轎車(chē)懸架系統(tǒng)設(shè)計(jì)【麥弗遜懸架】
轎車(chē)懸架系統(tǒng)設(shè)計(jì)【麥弗遜懸架】,麥弗遜懸架,轎車(chē)懸架系統(tǒng)設(shè)計(jì)【麥弗遜懸架】,轎車(chē),懸架,系統(tǒng),設(shè)計(jì),麥弗遜
附錄A
懸架幾何
引言
無(wú)論設(shè)計(jì)乘用車(chē)還是賽車(chē)的懸架系統(tǒng)都需要各種學(xué)科的知識(shí)。這一章只涉及到其中的一方面，懸架幾何的學(xué)習(xí)，沒(méi)有設(shè)涉及到懸架元件在載荷變化下，產(chǎn)生位移與變形所產(chǎn)生的影響。這些影響將在23章討論。
當(dāng)我們談?wù)搼壹軒缀螘r(shí)，它表示的是怎樣把整車(chē)的非簧載質(zhì)量與簧載質(zhì)量聯(lián)系起來(lái)。這些聯(lián)系不僅決定著它們之間的相對(duì)運(yùn)動(dòng)也影響著它們之間力的傳遞。
每種懸架幾何結(jié)構(gòu)的設(shè)計(jì)必須滿足你對(duì)應(yīng)的車(chē)型的要求。所以不會(huì)存在唯一的最好的懸架幾何結(jié)構(gòu)。
17.1自由度與運(yùn)動(dòng)路徑
對(duì)于獨(dú)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(chē)輪相對(duì)車(chē)身的運(yùn)動(dòng)在一個(gè)理想的路徑。由于設(shè)計(jì)者的設(shè)計(jì)，外傾角，內(nèi)傾角，前束角在這個(gè)路徑中會(huì)產(chǎn)生各種變化，但是當(dāng)上下運(yùn)動(dòng)時(shí)，懸架運(yùn)動(dòng)仍然遵循這個(gè)路徑。換句話說(shuō)，車(chē)輪相對(duì)車(chē)架有一個(gè)固定的運(yùn)動(dòng)路徑。相對(duì)這個(gè)路徑不會(huì)發(fā)生前后或水平方向的改變。轉(zhuǎn)向節(jié)不會(huì)隨意轉(zhuǎn)動(dòng)，而是由這個(gè)路徑?jīng)Q定.懸架在彈簧減震器的作用下上下運(yùn)動(dòng)時(shí)，通過(guò)懸架橫臂的相互連接來(lái)精確控制，轉(zhuǎn)向節(jié)在各個(gè)方向上的位置。在前懸架中，只有在人為的控制轉(zhuǎn)向器的是時(shí)候，才有一定轉(zhuǎn)向的自由度。
在空間上對(duì)于任何一物體相對(duì)于另一物體的運(yùn)動(dòng)而言，它的相對(duì)運(yùn)動(dòng)可分為三個(gè)位移運(yùn)動(dòng)的組成合三個(gè)轉(zhuǎn)動(dòng)運(yùn)動(dòng)的組成（見(jiàn)圖17.1）。在三維的空間里一個(gè)物體擁有6個(gè)自由度。我們以上所說(shuō)的任何一種獨(dú)立懸架上的轉(zhuǎn)向節(jié)相對(duì)于車(chē)架只有一種運(yùn)動(dòng)路徑，換句話說(shuō)，限制了懸架的五個(gè)方向上的自由度。實(shí)際上，機(jī)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(dú)立懸架幾何的學(xué)習(xí)是來(lái)控制怎樣約束轉(zhuǎn)向節(jié)來(lái)限制其他五個(gè)方向上的自由度。
在設(shè)計(jì)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(gè)自由度當(dāng)中的一個(gè)約束，換句話說(shuō)，提供五個(gè)自由度的約束需要五個(gè)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(guò)圖17.2我們可以看到，一個(gè)A臂是由兩個(gè)帶有桿端的球鉸副的直桿組成。一個(gè)麥弗遜的支柱是以是一個(gè)滑動(dòng)運(yùn)動(dòng)機(jī)構(gòu)，在一定的角度滑動(dòng)行程內(nèi)，它等同于一個(gè)A臂。
現(xiàn)在，帶著這種思想，我們來(lái)看一下大多數(shù)的獨(dú)立懸架，想出五個(gè)桿件的連接的各種方案。（見(jiàn)圖17.3）。
標(biāo)準(zhǔn)的賽車(chē)的雙橫臂懸架有兩個(gè)A臂，再加上一個(gè)橫拉桿。如此每個(gè)A臂有兩個(gè)桿，一個(gè)橫拉桿總共5個(gè)。一個(gè)麥弗遜的滑柱看做有兩個(gè)桿，下橫臂有兩個(gè)，加上橫拉桿也是五個(gè)。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(dá)到對(duì)運(yùn)動(dòng)的約束。其中一個(gè)例子是拖曳臂式后懸架。只有一個(gè)橫臂起到五個(gè)桿的作用，為了達(dá)到要求，必須足夠結(jié)實(shí)以承受三個(gè)轉(zhuǎn)動(dòng)方向上的轉(zhuǎn)矩與扭矩。
