Create a 3-speed transmission for Motorized Bike

What you need

・AA01 Super Cub 3-speed transmission assembly, 1 set

1 set includes:

1 x main shaft assembly

1 x countershaft assembly

1 x shift drum assembly

1 x shift drum stopper

1 x shift fork

Note: There are multiple variations of the Super Cub with the model number AA01, some of which are even 4-speed. There is no guarantee that the gear I used in my design will be the same as the gear you purchase. Part details are provided in the Details section, so please check carefully. Also, since I am not a Super Cub expert, I cannot answer questions such as "Can this mission be used?" Do everything at your own risk.

・AA01 Super Cub countershaft assembly, 1 piece

Used for power input. Hereafter, this will be referred to as the primary shaft.

・6001 bearing, 3 pieces

・6003 bearing, 3 pieces

・Oil seal for Super Cub engine

Use two for the countershaft output section and one for the shift arm. In many cases, for overhaul purposes, three pieces for the countershaft, shift arm, and kick pedal are sold as a set, so it would be a good idea to purchase two sets.

・415 sprocket, 2 pieces

I used a 12-tooth one, but they don't necessarily have to be the same. Adjust the number of teeth according to the required torque.

・Iron plate

I used a 3mm iron plate.

The required parts are shown in the image below.

The color of the explanatory text for each part corresponds to the parts arrangement shown later.

Cub mission assembly details

I used a 2008 Super Cub standard model, fuel injection specification, frame number AA01-1778818, 3-speed transmission.

The number of gear teeth on the main shaft is 11T, 17T, and 21T, and the number of gear teeth on the counter shaft is 25T, 31T, and 36T.

The diameter of the spline shaft is 17mm, and the diameter of the narrow part where the bearing is installed is 13mm.

What is important is that the maximum gear on the main shaft is 21T, and the 31T gear on the primary shaft is applied to drive it, so if a transmission assembly with a different number of teeth (=diameter) is used, the primal shaft will not mesh properly.

This can be handled by changing the mounting position of the primal shaft, but please design it yourself.



Printing these two images on A4 paper should serve as a template for the main plate to be cut out. I'm not familiar with the printing specifications, so if it doesn't work, please draw it according to the dimensions written. The unit is millimeters.

What is important is the positional relationship between the holes, not the shape of the outer periphery. Make it in any shape you like without interfering with the parts.

Please note that the 13mm and 34mm holes on the right side should be drilled very precisely.

The shift drum comes into direct contact with the inner wall of this hole without a bearing, so if there is play, the shift drum will move during a shift operation, inhibiting rotation and causing it to stop moving.

It is most ideal to use a milling machine or lathe.

A flange structure is provided on the left and right plates, and bearings are installed.

Place the bearings in the positional relationship as shown in the diagram.

Each part is placed like this. Please use this diagram to understand the parts placement.

Notice that the main shaft has been cut off. Originally this is the part where the clutch is attached, but it is unnecessary so we cut it off.

By adding one yellow primal shaft to the Cub's transmission, we create a power input point on the left side of the transmission. This makes it possible to add this transmission directly to a Motorized Bike engine, as the input and output shafts rotate in the same direction.

The shift drum has an M6 bolt hole, so use this for fixing.

Sandwich main plate A with washers. The shift drum will only be fixed at this one point, so use a shim ring, etc. to remove any looseness.

Primal Shaft

Completely disassemble the countershaft, leaving only the 31T gear.

At this time, you may or may not cut off the dog clutch part.

Find the position where the 31T gear and the 21T gear on the main shaft mesh.

Since the 21T gear moves left and right when shifting, the 31T gear must be positioned and fixed so that it will engage in either position.

The easiest way to secure the snap ring is to use a grinder or lathe to cut a new groove in the shaft for the snap ring.

Shift arm, drum stopper related

Note: There is no guarantee that the dimensions that follow are accurate. Don't be fooled by the numbers written on it; instead, please match the actual parts. This inaccuracy is why I was hesitant to reveal how I made the transmission. In order to correct the inaccuracy, I would need to purchase and measure another set of transmission parts, and I don't have enough money.

It is also a good idea to start making the transmission casing at this point and fix the A and B main plates. Of course, provide a flange and secure it with bolts so that it can be disassembled.

Weld the M6 nut and install the shift drum stopper. The shift drum rotates in 90 degree increments, and at each angle the gears are rearranged to become neutral, 1st gear, 2nd gear, and 3rd gear. Position the shift drum stopper so that the shift drum is fixed at just the right angle.

After determining the position of the stopper, the next step is to create a plate to attach the shift arm.

The position of the shift arm is very critical and greatly affects shifter operation. Please find the appropriate location by matching the actual item.

Below is a measurement of the 3D model. Please use it as a reference only.


Please refer to the image below for the positional relationship.

Oil seals are required on the cross-sections of the primary shaft and countershaft.

This can be used as is from Super Cub. Create a flange like the one shown below.

Similarly, create a flange at the base of the shift fork and install an oil seal.

Erect boards around the main plate to create an transmission case and seal everything up. 
Create flanges and bolt holes to create a structure that can be disassembled.
Cap the end of each bearing flange to prevent the bearing from falling out.

Fill all the holes with paper gaskets, gasket makers, JB Weld, whatever you can find. Install a drain plug and pour gear oil.

I use 300ml of 10W-30 oil (not sure if this is appropriate). An abnormal noise occurred when the volume was below 100ml.

3D model

Finally, I will upload the 3D model that I used for the design.

It should be possible to print on a 3D printer with a bed size of 200x200 or larger. 

Bolts and flanges for fixing the left and right sides are not included. Please do not put this into a metal 3D printer or CNC machine.

This is what happens when you print 4 files. Please use it as an aid to your build.