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THE BRAKE BIBLE
Brakes - what do they do?
The simple answer: they slow you down.
The complex answer: brakes are designed to slow down your vehicle but probably not by the means that you think. The common misconception is that brakes squeeze against a drum or disc, and the pressure of the squeezing action is what slows you down. This in fact is only part of the equation. Brakes are essentially a mechanism to change energy types. When you're traveling at speed, your vehicle has kinetic energy. When you apply the brakes, the pads or shoes that press against the brake drum or rotor convert that energy into thermal energy via friction. The cooling of the brakes dissipates the heat and the vehicle slows down. It's the First Law of Thermodynamics, sometimes known as the law of conservation of energy. This states that energy cannot be created nor destroyed, it can only be converted from one form to another. In the case of brakes, it is converted from kinetic energy to thermal energy.
Angular force. Because of the configuration of the brake pads and rotor in a disc brake, the location of the point of contact where the friction is generated also provides a mechanical moment to resist the turning motion of the rotor.
Thermodynamics, brake fade and drilled rotors.
If you ride a motorbike or drive a race car, you're probably familiar with the term brake fade, used to describe what happens to brakes when they get too hot. A good example is coming down a mountain pass using your brakes rather than your engine to slow you down. As you start to come down the pass, the brakes on your vehicle heat up, slowing you down. But if you keep using them, the rotors or drums stay hot and get no chance to cool off. At some point they can't absorb any more heat so the brake pads heat up instead. In every brake pad there is the friction material that is held together with some sort of resin and once this starts to get too hot, the resin starts to vapourise, forming a gas.
Because the gas can't stay between the pad and the rotor, it forms a thin layer between the two whilst trying to escape. The pads lose contact with the rotor, reducing the amount of friction and voila. Complete brake fade.
The typical remedy for this would be to get the vehicle to a stop and wait for a few minutes. As the brake components cool down, their ability to absorb heat returns and the next time you use the brakes, they seem to work just fine. This type of brake fade was more common in older vehicles. Newer vehicles tend to have less out gassing from the brake pad compounds but they still suffer brake fade. So why? It's still to do with the pads getting too hot. With newer brake pad compounds, the pads transfer heat into the calipers once the rotors are too hot, and the brake fluid starts to boil forming bubbles in it. Because air is compressible (brake fluid isn't) when you step on the brakes, the air bubbles compress instead of the fluid transferring the motion to the brake calipers. Voila. Modern brake fade.
So how do the engineers design brakes to reduce or eliminate brake fade? For older vehicles, you give that vapourised gas somewhere to go. For newer vehicles, you find some way to cool the rotors off more effectively. Either way you end up with cross-drilled or grooved brake rotors. While grooving the surface may reduce the specific heat capacity of the rotor, its effect is negligible in the grand scheme of things. However, under heavy braking once everything is hot and the resin is vapourising, the grooves give the gas somewhere to go, so the pad can continue to contact the rotor, allowing you to stop.
The whole understanding of the conversion of energy is critical in understanding how and why brakes do what they do, and why they are designed the way they are. If you've ever watched Formula 1 racing, you'll see the front wheels have huge scoops inside the wheel pointing to the front (see the picture above). This is to duct air to the brake components to help them cool off because in F1 racing, the brakes are used viciously every few seconds and spend a lot of their time trying to stay hot. Without some form of cooling assistance, the brakes would be fine for the first few corners but then would fade and become near useless by half way around the track.
Rotor technology.
If a brake rotor was a single cast chunk of steel, it would have terrible heat dissipation properties and leave nowhere for the vapourised gas to go. Because of this, brake rotors are typically modified with all manner of extra design features to help them cool down as quickly as possible as well as dissapate any gas from between the pads and rotors. The diagram here shows some examples of rotor types with the various modification that can be done to them to help them create more friction, disperse more heat more quickly, and ventilate gas. From left to right.
1: Basic brake rotor.
