指出正确的方法 前束,外倾角和后倾角.docx

BJ1042轻型载货汽车前悬架设计【前双横臂独立悬架】

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BJ1042轻型载货汽车前悬架设计【前双横臂独立悬架】,BJ1042,轻型,载货,汽车,悬架,设计,前双横臂,独立
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附录A:英文翻译附录A:英文翻译指出正确的方法外倾角,后倾角和前束:它们是什么意思? 汽车上三个主要校正参数是前束,外倾角和后倾角。大多数爱好者都能很好的了解这些设置的含义以及它们所涉及到的。但是很多人可能不知道为什么要设置这些特定的参数,或者它们是如何工作的。让我们简单的了解一下悬架调整的一些基本的方面。了解前束: 如果车轮设置时将它们的前面的边缘互相靠近一点,这对车轮就叫做车轮前束。如果前缘互相远离,这对车轮就叫做负前束。前束的大小用车轮与车轮的平行线的夹角的大小表示。或者更通俗的说,就是轮距在前缘和后缘的宽度变化的大小表示。前束的设置影响三个主要的方面:车轮的磨损,直线行驶的稳定性和转弯操纵特性。为了减小轮胎磨损和动力损失,当汽车直线行驶时安在车轴上的车轮应该指向正前方。过多的前束或者负前束导致轮胎的磨损,因为这对车轮总是相对行驶。过大的前束加速轮胎外侧边缘的磨损,相反的过大的负前束时导致车轮内边缘磨损。所以如果零度的前束能减小轮胎的磨损和动力,为什么还要前束角呢?答案示前束的设置对直线行驶的稳定性有主要影响,上面的图示展示了有关的机构。由于车轮中心的控制,前束使车轮趋于向前交叉转动。在这种控制下车轮之间彼此之间运动不协调,就没有了翻转的后果。当一侧车轮受到外界干扰时,这个车轮受到相对于车轴向后拉的力。这个运动同时也拉着另一侧的车轮向相同的方向运动。如果干扰较小,这个受干扰的车轮将会变化很小,或者导致直线行驶而不是轻微的前束。但是注意轻微的转向输入,车轮的旋转轨迹仍然不能描述变化。车轮吸收了路面的干扰而不使车辆改变行驶方向。这样,前束提高了汽车直线行驶稳定性。如果汽车设置成负前束,但是,前轮平行以便轻微的干扰使这对车轮呈现转动的趋势。并且每一瞬间中心位置的角度变换都将使内侧车轮比外侧车轮有更小的转弯半径。因此汽车总是能转弯进入想转弯进入的地方,而不是直线行驶。所以很显然负前束增强转弯的趋势,而前束减小这种趋势。前束汽车(左)悬架的偏转不能像负前束的汽车(右)使车轮产生转动:在特定的汽车上前束设置是在直线行驶稳定性和快速转弯能力之间的平衡。没有人想让他的车在柏油路上疯狂左右徘徊的行驶并且不停的调整方向。但是赛车手们希望牺牲一点稳定性而能够快速的转弯。因此城市汽车一般都设置成前束,而赛车都设置成负前束。对于独立悬架的汽车,前束也必需设置在汽车的后轮。前束设置在后轮上本质上也会使轮胎磨损,产生方向稳定性和转弯特性,就像在前轮一样。但是很少遇见在后论驱动的赛车上在后论设置负前束的,因为这样做会产生过多的过度转向,尤其是当动力提供时。前轮驱动的赛车,另一方面,经常设置成一点负前束,这样导致的过度转向和过大的不足转向相抵消。同样需要记住前束在汽车从静止到运动会有轻微的变化。在独立悬架前轮驱动的汽车上应该注意。当驱动转矩加到车轮上时,车轮推着自己前进并产生前束。这是另外一个原因为什么前轮驱动汽车在前轮设置成负前束。同样的,当在公路上行驶时,非驱动轮将产生负前束的趋势。在后轮驱动汽车上应该被重视。给定汽车上设置前束和负前束的大小由所属的悬架和所期望的操纵特性决定。为了改善乘坐舒适性在公路汽车的悬架链接处安装了有关的软橡皮套,因此当运载时链接处的变化很均匀。相反的,赛车连接用钢制的球形支承或者非常硬的氨甲酸乙酯。金属或塑料的套管提供最适宜的硬度。和控制悬架连接刚度。因此,城市汽车比赛车要求更多的静态前束。这样在任何时候套管都能允许车轮呈现前束的情形。应该注意,设计师已经用套管装在普通汽车上来提升性能。为了使瞬间反应最合适,有一点前束在后轮上来促使产生偏离角而在后轮上产生转弯力是很好的。通过允许在A-arm悬架上侧面链接的顺从,后轴会有一点前束当汽车进入直角时,直线行驶没有转弯的道路上,套管仍然保持状态并且允许前束设置一个角度使汽车车轮加速磨损和提高稳定性参数。这种设计使一种被动四轮转向操纵系统。后倾角的影响:后倾角是一个主销轴线相对于铅垂线向前或向后倾斜的角度。如果主销向后倾(上部被设置的比下部靠后),那么这个后倾角是有利的,如果向前则是不利的当汽车直线行驶时有利的后倾角有使车轮沿直线运动的趋势,因此后倾角被用来提高行驶稳定性。购物车的车轮倾角用图例更清晰的解释这种装置造成的趋势。购物车的转向轮设置的车轴比车轮接地点靠前。当车被推动向前时,转向轴拉着车轮前进,当车轮制动时它就直接停在转动轴的后面。使车轮跟随转向轴的力是成比例的根据转向轴和接地点距离距离越远,力越大。这个距离被认为是“主销拖距”。由于许多设计考虑因素,将汽车车轮转向轴安装在轮毂的正确位置是合理的。如果转向轴设计成垂直的,轴线就与车轮接地点重合。拖距就是零,就没有倾角。车轮本质上就可以绕接地点自由旋转(事实上,轮胎自己会产生一定的后倾角影响根据“气胎拖距”现象但这种影响比机械式后倾角产生的影响小很多,因此忽略不计)幸运的是只要倾斜前轴至有利的位置就可以产生后倾角。由于这样的安排前轮的横断面与地面的交点在轮胎接地点之前,因此会产生与主销后倾角一样的效果。从几何学上考虑倾斜的前轴还有另外一个对悬架来说很重要的作用。因为车轮相对于一个倾斜的轴旋转,车轮当其旋转时会增加后倾角。这种效果最好想象成极限的情况当前轴水平而转向轮转动,车轮就很容易改变后倾角而不是保持方向不变。这种效果引起外侧车轮转动产生不利的倾角,而内侧车轮获得有利的倾角。这些倾角改变在转弯时是有利的,虽然有时可能过大。大多数汽车对倾角设置并不是很敏感。不过必须保证两侧车轮有同样大小的倾角,从而避免汽车往一个方向行驶的倾向,这是很重要的。当作用较大的倾角来提升汽车的直线行驶稳定性,它们也同样会提升转向性能。3到5度的后倾角是典型的设置,而小角度后倾角用在重型车辆上来保持合理的转向性能。像购物车的车轮的拖距是由转向轴的后倾拉着车轮直线行驶形成的。外倾角是什么:外倾角时车轮相对于汽车垂直线的向前或向后的夹角。如果车轮向汽车方向倾斜,是车轮负外倾角,如果远离汽车倾斜是正外倾角。(请看下一页)。车轮所能提供的回转力在很大程度上取决于相对于地面的角度,所以对于汽车行驶性能来说外倾角是一个主要影响参数。一个很小的负外倾角就会产生很大的回复力矩是很有意思的,主要在负1/2度左右时。它是由外倾角的推力提供的,这个力是由轮胎橡胶面与地面的接触处的橡胶变形产生的侧向力导致的为了在转弯时使轮胎的性能最优化,悬架设计师们的工作需要设定轮胎的外倾角总是在一个负的小角度。这将是一个非常难的工作,因为在转弯处底盘转动时悬架必需垂直地倾斜一定距离。因为车轮通过一些链接与底盘相连,而这些链接必需转动才能使车轮转动,当底盘上下跳动时车轮会产生很大的外倾角变化。由于这个原因,车轮相对于静止时的倾斜越多,保持一个稳定的外倾角就越困难。因此,当车轮较大或轮毂刚性较软时对于设计者来说在客车上提供平顺的乘坐感是一个挑战,而小车轮和高刚度的赛车就不会使设计师头疼。了解外倾角与路面和外倾角与底盘的大小的区别是很重要的。为了保持相对于地面理想的外倾角,悬架必需被设计成当底盘向上倾斜时车轮相对于底盘的外倾角才能更加趋于负值。在46页 下部的例子告诉了为什么是这样的。如果悬架被设计成保持外倾角相对于底盘不变,那么车身转动就使外倾角相对于路面的正外倾角减少。因此,为了减少车身转动的影响,悬架必需被设计成靠近车轮的上部(也就是,获得负外倾角)当底盘向上倾斜。当悬架运动时保持理想的外倾角使轮胎有最高的效率,设计师们经常配置乘用车的前悬架使获得正外倾角当车轮向上运动。