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What does a clutch in a manual car do and how it works?

  1. Jul 24, 2011 #1
    Hello everyone,

    I don't know if this is the right section to post this but I want to know what does the clutch in a manual car do. I have absolutely zero knowledge on car mechanics and would be greatful if someone can explain this to me simply.

    1. Why do you need a clutch?
    I don't understand what they are saying in this. Also why auto cars don't kill the engine, they have no clutch.

    2.A clutch is that part of engine which engages or disengages power from the engine crankshaft to transmission?
    I don't understand these technical terms what is crankshaft and what is transmission? Is there any ebook or video or something for me to learn these basic terms.

    3. What is a clutch plate?

    I found this good pic, but I don't understand it much. So you have flywheel attached to engine, then clutch plate and pressure plate. Where is the wheel in this pic is it attached to drive shaft. Are there 4 clutch plates for a car with 4 wheels?

    4. Basically how a clutch works?

    *I don't really want to know too much on mechanical side but if I want to drive a manual car, the basic knowledge I must have on clutch. Excuse my poor knowledge.

    Thanks :smile:
  2. jcsd
  3. Jul 24, 2011 #2


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    hello sameeralord! :smile:

    a clutch works much like a disc brake …

    a clutch has two discs (or cones), which the clutch pedal (and clutch lever) can move apart or together

    when you start the car, the disc closest to the wheels is stationary, and the disc closest to the engine is going round like mad

    if you used cogwheels instead of discs, the jerk would be so great that it would damage the engine (and probably stall it)

    so you have to avoid a jerk, and you do so by gradually moving the discs together so that the friction gradually and smoothly accelerates the stationary disc to match the rotational speed of the engine

    (same for changing between gears … the two discs are going at different non-zero speeds, and you need to change one smoothly to match the other)

    when you gradually :wink: raise the clutch pedal, you are gradually moving the two discs together

    the clutch pedal is attached to the clutch lever shown in your diagram … that arrow is your foot, and it moves the right-hand disc away from the left-hand disc​
    the important word is "smooth" … if you used cogs, or if you used discs but couldn't put them together gradually, you'd damage the engine

    (and automatic cars have an automatic clutch … a clever bit of machinery does the same thing that raising the clutch pedal does)
    the crankshaft goes from the engine to one clutch disc, the transmission goes from the other clutch disc to the wheels

    one of the discs (i forget which one) is called the clutch plate

    1 engine, 1 clutch plate :wink:
  4. Jul 24, 2011 #3

    D H

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    You missed a question, call it question zero, the answer to which is essential to understanding your other questions.

    0. Why do you need gears?

    Both manual and automatic transmissions have multiple gearing ratios, the ratio of the rate at which the crankshaft is rotating to the rate at which the driveshaft is rotating. A car needs multiple gears because the physics of internal combustion engines combined with speeds at which a car must operate precludes the use of a single gearing ratio. Operate a car well below its peak torque RPM and the engine is likely to stall when you press on the gas pedal instead of making the car accelerate. Operate a car well above its peak power RPM and the engine is likely to blow up.

    1. Why do you need a clutch?

    Because you need to shift gears. The mechanism by which engine rotation is transferred from the crankshaft to the driveshaft in a manual transmission is solid-to-solid. You need to disengage that mechanism to switch out of and into gear. That you need to be able to have the engine running while the car is standing still is secondary to this primary purpose. This latter goal could be achieved by switching to neutral, something drivers typically don't do (and definitely don't want to be forced to do). Besides, you still need a clutch to shift into neutral and to switch gears. Forcing a manual transmission car out of gear without using the clutch is a very bad idea. Trying to put the car into a different gear without using the clutch is an even worse idea.

    An automatic transmission doesn't need a clutch because the hydraulic mechanisms used in an automatic to transfer crankshaft rotation to the driveshaft lose energy. A car with an automatic can stand still with the engine running thanks to this energy loss. Note that while a car with an automatic transmission doesn't need a clutch, it still has a clutch because without some kind of locking mechanism that energy loss would constantly sap the engine of some of its power.

    I found this good pic, but I don't understand it much.
    You found a lousy picture. There are plenty of good ones on the 'net.
  5. Jul 24, 2011 #4
    Thanks Tiny Tim and DH for their replies :smile: Well I'm trying to understand the replies but finding it bit difficult I think it is because I have no basic knowledge on the car. I'll take this step by step. Ok now the engine, crankshaft and clutch plate are together and at the front of the car. Then via transmission this is transmitted to the wheels, how is this transmitted to the wheels which are located much behind the engine. I think I'm struggling to understand where the clutch is located. I know where the brake pad is, is it located in the wheel like that or is it connected to the engine? So is there only 1 clutch plate for one car and is it located in the engine? Does the clutch give power to all 4 wheels or rear wheels only? In my pic does the arrow point to the rear of the car. Any diagrams or links would be great, I'm searching on net but finding it difficult to understand. Thanks!!
    Last edited: Jul 24, 2011
  6. Jul 24, 2011 #5
    [PLAIN]http://www.cazaautoparts.ca/images/guides/drivetrain.jpg [Broken]
    Last edited by a moderator: May 5, 2017
  7. Jul 24, 2011 #6


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    Click on item 2 and it shows all parts of a clutch.