2: Grooved rotor - the grooves give more bite and thus more friction as they pass between the brake pads They also allow gas to vent from between the pads and the rotor.
3: Grooved, drilled rotor - the drilled holes again give more bite, but also allow air currents (eddies) to blow through the brake disc to assist cooling and ventilating gas.
4: Dual ventilated rotors - same as before but now with two rotors instead of one, and with vanes in between them to generate a vortex which will cool the rotors even further whilst trying to actually 'suck' any gas away from the pads.
An important note about drilled rotors: Drilled rotors are typically only found (and to be used on) race cars. The drilling weakens the rotors and typically results in microfractures to the rotor. On race cars this isn't a problem - the brakes are changed after each race or weekend. But on a road car, this can eventually lead to brake rotor failure - not what you want. I only mention this because of a lot of performance suppliers will supply you with drilled rotors for street cars without mentioning this little fact.
Big rotors.
How does all this apply to bigger brake rotors - a common sports car upgrade? Sports cars and race bikes typically have much bigger discs or rotors than your average family car. A bigger rotor has more material in it so it can absorb more heat. More material also means a larger surface area for the pads to generate friction with, and better heat dissipation. Larger rotors also put the point of contact with the pads further away from the axle of rotation. This provides a larger mechanical advantage to resist the turning of the rotor itself. To best illustrate how this works, imagine a spinning steel disc on an axle in front of you. If you clamped your thumbs either side of the disc close to the middle, your thumbs would heat up very quickly and you'd need to push pretty hard to generate the friction required to slow the disc down. Now imagine doing the same thing but clamping your thumbs together close to the outer rim of the disc. The disc will stop spinning much more quickly and your thumbs won't get as hot. That, in a nutshell explains the whole principle behind why bigger rotors = better stopping power.
The different types of brake.
All brakes work by friction. Friction causes heat which is part of the kinetic energy conversion process. How they create friction is down to the various designs.
I thought I'd cover these because they're about the most basic type of functioning brake that you can see, watch working, and understand. The construction is very simple and out-in-the-open. A pair of rubber blocks are attached to a pair of calipers which are pivoted on the frame. When you pull the brake cable, the pads are pressed against the side or inner edge of the bicycle wheel rim. The rubber creates friction, which creates heat, which is the transfer of kinetic energy that slows you down. There's only really two types of bicycle brake - those on which each brake shoe shares the same pivot point, and those with two pivot points. If you can look at a bicycle brake and not understand what's going on, the rest of this page is going to cause you a bit of a headache.
Drum brakes - single leading edge
The next, more complicated type of brake is a drum brake. The concept here is simple. Two semicircular brake shoes sit inside a spinning drum which is attached to the wheel. When you apply the brakes, the shoes are expanded outwards to press against the inside of the drum. This creates friction, which creates heat, which transfers kinetic energy, which slows you down. The example below shows a simple model. The actuator in this case is the blue elliptical object. As that is twisted, it forces against the brake shoes and in turn forces them to expand outwards. The return spring is what pulls the shoes back away from the surface of the brake drum when the brakes are released. See the later section for more information on actuator types.
The "single leading edge" refers to the number of parts of the brake shoe which actually contact the spinning drum. Because the brake shoe pivots at one end, simple geometry means that the entire brake pad cannot contact the brake drum. The leading edge is the term given to the part of the brake pad which does contact the drum, and in the case of a single leading edge system, it's the part of the pad closest to the actuator. This diagram (right) shows what happens as the brakes are applied. The shoes are pressed outwards and the part of the brake pad which first contacts the drum is the leading edge. The action of the drum spinning actually helps to draw the brake pad outwards because of friction, which causes the brakes to "bite". The trailing edge of the brake shoe makes virtually no contact with the drum at all. This simple geometry explains why it's really difficult to stop a vehicle rolling backwards if it's equipped only with single leading edge drum brakes. As the drum spins backwards, the leading edge of the shoe becomes the trailing edge and thus doesn't bite.