这样设计的目的是为了减少前边缘相对于后边缘的侧向力,这样汽车将会在得到最大的支撑时具有转向不足。转向不足比过多转向更安全更稳定,因此这样对于量产的汽车时可取的。因为多数独立悬架被设计成当车轮相对于底盘上下运动时外倾角可变的,我们设置的外倾角当我们校正的汽车不是典型的那种转弯的汽车。然而,它真的是我们校正外倾角的唯一参考。对于比赛,在静态下设置外倾角是必须的,测试汽车,在那时在方向上的静态参数改变在测试结果中会显示出来。为了比赛选择合适的外倾角的就最好办法就是当轮胎还是热的时候立即测量胎面的温度曲线。一般而言,内侧轮胎边缘比外侧轮胎边缘热一点是合适的。但是,轮胎要能承受比理想的温度曲线高的工作温度是很重要的。因此,可以提供有利的额外的负外倾角使轮胎承受这个温度。(右上图)正的外倾角:车轮的下部比上部靠的更进。(左上图)复外倾角:车轮的上部比下部靠的更进(中间图)当悬架倾斜时没有外倾角,当汽车转弯倾斜时这会造成很大的正外倾角。这能使驾驶员操纵紧张。(下图)没有紧张感:在倾斜时获得外倾角的悬架将补偿车身的转动。协调动力上的外倾角是悬架调整的神奇效果。附录B:英文原文Pointed the Right WayCamber, Caster and Toe: What Do They Mean?The three major alignment parameters on a car are toe, camber, and caster. Most enthusiasts have a good understanding of what these settings are and what they involve, but many may not know why a particular setting is called for, or how it affects performance. Lets take a quick look at this basic aspect of suspension tuning.UNDERSTANDING TOEWhen a pair of wheels is set so that their leading edges are pointed slightly towards each other, the wheel pair is said to have toe-in. If the leading edges point away from each other, the pair is said to have toe-out. The amount of toe can be expressed in degrees as the angle to which the wheels are out of parallel, or more commonly, as the difference between the track widths as measured at the leading and trailing edges of the tires or wheels. Toe settings affect three major areas of performance: tire wear, straight-line stability and corner entry handling characteristics.For minimum tire wear and power loss, the wheels on a given axle of a car should point directly ahead when the car is running in a straight line. Excessive toe-in or toe-out causes the tires to scrub, since they are always turned relative to the direction of travel. Too much toe-in causes accelerated wear at the outboard edges of the tires, while too much toe-out causes wear at the inboard edges.So if minimum tire wear and power loss are achieved with zero toe, why have any toe angles at all? The answer is that toe settings have a major impact on directional stability. The illustrations at right show the mechanisms involved. With the steering wheel centered, toe-in causes the wheels to tend to roll along paths that intersect each other. Under this condition, the wheels are at odds with each other, and no turn results.When the wheel on one side of the car encounters a disturbance, that wheel is pulled rearward about its steering axis. This action also pulls the other wheel in the same steering direction. If its a minor disturbance, the disturbed wheel will steer only a small amount, perhaps so that its rolling straight ahead instead of toed-in slightly. But note that with this slight steering input, the rolling paths of the wheels still dont describe a turn. The wheels have absorbed the irregularity without significantly changing the direction of the vehicle. In this way, toe-in enhances straight-line stability. If the car is set up with toe-out, however, the front wheels are aligned so that slightdisturbances cause thewheel pair to assume rolling directions that do describe a turn. Any minute steering angle beyond the perfectly centered position will cause the inner wheel to steer in a tighter turn radius than the outer wheel. Thus, the car will always be trying to enter a turn, rather