汽车设计--车架设计(外文原文及译文)

汽车设计--车架设计(外文原文及译文) 汽车设计----车架设计 车架是汽车最基本的台架，所有的悬架和转向连接部件都安装在车架上面。如果汽车车架柔性过大，会使汽车既无法转向，也无法进行正常操纵。而如果汽车车架结构刚性过大，又会引起不必要的震动传递给驾驶员和乘客的座舱室。汽车车架和悬架的结构设计不仅决定了汽车噪声大小和震动的幅度强度，而且也将影响到汽车的质量和车辆的正常操纵。 汽车制造厂商们在他们生产的汽车上都使用了几种不同的车架结构。其中，整个七十年代最常使用的是壳体和大梁的分体结构。目前它仍然在大型货车、小吨位货车和卡车上应用着。在汽车壳体和大梁的分体结构里，发动机、传动装置、传动齿轮和车壳都是通过绝缘装置固定在车身大梁上。车架内部的绝缘装置是人造橡胶缓冲垫，能够阻止道路不平和发动机工作引起的噪音和震动传到驾驶员和乘客的座舱里。 第二种是汽车车架的单体结构。这种设计到目前为止在现代汽车上是最常用。单体车架按所需的强度来分，设计有轻型结构。在这种汽车结构中大梁作为车架的一部分被直接焊接到壳体上。底盘的重量增加了大梁的强度。传动齿轮和传动装置经由大而软的人造橡胶绝缘垫安装在单体车架上。绝缘垫减弱了噪声的传动和震动。若绝缘垫太软，将会引起传动齿轮和传动装置位移。这种位移称为柔量，它会影响到汽车的操纵性能和控制性能。若绝缘垫太硬，则不能起到应有的隔绝噪音和减小震动的作用。汽车制造厂商们精心地设计绝缘垫，把它们装置在汽车适当的地方，以降低噪声，缓冲震动的传送，使汽车便于驾驶，驾驶员和乘客乘坐舒适。绝缘垫的性能随使用年限发生变化，当汽车变旧时原先的性能也随之改变。 第三种结构是把前两种结构的主要特点结合在一起。它在汽车前舱使用了短车梁，在汽车后舱使用了单体车架。单体部分刚性很大，而短的车梁增强了绝缘作用。 汽车制造厂家们在汽车上选择那种生产成本低而同时又符合对噪音震动，驾驶操纵性能要求很高的车架结构。老式的大型的车辆、货车、和卡车通常使用壳体和大梁的分体结构。较新的，较小型的车辆通常使用单体结构的车架。 发动机活塞连杆组 活塞连杆组由活塞、活塞环、活塞销、连杆、连杆轴瓦等组成。 功用:活塞的功用是承受气体压力，并通过活塞销传给连杆驱使曲轴旋转，活塞顶部还是燃烧室的组成部分。 工作条件:活塞在高温、高压、高速、润滑不良的条件下工作。活塞直接与高温气体接触，瞬时温度可达2500K以上，因此，受热严重，而散热条件又很差，所以活塞工作时温度很高，顶部高达600,700K，且温度分布很不均匀;活塞顶部承受气体压力很大，特别是作功行程压力最大，汽油机高达3,5MPa，柴油机高达6,9MPa，这就使得活塞产生冲击，并承受侧压力的作用，因此，活塞应有足够的耐热性，要尽量减小活塞的受热面，加强活塞的冷却，适当增大传热面，使活塞顶部的最高温度下降。活塞在气缸内以很高的速度(8,12m/s)往复运动，且速度在不断地变化，这就产生了很大的惯性力，使活塞受到很大的附加载荷。活塞在这种恶劣的条件下工作，会产生变形 并加速磨损，还会产生附加载荷和热应力，同时受到燃气的化学腐蚀作用。 为了减小往复惯性力，必须尽可能地减轻活塞的重量。活塞是在高温、高压、高速(活塞平均速度可达101115m/s)的条件下工作的，其润滑条件较差，活塞与气缸壁间摩擦严重。为减小摩擦，活塞表面必须耐磨。 要求: 1) 要有足够的刚度和强度，传力可靠; 2) 导热性好，耐高压、耐高温、耐磨损; 3) 质量小，重量轻，尽可能减小往复惯性力。 铝合金材料基本上满足上面的要求，因此，活塞一般都采用高强度铝合金，但在一些低速柴油机上采用高级铸铁或耐热钢。 构造:活塞可分为三部分，活塞顶部、活塞头部和活塞裙部。 1.活塞顶部 活塞顶部承受气体压力，它是燃烧室的组成部分，其形状、位置、大小都和燃烧室的具体形式有关，都是为满足可燃混合气形成和燃烧的要求，其顶部形状可分为四大类，平顶活塞、凸顶活塞、凹顶活塞和成型顶活塞。 加工简单平顶活塞顶部是一个平面，结构简单，制造容易，受热面积小，加工简单，顶部应力分布较为均匀，一般用在汽油机上，柴油机很少采用。 凸顶活塞多用于二冲程内燃机上，有利于改善换气过程。现代四冲程汽油机为了增强挤气效果或增大压缩比也有采用凸顶活塞。凸顶活塞顶部凸起呈球顶形，其顶部强度高，起导向作用，有利于改善换气过程，二行程汽油机常采用凸顶活塞。 凹顶活塞顶部呈凹陷形，凹坑的形状和位置必须有利于可燃混合气的燃烧，有双涡流凹坑、球形凹坑、U形凹坑等等。 2.活塞头部 活塞头部指第一道活塞环槽到活塞销孔以上部分。它有数道环槽，用以安装活塞环，起密封作用，又称为防漏部。柴油机压缩比高，一般有四道环槽，上部三道安装气环，下部安装油环。汽油机一般有三道环槽，其中有两道气环槽和一道油环槽，在油环槽底面上钻有许多径向小孔，使被油环从气缸壁上刮下的机油经过这些小孔流回油底壳。第一道环槽工作条件最恶劣，一般应离顶部较远些。 在一道气环的上方往往开有一道较窄的隔热槽，隔断由活塞顶部传向第一道气环的热流，使部分热量由第二、三道活塞环传出，从而减轻第一道气环的热负荷。有的发动机，在活塞顶面至第一道环槽之间，有时一直到以下几道环槽处，常加工出细小的环行槽。这种细小的环行槽可以因积碳而吸附润滑油，在失油工作时可防止活塞与气缸壁的咬合，故称之为积碳槽。 活塞顶部吸收的热量主要也是经过防漏部通过活塞环传给气缸壁，再由冷却水传出去。总之，活塞头部的作用除了用来安装活塞环外，还有密封作用和传热作用，与活塞环一起密封气缸，防止可燃混合气漏到曲轴箱内，同时还将(70,80)%的热量通过活塞环传给气缸壁。 3.活塞裙部 活塞裙部指从油环槽下端面起至活塞最下端的部分，它包括装活塞销的销座孔。活塞裙部对活塞在气缸内的往复运动起导向作用，并承受侧压力。裙部的长短取决于侧压力的大小和活塞直径。所谓侧压力是指在压缩行程和作功行程中，作用在活塞顶部的气体压力的水平分力使活塞压向气缸壁。压缩行程和作功行程气体的侧压力方向正好相反，由于燃烧压力大大高于压缩压力，所以，作功行程 中的侧压力也大大高于压缩行程中的侧压力。活塞裙部承受侧压力的两个侧面称为推力面，它们处于与活塞销轴线相垂直的方向上。 动力传动系统 动力传动系统包括从发动机直到驱动轮的所有部件。联动装置和后驱动装置传送着来自发动机的扭矩。其它部件则把部件与部件相互连接起来。加速时发动机的扭矩和制动时的扭矩则加载在悬架部位上。 修理悬架时，很可能需要拆卸传动系统的各零部件来进行修理。悬架移动时产生的噪音可能来源于传动系统的零部件。下边叙述一些不同的传动装置系统的基本知识，在进行悬架修理时可供参考。 使用前轮驱动的传动系统经常将联动装置和后轮传动装置结合成一个装置。这个对中置和后置发动机的汽车也是很适用的。这个装置称为转换轴。它为两端各带有一个万向节的短半轴，把转换轴和车轮连接起来。这些轴当悬梁移动和转向时把动力从后传动装置传送到车轮上。 后传动装置里的差速器分流输入的动力，每个驱动轮上各分一半。这就使驱动轮在转弯时会以不同的速度转动。 在前置发动机后轮驱动的汽车里，联动装置位于驾驶坐舱的前底板下。传动轴被用来把发动机动力传送到后桥上。传动轴每端各有一个万向节。当悬架移动时，万向节通过变化着的传动系统的角度传送动力。 驱动轮上带有独立悬架的汽车中有一个牢固地附加在车身大梁或发动机上的后传动装置。在加速时该装置在悬架部位上会产生动力，并不产生扭矩。如果刹车装置安装在车舱内，卡钳装到大梁上而不是悬架上，那么刹车装置也不会在悬架上产生扭矩。仅用于控制加速和减速扭矩的悬架与必须同时控制悬架力和扭矩的悬架在汽车设计上是完全不相同的。 悬架系统 悬架包括弹簧，避震器和控制连杆装置。它必须能够足以支撑车身自重和负载。