Drum brakes - double leading edge
The drawbacks of the single leading edge style of drum brake can be eliminated by adding a second return spring and turning the pivot point into a second actuator. Now when the brakes are applied, the shoes are pressed outwards at two points. So each brake pad now has one leading and one trailing edge. Because there are two brake shoes, there are two brake pads, which means there are two leading edges. Hence the name double leading edge.
Disc brakes
Some background. Disc brakes were invented in 1902 and patented by Birmingham car maker Frederick William Lanchester. His original design had two discs which pressed against each other to generate friction and slow his car down. It wasn't until 1949 that disc brakes appeared on a production car though. The obscure American car builder Crosley made a vehicle called the Hotshot which used the more familiar brake rotor and calipers that we all know and love today. His original design was a bit crap though - the brakes lasted less than a year each. Finally in 1954 Citro?n launched the way-ahead-of-its-time DS which had the first modern incarnation of disc brakes along with other nifty stuff like self-levelling suspension, semi-automatic gearbox, active headlights and composite body panels. (all things which were re-introduced as "new" by car makers in the 90's).
Disc brakes are an order of magnitude better at stopping vehicles than drum brakes, which is why you'll find disc brakes on the front of almost every car and motorbike built today. Sportier vehicles with higher speeds need better brakes to slow them down, so you'll likely see disc brakes on the rear of those too.
制動(dòng)器
制動(dòng)器：它們的作用？
簡(jiǎn)單的說(shuō)：它會(huì)使你的汽車慢下來(lái)。
