Bosch E-Assist on Tern Bikes

Understanding Bosch E-Assist on Tern E-Bikes

Most Tern e-bikes come equipped with a Bosch motor that assists you when riding under certain conditions. Understanding how this e-assist system works is key to getting the most out of your new e-bike. This article explains the basics of the Bosch e-assist system.

How do pedelecs work?

Tern e-bikes are pedal-electric bicycles, also known as pedelecs. That means the bike's motor system only provides power while the rider pedals. Our e-bikes do not have a throttle.

The Bosch motor system amplifies the power you put into the crank arms by pedaling. The system offers these levels of assistance: Eco, Tour, Sport, Auto or eMTB, and Turbo.

It's possible to ride our e-bikes with the e-assist set to "Off" (no extra assistance from the motor), but we wouldn't recommend riding that way for significant distances if you are carrying heavy cargo.

How does the motor know how much assistance to provide?

The Bosch e-bike system has three sensors that measure your torque (how hard you are pedaling), cadence (rpm, revolutions per minute, i.e., how fast you are turning the pedals), and speed. These three sensors work together to tell the motor controller how you're riding so that the motor can provide the right amount of extra power when needed.

For example, if you begin climbing a steep hill, you'll start pushing on the pedals harder, your pedaling will slow down, and your bike's speed will decrease. The Bosch system will interpret data coming from the three sensors, and the motor controller will provide extra power to help you climb the hill. How much additional power you'll receive depends on which level of assistance you've chosen (Eco, Tour, Sport, Auto, eMTB, or Turbo).

What happens when you reach the cutoff speed?

To comply with local regulations, your e-bike motor system is programmed to stop providing power once you reach a certain speed. The cutoff speed depends on where you bought your bike. For example, in the EU, the cutoff speed is 25 km/hr, while in the US, it's 20 mi/hr.

Your bike can still exceed these speeds if you pedal hard enough or are going downhill, but the motor system will only assist you once your speed falls under the cutoff again.

What happens if there's a problem with the sensors?

All three sensors described above need to be working correctly for the Bosch system to run. If one of your sensors is damaged, misaligned, or otherwise malfunctioning, your bike display may show an error message, and the motor may stop providing power.

One of the most common sensor problems happens when the speed sensor becomes misaligned. The speed sensor is located on a rear-wheel spoke, near the tire inflation valve. If this sensor is knocked out of place (for example, when pumping your tires or fixing a flat), the display will show "ERROR 503", and the system won't provide e-assist.

Refer to your Bosch display manual for help with troubleshooting this and other errors. If you need additional help, visit your local Tern dealer.