悬架也应能够承受发动机和制动对它的反作用力。悬架系统最重要的作用是使轮胎与路面接触的时间尽可能的长。在支撑车体和负载时，甚至在高低不平的道路上行驶时更加应如此。这四个轮胎的胎面是车与路面相接触的唯一的部位。发动机全部输出的动力，转向力和制动力都通过与路面相接触的轮胎的胎面起作用。每当轮胎不与路面接触或汽车开始打滑时，汽车的控制力(动力、转向力、制动力)就会减弱甚至丧失。 车体是靠弹簧支撑着，弹簧可分为螺旋型、钢板型、扭棒型和充气型。螺旋型弹簧是现代汽车中应用最为广泛的类型。螺旋型、扭棒型和充气型弹簧都需要用连杆和连杆臂以使车轮就位。钢板弹簧提供了对车体的横向和纵向控制，以防止汽车车轮在行驶时不必要的位移，它们通常用在载重货车和卡车上。 悬架系统是随着客运汽车的发展而变化和改进着。豪华轿车，特种车辆，小型汽车和轻型卡车的设计目的是截然不同的。现代轮胎的改进不断地改善了车辆的操作性能，它的改进是与避震器，转向系统和悬架控制装置一起同步改进的。 现代汽车在各种操纵条件下都需要轮胎与路面接触，以便安全、正确地控制并行驶汽车。要想要最大限度的安全驾车，要牢记这四个轮胎必须在任何时间都与路面相接触。同时需要考虑汽车操纵的灵活性，轮胎的抗耐磨性，汽车驾驶的舒适性和行车的安全性，以达到汽车的有效控制。 悬架系统分为前悬架和后悬架。前悬架的设计已得到了飞速发展。从较为粗糙的硬轴结构发展到了现代的轻型、高强度、支撑型独立悬架结构，并由于增加了连杆装置而使汽车的性能得到了改善。悬架结构的改进是随着路况的改善和驾驶员的需要而进行改进的。 大多数前置发动机，后轮驱动的汽车都采用一个简单的从属性后悬架。但后轮驱动的独立悬架结构复杂得多，而且成本极高，因而只用于少数客车上。 对于前置发动机前轮驱动的车辆，通过把传动装置移至前部，后悬架仅用来调节驾驶控制力和刹车时的反作用。这就导致了简化的非独立的悬架机构，半独立的悬架机构和独立的后悬架机构的应用，后者大量应用于新型车辆的结构设计上。 转向系统 汽车驾驶员通过对转向齿轮的控制汽车前轮的方向。现代的转向齿轮有两个主要的部分组成，转向杆和齿轮组。转向杆有一个被支撑的轴，它把驾驶员的方向盘与齿轮组连在了一起。齿轮组可将汽车驾驶员的转向力增大，以带动转向连杆装置。 后轮驱动汽车的前轮在一个心轴上转动。心轴是转向节的一部分。该转向节与带有球接头的前悬横梁相互连接。球接头在前悬架上下移动时可以进行转向。前轮驱动的汽车的轮毂在转向节里的轴承内的空心轴短轴杆上传动。 汽车方向盘控制转向齿轮装置。它依次通过转向连杆装置使转向节开始移动。现在使用两种转向齿轮的结构，即齿轮齿条式结构以及循环球式结构。 现代汽车设计了对速度敏感的转向结构。因此当汽车慢速行驶时需要较大的力才能使汽车转向。于是在很多汽车上装备了助力转向装置。 由于助力转向装置起了主要作用，所以转向比降低了，这样就能够轻微转动方向盘使得汽车转向。助力转向齿轮类似于 标准 excel标准偏差excel标准偏差函数exl标准差函数国标检验抽样标准表免费下载红头文件格式标准下载 的转向齿轮。它有承压面，液体压力加在其上，以增加汽车驾驶员的转向力。齿轮齿条式转向结构和循环球齿轮结构都有了动力辅助装置。 转向齿轮的动力是由发动机从动泵提供的。该泵使动力转向液体流过一个由阀体控制的系统。该控制阀能感知汽车驾驶员的转向力。把液体压力加到转向系统的承压面上。该液体压力承接了一些使汽车转向的力。 现在汽车的转向杆有很多个部件组成。它被用来分散、抵消汽车碰撞力以保护驾驶员的切身安全。在有些汽车上转向杆还可以倾斜和伸缩来调节方向盘的位置使驾驶员感觉更加舒适。为了减少驾驶员汽车被盗的机会，还安装有一个转向齿轮的保险锁。很多汽车还有一个变速器保险锁。因为处在驾驶员很容易触及的范围内，所以转向杆上还可以带有变速器换挡控制滑杆，转向信号开关，前大灯和变光开关，刮水器开关，紧急闪烁器开关和速度控制器。 制动系 使用中的制动器应能起到制动住车辆的作用。制动器能使汽车滑行时能防止行驶速度过快，在斜坡上制动时能将汽车停在适当的位置上。汽车刹车的设计应使驾驶员能调节制动力以控制汽车。汽车的控制不仅受悬架和转向系统影响，而且也受汽车刹车影响。制动系统的故障可导致汽车刹车时车轮滑脱。要修理悬梁系统，也可能需要将制动系统的部件拆卸开。为此本文将讨论制动系统。 制动系统应给予汽车驾驶员提供均匀平稳的制动力。刹车板上所需的力不应 太大，而使车轮不至于被瞬间刹死。为满足这些汽车刹车的要求，对于汽车制动已有了最低限度的刹车标准。 驾驶员通过机械装置、真空和液压装置控制制动力。制动力是随着附加在汽车刹车板上的踏板力的增加而增加的。这个力通过制动系统的传递以把固定的汽车刹车片推压到转动的制动器表面上。当它把动能(运动的能量)转化为热能(热)时，就使汽车减速。制动量的最大值就产生于车轮被瞬间闸死而引起的轮胎在路面上滑动之前。所以制动量的最大值取决于轮胎和路面之间的附着力。当轮胎在道路上滑动时，制动效果减弱，汽车的方向控制可能就不起作用了。 前刹车总成的固定构件安装在前悬架的转向节上，在后部，它们被安装在后桥壳或后心轴总成上，铸铁刹车鼓或车盘随车轮一起转动。 汽车的制动盘刹车时:汽车制动盘刹车有随车轮一起转动的圆盘。它通常被称为汽车刹车转子。在固定的卡钳里的液压控制的活塞被用来把汽车的刹车片加在转子的汽车刹车表面上。汽车刹车片和转子之间的摩擦力的大小会减慢或阻止车轮的转动。固定的卡钳壳体使垫圈被压在转动的汽车刹车盘上，使之不能转动。 汽车制动盘刹车垫圈的运动与刹车转子的表面垂直，这样会使它们卡在转子上减慢汽车的车轮运动。卡钳压的力与驾驶员加在汽车刹车板上的力成正比。 汽车制动鼓刹车:汽车制动鼓刹车使用带有摩擦片的固定的内胀式刹车块。他们被安装在转动的汽车刹车鼓内侧。汽车刹车鼓紧箍在轮胎总成和毂总成或轮轴法兰之间。当汽车刹车块的直径膨胀至使汽车刹车片与汽车刹车表面相接触时，汽车刹车块就减慢了汽车刹车鼓的转动。它是由液压操纵的汽车刹车分泵来完成的。来自刹车总泵的流体压力被施加到汽车刹车分泵上，使刹车分泵膨胀起来。汽车刹车分泵的膨胀使刹车块通过机械连杆进行移动，把汽车刹车片压到转动的刹车鼓上。当汽车刹车鼓的转动速度减慢时，就起到了制动作用。 Automobile Design----Frame Designs The vehicle frame is the basic platform to which all suspension and steering linkage parts attach. A vehicle will neither steer nor handle well if the frame is too flexible. A rigid frame structure may pass unnecessary vibrations into the passenger compartment. The frame and suspension design will affect the ride quality, handling, and durability, as well as the levels of both noise and vibration. Manufacturers use several different types of construction on their vehicles. Of these, separate body and frame construction was the most common through the 1970's. It is still used in large vans, pickups, and