復(fù)雜的說(shuō)：制動(dòng)器被用來(lái)讓你的車減速，但可能不是你所想的意思。普遍的誤解是，制動(dòng)器擠壓制動(dòng)鼓或制動(dòng)片，擠壓的壓力的作用使你的車慢下來(lái)。但這只是制動(dòng)的一部分。制動(dòng)系統(tǒng)本質(zhì)上是改變能量的類型。當(dāng)你在全速行駛時(shí)，你的汽車獲得動(dòng)能。當(dāng)你踩下剎車，墊子或鞋子對(duì)制動(dòng)鼓和轉(zhuǎn)子的作用轉(zhuǎn)化為摩擦熱能。剎車的冷卻使車的熱能消散，減慢車速。這是熱力學(xué)第一定律，有時(shí)被視為能量守恒定律。也是就說(shuō)：能量不能被創(chuàng)造也不能被消滅，只能由一種形式轉(zhuǎn)換成另一種。制動(dòng)情況下，它是動(dòng)能轉(zhuǎn)化為熱能。
角向力。 因?yàn)樵诒P式制動(dòng)器的剎車片和轉(zhuǎn)子的位置，摩擦產(chǎn)生的接觸點(diǎn)的位置也產(chǎn)生了一個(gè)機(jī)械的抵御轉(zhuǎn)子的回轉(zhuǎn)運(yùn)動(dòng)。
熱力學(xué)，制動(dòng)失效，鉆孔轉(zhuǎn)子。
如果你騎摩托車或駕駛一輛賽車，你或許熟悉制動(dòng)失效，描述當(dāng)制動(dòng)器太熱，他發(fā)生了什么。一個(gè)很好的例子就是從山上下來(lái)使用剎車制動(dòng),而不是你的引擎使你減速。當(dāng)汽車開始滑動(dòng)下來(lái)時(shí)，剎車使汽車產(chǎn)生熱能,使你減速。但是如果你持續(xù)使用他們， 轉(zhuǎn)子或鼓留熱并沒有機(jī)會(huì)冷卻。從某種意義上說(shuō)他們不能吸收更多的熱量，使剎車墊熱了起來(lái)。在每一個(gè)墊子的摩擦材料有某種共同的樹脂一旦開始變得太熱，該樹脂開始蒸發(fā)，形成氣。由于氣體之間不能待在墊層及轉(zhuǎn)子，而是形成薄薄的一層在兩個(gè)之間準(zhǔn)備排走。墊失去與轉(zhuǎn)子的接觸，減少摩擦和熱量。這是完全的制動(dòng)失效。
典型的補(bǔ)救辦法，將車停了下來(lái)，等待幾分鐘。由于制動(dòng)部件降溫，吸收熱量的原因，下一次您使用剎車的能力，似乎會(huì)好一點(diǎn)。這種類型的制動(dòng)失效在舊車輛更常見。新的車輛往往從剎車墊中減少排氣，但他們?nèi)杂兄苿?dòng)失效。為什么呢？它仍然因?yàn)閯x車墊太熱。猶由于新的剎車墊合成，襯墊的熱傳遞到卡鉗一旦轉(zhuǎn)子太熱了，制動(dòng)液開始沸騰冒泡。因?yàn)榭諝馐强蓧嚎s的(制動(dòng)液不是)當(dāng)你踩剎車，氣泡的壓縮代替了流體轉(zhuǎn)移到制動(dòng)卡鉗。這就是現(xiàn)代制動(dòng)失效。
工程師們是怎樣設(shè)計(jì)減少或消除剎車制動(dòng)失效的? 年長(zhǎng)的車輛，是使氣化的氣體有地方排掉。新的車輛，找到一些方式來(lái)冷卻轉(zhuǎn)子更為有效。無(wú)論如何你最終獲得交叉鉆孔或溝槽剎車盤。當(dāng)槽表面是可以減少比熱容量的轉(zhuǎn)子，其效果可以忽略不計(jì)的。然而當(dāng)大力剎車時(shí)一旦一切都是熱和樹脂材料蒸發(fā)，槽讓氣體排去， 所以墊可以繼續(xù)接觸轉(zhuǎn)子，讓車減速停下來(lái)。
整個(gè)的理解能量轉(zhuǎn)換的關(guān)鍵是，剎車他們?cè)撟鍪裁?以及為什么它們?cè)O(shè)計(jì)成這樣。如果你曾看過(guò)一級(jí)方程式賽車，你就可以看到向前的前輪里面有很大的洞（如上圖所示）。這是管道空氣剎車部件，以幫助他們冷卻下來(lái)，因?yàn)樵贔1賽車中，剎車每隔幾秒鐘頻繁使用，花很多時(shí)間預(yù)留熱量。如果沒有某種冷卻協(xié)助，剎車就可能在最開始的幾個(gè)轉(zhuǎn)角失靈，最后剎車失效賽車在一半路程出局。
