Torque Arms - Safety for Hub Motors
A torque arm is a critical safety component for powerful hub motors. It protects your frame and prevents axle rotation in the dropout, which can lead to serious failures. Here we cover what a torque arm is, why it is needed, the physics behind torque, and safe installation.
What is a torque arm?
A torque arm (English: torque arm) is a separate metal component mounted on the axle of a hub motor and transfers the motor's reaction torque to the bike frame or fork. When the motor drives the wheel forward, an equal but opposite torque appears on the motor housing, called reaction torque.
Without a torque arm, the dropout interface itself, where the wheel axle sits, must absorb all of this reaction torque. On low-power motors (under ~250 W), dropouts can often handle it, but at higher power levels, especially in modern aluminum or carbon fiber frames, the forces can exceed material strength. These frame areas were not designed by the manufacturer to absorb forces from a powerful hub motor.
The torque arm acts as a safety link between the motor axle and the frame. It is usually made of steel or aluminum and installed so it physically prevents motor rotation. This protects both the dropout/suspension interface and the motor cables from damage.
Why are torque arms needed?
As hub motors have become more powerful, the need for torque arms has increased dramatically. When you accelerate with a powerful hub motor, for example 1000 W or more, the motor produces high torque. At the same time, the motor housing experiences an opposite torque that tries to rotate it in the opposite direction.
Many standard hub motors have an axle with milled flat surfaces (so-called "flats") intended to bite into the dropout and prevent rotation. But at high power or with weaker materials, this is not enough:
- Aluminum dropouts tend to crack or deform permanently under high loads.
- Suspension forks often have weaker dropouts than rigid steel forks, increasing the risk of damage.
- Carbon fiber frames can develop micro-cracks under concentrated point loads, which can compromise structural integrity.
- Motor cables can be damaged or torn if the motor starts rotating, which in the worst case can lead to short circuits or wheel detachment while riding.
In well-designed specialty motors, torque handling can often be built into the motor design through wide flange mounts or other mechanical solutions. But for most standard hub motors retrofitted to MTB frames or similar bikes, an external torque arm is necessary for safe operation.
Warning
Running a powerful hub motor without a torque arm on an aluminum frame or suspension fork is a safety risk. The dropout can fail suddenly, which can lead to wheel detachment or loss of vehicle control.
The physics of torque
To understand why torque arms are so important, we need to look at the physics of torque. Torque is defined as force times lever arm:
Torque (M)
where F is force (N) and r is the lever arm length (m). The unit is newton meter (Nm).
When a hub motor drives the wheel forward, torque is created at the wheel. According to Newton's third law (actio = reactio), an equal but oppositely directed reaction torque appears on the motor stator, which is directly connected to the axle.
Example: force calculation
Assume a hub motor develops 40 Nm of torque and has an axle diameter of 12 mm (radius r = 6 mm = 0.006 m). The force acting on the dropout is then:
F = M / r = 40 Nm / 0,006 m ≈ 6667 N
That corresponds to a load of about 680 kg trying to spread or twist the dropout.
In reality, the force is distributed across both dropout sides (left and right), but even half of this force, around 340 kg per side, is enormous for a thin aluminum dropout. For even more powerful motors (100-200 Nm or more, GBIKE 1 produces over 150 Nm), the load is even higher.
A torque arm absorbs this counter-torque from the axle and then distributes it farther up the frame or fork, which normally eliminates the risk of deformation or failure.
Materials and design
Torque arms are usually made of steel or aluminum, where steel is stiffer and tougher but heavier, while aluminum is lighter but requires thicker dimensions for the same strength.
Design principles
- Axle mounting: The torque arm is attached to the motor axle, either by clamping around the axle or through the axle flats. A secure clamp is essential because a loose torque arm cannot transfer reaction torque effectively.
- Frame mounting point: The other end of the torque arm is attached to the frame or fork, often using a bolted joint or clamp. The mounting point should be solid and avoid loading vulnerable frame areas.
- Lever arm length: A longer torque arm gives lower force at the mounting point (because M = F · r), which can help avoid overloading the frame mount. But an excessively long arm can interfere with installation or reduce ground clearance.
- Dual-side vs single-side: Some motors benefit from torque arms on both sides of the wheel for extra safety, while others can work with a single-side arm. For very powerful motors (>2 kW), dual-side installation is often recommended.
Tips when choosing a torque arm
- Make sure the torque arm fits your axle diameter (often 12 mm or 14 mm).
- Ensure the frame mounting point is strong enough, ideally metal rather than plastic parts.