trucks. In this type of construction, the engine, drive line, running gear, and body mount to the frame through insulators. Insulators are synthetic rubber pads that keep road and engine noise and vibration from going into the passenger compartment. A second type of construction is the unitized body. This, design is by far the most popular in modern vehicles. The unitized design has a lightweight structure with the required strength. Tn this type of construction, the frame is welded into the body as part of the body structure. Body panels add strength to the frame pieces. The running gear and drive line are mounted to the unitized body through large, soft synthetic rubber insulators. The insulators minimize the transfer of noise and vibration. If the insulators are too soft, they will allow too much running gear and drive line movement. This movement, called compliance, affects vehicle handling and control. If the insulators are too hard, they will not insulate noise and vibration as they should. The manufacturer carefully designs the insulators and puts them where they will be in a vehicle with low noise and vibration transmission that still has proper handling and feel. Insulator properties change with age, changing original characteristics as the vehicle becomes older. A third type of construction combines the features of the first and second types. It uses a stub frame from the bulkhead forward and a unitized body from the bulkhead back. The unitized part is very rigid, while the stub frame provides a place for good insulation. Manufacturers select the type of construction .that is most economical to build,' while providing the noise, vibration, and ride and handling characteristics they want in the vehicle. Large older vehicles, vans, and trucks generally use separate body and frame construction. The newer, smaller' vehicles generally use unitized construction. The machine piston connecting rod set The piston connecting rod set is composed of the piston, piston wreath, piston pin, connecting rod, connecting rod axle bush, etc. effect: The effect of the piston is to bear the air pressure, and pass to connecting rod to drive the bent axle to revolve through connecting rod axle bush, the piston coping is still a part of the burning room. The work condition: Piston works under the condition of heat, high pressure, high speed, and bad lubrication .Piston directly contacts with the heat air. The temperature can amount to above 2500 Ks in a moment .The piston is heated severely, but the condition of spreading the hot is bad .So while the piston works, the temperature is very high and the coping is up to the 600-700 Ks: And the temperature distributes asymmetrically; The piston coping bears great air pressure, especially the pressure is greatest in the route of doing efficacy. The gasoline machine is up to the 3-5MPas, the diesel engine is up to the 6-9MPas.This makes the piston produce pound, and bear the function of the side pressure. Therefore, the piston should have enough heat-proof, try to decrease the heating area,, strengthen the cooling of the piston, to make the highest temperature of the coping descend .The piston moves at very high speed(8-12 ms/ s) back and forth in the air