轉(zhuǎn)子技術(shù)。
如果制動(dòng)轉(zhuǎn)子是一個(gè)單一的鋼鐵鑄塊，這將有嚴(yán)重的散熱性能和氣化氣無(wú)法排去。因此，剎車盤通常使用各種額外的設(shè)計(jì)特點(diǎn)的方式來(lái)改進(jìn)幫助他們冷卻下來(lái)，盡快使墊和轉(zhuǎn)子之間的任何氣體排走。 這里的圖表顯示了轉(zhuǎn)子類型的各種修改，可以改進(jìn)幫助他們創(chuàng)造更多的摩擦力，更迅速地驅(qū)散更多的熱量，通風(fēng)氣體的一些例子。 從左至右。
1：基本制動(dòng)轉(zhuǎn)子。2：溝槽轉(zhuǎn)子-溝槽給予更多口，他們之間產(chǎn)生更多的摩擦，還允許氣體從墊和轉(zhuǎn)子之間的排走。3：溝槽鉆孔轉(zhuǎn)子-再給多一點(diǎn)口，但也讓氣流(渦旋)通過(guò)制動(dòng)盤協(xié)助冷卻和通風(fēng)。4：雙通風(fēng)轉(zhuǎn)子-以前一樣，然而現(xiàn)在有了兩個(gè)轉(zhuǎn)子而不是一個(gè)，和他們之間葉片產(chǎn)生渦流將進(jìn)一步冷卻轉(zhuǎn)子同時(shí)試圖實(shí)際上從襯墊中排掉任何氣體。
重要的一點(diǎn)：鉆孔轉(zhuǎn)子通常只使用于賽車。鉆孔使得轉(zhuǎn)子變?nèi)?，通常?huì)導(dǎo)致轉(zhuǎn)子產(chǎn)生各類裂縫。在賽車中這不是一個(gè)問題——在每場(chǎng)比賽或者每周都會(huì)更換剎車盤。但在路上的車，最終會(huì)導(dǎo)致剎車轉(zhuǎn)子失靈的，不是你能想象的。我只提這件事，因?yàn)橛性S多供應(yīng)商將為您提供鉆孔轉(zhuǎn)子，沒有直接提到這個(gè)事實(shí)。
這是如何適用于更大的剎車轉(zhuǎn)子-一種普遍的跑車升級(jí)？汽車和自行車運(yùn)動(dòng)比賽通常有比一般的家庭汽車更大的盤或轉(zhuǎn)子。一個(gè)更大的轉(zhuǎn)子有更多的材料在里面，因此它可以吸收更多的熱量。更多的物質(zhì)也意味著更大的表面積，墊片產(chǎn)生摩擦,和更好的散熱。較大的角度也將轉(zhuǎn)子接觸墊進(jìn)一步遠(yuǎn)離軸旋轉(zhuǎn)。這提供了一個(gè)更大的機(jī)械優(yōu)勢(shì)抵抗旋轉(zhuǎn)的轉(zhuǎn)子本身。這個(gè)工作最好的說(shuō)明，設(shè)想一種紡紗鋼軸上的閥瓣在你的面前。如果你夾緊你的大拇指任何一方的閥瓣靠近中間，你的大拇指將熱得非?？?，你會(huì)需要推動(dòng)相當(dāng)大的摩擦力使閥瓣慢下來(lái)?，F(xiàn)在想象做同樣的事情，但是你的大拇指夾在一起接近外緣的閥瓣。閥瓣將停止旋轉(zhuǎn)得特別快，你的大拇指也不會(huì)很熱。簡(jiǎn)單地說(shuō)解釋整個(gè)原理就是更大轉(zhuǎn)子=更好的制動(dòng)原則。
不同類型的制動(dòng)器。
所有制動(dòng)器都產(chǎn)生摩擦力。摩擦力是熱的一部分動(dòng)能轉(zhuǎn)換過(guò)程。他們是如何不同的設(shè)計(jì)產(chǎn)生了摩擦的。
我想我覆蓋這些，因?yàn)樗鼈兪亲罨绢愋偷闹苿?dòng)方式，你可以看到，看工作了解。設(shè)計(jì)非常簡(jiǎn)單，在外部。一雙橡膠塊連接到一雙卡鉗，能在機(jī)架上旋轉(zhuǎn)。當(dāng)你拉剎車線，剎車墊壓向一側(cè)或自行車輪輞的內(nèi)側(cè)邊緣。 橡膠產(chǎn)生摩擦，產(chǎn)生熱量，這是動(dòng)能轉(zhuǎn)移使車慢下來(lái)。 自行車制動(dòng)實(shí)際上只有兩個(gè)類型 - 自行車剎車制動(dòng)蹄上有相同的摩擦點(diǎn)，并有兩個(gè)摩擦點(diǎn)。 如果你可以看了自行車制動(dòng)，不明白發(fā)生了什么事情，本頁(yè)面的其余部分你理解起來(lái)有麻煩了。