    A rear wheel drive: looking under the car you can see the bell housing right after the engine - if you can see the starter you are in the correct location.. Four wheel drive and front wheel drive - well everything just looks more complicated, and is.

    rear wheel drive:
    The "clutch plate" is between the engine flywheel and the pressure plate. By using friction, ( ie when you release the clutch as they say when driving) the clutch smoothly (let's hope whith practice) transfer rotating motion from the engine and flywheel to the non rotating pressure plate, which is attached to the transmission, driveshaft, the differential, rear axle, and rear wheels. When you engage/disengage the clutch, you are moving the pressure plate back and forth, and compressing the clutch plate between the pressure plate and flywheel. With a fully dis-engaged clutch, the clutch plate has no contact with the flywheel or pressure plate and there is no power transfer from the engine to the power train( transmission, etc ). Fully engaged, all three move in unison at the same rotaional speed and you can be in 1st gear, 2nd, 3rd etc or reverse, and driving down the road.
  8. Jul 24, 2011 #7
    Thanks cdotter, that was the exact picture I needed. I didn't know which key words to put in search to get that. Thanks a lot I think I'm understanding this now :smile:

    @256 bits: Hey thanks for the reply :smile: I'm now understanding better due to the pic. Ok I have few questions. So if the clutch was not there, engine would always be connected to transmission and the wheels would be spinning all the time, so that means to stop you must kill the engine, this is not good for the engine so you need something like a clutch to engage and disengage wheels from engine. Is this why you need a clutch did I get it right? Other question since I'm really very ignorant with cars, I'm assuming that normal cars have a rear wheel drive engine, does that mean the front 2 wheels are working using the momentum of the rear 2 wheels? Thanks :smile:
  9. Jul 24, 2011 #8


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    correct an correct.
    You are now a car guru! :)
  10. Jul 24, 2011 #9
    Yes. You also need the clutch to shift gears unless you're very good at driving manual transmissions or you don't mind destroying your transmission in an afternoon.
  11. Jul 25, 2011 #10
    Thanks guys now I can move to the next question I have. So what are gears? What is 1st gear, 2nd gear ect. Do they change the rotation speed of the fly wheel? Does gear 1 rotate it faster or something. I can check how stuff works, but they just complicate stuff for me, you guys are better. Thanks :smile:
  12. Jul 25, 2011 #11

    D H

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    Think of a multi-speed bicycle that uses derailleur gears. Your legs rotate the crankset at one rate while the tires rotate at a different rate thanks to the gears. The ratio of the rate at which your legs are rotating to the rate at which the tires are rotating is the gearing ratio. You switch gears by moving the chain from one sprocket to another.

    The reason you need to switch gears is simple: Your legs have a rather narrow dynamic range over which they can operate. Example: You need a significant mechanical advantage to climb a hill with a bicycle. Once you get past the peak, if you don't change gears your speed will be limited to a rather low value because the rate at which you can efficiently rotate the crankset is limited.

    The same concept applies to a car. We want our cars to be able to go slow and climb hills without stalling. We also want our cars to be able to go fast on the open road. A car engine, like your legs, has a limited dynamic range. That limited dynamic range precludes a single gearing ratio. A car needs multiple gearing ratios to enable the use of that car in urban traffic, on hilly terrain, and on highways.
  13. Jul 25, 2011 #12
    Thanks D.H. I understand how bicyle gears work at least externally, I want to know if it works same for cars and how can I relate that concept to cars. In a bike if I want to go up hill, I put to gear 1 so easier to pedal and I travel less distance. In a car also if I have to go up a hill I have to put to gear 1. Now how can I relate bicycle concept to this. What is the equivalent of easier to pedal mean in a car? Thanks :smile:
  14. Jul 25, 2011 #13
    Just like you can't pedal up a steep hill in a high gear on a bicycle because it's too hard to pedal, you can't drive up a steep hill in a high gear on a car because the engine will stall from turning too slow.

    If you're on a level surface and want to go fast on a bicycle you shift to a higher gear to slow your legs down from pedaling too fast. You do the same in a car to prevent engine damage from revving it too high.
  15. Jul 27, 2011 #14
    Also, the clutch can "slip". This means that the clutch pedal is depressed just enough to take pressure off the point of contact. This causes the clutch to be partially engaged, and the shaft rotates some, but not as fast as the engine.
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