cylinder, and speed changes constantly, This produces very great inertial dint, making the piston bear great additional load working under such bad condition, the piston will become deformed heating power. At the same time ,it slitters the chemical corrosive power of the burning gas .In order to descend the inertial dint of back and forth, we must ease the weight of the piston as possible .Piston works under the condition of the heat, high pressure, high speed(the average speed can amount to the 101115 m/ s), and its lubricant condition is bad and the frication between the piston and the air cylinder wall is very great. In order to descend the friction, the surface of the piston surface must be wear-resistant.. Request: 1) To have enough rigidity and strength, and the reliable dint; 2) Transmit heat well, bear the high pressure, bear the heat and bear to wear away; 3) the quantity is small, the weight is light, descend the inertial dint of back and forth as possible The aluminum metal alloy material satisfies the top requests basically, therefore, the piston generally adopts the high strength aluminum metal alloy, but some low speed diesel engines adopt high class iron casting or heat-proof steel structure: The piston can be divided into three parts, piston coping, the piston head and piston skirt departments. 1. The piston coping The piston coping bears the air pressure, it is a part of the burnable room .Its shape, position, size are relevant to the concrete from of the burnable room. They are made to satisfy the combustible hybrid spirit formation and burnable requests. Its coping shape can be divided into four major types, a flat coping piston, a convex coping piston, a concave coping piston and model piston. A convex coping piston is usually used on the two blunt distance I.C. engines, It is good to improve the process exchanging the gas .Modern four blunt the distance gasoline machine also adopts the convex coping piston in order to strengthen the effect of pushing the gas or extend the ratio of compressing .Convex of a piston coping presents a form of ball, its coping strength is high, having an effect of leading, being advantageous to improve the process of exchanging the gas, two route of travel gasoline machines often adopt the convex coping piston A piston coping presents the hollow form, the shape and positions of the cave pit must be advantageous to the combustion of the combustible and hybrid gas, having a pair of eddies concave pit ball, concave pit, U concave pit, and so on. 2. Piston head The piston refers to the first piston wreath to the part above the piston pin.It has several wreath slots, which are used to install the piston wreath and have an effect of sealing completely. It is also called the leak proof department .The diesel engine’s compress ratio is high, and generally have four wreath slots, The three upper wreaths are used to install, the lower part installs the oil wreath. The gasoline machine has three wreath slots generally, which are two jet of gas wreath slots and an oil wreath slots. At the bottom of oil wreath slot many paths toward eyelet are drilled to make the quilt oil wreath flow from the air cylinder wall to the oil bottom hull through these eyelets. The