00:00In motorcycles, the torque and RPM of a wheel is controlled by manually shifting gear levers up or down with our left foot.
00:09Motorcycle gearbox comprises of a series of different gear steps, which can be switched according to the requirement.
00:17The gear lever is connected to a shifter shaft, and the other end of the shifter shaft is connected to a ratchet mechanism.
00:24This ratchet mechanism activates, as shown in the animation.
00:30The purpose of this ratchet mechanism is to rotate the shifting drum at specific angles.
00:51The shifting drum has special shaped channels cut into it.
00:55Special attachments, known as shifting forks, fit into these channels.
01:02Gears, known as gear dogs or dog clutches, fit into these shifting forks.
01:08As the shifting drum rotates, the shifting fork, along with gear dogs, slides sideways as shown in the animation.
01:16The purpose of sliding dog clutches will be explained shortly.
01:20First, let's understand the arrangement of different gears in gearbox.
01:23There's an input shaft and an output shaft.
01:28The gears are arranged with their axes aligned to their respective shafts.
01:33Some gears are internally splined.
01:37These splines perfectly mesh with external splines of their respective shafts.
01:42These splines allow gears to slide from side to side while maintaining the same angular speed as the shaft.
01:51All gears, which are shown here in red color, are splined.
01:58Some gears are freewheeling gears.
02:01These gears can freely spin about their axes, but cannot slide sideways along the shaft.
02:06The remaining gear is a static gear, which is attached to the input shaft.
02:18Let's number all these gears from 1 to 5 for better visualization.
02:22Please note that all freewheeling gears adjacent to dog clutches have recesses.
02:28The dog teeth can slide into these recesses and lock these two gears with each other.
02:35Now let's see how power is transferred with different gear ratios in a gearbox.
02:39One thing you can observe here is that every splined gear is meshed with a freewheeling gear.
02:46In this condition, if we spin the input shaft, no power has transferred to the output shaft.
02:52This is the neutral position of the motorcycle.
02:55To change into first gear, the gear dog number 5 in the output side slides toward the freewheeling gear number 1 and locks with it.
03:09Since the gear number 5 is splined, the output shaft will now spin with this gear.
03:16This is the first gear.
03:17When we change to second gear, the gear dog number 5 in output shaft returns to neutral position,
03:27and gear dog number 4 slides toward number 2 gear and locks with it.
03:34The technique is similar when we shift to third gear.
03:43The fourth gear.
03:47And the fifth gear.
03:51Please note here that before shifting from one gear to the other,
03:55the previously locked gear dog returns to neutral position first.
03:59Only then it slides into another recess to achieve power transmission.
04:04This is why there is a neutral in between every successive gear.
04:08But in real life, we find the neutral position only in between first and second gears.
04:13To understand how this happens, let's go back to our ratchet mechanism.
04:19A star shift detent mechanism is responsible for proper shifting of gears.
04:25As we can see, the star plate has ridges and valleys.
04:29A spring-biased roller rolls onto these ridges and valleys.
04:34The position of the roller at each ridge of the star corresponds to neutral condition.
04:38As we can visualize, the roller cannot rest on the ridges of the star
04:43and immediately disposes to the appropriate valley,
04:46preventing the shifting drum to stop at neutral position.
04:49But one of the ridges of this plate is ground off, as we can see here.
04:54This is done in between first and second gears so that the roller can briefly rest on this ground off surface,
05:02thus enabling the shifter drum to stop at neutral position,
05:05instead of shifting directly to second gear.
05:07The transmission system we showed here is known as Constant Mesh Transmission System
05:14because all gears remain engaged with each corresponding gear mates at all times.
05:20In conventional sliding mesh transmission,
05:23gear shifting is done by completely disengaging a gear with one gear
05:26and re-engaging it with another.
05:28It involves complex mechanisms, and drivers need tremendous skill to operate them.
05:33In addition, imperfect meshing causes gear teeth to grind and wear.
05:37The constant mesh transmission completely eliminates this problem.
05:44As you might have noticed, that some gears are only partially engaged sometimes.
05:49But this is not a problem because they are not transmitting power at this moment.
05:53Only those gears which are transmitting power
05:56need to be fully engaged at that instant.
05:58We hope you learned something today.
06:03Thanks for watching.
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