Bosch Pedal-Assist Bicycle Mid-Motor Description And Specs

We take a look at Bosch eBike mid-motors to see how they work.

Bosch Active Line Mid Motor
Bosch’s Active Line is intended for upright urban riding.Bosch

Bosch is a huge German manufacturer of automotive equipment, a pioneer in ABS braking, electronic fuel injection, and electrical systems. It came into the electric-bike motor field in 2009 and thanks to its capable in-house R&D resources has greatly expanded its market, earning the respect of bicyclists.

Like the other mid-motors, Bosch pedal-assist machines measure the power you are delivering to the pedals (just as a stationary exercise bike does) and then adds to it in four levels or modes of assist. Bosch calls its Eco, Tour, Sport, and Turbo. Like the others, it electronically measures the torque you deliver to the pedals, pedal crank cadence (rpm), and bicycle speed, then employs software on a built-in circuit board to deliver the power-assist percentage you choose. Electrical power from the battery is then delivered through power transistors to the permanent-magnet motor, which converts it into mechanical power. Because the motor must spin rapidly to develop best power, there is internal gearing to drive the pedal crank through two reductions (in one case the ratio is 35.5 to one). All these parts are densely packaged into a cast-aluminum housing, allowing a Q-factor (width across the outer faces of the pedal crank arms) of 180mm (slightly more than 7 inches). Compare this with a typical mountain bike Q of 170mm—the difference is small.

Two lithium-ion batteries are offered—PowerPack 400 and PowerPack 500—each weighing about 5.5 pounds. A full charge for the 400 watt-hour (WH) unit takes about 3.5 hours, and a PowerPack 500 4.5 hours. Realistically, batteries are seldom discharged that much, so a half-charge for the 400 takes 1.5 hours, and for the 500, about 2 hours.

These batteries are protected from such evils as excessive temperature rise or complete discharge by an integral electronic management system. As we all know from feeling the warmth of laptop computers, battery charge and discharge generate heat. Battery life is typically given in terms of number of cycles of full charge and discharge, which is harsher than the manner in which most riders will use their batteries. Quoting from the website, “Bosch PowerPacks are designed for many tours, distances, and years of use.”

Bosch’s Active Line is intended for upright urban riding. Active Line Plus provides somewhat higher torque assist. Performance Line aims at paved roads and sporty riding over longer distances. For the mountain biker, Performance Line CX offers high capabilities for unpaved surfaces and mountain riding.

Bosch Active Line Plus motor on white background.
Hidden inside the Bosch Active Line Plus motor housing is a complex array of gears, circuitry and a permanent magnet motor. Despite its complex nature, it still has Q factor, or width between cranks just 10mm greater than a standard bicycle.Bosch

Here is Bosch’s official technical information:

Active Line
MODE Eco Tour Sport Turbo
ASSIST % 40% 100% 170% 250%
MAXIMUM DRIVE TORQUE 30 Nm 35 Nm 38 Nm 40 Nm
MOTOR WEIGHT 6.3 lb. (2.8kg)
Active Line Plus
MODE Eco Tour Sport Turbo
ASSIST % 40% 100% 180% 270%
MAXIMUM DRIVE TORQUE 35 Nm 40 Nm 45 Nm 50 Nm
MOTOR WEIGHT 7.1 lb. (3.2kg)
Performance Line
MODE Eco Tour Sport Turbo
ASSIST % 50% 120% 190% 275%
MAXIMUM DRIVE TORQUE 40 Nm 50 Nm 55 Nm 63 Nm
MOTOR WEIGHT 8.8 lb. (4.0kg)

The Performance Line also offers a 28-mph option.

Performance Line CX
MODE Eco Tour Sport Turbo
ASSIST % 55% 120% 120–300% 300%
MAXIMUM DRIVE TORQUE 40 Nm 50 Nm 75 Nm 75 Nm
MOTOR WEIGHT 8.8 lb. (4.0kg)
Bosch Active Line Mid Motor
The circuit board within the Bosch mid-drive motor senses the rider's power input into the pedals and then considers the speed of the bike to decide how much power the motor is going to contribute to the cranks. This is all done instantly and seamlessly.Bosch

All of the above have shift detection—momentarily easing off the assist to enable smooth shifting. I’ve given torque in Nm (Newton-meters) because that is what you will see on manufacturer spec sheets. To convert to pound-feet, multiply by 0.74.

Motor power is limited to 250 watts, averaged over half hour of operation, to conform to the various national legal definitions of where electric-assist bicycles end and outright electric motorcycles begin. This is done for licensing and tax considerations.

The maximum speed of 20 mph is the point beyond which electric assist is no longer given (in order to maintain legal status). Read tests and comparisons to get an idea of each make’s cutoff; some are less abrupt than others.

The question of range always comes up, and the rule is that the more assist you choose, the shorter the range—it’s a matter of common sense. For example, in the Active Line Tour mode, with a combined bicycle-and-rider maximum weight of 220 pounds, at a cadence of 60 and at 14 mph in little wind and on smooth and level roads, range could be as much as 54 miles. In Eco mode, 92 miles; in Turbo, 36 miles. Keep track of remaining range and battery power by looking at the screen of your handlebar-mounted control module.