breakaway signal”will be swamped out by mechanical trail if the mechanical trail is large compared to the pneumatic trail.5. Sometimes the trail is measured in a direction perpendicular to the steering axis (rather than horizontal as shown in Figure 19.1)because this more accurately describes the lever(moment )arm that connects the tire lateral forces to the kingpin.Tie rod location Note that in Figure 19.1a shaded area is shown for the steering tie rod location.Camber comploance under lateral force is unavoidable and if the tie rod is locater as noted ,the effect on the steering will be in the understeer(steer out of the turn)direction.If the suspension and rack are mounted on som sort of flesible subframe,the situation becomes much more complex than can be covered here.19.2Ackermann steering geometryAs the front wheels of a vehicle are steered away from the straight-ahead position,the design of the steering linkage will determine if the wheels stay parallel or if one wheel steers more than the other.This difference in steer angles on the left and right wheels should not be confused with toe-in or toe-out which are static adjustments and add to (or subtract from)Ackermann geometric effects.For low lateral acceleration usage (street cars)it is common to use Ackermann geometry.As seen on the left of Figure 19.2,this geometry ensures that allthe wheels roll freely with no slip angles because the wheels are steered to track a common turn center .Note that at low speed all wheels are on a significantly different radius,the inside front wheel must steer more than the outer front wheel.A reasonable approximation to this geometry may be made as shown in Figure 19.3.According to Ref.99,Rudolf Ackermann patented the double pivot steering system in 1817 62and,in 1878,Charles Jeantaud added the concept mentioned above to eliminate wheel scrubbing when cornering.Another reason for Ackermann geometry,mentioned by Maurice Olley,was to keep carriage wheels from upsetting smooth gravel driveways.High lateral accelerations change the picture considerable.Now the tires all operate at significant slip angles and the loads on the inside track are much less than on the outside track.Looking back to the tire performance curves,it is seen that less slip angle is required at lighter loads to reach the peak of the cornering force curve.If the car has lowspeed geometry (Ackermann),the inside front tire is forcee to a higher slip angle than required for maximum side force.Dragging the inside tire along at high slip angles (above the peak lateral force )raises the tire temperature and slows the car down due to slip angle(induced)drag.For racing, it is common to use parallel steering or even reverse Ackermann as shown on the center and right side of Figure 19.2.It is possible to calculate the correct amount of reverse Ackernann if the tire properties and loads are known.In most cases the resulting geometry is found to be too extreme because the car must also be driven(or pushed)at low speeds,for example in the pits.Another point to remember is that most turns in racing have a fairly large