working condition of the wreath slot is the worst and should leave the coping generally a little farther. Above the gas wreath, a narrow insulating slot is usually set to cut off the heat flow which is spread from the piston coping to the first gas wreath and make parts of calories from a piston wreath spread, thus easing the hot burden of the first gas wreath. On some engines small wreathe slots are often made between the coping head and the first gas wreathe, sometimes till a few more wreath. This kind of small wreath can adsorb the lubricant because it accumulates the carbon. It can keep piston and the air cylinder walls from biting to match when it work in the condition of losing oil, so it is called accumulating the carbon slot. The calories that the piston coping absorbs also mainly passes the air cylinder wall through the piston wreath to leak proof department, again spread by the cool water.In a word, the function of the piston head is in addition to install the piston wreath, still seal completely function and transmit heat function, sealing completely the air cylinder together with the piston wreath, keeping combustible admixture spirit from leak crankcase, at the same time pass the(70-80)%calories to the air cylinder wall through the piston wreath. 3. Piston skirt department The piston skirt department refers to the parts from the bottom of the oil wreathe slot. It includes the pin which is used to pack the piston. The piston skirt department exercises to rise to lead to the function to the piston in the back and forth in the air cylinder, and bear the side pressure. The length of the skirt department is decided by the size and the piston diameter of the side pressure. The so-called side pressure mean in the compression route of travel and make route of travel of efficacy .The level component of the gas pressure which take effect on the piston coping presses the piston to the air cylinder wall. Compress the route of travel and make the side pressure direction of the efficacy route of travel air exactly the opposite, because of the combustion pressure consumedly high in compress the pressure, so, make the side in the route of travel of efficacy pressure also consumedly high in compress the side in the route of travel pressure.Two on the sides that bear the side pressure of the piston skirt department be called to push the dint to face, they be placed in to sell the mutually perpendicular direction of the stalk line with piston up. Drive Lines The drive line includes all the parts from the and final drive carry the torque from the engine, the other. The engine torque during acceleration and the torque during braking place loads on the suspension parts. During suspension repair, it may be essary to disassemble parts of the drive line. Noises produced when the suspension moves may originate from drive line parts. A basic understanding of different drive line assemblies is presented here to give you a working knowledge so that you can do suspension repair. Drive lines with front-wheel drive often combine the transmission and the final drive into one assembly. This is also true of mid-and rear-engine vehicles. The assembly is called a transaxle, Short half-shafts with universal joints at