轎車懸架系統(tǒng)設計【麥弗遜懸架】

轎車懸架系統(tǒng)設計【麥弗遜懸架】,麥弗遜懸架,轎車懸架系統(tǒng)設計【麥弗遜懸架】,轎車,懸架,系統(tǒng),設計,麥弗遜 附錄A 懸架幾何 引言 無論設計乘用車還是賽車的懸架系統(tǒng)都需要各種學科的知識。這一章只涉及到其中的一方面，懸架幾何的學習，沒有設涉及到懸架元件在載荷變化下，產生位移與變形所產生的影響。這些影響將在23章討論。 當我們談論懸架幾何時，它表示的是怎樣把整車的非簧載質量與簧載質量聯(lián)系起來。這些聯(lián)系不僅決定著它們之間的相對運動也影響著它們之間力的傳遞。 每種懸架幾何結構的設計必須滿足你對應的車型的要求。所以不會存在唯一的最好的懸架幾何結構。 17.1自由度與運動路徑 對于獨立懸架，不管前懸架還是后懸架，其控制臂的作用就是控制車輪相對車身的運動在一個理想的路徑。由于設計者的設計，外傾角，內傾角，前束角在這個路徑中會產生各種變化，但是當上下運動時，懸架運動仍然遵循這個路徑。換句話說，車輪相對車架有一個固定的運動路徑。相對這個路徑不會發(fā)生前后或水平方向的改變。轉向節(jié)不會隨意轉動，而是由這個路徑決定.懸架在彈簧減震器的作用下上下運動時，通過懸架橫臂的相互連接來精確控制，轉向節(jié)在各個方向上的位置。在前懸架中，只有在人為的控制轉向器的是時候，才有一定轉向的自由度。 在空間上對于任何一物體相對于另一物體的運動而言，它的相對運動可分為三個位移運動的組成合三個轉動運動的組成（見圖17.1）。在三維的空間里一個物體擁有6個自由度。我們以上所說的任何一種獨立懸架上的轉向節(jié)相對于車架只有一種運動路徑，換句話說，限制了懸架的五個方向上的自由度。實際上，機械元件提供的約束就限制自由度方面而言并不是完美的。因此獨立懸架幾何的學習是來控制怎樣約束轉向節(jié)來限制其他五個方向上的自由度。 在設計懸架幾何中，如果你所能用的為唯一的元件是帶有桿端軸承的桿件的話，便能夠提供五個自由度當中的一個約束，換句話說，提供五個自由度的約束需要五個張緊受力的桿件。 為了聯(lián)系概念，以便更了解的更加清楚，我們需要明白，典型的懸架原件是怎樣提供約束的。通過圖17.2我們可以看到，一個A臂是由兩個帶有桿端的球鉸副的直桿組成。一個麥弗遜的支柱是以是一個滑動運動機構，在一定的角度滑動行程內，它等同于一個A臂。 現在，帶著這種思想，我們來看一下大多數的獨立懸架，想出五個桿件的連接的各種方案。（見圖17.3）。 標準的賽車的雙橫臂懸架有兩個A臂，再加上一個橫拉桿。如此每個A臂有兩個桿，一個橫拉桿總共5個。一個麥弗遜的滑柱看做有兩個桿，下橫臂有兩個，加上橫拉桿也是五個。有一些懸架用的桿件比較少可能不是那么明顯，但是最終的目的是達到對運動的約束。其中一個例子是拖曳臂式后懸架。只有一個橫臂起到五個桿的作用，為了達到要求，必須足夠結實以承受三個轉動方向上的轉矩與扭矩。 對于非獨立懸架的后軸來說，兩個車輪聯(lián)系在一起，所以其中一個車輪運動會影響到另一個（見圖17.4）。 當兩個車輪聯(lián)系在一起，它們相對于車身有兩種不同的運動，它們可以一起上下運動，也可以反方向一個向下一個向上運動。在運動過程中，這個軸相對車身有兩個自由度。在空間中，總共有六個自由度，當我們設計非獨立懸架時，我們需要限制其中的四個?？梢酝ㄟ^一個梁代表四個受力的桿來實現。 17.2 瞬時中心的定義 在接下來的這一章里，瞬時中心將用來描述和決定一些懸架的基本參數。為了清楚地明白這些討論，關于瞬時中心的定義將按順序來說明?！八矔r”的意思是桿的連接在那一確切的位置。“中心”代表的是假想的，桿的連接處的瞬時轉動點的有效投影點。圖17.5表明了怎樣一個長桿代替兩個短桿。隨著桿的連接時移動的，瞬時中心也是動的，所以合適的幾何設計不僅建立在所有的瞬時中心隨離地間隙的變化出現在它們期望的位置，也要隨懸架的行程的變化，控制瞬時中心位置的變化與變化的快慢。 瞬時中心來源于在二維平面內的動態(tài)的學習。這樣形象的表達出了兩個物體之間的運動關系。在懸架設計中，將三維問題轉化為二位問題可以變得很方便。這樣我們討論前視圖和側視圖。我們做出經過車輪中心的鉛垂面，一個平行于汽車的中心線，另一個垂直于汽車中心線。然后我們把懸架的關鍵點投影到這兩個平面上。 當我們用一條線連接球鉸接點和控制臂之間的軸套，把它投影到包含上下橫臂的平面，然后這兩條先將在某點相交，這個交點便是桿的瞬時連接點。如果在前視圖里做投影，得到的瞬時中心影響著外傾角的變化率、側傾中心的某些信息、磨胎運動和一些決定著轉向特性的某些數據。如果你在側視圖里作投影，得到的瞬時中心，將影響著車輪的運動路徑，抗俯仰特性，主銷后傾角的變化率。在三維空間里，三個正交視圖中，俯視圖得到的有關輪胎路徑變化的信息是最少的。 瞬時軸線 在真正的三維空間里瞬時中心被瞬時軸線所代替。如果我們把前視圖和后視圖的瞬時中心相連，便得到一條線。這條線可以看做相對車身的瞬時轉動軸線（見圖17.6）。 獨立懸架有一個運動的瞬時軸線，這是因為它們有五個約束：當然，這條瞬時軸線隨離地間隙的變化而變化。后軸有兩個瞬時軸心，一個對應懸架的上下跳動，一個對應側傾運動。它們也隨著離地間隙的變化而運動。所以無論何時我們學習一個懸架系統(tǒng)，都需要完成它的瞬時中心和瞬時軸先。這章的余下部分將涉及到常見的前后懸架類型的這些軸線的決定因素，此外也設計到它們的一些調整，來滿足賽車的需求。 17.3獨立懸架 對于所有的獨立懸架它們都有兩個瞬時中心，來完成懸架特性的設計。側視圖的瞬時中心控制與前后加速度有關的力與運動，前視圖的瞬時中心控制與水平方向的加速度有關的力與運動。 前視圖中的等視擺臂幾何。 前視圖中控制臂瞬時中心的位置控制著側傾中心的高度，外傾角變化率，輪胎水平方向上的磨胎運動。瞬時中心可以在車輪的內側，也可以在車輪的外側。它可以在水平面以上或水平面以下。它確切的位置取決于設計者的要求。 側傾中心高度 側傾中心的高度是在前視圖中，由輪胎的接地點與瞬時中心的連線與汽車中心線的投影的交點測量而得的（見圖17.7（a）。通過在汽車的兩側作圖而得，這兩條線的交點便是車的簧載質量相對地面的轉動中心。這也并不一定是在汽車的中心線，尤其是對于非對稱的懸架幾何結構（見圖17.7（b）），或者是汽車在轉彎的時候。很顯然瞬時中心距離地面的高度，與輪胎的距離，在車輪的內側還是外側決定著側傾中心的位置。 現在你知道怎樣找到側傾中心，那它代表什么意思呢？ 在簧載質量與非簧載質量之間，由側傾中心建立了離心力的作用點。當一輛車轉彎時，作用于中心的離心力，被輪胎與地面的摩擦力所抵消。如果適當的力與力矩（有關側傾中心的）被顯示出來，作用于CG的水平力可以轉移到側傾中心上來。側傾中心越高圍繞側傾中心的側傾力矩就越小。側傾中心越低的話，側傾力矩就越大。你也會注意到，側傾中心越高的話，作用于側傾中心的水平力，也就力地面越高。這種水平作用力與它到地面的距離的乘積被認為是非側傾力矩。所以側傾中心的高度是權衡側傾力矩與非側傾力矩的相對影響的結果。（見18章關于這些影響的另一個解釋） 以上部分簡單而直接。然而在建立一個期望的側傾中心高度的時候有另外一個影響因素，那就是橫縱向的耦合效應。如果側傾中心高于水平面，來自于輪胎的橫向力形成了關于瞬時中心的力矩。這個力矩向下壓輪胎，向上抬升簧載質量，叫做千斤頂效應。（見圖17.8（a））。如果側傾中心低于水平面，這個力矩將會向下壓簧載質量。無論哪種情況，由于水平力的作用，將會使簧載質量收到垂直方向上的力的作用。在帶有非獨立懸架的老式車上很常見。另一個分析這種方案的方法見圖17.8（b）。這里作用與接觸點的所有力在瞬時中心這個作用點被分解成水平方向與垂直方向上的力，圖中所示的垂直力將會抬升簧載質量。 外傾角變化率 側傾中心與等效擺臂的長度與高度有關。外傾角的變化率僅與等效擺臂的長度有關（見圖17.9）當我們可以把懸架的橫臂簡化為一個擺動的桿時，這個輪胎外傾角的變化率就可以由這個式子求出了arctan(l/fvsa length) ，即車輪每運動一英寸對應的車輪外傾角變化。由圖2-3我們可知，短的臂長會造成大的外傾變化，長的臂長造成小的外傾變化。注：這個是不同于靜態(tài)車輪外傾設置和定位的。  