radius and the Ackermann effect is very small.In fact,unless the steering system and suspension are very stiff,compliance(deflection)under cornering loads may steer the wheels more than any Ackermann(or reverse Ackermann)built into the geometry.The simplest construction that generates Ackermann geometry is shown in Figure 19.3 for “rear steer”.Here, the rack(cross link or relay rod in steering box systems)is located behind the front axle and lines starting at the kingpin axis,extended through the outer tie rod ends,intersect in the center of the rear axle.The angularity of the steering knuckle will cause the inner wheel to steer more than the outer(toe-out on turning)and a good approximation of “perfect Ackermann”will be achieved.The second way to design-in differences between inner and outer steer angles is by moving the rack ( or cross link)forward or backward so that it is no longer on a line directly connecting the two outer tie rod ball joints.This is shown in Figure 19.4.With “rear steer”,as shown in the figure,moving the rack forward will tend more toward parallel steer( and eventually reverse Ackermann),and moving it toward the rear of the car will increase the toe-out turing.A third way to generate toe with steering is simply to make the steering arms different lengths.A shorter arm (as measured from the kingpin axis to the outer tie rod end ) will be steered through a larger angle than one with a longer knuckle.Of course this effect is asymmetric and applies only to cars turning in one direction_oval track cars.RecommendationWith the conflicting requirements mentioned above, the authors feel that parallel steer or a bit of reverse Ackermann is a reasonable compromise.With parallel steer,the car will be somewhat difficult to push through the pits because the front wheels will be fighting each other.At racing speeds,on large-radius turns, the front wheels are steered very little,this any Ackermann effects will not have a large effect on the individual wheel slip angles, relative to a reference steer angle, measured at the centerline of the car.19.3Steering gearsThe steering rack or steering box translates rotary motion of the steering wheel to linear motion at the tie rods.In turn, the tie rods translate this linear motion back to rotary motion about the kingpin axis (steering axis)resulting in steer of the front wheels.The first thing that should be obvious is that there are a lot of connection in the steering system.All of these connections are a source of compliance (bending or deflecting)or lost motion (looseness or slop),any of which will make the steering imprecise-the driver will not know exactly in which direction the front wheels are aimed.The steering system components must all be tight and mounted securely for both safety and control.Steering ratio The overall steering ratio is defined as degrees of steering wheel angle divided by corresponding front wheel angle.For race cars it varies from over 20:1(slow)for Superspeedway cars to less than 10:1(very fast)for Formula One cars on tight street circuits,Of course, the ultimate in fast steering is the