than maintaining a straight line of travel. So its clear that toe-out encourages the initiation of a turn, while toe-in discourages it.With toe-in (left) a deflection of the suspension does not cause the wheels to initiate a turn as with toe-out (right).The toe setting on a particular car becomes a tradeoff between the straight-linestability afforded by toe-in and the quick steering response promoted by toe-out. Nobody wants their street car to constantly wander over tar strips-the never-ending steering corrections required would drive anyone batty. But racers are willing to sacrifice a bit of stability on the straightaway for a sharper turn-in to the corners. So street cars are generally set up with toe-in, while race cars are often set up with toe-out.With four-wheel independent suspension, the toe must also be set at the rear of the car. Toe settings at the rear have essentially the same effect on wear, directional stability and turn-in as they do on the front. However, it is rare to set up a rear-drive race car toed out in the rear, since doing so causes excessive oversteer, particularly when power is applied. Front-wheel-drive race cars, on the other hand, are often set up with a bit of toe-out, as this induces a bit of oversteer to counteract the greater tendency of front-wheel-drive cars to understeer.Remember also that toe will change slightly from a static situation to a dynamic one. This is is most noticeable on a front-wheel-drive car or independently-suspended rear-drive car. When driving torque is applied to the wheels, they pull themselves forward and try to create toe-in. This is another reason why many front-drivers are set up with toe-out in the front. Likewise, when pushed down the road, a non-driven wheel will tend to toe itself out. This is most noticeable in rear-drive cars.The amount of toe-in or toe-out dialed into a given car is dependent on the compliance of the suspension and the desired handling characteristics. To improve ride quality, street cars are equipped with relatively soft rubber bushings at their suspension links, and thus the links move a fair amount when they are loaded. Race cars, in contrast, are fitted with steel spherical bearings or very hard urethane, metal or plastic bushings to provide optimum rigidity and control of suspension links. Thus, a street car requires a greater static toe-in than does a race car, so as to avoid the condition wherein bushing compliance allows the wheels to assume a toe-out condition.It should be noted that in recent years, designers have been using bushing compliance in street cars to their advantage. To maximize transient response, it is desirable to use a little toe-in at the rear to hasten the generation of slip angles and thus cornering forces in the rear tires. By allowing a bit of compliance in the front lateral links of an A-arm type suspension, the rear axle will toe-in when the car enters a hard corner; on a straightaway where no cornering loads are present, the bushings remain undistorted and allow the toe to be set to an angle that enhances tire wear and stability characteristics. Such a design is a type of passive four-wheel steering system.THE EFFECTS OF CASTERCaster is the angle to which the steering pivot axis is tilted forward or rearward from vertical, as