對(duì)于非獨(dú)立懸架的后軸來(lái)說(shuō)，兩個(gè)車(chē)輪聯(lián)系在一起，所以其中一個(gè)車(chē)輪運(yùn)動(dòng)會(huì)影響到另一個(gè)（見(jiàn)圖17.4）。
當(dāng)兩個(gè)車(chē)輪聯(lián)系在一起，它們相對(duì)于車(chē)身有兩種不同的運(yùn)動(dòng)，它們可以一起上下運(yùn)動(dòng)，也可以反方向一個(gè)向下一個(gè)向上運(yùn)動(dòng)。在運(yùn)動(dòng)過(guò)程中，這個(gè)軸相對(duì)車(chē)身有兩個(gè)自由度。在空間中，總共有六個(gè)自由度，當(dāng)我們?cè)O(shè)計(jì)非獨(dú)立懸架時(shí)，我們需要限制其中的四個(gè)?？梢酝ㄟ^(guò)一個(gè)梁代表四個(gè)受力的桿來(lái)實(shí)現(xiàn)。
17.2 瞬時(shí)中心的定義
在接下來(lái)的這一章里，瞬時(shí)中心將用來(lái)描述和決定一些懸架的基本參數(shù)。為了清楚地明白這些討論，關(guān)于瞬時(shí)中心的定義將按順序來(lái)說(shuō)明?！八矔r(shí)”的意思是桿的連接在那一確切的位置?！爸行摹贝淼氖羌傧氲模瑮U的連接處的瞬時(shí)轉(zhuǎn)動(dòng)點(diǎn)的有效投影點(diǎn)。圖17.5表明了怎樣一個(gè)長(zhǎng)桿代替兩個(gè)短桿。隨著桿的連接時(shí)移動(dòng)的，瞬時(shí)中心也是動(dòng)的，所以合適的幾何設(shè)計(jì)不僅建立在所有的瞬時(shí)中心隨離地間隙的變化出現(xiàn)在它們期望的位置，也要隨懸架的行程的變化，控制瞬時(shí)中心位置的變化與變化的快慢。
瞬時(shí)中心來(lái)源于在二維平面內(nèi)的動(dòng)態(tài)的學(xué)習(xí)。這樣形象的表達(dá)出了兩個(gè)物體之間的運(yùn)動(dòng)關(guān)系。在懸架設(shè)計(jì)中，將三維問(wèn)題轉(zhuǎn)化為二位問(wèn)題可以變得很方便。這樣我們討論前視圖和側(cè)視圖。我們做出經(jīng)過(guò)車(chē)輪中心的鉛垂面，一個(gè)平行于汽車(chē)的中心線，另一個(gè)垂直于汽車(chē)中心線。然后我們把懸架的關(guān)鍵點(diǎn)投影到這兩個(gè)平面上。
當(dāng)我們用一條線連接球鉸接點(diǎn)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(diǎn)相交，這個(gè)交點(diǎn)便是桿的瞬時(shí)連接點(diǎn)。如果在前視圖里做投影，得到的瞬時(shí)中心影響著外傾角的變化率、側(cè)傾中心的某些信息、磨胎運(yùn)動(dòng)和一些決定著轉(zhuǎn)向特性的某些數(shù)據(jù)。如果你在側(cè)視圖里作投影，得到的瞬時(shí)中心，將影響著車(chē)輪的運(yùn)動(dòng)路徑，抗俯仰特性，主銷(xiāo)后傾角的變化率。在三維空間里，三個(gè)正交視圖中，俯視圖得到的有關(guān)輪胎路徑變化的信息是最少的。
瞬時(shí)軸線
在真正的三維空間里瞬時(shí)中心被瞬時(shí)軸線所代替。如果我們把前視圖和后視圖的瞬時(shí)中心相連，便得到一條線。這條線可以看做相對(duì)車(chē)身的瞬時(shí)轉(zhuǎn)動(dòng)軸線（見(jiàn)圖17.6）。
獨(dú)立懸架有一個(gè)運(yùn)動(dòng)的瞬時(shí)軸線，這是因?yàn)樗鼈冇形鍌€(gè)約束：當(dāng)然，這條瞬時(shí)軸線隨離地間隙的變化而變化。后軸有兩個(gè)瞬時(shí)軸心，一個(gè)對(duì)應(yīng)懸架的上下跳動(dòng)，一個(gè)對(duì)應(yīng)側(cè)傾運(yùn)動(dòng)。它們也隨著離地間隙的變化而運(yùn)動(dòng)。所以無(wú)論何時(shí)我們學(xué)習(xí)一個(gè)懸架系統(tǒng)，都需要完成它的瞬時(shí)中心和瞬時(shí)軸先。這章的余下部分將涉及到常見(jiàn)的前后懸架類(lèi)型的這些軸線的決定因素，此外也設(shè)計(jì)到它們的一些調(diào)整，來(lái)滿足賽車(chē)的需求。
17.3獨(dú)立懸架