附錄B Suspension Geometry Introduction Designing suspension systems for production or racing cars requires technical knowledge in several disciplines. This chapter will cover only one of those disciplines-the study of suspension kinematics or "geometry."This chapter does not cover the effects of compliance or deflections of structural components under load ; these effects are discussed in Chapter 23. When we talk about suspension geometry it means the broad subject of how the unsprung mass of a vehicle is connected to the sprung mass. These connections not only dictate the path of relative motion, they also control the forces that are transmitted between them. Any particular geometry must be designed to meet the needs of the particular vehicle for which it is to be applied. There is no single best geometry. 17. 1 Degrees of Freedom and Motion Path For an independent suspension, be it front or rear, the assemblage of control arms is intended to control the wheel motion relative to the car body in a single prescribed path. That path may have camber gain, caster change, and toe change as prescribed by the designer but it still follows only one path as it moves up and down. In engineering terms we could say that the wheel has a fixed path of motion relative to the car body. It is not allowed to move fore and aft laterally relative to this path. The knuckle is not allowed to rotate other than as determined by this fixed path (of course the wheel is allowed to roll around the spindle axis). The suspension linkages are expected to position the knuckle (wheel) very accurately in all directions while allowing it to move up and down against the spring and shock. In front suspension we do have a steer rotation degree of freedom but only when it is demanded from the steering system. For any body moving in space relative to another body. Its motion can be completely defined by three components of linear motion and three components of rotational motion (see Figure 17. 1). A single body is said to have six degrees of freedom of motion in a three-dimensional world. We said above that any independent suspension allows only one path of motion of the knuckle relative to the body. Another way to say the same thing is that the suspension provides five degrees of restraint (D. O. R.), i. e. It severely limits motion in five directions. In the real world, the mechanical components that supply the restraints are not "perfect” in the sense of restraining the motion to a particular degree of freedom. Therefore the study of independent suspension geometries is to determine how to restrain the knuckle to limited motion in live directions. If the only components you could use to design a suspension geometry were straight links with rod ends (spherical joints) on each end, the required restrains can be provided with five of them. In other words to obtain five degrees of restrains requires exactly five tension-compression links. To relate this concept to more familiar hardware, we need to understand how typical suspension components provide their restraining function. Looking at Figure 17.2 You can see that an A-arm is really equivalent to two straight links with their outer ends coming together at the ball joint. A Macpherson Strut is kinematically a "slider” mechanism which is equal to an A arm that is infinitely long at right angles to the slider travel. Now, with this in mind, we can look at most independent suspensions and come up with a count of five links in every case(see Figure 17. 3), The standard racing double wishbone suspension has two A-arms plus a tie rod Thus two links for each A-arm and one link for the tie rod adds up to five. A Macpherson Strut suspension has two for the strut, two for the lower A · arm and the tie rod makes live. There are some suspensions that are less obvious because they have fewer links, but what they are usually doing is introducing a bending requirement to achieve restraint of motion. An example of this is a semi trailing arm rear suspension. There is one arm that does the job of live links, but in order to do it, it must be strong in bending and torsion in the three directions of rotation. For solid axle (or beam type) rear (and occasionally front) axles, the two wheels are tied together, so motion of one affects the other (see Figure 17. 