each end connect between the transaxle and the wheels. These shafts carry power from the final drive to the wheels even when the suspension moves and steers. A differential in the final drive splits incoming power, sending half to each drive wheel. This allows the drive wheels to turn at different speeds while rounding corners. The transmission Other parts form the link from one part to while cornering. In front-engine, rear-wheel drive vehicles, the transmission is located under the front floor of the passenger compartment. A drive shaft is used to carry engine power to the rear axle. The drive shaft has a universal joint at each end. It carries power through the changing drive line angles as the suspension moves. A vehicle with independent suspension at the drive wheels has the final drive attached rigidly to the vehicle frame or the engine. This drive arrangement produces forces, without any torques, on the suspension parts during acceleration. If the brakes are mounted inboard so the caliper mounts to a frame piece and not to a suspension, the brake will also not produce a torque on the suspension. A suspension designed to handle only acceleration and braking torques can be designed differently than one that must handle both suspension forces and torques. Suspension Systems The suspension includes springs, shock absorbers, and control linkages. It must be strong enough to support the vehicle body and load. The suspension must also resist engine and brake reactions. The most important job of the suspension is to keep the tires in contact with the road as much of the time as possible. This is done while supporting the vehicle and its load, even while traveling over rough roads. The four tire footprints are the only place the vehicle touches the road. All of the engine power, steering, and braking forces operate through the tire-to-road footprints. Control of the vehicle ( power, steering and braking) is reduced or lost any time a tire does not stay on the road or when skidding begins. The vehicle body is supported by springs. The springs can be of the coil, leaf, torsion bar, or pneumatic type. Coil springs are the most popular design used in the modern automobile. Coil, torsion bar, and pneumatic springs all require links and arms to hold the wheel in position. Leaf springs provide lateral and longitudinal control to prevent unwanted wheel motions. They are commonly found on vans and trucks. Suspension systems have been changed and refined as the passenger automobile has developed. Design objectives differ between luxury sedans, performance vehicles, small compact vehicles, and light trucks. Tire improvements, along with improvements in shock absorbers, steering systems, and suspension control devices, have continually upgraded vehicle handling characteristics. Tire-to-road contact is needed for safe, positive vehicle control under all operating conditions. Keep in mind that all four tires must stay in contact with the road at all times for maximum vehicle control. Compromises are made in handling response, tire wear, driver comfort, and ride harshness to achieve positive vehicle control. Suspension systems are divided into front suspension and rear suspension. Front suspension designs have