Regular bicycle forks are designed for lightweight wheels which do not apply nearly as much torque to the bicycle dropouts and forks compared to an ebike. They are not designed to resist wheel torque, their purpose is simply to hold the wheels in place. When you convert your regular bike into an electric bicycle and install a high hub motor torque to forks/dropouts, more than a bike was designed for, it may not handle potential safety risks may arise for riders. This may be a special concern for those who run hub motors on the front as front forks are usually weaker than the rear ones.
Let’s get some idea about how a hub motor works. When a hub motor powers up running, it generates some amount of power, torque to rotate the wheel forward so you can go and an equal torque applied to the axle to rotate it in the opposite direction of the wheel. Imagine what would happen if the axle could spin easily? Of course the bike would not go. That’s why both fork ends and the wheel axles have two flat sides, to prevent the axle spin normally.
Now, think about a much higher motor power which forces the axle to spin! You may end up with distorted or completely damaged slots which can lead to the wheel falling out of the bike when a continuous torque applied exceeds the limit the fork in which it can handle. The greater the hub motor power, the higher the risk.
This is where a torque arm comes in. It’s stainless steel piece engineered to help bicycle dropouts resist the torque of hub motor by transferring the power further up the frame and prevent wheel axle spin within the dropouts of the bike. Torque arm is there to save your bike, your motor, and to keep you safe from unexpected accidents.
So do you need one? Our answer is YES, especially if it’s a front hub motor and your bike’s fork is not stainless steel.
It’s better to be safe than sorry.