A torque arm is recommended for front hub installations that are more than 500 watts (36V with 20A or 25A controller), or for any installation done on aluminum forks. It slides over the non-wire side of the motor axle, engaging with the 10mm axle flats to prevent the axle from spinning out inside the dropouts.
We have 3 torque arm models in production, two for front forks and one for rear dropouts. All are made with thick ¼” stainless steel for the slotted plate and will work with both 12mm and 14mm axles. For more information on proper torque arm installation and other details see the torque arm homepage.