- Choose an arm with sufficient thickness and stiffness for your motor power level.
- Consider dual-side installation for motors above 2000 W.
Installation and safety
Correct installation of the torque arm is essential for it to work as intended. A poorly installed torque arm can slip, making it ineffective, or in the worst case come loose and cause damage.
Installation steps (general)
- Prepare the motor axle: Clean the axle and make sure the flat surfaces (flats) are clean and free from dirt or grease.
- Install the wheel in the dropout: Insert the wheel into the dropout.
- Position the torque arm: Place the torque arm on the axle so it sits firmly on the flats under nuts and washers. Some arms have a clamp screw that must be tightened firmly.
It is important to orient the torque arm correctly. The axle will try to rotate opposite the wheel direction, so mount the arm in a way that counters this. - Install washers and nuts: Install washers and nuts on the axle so the motor is secured firmly.
- Secure the other end of the torque arm: Use a suitable mounting method to secure the torque arm to the frame or fork.
- Check tightening: Tighten all bolts to the recommended torque. Overtightening can damage threads, while insufficient tightening can allow the arm to slip.
- Test before riding: Lift the wheel off the ground and apply gentle throttle. Verify that the motor does not rotate in the dropout and that the torque arm remains secure.
Safety warning
Check regularly (after every 100-200 km or after hard riding) that the torque arm and all bolts are still properly tightened. Vibrations can loosen bolts over time, even with thread locker.
Common installation mistakes
- Clamp too loose on the axle: If the torque arm is not firmly seated on the axle, it can slip and become ineffective.
- Mounting point on a weak frame area: Attaching the torque arm to thin sheet metal, plastic, or carbon fiber surface without reinforcement can damage the frame.
- Missing thread locker: Without thread locker, bolts can vibrate loose, which can cause the torque arm to detach while riding.
- Torque arm too short: An arm that is too short creates higher force at the mounting point and can overload the frame mount.
Testing and sizing
Extensive tests of torque arms and dropouts have been conducted by the EV community. These tests provide valuable insight into what forces are required to damage different dropout types and how effective torque arms are.
Test results (from around 2009)
- Standard fork (aluminum): Axle rotation occurs at around 60-90 Nm of torque. Aluminum dropouts tend to crack or deform permanently.
- Steel fork: Somewhat tougher, but even steel dropouts can bend under high loads (above 80-100 Nm).
- With a correctly installed torque arm: The system can often handle over 150 Nm without issues, because the load is spread over a larger area and absorbed by the frame's main structure.
These results clearly show that a torque arm can double or triple the safe torque capacity of a standard dropout. For modern high-power motors (1000-3000 W or more), a torque arm is not a "nice to have" but a necessity.
Sizing by power level
As a rule of thumb, the following guidelines can be used:
- Under 250 W: A torque arm is often not required on steel frames, but recommended on aluminum.
- 250-750 W: Torque arm strongly recommended, especially on aluminum frames or suspension forks.
- 750-2000 W: Torque arm required. A single-side arm may be enough on robust frames.
- Over 2000 W: Dual-side torque arm strongly recommended.
Note that these are general guidelines. Actual loads also depend on riding style, terrain, and motor Kv/Kt values (see the motor page for more on this). A motor with low Kv and high Kt can produce higher torque than a high-rpm motor at the same power.
GBIKE and torque arms
GBIKE kits use powerful direct-drive hub motors that can deliver up to nearly 6000 W peak power. At these power levels, a robust torque arm is absolutely necessary for safe operation when the kit is mounted on a "regular" bicycle frame.
We recommend that all GBIKE users install a high-quality torque arm on both aluminum and steel frames. For GBIKE 1 and the upcoming GBIKE 2, we offer torque arms specifically sized for our motor axle diameters and power specifications.
Tips for GBIKE users
Stop riding immediately if you notice abnormal noise, vibration, or axle movement in the dropout. Then inspect the torque arm and all mounting points carefully. A loose torque arm is a major safety risk that can rapidly increase the chance of serious damage to both motor and frame, and may lead to accidents.
Conclusion
A torque arm is a small but critical component that can make the difference between safe riding and serious damage. The physics is simple: high torque requires robust safety solutions. For all powerful hub motors, and especially for high-power kits like GBIKE, a properly installed torque arm is non-negotiable if you mount the kit on a frame that was not originally designed to handle these forces.
Invest time in correct installation, use high-quality components, and check regularly that everything stays tight. Your safety, and your bike's service life, are worth this simple step.