For my solar powered bicycle, the Maxun One, I designed a very powerful and lightweight mid-drive motor. I also wanted to investigate whether I could use a Bosch Active Line 3 motor. The Bosch motor weight is 2975g, this is relatively lightweight for a mid-drive motor, but it is still too heavy for my solar bike.

So, I wanted to investigate if the Bosch motor can be reduced in weight, so I disassembled the motor. This is also a great opportunity to see how it works and which electronic parts are used.

Bosch active line 3 weight
Bosch active line 3 weight

Bosch active line 3 typenumber
Bosch active line 3 typenumber

Bosch Active Line 3

  • On the market since end 2017
  • 250W / 40Nm
  • Weight 2975g without plastic covers, metal holder and bolts
  • Weight 3345g with plastic covers, metal holder and bolts

What is striking after opening the motor is that there are only a few parts inside, that is a great achievement of the Bosch engineers:

Bosch Active Line 3 mid-drive motor tear down
Bosch Active Line 3 mid-drive motor tear down

Bosch Active Line 3 mid-drive motor tear down
Bosch Active Line 3 mid-drive motor tear down

Bosch Active Line 3 mid-drive motor tear down
Bosch Active Line 3 mid-drive motor tear down

Freewheel

The motor has just one freewheel, used for pedaling without turning the motor. But the motor can’t drive the rear wheel without the pedals turning, this requires an extra freewheel.

Bosch active line 3 freewheel
Bosch active line 3 freewheel
Freewheel
Freewheel


Torque sensor

Bosch active line 3 torque sensor
Bosch active line 3 torque sensor

Torque sensor plain bearing
Torque sensor plain bearing
Torque sensor plain bearing
Torque sensor plain bearing

TODO……………………….....................

Helical gear

The motor must have a speed above 1000 rpm, otherwise the motor losses will be too high, see the article motor tuning. So, we always need a gear. The previous Bosch mid drive motors have four gears wheels, but the Bosch Active Line 3 has just two gear wheels. That limits the transmission ratio, and so the motor efficiency, unless we take a motor gear wheel with very few teeth. To avoid gear undercutting, the minimum number of teeth of a gear is commonly 12, but that is too much in our situation. The reason that Bosch uses a helical gear is not just noise reduction, more important is that helical gears allows a low number of teeth without undercutting.

The motor gear has just 5 teeth, which is very rare.

Bosch active line 3 motor gear wheel
Bosch active line 3 motor gear wheel

The large gear is made of nylon:

  • Teeth number 71
  • Diameter 115mm
  • Width 18.5mm
  • Helix angle 15º
  • Module m = 112mm / 71 = 1.6

Note that the gear wheel is not standard: https://nl.wikipedia.org/wiki/Modulus_(tandwielen).
The gear ratio is 71 / 5 = 14.2. At a cadence of 80rpm, the motor speed is about 1100rpm which is a common practice.

Lubrication

It is remarkable that the gears are nearby the PCB but the electronics remain free of grease. The grease is apparently not being thrown around. This requires a special grease with a certain thickness. Used is “Renolit WTF 2B Bosch Drive Unit Fett” but I don’t know where you can buy it. The small motor gear is fitted with a plastic cap, which ensures that the grease stays on the gear wheels. The nylon plate between the large gear and the cover has the same purpose.

Motor parameters

  • Resistance 0.109Ω
  • Stator 18 pole
  • Rotor 14 pole
  • Speed = 1100rpm at a cadence of 80rpm

Note that there is some permanently sticky glue on the rotor. I suppose, this attracts dirt particles to prevent them from entering the motor.

Permanent sticky glue
Permanent sticky glue

Thermal grease

There are four locations on the PCB where thermal grease is applied for cooling electronic parts to the aluminum housing. The used thermal grease is a dry blue paste, I do not know what it is and where you can buy it. Because it is dry, I suppose that you have to apply some extra paste after repair.

Bosch active line 3 PCB thermal grease
Bosch active line 3 PCB thermal grease

Part 2

Bosch Active Line 3 printed circuit board explained

Bosch active line 3 PCB explained

Do you have any comments? Please let me know.
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