viewed from the side. If the pivot axis is tilted backward (that is, the top pivot is positioned farther rearward than the bottom pivot), then the caster is positive; if its tilted forward, then the caster is negative.Positive caster tends to straighten the wheel when the vehicle is traveling forward, and thus is used to enhance straight-line stability. The mechanism that causes this tendency is clearly illustrated by the castering front wheels of a shopping cart (above). The steering axis of a shopping cart wheel is set forward of where the wheel contacts the ground. As the cart is pushed forward, the steering axis pulls the wheel along, and since the wheel drags along the ground, it falls directly in line behind the steering axis. The force that causes the wheel to follow the steering axis is proportional to the distance between the steering axis and the wheel-to-ground contact patch-the greater the distance, the greater the force. This distance is referred to as trail.Due to many design considerations, it is desirable to have the steering axis of a cars wheel right at the wheel hub. If the steering axis were to be set vertical with this layout, the axis would be coincident with the tire contact patch. The trail would be zero, and no castering would be generated. The wheel would be essentially free to spin about the patch (actually, the tire itself generates a bit of a castering effect due to a phenomenon known as pneumatic trail, but this effect is much smaller than that created by mechanical castering, so well ignore it here). Fortunately, it is possible to create castering by tilting the steering axis in the positive direction. With such an arrangement, the steering axis intersects the ground at a point in front of the tire contact patch, and thus the same effect as seen in the shopping cart casters is achieved.The tilted steering axis has another important effect on suspension geometry. Since the wheel rotates about a tilted axis, the wheel gains camber as it is turned. This effect is best visualized by imagining the unrealistically extreme case where the steering axis would be horizontal-as the steering wheel is turned, the road wheel would simply change camber rather than direction. This effect causes the outside wheel in a turn to gain negative camber, while the inside wheel gains positive camber. These camber changes are generally favorable for cornering, although it is possible to overdo it.Most cars are not particularly sensitive to caster settings. Nevertheless, it is important to ensure that the caster is the same on both sides of the car to avoid the tendency to pull to one side. While greater caster angles serve to improve straight-line stability, they also cause an increase in steering effort. Three to five degrees of positive caster is the typical range of settings, with lower angles being used on heavier vehicles to keep the steering effort reasonable.Like a shopping cart wheel (left) the trail created by the castering of the steering axis pulls the wheels in line.WHAT IS CAMBER?Camber is the angle of the wheel relative to vertical, as viewed from the front or the rear of the car. If the wheel leans in towards the chassis, it has negative camber; if it leans away from the car, it has positive camber (see next page). The cornering