Torque Arm: When You Need One

A torque arm stops the axle of a hub motor from turning in the dropout. With aluminium dropouts and with front motors it is always sensible. Without one, the motor's torque can let the axle walk in the dropout, and that is a real safety risk.

What the arm does

A hub motor turns the wheel by pushing against the axle. The axle takes the full reaction torque. Normally the flat-milled axle holds in the dropout, but only if the flats fit cleanly and the material is strong enough. The arm takes part of this force through a second fixing on the frame or fork.

Hub motor axles are usually 10 mm, sometimes 12 mm thick, and flattened. More on axles is under hub spacing.

When it is needed

The assessment comes from experience and is an assumption Fabio is still checking.

Why the front is especially touchy

The front fork has open dropouts pointing downwards. If the axle turns and slips out there, the front wheel can drop out. That happens at the handlebars, not the rear, and the consequences are accordingly serious. That is why many dealers add retaining washers on front motors in addition to the arm. Ask whether your kit includes some.

On carbon forks or very light aluminium forks a front motor is not recommended anyway, see motor types compared.

Fitting step by step

1. Check the axle. The flats must sit cleanly in the dropouts. Do not widen tight dropouts if you do not know what you are doing.
2. Choose an arm. It has to suit axle thickness and frame. Many kits include one, otherwise universal arms exist.
3. Slide on the arm. It sits outside or inside the axle depending on the model and is secured with nut and toothed washer.
4. Fix it to the frame. With the clamp on the fork or chainstay. It must not touch the brake area or the cables.
5. Tighten nuts to torque. With a torque wrench to the manufacturer's value. Check the figure in the kit's manual.
6. Route the cable cleanly. The motor cable must not be under tension or chafe on the arm.
7. Test ride and recheck. After the first kilometres, retighten all bolts.

Which tools you need for this are under tools.

Risks and typical mistakes

• Nuts too loose: The axle walks. A drop of thread locker helps if the manufacturer allows it.
• Clamp too tight: It can dent an aluminium tube. Tighten with feel.
• Arm in the wrong place: It has to catch the direction of rotation, not just sit there.
• Worn-out dropouts: Replace the frame or fork if they are widened.

A turned axle is among the common problems, see common problems.

If it does not fit

If you cannot find an arm that suits your bike, or the dropouts are too soft, that is a reason to move to a mid-drive or to leave the conversion. Honestly: an arm is no stopgap for an unsuitable bike. That workshops refuse conversions often has to do with exactly these questions (Fahrrad XXL).

You find the whole process in the conversion overview.

How reliable is this?

Statements on hub motor standards partly come from the source named; the rule on torque arms with aluminium dropouts and front motors, the fitting advice and the risks are experience or assumptions that Fabio is still checking.

Does a conversion fit your bike?