Clutch What is the Clutch?

Discussion in 'Transmission / Drivetrain' started by Falkor, Apr 25, 2015.

  1. Falkor

    Falkor New Member

    I am about to build a 80 cc motor bike so I want to know what a clutch does on a bike, the purpose of it, and how to work it. Thanks

  2. bluegoatwoods

    bluegoatwoods Well-Known Member

    The purpose of a clutch is to give the rider the ability to 'turn on' and 'turn off' the bike's drive whenever he chooses. Imagine if there were no clutch. The rider would have to actually start up the engine in order to make the bike move and he would have to actually shut off the engine in order to make the bike stop moving.

    (To meander just a can imagine a vehicle that would be stopped and started just this way with modern technology. I think Prius's do something like this. The gas engine will start up and shut down automatically when conditions are right. But the mechanical clutch goes back way before such technology was developed. It goes back to the days of the steam engine. The steam engine simply wouldn't have worked in any practical manner if the only way to start up the output was to fire up the engine, which took some time. and if the only way to stop the output was to let the engine cool off.)

    To put it a bit more briefly, the clutch acts as a 'disconnect' between the engine and the final drive. In our case, that's the rear wheel. This is what allows you to stop at a red light, leaving the engine running but with that bike not pulling you down the road. And the clutch is also what allows you to send the engine's power to your rear wheel when the light turns green.

    When the drive train is 'complete' from running engine to spinning rear wheel, we say that the clutch is 'engaged'. (Another way of looking at that is to think of the entire drive train as being engaged.) When you're sitting at a red light, with the engine running but your rear wheel not turning, then we say that the clutch is 'dis-engaged'.

    The clutches on these bikes are really pretty simple. (Automotive clutches are more complex, but the basic mechanism is the same). This mechanism consists of two disks that are pulled together by a spring. One of the disks is driven directly off of the engine's crankshaft. The other is attached to the chain driving your rear wheel. When you pull on the clutch lever, the cable pulls the second disk away from the first disk. (first disk, in this case, being the one that is driven by the engine.) Now your engine can run freely without driving the rear wheel. When you let go of that lever, then the spring pulls the second disk into contact with the first disk. So now that power that is coming from the engine gets directed to the chain and that drives your rear wheel.

    So to operate the clutch you simply get yourself pedaling that bike a bit. Up to five-to-ten miles per hour. Now you let go of the clutch lever. you can get away with doing it pretty quickly, as a matter of fact. (This is different in a car or motorcycle. But that doesn't matter to us right now.) The 'oomph' from the engine now goes into the chain. From there it goes into the rear wheel. And you're off and running.

    I wonder if I've described things really clearly? Oh well, if not I can try again. Or maybe someone else will come along who can do better.
    Falkor likes this.
  3. butre

    butre Well-Known Member

    another important feature of a clutch is that when it's slipping it transmits torque, but not RPM, which when operated correctly smooths out starting from a standstill.
    Falkor likes this.
  4. Falkor

    Falkor New Member

    This was such a great help, thanks! I will be getting my kit this week and then building so I should be cruising in about a week.
  5. Falkor

    Falkor New Member

    Thanks, I needed info so I get to use my bike correctly.
  6. MotorBicycleRacing

    MotorBicycleRacing Well-Known Member

    Have you ever driven a car with a stick shift or a motorcycle?

    When you ride your new 80 which is actually a 66 cc below a certain slow
    speed like when you are making a tight U turn pull the clutch in and coast
    around the corner or your motor will be jerking and on the edge of stalling.

    To get back up to speed pedal then let the clutch back out.