developed from relatively rugged solid-axle designs to the modern lightweight, high-strength , strut-type independent designs. These have been upgraded with added linkage. Suspension design improvements have followed improvements in roadways and driver expectations. Most front-engine, rear-wheel-drive vehicles use a simple dependent rear suspension . Rear-wheel-drive independent suspension is much more complex and expensive. As a result, it is only used on a few passenger vehicles. To front-engine, front-wheel-drive vehicles by moving the drive train to the front, only ride control and braking reactions are controlled by the rear suspension. This has led to the use of simplified dependent suspension , semi-independent suspension and independent rear suspension. The latter is used in a larger number of new vehicle designs. Steering Systems The driver controls the direction of the front wheels of the vehicle through the steering gear. Modern steering gears have two major units* a steering column and a gear unit. Tin-steering column has a supported shaft that connects the driver's steering wheel to the gem unit. The gear unit multiplies the driver's steering effort to move the steering linkage. The front wheels of rear-wheel-drive vehicles rotate on a spindle. The spindle is part ol the steering knuckle . The knuckle is connected to the front suspension members with ball joints. The ball joints allow for steering as the suspension moves up and down. The wheel hubs on front-wheel-drive vehicles rotate on hollow axle stub shafts inside bearings within the steering knuckles. The steering wheel controls the steering gear assembly. This, in turn, moves the knuckle through the steering linkage. Two steering gear designs are in use today, the rack and pinion and recirculating ball. vehicles are designed with responsive steering. As a result, more effort is needed to steer the vehicle when it is moving slowly. Power steering supplies this effort on many vehicles. With power steering doing most of the work, steering ratios are decreased so that the vehicle can be steered with small steering wheel movements. The power steering gear is similar to the standard steering gear. It includes surfaces upon which fluid pressure is applied to aid the driver's steering effort. Both rack and pinion and recirculating ball gears may have power assist. Power for the steering gear is provided by an engine-driven pump. The pump forces power steering fluid through a system controlled by a valve. This control valve can sense the driver's steering effort. It puts fluid pressure against a pressure surface in the steering system. This fluid pressure takes over some of the effort needed to steer the vehicle. The steering column in the modern vehicle has many parts. It is designed to collapse or fold in a vehicle collision to protect the driver. In some installations, it may be tilted and telescoped to adjust the position of the steering wheel for the comfort of the driver. To reduce the chance of theft, it also has a steering gear lock. On many vehicles, it has a transmission lock. Because it is within easy reach of the driver, the steering column may carry the transmission shift control lever, turn signal switch, headlight and