force that a tire can develop is highly dependent on its angle relative to the road surface, and so wheel camber has a major effect on the road holding of a car. Its interesting to note that a tire develops its maximum cornering force at a small negative camber angle, typically around neg. 1/2 degree. This fact is due to the contribution of camber thrust, which is an additional lateral force generated by elastic deformation as the tread rubber pulls through the tire/road interface (the contact patch).To optimize a tires performance in a corner, its the job of the suspension designer to assume that the tire is always operating at a slightly negative camber angle. This can be a very difficult task, since, as the chassis rolls in a corner, the suspension must deflect vertically some distance. Since the wheel is connected to the chassis by several links which must rotate to allow for the wheel deflection, the wheel can be subject to large camber changes as the suspension moves up and down. For this reason, the more the wheel must deflect from its static position, the more difficult it is to maintain an ideal camber angle. Thus, the relatively large wheel travel and soft roll stiffness needed to provide a smooth ride in passenger cars presents a difficult design challenge, while the small wheel travel and high roll stiffness inherent in racing cars reduces the engineers headaches.Its important to draw the distinction between camber relative to the road, and camber relative to the chassis. To maintain the ideal camber relative to the road, the suspension must be designed so that wheel camber relative to the chassis becomes increasingly negative as the suspension deflects upward. The illustration on the bottom of page 46 shows why this is so. If the suspension were designed so as to maintain no camber change relative to the chassis, then body roll would induce positive camber of the wheel relative to the road. Thus, to negate the effect of body roll, the suspension must be designed so that it pulls in the top of the wheel (i.e., gains negative camber) as it is deflected upwards.While maintaining the ideal camber angle throughout the suspension travel assures that the tire is operating at peak efficiency, designers often configure the front suspensions of passenger cars so that the wheels gain positive camber as they are deflected upward. The purpose of such a design is to reduce the cornering power of the front end relative to the rear end, so that the car will understeer in steadily greater amounts up to the limit of adhesion. Understeer is inherently a much safer and more stable condition than oversteer, and thus is preferable for cars intended for the public.Since most independent suspensions are designed so that the camber varies as the wheel moves up and down relative to the chassis, the camber angle that we set when we align the car is not typically what is seen when the car is in a corner. Nevertheless, its really the only reference we have to make camber adjustments. For competition, its necessary to set the camber under the static condition, test the car, then alter the static setting in the direction that is in
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本文标题:BJ1042轻型载货汽车前悬架设计【前双横臂独立悬架】
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