Motorcycle Transmission – How and Why It Works

6 Speed Motorcycle Transmission

Understanding The How and Why It Works

A Transmission is defined as a mechanical assembly (machine) that transmits power from the Engine to the Final Drive. The simplest transmissions can have just one, fixed ratio and no shifting. This article will expose the detailed workings of the 6-speed motorcycle transmission, but the 4 and 5 speeds work the same way.

Power is applied to the Input Shaft (pinion/drive shaft) of the transmission by the Clutch when it is engaged (plates and disc sandwiched together) and the Engine is running. The input/drive shaft turns anytime and all the time that the clutch is engaged. There is a drive/pinion gear for each ratio (gear change).

The input shaft gears mesh with their corresponding Driven Gears that ride on the Output/Driven Shaft. Motorcycles use Constant Mesh type transmissions. This name comes from the fact that the “gear pairs” are always meshed. These gear Sets/Pairs provide the gearing ratio changes that we use to go from stand-still to top speed, and in our case a high transmission ratio of 3.1:1 for our 1st gear and 0.8:1 for our 6th gear.

The 3.1:1 Ratio of 1st means that the Input Shaft must make 3.1 revolutions for every 1 revolution of the Output Shaft (shaft where the front sprocket attaches).
The 0.8:1 Ratio of 6th means that just .8 revolutions of the Input Shaft results in 1 full revolution of the Output Shaft.

1st Gear RatioNeutral Motorcycle Transmission

2nd Gear Ratio and Power Flow3rd Gear Ratio and Power Flow

4th Gear Ratio and Power Flow5th Gear Ratio and Power Flow

6th Gear Ratio and Power Flow

Some of the gears in our transmission are keyed/splined to the shafts they ride on and some spin freely. No two gear “Pairs” can be locked-in at the same time. Doing so would have disastrous results since they would each try to turn the output shaft at different speeds. The job of motorcycle gear shifting falls on the Shift Forks and Shift Drum to ensure that absolute control and placement of the splined gears be precise and properly timed.

Shift Drum Fork Control Channels

The Shift Drum has “worm channels” cast or machined into the surface. These channels provide and manage the directional control for each of the Shift Forks. As the Drum rotates, the channels hold or move the Forks right or left to engage and disengage the different splined gear sets.

The Shift Drum moves in a rotational manner, but never a full 360 degrees. The Drum is limited and rotation stopped at it’s lowest and it’s highest positions (usually 1st, but some motorcycles do have Neutral at the bottom and some at the top). Neutral is achieved when all gear pairs are unlocked and the Output/Driven shaft is not powered. Most Neutrals are now designed between 1st and 2nd.

Shifter Shaft and Drum Ratchet

The Shift Drum has the mechanical Stop for a reason. If allowed to continue rotating, the shift forks would lock-in 1st gear immediately after 5th or 6th – locking the tire, damaging the transmission, and over-speeding the engine – all of which could have catastrophic results. For that reason, Shift Drums have the rotational limits built-in. While Neutral could be designed to fall between every gear change, that function would be undesirable and a real nuisance. The choice to have it closest to and in-between the gear change/selection most used when starting off fell squarely between 1st and 2nd Gears.

Shifter Drum and Shaft Ratchet Detent Assembly

The Shift Drum is rotated forward and backward by a “ratchet” system, When your foot presses down on the shifter pedal, the Shifter Shaft is rotated and the Ratchet tooth pulls on the Shifter Drum Pins to rotate the Drum by one Detent. Only when the shifter pedal is released does the ratchet move back and is set to move the Drum again. The travel of the ratchet is limited to only 1 pin/detent move. This same ratchet action is used when the shifter pedal is pulled upward by the foot. The Ratchet tooth then pushes the Shift Drum Pins to rotate the Drum by one detent. It is this “press/release” Ratcheting action that keeps us from making more than 1 Gear Ratio Change at the time. 1-N-2-3-4-5-6 and 6-5-4-3-2-N-1.

The precise rotation and positioning of the Shift Drum is controlled by the Shift Detent Roller/Spring assembly. This spring-loaded roller rides on a cam assembly that maintains “next gear only” control and prevents the catastrophic results if 2 pairs were to be engaged at once. I think of this one device/mechanism as the single most important part of the entire engine/transmission assembly – without it, failure is immediate and certain.

Join us next time and we’ll take a detailed look at Clutch Design, the Components, and How They Work.

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