鼓式制動(dòng)器-單前沿
下一個(gè)，更加復(fù)雜的類型的制動(dòng)是鼓式制動(dòng)器。這是簡(jiǎn)單的概念。兩個(gè)半圓形的剎車片裝在里面連接一個(gè)旋轉(zhuǎn)的車輪的鼓。當(dāng)你踩下剎車,剎車片向外擴(kuò)大擠壓內(nèi)側(cè)的鼓。這造成了摩擦，產(chǎn)生熱量，轉(zhuǎn)移動(dòng)能，這將使車減速。下面的例子顯示了一個(gè)簡(jiǎn)單的模型。制動(dòng)器在這種情況下是藍(lán)色橢圓形的對(duì)象。因?yàn)檫@是扭曲的，它的力使剎車片迫使他們向外擴(kuò)張。當(dāng)松開剎車，回位彈簧從制動(dòng)鼓的表面拉回剎車片?？吹秸鹿?jié)后面更多信息。
"單前沿"是指實(shí)際接觸的旋轉(zhuǎn)鼓輪制動(dòng)蹄部件的數(shù)量。因?yàn)橹苿?dòng)蹄片在一端，簡(jiǎn)單的幾何意味著整個(gè)剎車片無(wú)法都接觸到制動(dòng)鼓。單前沿就是部分剎車片的術(shù)語(yǔ)，那些接觸制動(dòng)鼓，在單一制動(dòng)情況下的方法，在最接近制動(dòng)器的襯墊。此圖 （右側(cè)） 顯示當(dāng)剎車時(shí)，會(huì)發(fā)生什么情況。這剎車片向外壓和制動(dòng)襯墊的最初接觸制動(dòng)鼓的部分剎車片就是前沿。制動(dòng)鼓旋轉(zhuǎn)實(shí)際上有助于制動(dòng)片向外加壓，因?yàn)閯x車片向口子的摩擦力。后沿的制動(dòng)蹄片與制動(dòng)鼓幾乎沒有接觸。這個(gè)簡(jiǎn)單的幾何解釋了，為什么汽車是很難停止向后滾動(dòng)，如果它只配單前緣沿鼓式制動(dòng)器。由于制動(dòng)鼓向后旋轉(zhuǎn)，前沿的剎車片成為了后沿，因?yàn)橹苿?dòng)不會(huì)咬合。
鼓剎車-雙前沿
可以通過(guò)添加回位彈簧和旋轉(zhuǎn)第二個(gè)制動(dòng)器中心點(diǎn)來(lái)消除鼓式制動(dòng)器的單個(gè)前沿的缺點(diǎn)。踩下剎車時(shí)，剎車片在兩個(gè)點(diǎn)向外壓。所以每個(gè)剎車片現(xiàn)在有一個(gè)前沿的和一個(gè)后沿。因?yàn)橛袃蓚€(gè)剎車蹄，那里有兩個(gè)剎車片，這意味著有兩個(gè)邊沿。因此名稱雙前沿。
盤式制動(dòng)器一些背景。
盤式制動(dòng)器在 1902 年被發(fā)明，伯明翰汽車制造商檢基威廉 · 蘭徹斯特的專利。他原先的設(shè)計(jì)了兩個(gè)光盤，緊貼彼此產(chǎn)生摩擦來(lái)使車減速。直到 1949 盤式制動(dòng)器的量產(chǎn)車上使用。在美國(guó)汽車創(chuàng)始人克羅斯利發(fā)明了我們目前熟知和喜愛的快車，就是使用了很多類似的盤動(dòng)制動(dòng)器和卡鉗。他原先的設(shè)計(jì)雖然有點(diǎn)缺陷-制動(dòng)器持續(xù)不到一年。終于在 1954 年雪鐵龍推出先進(jìn)的DS，成就了像自流平懸浮、 半自動(dòng)變速箱、 活動(dòng)前燈和復(fù)合車身盤式制動(dòng)器的第一次現(xiàn)代化身。（所有事情，在 90 年代的汽車制造商都重新作為"新型"）。
盤式制動(dòng)器比鼓式制動(dòng)器好了一個(gè)數(shù)量級(jí)來(lái)使車輛制動(dòng)，這就是為什么你會(huì)發(fā)現(xiàn)的現(xiàn)代幾乎所以汽車和摩托車都使用的是盤式制動(dòng)器。運(yùn)動(dòng)型車輛具有更高的速度需要更好的制動(dòng)減速，所以您會(huì)明白盤式制動(dòng)器在這些車上的使用。