4). When two wheels are tied together, they have two different motions relative to the body ; they can go up and down together (parallel bump motion) or they can move in opposite directions one up and one down (roll motion). In kinematic terms the axle has two degrees of freedom of motion relative to the body. There is a total of six degrees of motion in space ; we must restrain four when we design a beam-type rear suspension. This can be accomplished with a linkage having just four tension-compression links. 17. 2 Instant Center Defined throughout the rest of this chapter the term instant center (IC) will be used in describing and determining several common suspension parameters, To help achieve clarity in these discussions some comments about what is an instant center, are in order. The word “instant" means at that particular position of the linkage. “Center” refers to a projected imaginary point that is effectively the pivot point of the linkage at that instant. Figure 17. 5 suggests how two short links can be replaced with one longer one. AB the linkage h moved, the center moves, so proper geometric design not only establishes all the instant centers in their desired position at ride height, but also controls how fast and in what direction they move wide suspension travel Instant centers come from the study of kinematics in two dimensions (in a plane). They are a convenient graphic aid in establishing motion relationships between two bodies. In suspension design it is convenient to break down this three-dimensional problem into two, two-dimensional problems. We talk about the front view and the side view geometry. What we are doing is cutting vertical planes (9oe to the ground) through the wheel center, one parallel to the centerline of the car, and the other at a right angle to the vehicle centerline. We then project all the suspension points onto these planes. when we connect a line between the ball joint and the control arm bushing and project it across e plane both for the upper and lower control arms they will usually intersect at some point This intersect is an instantaneous linkage center. If you do the projection in the front view the instant center defines the camber change rate, part of the roll center information scrub motion, and data needed to determine the steer characteristics. If you are working with the side view, the instant center will define the wheel fore and aft path, anti-lift and anti-dive/squat information, and caster change rate. As with any three-dimensional objects, three orthogonal views are possible : because the third view (top view) is approximately along the single (ride) degree of freedom it contains little useful information about the path of the wheel. Instant Axis In true three dimensional space, instant centers are replaced by instant axes. If we take the instant centers defined in the side view and the rear view and connect them together we get a line. This line can be thought of as the instant axis of motion of the knuckle relative to the body (see Figure 17. 6). Independent suspensions have one instant axis of motion because they have five restraints ; of course, this instant axis moves with changes in ride height. Rear axles have two instant axes, one for parallel bump and one for roll ;these also may move with changes in ride height So whenever we are studying a particular suspension system we need to establish the instant centers and/or the instant axes. The remainder of this chapter will be devoted to the determination of these axes for many common types of front and rear suspensions, with additional comments in regard to their adjustability to meet the needs of race cars. 17. 