對(duì)于所有的獨(dú)立懸架它們都有兩個(gè)瞬時(shí)中心，來(lái)完成懸架特性的設(shè)計(jì)。側(cè)視圖的瞬時(shí)中心控制與前后加速度有關(guān)的力與運(yùn)動(dòng)，前視圖的瞬時(shí)中心控制與水平方向的加速度有關(guān)的力與運(yùn)動(dòng)。
前視圖中的等視擺臂幾何。
前視圖中控制臂瞬時(shí)中心的位置控制著側(cè)傾中心的高度，外傾角變化率，輪胎水平方向上的磨胎運(yùn)動(dòng)。瞬時(shí)中心可以在車(chē)輪的內(nèi)側(cè)，也可以在車(chē)輪的外側(cè)。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(shè)計(jì)者的要求。
側(cè)傾中心高度
側(cè)傾中心的高度是在前視圖中，由輪胎的接地點(diǎn)與瞬時(shí)中心的連線與汽車(chē)中心線的投影的交點(diǎn)測(cè)量而得的（見(jiàn)圖17.7（a）。通過(guò)在汽車(chē)的兩側(cè)作圖而得，這兩條線的交點(diǎn)便是車(chē)的簧載質(zhì)量相對(duì)地面的轉(zhuǎn)動(dòng)中心。這也并不一定是在汽車(chē)的中心線，尤其是對(duì)于非對(duì)稱的懸架幾何結(jié)構(gòu)（見(jiàn)圖17.7（b）），或者是汽車(chē)在轉(zhuǎn)彎的時(shí)候。很顯然瞬時(shí)中心距離地面的高度，與輪胎的距離，在車(chē)輪的內(nèi)側(cè)還是外側(cè)決定著側(cè)傾中心的位置。
現(xiàn)在你知道怎樣找到側(cè)傾中心，那它代表什么意思呢？
在簧載質(zhì)量與非簧載質(zhì)量之間，由側(cè)傾中心建立了離心力的作用點(diǎn)。當(dāng)一輛車(chē)轉(zhuǎn)彎時(shí)，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(dāng)?shù)牧εc力矩（有關(guān)側(cè)傾中心的）被顯示出來(lái)，作用于CG的水平力可以轉(zhuǎn)移到側(cè)傾中心上來(lái)。側(cè)傾中心越高圍繞側(cè)傾中心的側(cè)傾力矩就越小。側(cè)傾中心越低的話，側(cè)傾力矩就越大。你也會(huì)注意到，側(cè)傾中心越高的話，作用于側(cè)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(rèn)為是非側(cè)傾力矩。所以側(cè)傾中心的高度是權(quán)衡側(cè)傾力矩與非側(cè)傾力矩的相對(duì)影響的結(jié)果。（見(jiàn)18章關(guān)于這些影響的另一個(gè)解釋）
以上部分簡(jiǎn)單而直接。然而在建立一個(gè)期望的側(cè)傾中心高度的時(shí)候有另外一個(gè)影響因素，那就是橫縱向的耦合效應(yīng)。如果側(cè)傾中心高于水平面，來(lái)自于輪胎的橫向力形成了關(guān)于瞬時(shí)中心的力矩。這個(gè)力矩向下壓輪胎，向上抬升簧載質(zhì)量，叫做千斤頂效應(yīng)。（見(jiàn)圖17.8（a））。如果側(cè)傾中心低于水平面，這個(gè)力矩將會(huì)向下壓簧載質(zhì)量。無(wú)論哪種情況，由于水平力的作用，將會(huì)使簧載質(zhì)量收到垂直方向上的力的作用。在帶有非獨(dú)立懸架的老式車(chē)上很常見(jiàn)。另一個(gè)分析這種方案的方法見(jiàn)圖17.8（b）。這里作用與接觸點(diǎn)的所有力在瞬時(shí)中心這個(gè)作用點(diǎn)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(huì)抬升簧載質(zhì)量。
外傾角變化率
側(cè)傾中心與等效擺臂的長(zhǎng)度與高度有關(guān)。外傾角的變化率僅與等效擺臂的長(zhǎng)度有關(guān)（見(jiàn)圖17.9）當(dāng)我們可以把懸架的橫臂簡(jiǎn)化為一個(gè)擺動(dòng)的桿時(shí)，這個(gè)輪胎外傾角的變化率就可以由這個(gè)式子求出了arctan(l/fvsa length) ，即車(chē)輪每運(yùn)動(dòng)一英寸對(duì)應(yīng)的車(chē)輪外傾角變化。由圖2-3我們可知，短的臂長(zhǎng)會(huì)造成大的外傾變化，長(zhǎng)的臂長(zhǎng)造成小的外傾變化。注：這個(gè)是不同于靜態(tài)車(chē)輪外傾設(shè)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
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