The performance of an ebike is determined by a number of factors, including:
- The leg power being used by the rider;
- The rider’s ability to use the mechanical gears correctly;
- The torque (turning force) of the motor;
- The peak watts the motor can deliver on demanding hills;
- Gear ratios
The leg power of the rider
Most pro cyclists produce about 200 to 300 watts on average during a four-hour tour stage. The recreational rider, on the other hand, might only be able to sustain this wattage during a 45-minute or hour-long spin class. However, when you consider that most ebike motors are 250-300 watts, it’s easy to see the importance of leg energy when it comes to ebike performance. Add 200 watts of leg energy to 250 watts of motor and you get 450 watts!
The rider’s ability to use the mechanical gears correctly
Many ebike newbies have not been on a bike in 20 or 30 years, and have forgotten how to use the mechanical gears – if their bike from back in the day even had them! What they tend to do then, is leave the mechanical gears in about 5th gear, and just use the pedal assist button to boost or reduce the motor power. This approach will not only affect performance, but it will also have a dramatic impact on the range of the bike. In other words, using the mechanical gears correctly has a huge impact on how far you can ride on a single battery charge.
The torque (turning force) and gear ratios
Different motors vary widely in the turning force they produce. Some motors can produce 40 newton-metres of turning force, while others can produce 80+ newton-metres of turning force. The more turning force the motor delivers, the less turning force your legs need to deliver – an important consideration, especially for baby boomers with old legs!
Torque on a bike with a mid-drive motor is a bit more complex though. The torque of a mid-drive motor is transferred to the back wheel through the chain, which has variable gearing. So, let’s say the bike has a 42 tooth chainring on the front, and that the largest sprocket (lowest gear) on the gear cluster has 32 teeth. And, let’s assume the motor has 80nm of torque. In that setting, the turning force delivered to the wheel will be 80 x 32/42 = 61 newton metres of torque. Change the chainring on the front to 38 teeth, and the formula is 80 x 32/38 = 67 newton metres of torque. Change the chainring on the front to 38 teeth, and the largest sprocket on the gear cluster to 36 teeth, and the formula is 80 x 36/38 = 76 newton metres of torque.
The peak watts the motor can deliver in short bursts
The most common ebike motor sizes are either 250 watts or 350 watts. Some 350 watt motors are limited to 300 watts to comply with NZ regulations. What the 250 or 350 represents is the continuous wattage the motor can deliver – so it can sustain that level of output for a long time. There is another measure of wattage however, known as peak watts. Peak watts are measured by multiplying the voltage of the bike (most are 36 volts) by the number of amps in the controller. Some controllers are 15 amps, some are 18 amps, some are 22 amps. A 36 volt bike with a 15 amp controller can output 540 peak watts (36 x 15). A 36 volt bike with a 22 amp controller can output 792 peak watts – 47% more peak watts than the 15 amp controller. This only matters in steeper hill-climbing situations where you need a burst of peak power for a short amount of time.