go-kart with nearly1:1.Common values in road racing are 16:1to 18:1.With Ackermann(or reverse Ackermann)geometry,the steering ratio will be different side to side.Depending on the linkage configuration the steering may be nonlinear,that is ,the ratio may vary with wheel angle.Steer-steer testA straightforward way to measure the overall steering ratio is to set the front end on slignment tables(Weaver plates)with a steering scale.A circular protractor is mounted(centered) on the steering wheel and a suitable pointer is attached so that the steering wheel angle can be measured.This test is called steer-steer and should be performed with the car at known load and ride height.The steering wheel is turned to the right in even intervals,perhaps 45,90,etc,and the steer angles of both front wheels are noted.The test continues by rotating the steering wheel back to center,stopping at each angle,and checking the front wheel steer angles again to look for any slop (or hysteresis).Continue past center, steering to the left, and finally return to center.Data and a plot of the results of this test are shown in Figure 19.5.From the plot,the average slope of the data points is called the overall steering ratio.Note that the data plots make loops;this is called hysteresis and means that there is some compliance and/or lost motion in this steering system.Also note that the plot is not straight;this nonlinear characteristic indicates that the linkage is not”perfect”,common in many steering systems.For racing, it is appropriate to take data points only in the range of steering wheel angles that are normally used.Steering ratio data near full lock will reflect performance only during low speed mancuvers. Steering ratio partialIy determines the steering effort that is required for a manual steering system in conjunction with the kingpin geometry (trail and scrub). Higher (20: 1) ratios will require less effort than lower (quicker) ratios. When interpreting driver comments，be aware that a quick steering ratio can often be confused with a fast vehicle transient response time, as discussed in the chapters on vehicle dynamics.Steering ratio can be calculated as described in the following sections Rack-and-Pinion Steering Box Ratio.Rack -and-pinion gearsets convert rotary motion at the steering wheel to linear motion at the inner tie rod ball joint. The steering ratio is calculated using the rack c-factor and thesteering arm 1ength(as measured from the outer ball joint to the kingpin axis).c-factor = travel (in. )/3600 pinion rotationOften, a steering rack will be described as a 1-7/8-inch rack or a 2-inch rack; this dimensionis the amount the rack moves for one rotation of the steering wheel-the c-factor.Once the c-factor is known for the rack, the steering ratio can be calculated approximately bySteer ratio = sn-1(c-factor/steering arm length)/360where dimensions are in in.angles are in deg.Sin-1 is the same as the angle with sin of, or arcsinThe approximation will be good as long as the angularity in the. system is minimal, that is, the tie rod is nearly perpendicular to the steering arm in top and front view. For designs with high angularity ， a layout is required to determine the steering ratio.