dimmer switches, wiper switch, emergency flasher switch, and speed control. Brake Systems Service brakes must be able to stop the vehicle, prevent excess speed when coasting, and hold the vehicle in position while it is stopped on grades. They are designed so the driver can adjust the braking effort to maintain vehicle control. Vehicle control is influenced by brakes as well as the suspension and steering systems. Faults in the brake system can lead to wheel pull during braking. To repair suspension systems, parts of the brake system may require disassembly. For these reasons, the brake system will be discussed briefly in this text. The brake system must provide smooth stopping power that can be controlled by the driver. The force required on the brake pedal must not be so high that the wheels cannot be locked. To meet these braking requirements, minimum braking standards have been set for vehicle brakes. The driver controls the braking force through mechanical, vacuum, and hydraulic mechanisms. The amount of braking increases as more force is placed on the brake pedal. This force is transferred through the brake system to push stationary brake linings against the rotating brake surface. This slows the vehicle as it turns kinetic energy (energy of motion) into thermal energy (heat). Maximum braking occurs just before the wheels lock to cause the tires to slide on the road surface. Maximum braking, therefore, depends on the adhesion between the tire and the road surface. When the tire slides on the road, braking effect is reduced and directional control of the vehicle may be lost. The stationary parts of the front brake assemblies are mounted on the steering knuckle of the front suspension. In the rear, they are mounted on the axle housing or the rear spindle assembly. The cast-iron brake drum or disc rotates with the wheel . Disc Brake. Disc brakes have discs that rotate with the wheel . The brake disc is usually called a brake rotor. A hydraulically operated piston in a stationary caliper is used to force the lining of the brake pad against the braking surface of the rotor. The friction between the lining and rotor is used to slow or stop wheel rotation. The stationary caliper housing keeps the pads from rotating when they are being forced against the rotating brake disc. Disc brake pads move perpendicular to the face of the brake rotor. In this way, they clamp on the rotor to slow the vehicle motion. The clamping force is proportional to the force the driver puts on the brake pedal. Drum Brakes. Drum brakes use stationary, internal expanding brake shoes with linings. They are mounted inside a rotating brake drum. The brake drum is fastened between the wheel-tire assembly and the hub assembly or the axle flange. The brake shoes slow drum rotation when the diameter of the shoes is expanded to bring the lining in contact with the brake surface. This is done by a hydraulically operated wheel cylinder. Fluid pressure from the master cylinder is forced into the wheel cylin-ders, expanding them. The expansion of the wheel cylinder moves the brake shoe through mechanical linkage to press the-linings against the rotating brake drum. This provides braking action as it slows the rotation of the drum.