3 Independent Suspensions For all independent suspensions there are the two instant centers (which change with bump and droop) that establish the properties of that particular design. The side view instant center controls force and motion factors predominantly related to fore and aft accelerations, while the front view instant (or swing) center controls force and motion factors due to lateral accelerations. Front View Swing Arm Geometry The front view swing arm (fvsa) instant center location controls the roll center height (RCH), the camber change rate, and tire lateral scrub. The IC can be located inboard of the wheel or outboard of the wheel. It can be above ground level or below ground. The location is up lo the designers' performance requirements. Roll Center Height The roll center height is found by projecting a line from the center of the tire-ground contact patch through the front view instant center shown in Figure 17. 7 (a). This is repeated for each side of the car, where these two lines intersect is the roll center of the sprung mass of the car, relative to the ground. It is not necessarily at the centerline of the car, especially with asymmetric suspension geometry (Figure 17. 7 (b)) or once the car assumes a roll angle in a turn. It is obvious that the roll center location is controlled by the instant center heights above or below ground, the distance away from the tire that the instant center is placed, and whether the instant center is inboard or outboard of the tire contact patch. Now that you know how to find the roll center, what does it mean? The roll center establishes the force coupling point between the unsprung and sprung masses. When a car comers, the centrifugal force at the center of gravity is reacted by the tires. The lateral force at the CG can be translated to the roll center if the appropriate force and moment (about the roll center) are shown. The higher the roll center the smaller the roll moment about the toll center (which must be resisted by the springs) ; the lower the roll center tile larger the rolling moment. You will also notice that with higher roll centers the lateral force acting at the roll center is higher off the ground. This lateral force the distance to the ground can be called the nonrolling over moment. So roll center heights are trading off the relative effects of the rolling and nonrolling moments. (See Chapter 18 for another explanation of these effects.) The above part is simple and straight forward. There is however, another factor in establishing the desired roll center height, and this is the horizontal-vertical coupling effect. If 1he roll center is above ground level the lateral force from the tire generates a moment about the instant center (IC). This moment pushes the wheel down and lifts the sprung mass; it is called jacking (see Figure 17. 8(a)). If the roll center is below the ground level (possible with SLA suspension) then the force will push the sprung mass down. In either case the sprung mass will have a vertical deflection due to a lateral force! This is most apparent on older cars with swing axle rear suspensions such as the Formula Vee. An alternate way to analyze this situation is shown in Figure 17. 8 (b). Here the total force at the contact patch is drawn to its reaction point at the instant center and the lateral and vertical components are indicated ; the vertical component in the case shown will lift U)e sprung mass. Camber Change Rate While the roll center is a function of the fvsa length and height, the camber change rate is only a function of fvsa length (sec Figure 17. 9). If you replace the control arms of the suspension with a single link that ran from the knuckle to the instant center, the amount of camber change that was achieved per inch of ride travel would be camber change (deg./h) = arctan(l/fvsa length). Note : This is different from the static camber setting or alignment 19