序言：本章以转向几何参数的讨论为开始，包括主销后倾角，后倾拖距，主销内倾角，主销偏置量。接下来的部分讨论了转向齿轮齿条以及阿克曼转向几何关系。跳动转向和侧倾转向之间是紧密相关的，如果没有柔性这两种情况是等同的。最后讨论了车轮的调整。这一章与第17章的悬架几何形状密切相关，在设计新的底盘系统时，转向和悬架几何参数是优先考虑的因素。19.1 转向几何关系（定位参数） 在整体式车桥上转向节主销是转向时的枢轴。1932年Maurice Olley在Cadillac首次提出了现在的非独立悬架，主销因此而被两个球绞连接定义的转向轴线代替。因为各种原因这根轴并不是垂直的也不在轮胎接地中心处。主销的位置表示见图19.1。在前视图中，主销偏转的角度被称为主销内倾角，转向主销与地面的交点至车轮中心平面与地面相交处的距离称之为主销偏置量。在前轴所在水平面内，从主销轴心到车轮中心平面的距离称为主销偏距（spindle length）。在侧视图中，主销偏转角度称为主销后倾角。如果主销轴线没有通过车轮中心那么就有了侧视的主销偏距（side view kingpin offset），就像大部分的摩托车前轮一样。在地平面内测量从主销到轮胎接地点中心的距离称为主销后倾拖距。前视图中的主销定位参数正如在17章中提到，主销内倾角，主销偏距还有主销偏置量在装配以及性能满足时往往是互相妥协的。一些需要考虑的因素包括以下：1. 当主销偏距是正的时（一般的车都是正主销偏距，如图19.1中一样）那车轮转离中心位置的时候车会有一个抬升效果。主销内倾角偏离竖直平面越大前轮转向时车被抬起的效果越明显。不管车轮往哪个方向转都会是一个抬升的效果，除非主销是完全垂直的。这个效果只有在主销后倾角为零时才是两边对称的。见后面关于主销后倾角部分。对于一个给定的主销内倾角来说，主销偏距越大转向时的抬升量也越大。2. 主销内倾角和主销偏距将车子前端抬起的效果对于自身来说是有助于低速转向的。在高速转向时，只要有主销后倾拖距就可能会掩盖掉转向时抬升和下落的效果。3. 主销内倾角影响转向时车轮的外倾角特性。如果主销向内倾斜（主销上端倾向车辆中心）当车轮转向的时候，车轮上端将会向外倾斜，趋向正的车轮外倾角。左右转向都会导致正的车轮外倾。如果跑道有比较紧的弯这个作用效果是比较小但却是有重要意义的。4. 当车轮滚过颠簸不平的路面时，滚动半径是不断变化的，将会导致轮速的改变。这将会增加车轮中心的纵向力。这些力的反作用与主销偏距的大小成比例，成为反冲效果进入转向系统。如果主销偏距为零，那么将不会有由此引起的反冲。在前面提到的一辆通用“P”型车（菲罗车）中做出设计改动，与较早的一辆“P”型车模型相比，减小了主销偏距，因此而减少了不平路面上的反冲。5. 如图19.1中所示的主销偏置量是负的，正如下面这辆前轮驱动车用的一样。来自地面的驱动和制动力与主销偏置量成比例的转化成转向力矩。如果左右轮的制动或者驱动力是不等的，那么驾驶者将会感受到的到这个转向力矩（假设转向器有较高的逆效率）。只有在主销偏置量为零时才不会有这个力矩产生因为此时制动力或驱动力对主销的作用力臂为零。如果轮胎比较宽的话轮胎力通常并不是作用在轮胎中心平面内的，因为轻微的外倾角变化、路面不平、轮胎有一定圆锥度、或者其他的不对称因素存在。这些不对称因素可能导致转向反冲，即使没有前轮的各个定位参数作用。装配要求通常会与中心点转向要求冲突因而很多赛车在较平整的赛道上是采用较大的主销偏置量也是可以的。6. 对于前轮驱动来说，一个负的主销偏置量有两个重要的稳定作用：第一， 固定方向盘，如果一个驱动轮打滑，另外一个轮将会外张一定角度，因为 转向系统内有变形。即使两侧的牵引力不等，不同的牵引力使车辆产生一个偏航角，这个负的主销偏置量作用也会使车辆回复到直线行驶。第二， 有良好的反馈作用情况下驾驶员从来不会真正的固定住方向盘。在这种情况下方向盘可能在不等的车轮纵向牵引力作用下而转动，因此而增加了负主销偏置量的稳定效果。制动的情况同样适用。负的主销偏置量能使车子回正，即使是在左右轮制动力不等的情况下（左右轮的制动情况或者路面情况不同时）。主销后倾角和后倾拖距如图19.1中所示，在有后倾拖距时，侧视图中轮胎接地点是在主销之后的。或许最简单的例子就是办公室座椅上的小脚轮（？）不管移动多远，轮子总会校正使其自身在枢轴之后。主销拖距越大意味着轮胎侧向力在主销轴上作用有更大的力臂。这会产生更明显的回正作用，并且是作用在主销上最主要的回正力矩。在选择主销后倾角和主销拖距时需要考虑的因素如下：1. 主销后倾拖距越大转向力也越大。对于所有的车来说，小的后倾拖距都将会减小转向力。在某些情况下，如果后倾拖距减小接近零的话，人力转向也可能被用于重型轿车（代替助力转向）。2. 像主销内倾角一样，主销后倾角伴随着转向过程也会引起车轮的抬起和回落。与内倾角不同的是，后倾角对两侧的影响是相反的。在有对称定位参数时（包括左右轮有相等的正的主销后倾角），左转的效应是使车向右侧倾，导致一个对角线的重量转移。在这种情况下，左前右后对角线会承受更大的载荷，有一个左转时的过度转向效应。使用的弹簧越硬对角线的重量转移效果也会越明显因为这个是几何效应。每个车轮被抬起（或者下落）的距离是恒定的但是重量抬起量和底盘侧倾角是前后侧倾刚度的作用结果。这个对角线的载荷转移可以通过把车放在秤上和定位板上来测量。记住在实际比赛中前轮并没有转过很大的角度，除非是非常紧的发夹弯。例如，在一个半径是100英尺（时速在40-50英里）的弯，一个10英尺的轴距的中性转向车辆转弯时前轮只需要转过0.1rad（5.7）（转向传动比是16：1时方向盘的转角大概在90）。对于只往一个方向转的车来说，因为整车为了寻求最低的最小离地间隙，可以使主销后倾角交错（左右主销后倾角不同）来把车拉到一边。主销后倾角的交错也会影响上面提到的对角线重量抬升效应。如果两侧主销后倾角是相反的（一侧为正一侧为负且两侧角度大小相等）那么在转向时车的前端只会抬升和下落，而不会有对角线的重量抬升。3. 主销后倾角也会影响转向外倾角，但是不像内倾角一样，这个效应是有利的。当有正的后倾角时将会导致外侧车轮内倾（车轮的上部指向车的中心）同时内测轮外倾角为正，两轮都向弯内倾。 在侧滑恢复的时候，反打方向（出弯），后倾角引起的外倾角变化会使前轮抓地力减小。而此时后轮抓地力也很小并不需要很大的前轮抓地力。4. 如第2章提到，轮胎本身的轮胎拖距会使实际主销后倾拖距明显增加（有大的侧偏角时会减小）。这个效应并不是随着侧向力变化而线性变化的，并且会影响转向力矩和驾驶感。特别是轮胎到极限时轮胎拖距会接近零，这时回正力矩会减小，并给车手一个信号轮胎就要侧滑了。如果主销后倾拖距相对轮胎拖距很大的话，轮胎拖距给出的这个信号会被掩盖。5. 有时主销后倾拖距是在垂直于主销轴心的方向上进行测量的（而不是像19.1中在水平面内测量的），因为这能更准确的描述轮胎侧向力对主销作用的力臂。拉杆位置注意在19.1中的阴影部分就是转向拉杆的合适位置。侧向力引起的外倾角是不可避免的，如果拉杆的位置如
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