Why You Need a Torque Arm for Your Conversion Kit

This guide expands on the torque arm section from our how much cargo can your conversion kit handle article — there we touched on it briefly in the context of heavy loads; here we go deeper: what it actually is, how to check if you need one, and how to install it correctly, whether you carry cargo or just ride normally.

What a torque arm is and what problem it solves

A brushless hub motor generates torque directly on the wheel axle with every acceleration. At 250W and 36V, this force is roughly 10-20 N·m — small enough that the flat surfaces of a standard axle can handle it alone, especially on steel frames with thick dropouts. At 1000W and 48V, however, torque can exceed 40-80 N·m applied directly to the rotating axle.

The axle nut keeps the wheel fixed vertically in the dropout, but it offers no resistance to the axle rotating around its own axis — exactly the direction the motor's force acts in. Without an extra part, this force is absorbed entirely by the contact between the axle's flat surfaces and the dropout walls. A torque arm is the metal piece that mounts on the axle and clamps to the frame, transferring this tangential force straight to the frame instead of leaving the dropout to absorb it alone.

What happens when the axle starts spinning ("spun axle")

The phenomenon is called a spun axle: without a torque arm, at high power or on aluminium frames, the motor axle starts rotating slightly inside the dropout slot with every hard acceleration. It doesn't happen all at once — it's a gradual process, hard to notice until it becomes a serious problem.

The end result, on a sufficiently damaged aluminium frame: the frame needs replacing. Not just the missing torque arm — the whole bike. On steel frames the process is slower, but it reaches the same outcome if you ignore the warning signs.

Do you need a torque arm? Check power and frame material

There's no single rule for everyone — it depends on the combination of motor power, frame material, and whether the system has regenerative braking:

For 1500W kits and above, torque exceeds the capacity of any standard dropout regardless of material — at this point it's not a question of "good frame or weak frame," a torque arm is simply necessary on any frame.

Compatibility: what to check before you order

Not every torque arm fits every bike. Before ordering, check three things:

All All4eBikes MTX kits have a hub motor with a standard 135mm QR axle, compatible directly with the torque arm's 10mm x 14mm opening. If you're fitting the kit on a bike with a thru-axle or a carbon frame, a classic torque arm isn't compatible — check your axle type before ordering.

The 4mm dropout thickness rule

A standard torque arm has an arm thickness of 3-4mm and provides roughly 20-30 N·m of resistance, enough for 500W. If your frame's dropout thickness is under 4mm (common on modern aluminium MTB or road frames), mount the torque arm "sandwich" style — one on the outside and a steel plate on the inside — to double the clamping surface. Measure your dropout thickness with calipers before ordering if you're not sure.

How to install a torque arm

Installation doesn't require special tools, just the right wrench for the axle nut and, ideally, calipers to check the dropout:

1. Loosen the motor axle nut and remove the wheel from the frame
2. Slide the torque arm onto the axle, with the 10x14mm opening seated on the axle's flat surfaces
3. Refit the wheel into the frame and tighten the axle nut to the recommended torque
4. Clamp the torque arm's tab to the frame using the included C-clamp or an existing frame eyelet
5. Check that the arm isn't parallel to the axle — it needs to form an angle, otherwise it won't distribute force correctly
6. Repeat on both sides of the axle for 1000W+ systems, or any system with regenerative braking

How to check you installed it correctly

After installation, a few simple checks confirm the torque arm is doing its job:

• Test with a hard acceleration with the wheel off the ground — the axle shouldn't move at all inside the dropout
• Check the tightness periodically of the axle nut and the torque arm's bolts, especially during the first month of use
• Visually inspect the dropout after the first 100km of intense use — you shouldn't see any new wear marks or widening of the slot

Signs something isn't installed correctly or you need a second arm

If you notice any of these signs, add the second torque arm from the set on the opposite side of the axle, even if your kit's power is under 1000W — it's cheaper than a damaged dropout.

What it costs to skip one

The comparison is simple: a set of 2 torque arms costs about as much as a small accessories order. An irreversibly damaged dropout on an aluminium frame usually means a new frame — that is, a new bike, not just a replaced part. For your conversion's full budget, see our complete cost breakdown of converting a bike to electric; the torque arm is, percentage-wise, the smallest line item in the budget and the one with the biggest impact on protecting your investment.

Frequently asked questions about torque arms

Conclusion: a small part, a big risk if it's missing

A torque arm isn't an optional accessory if you're taking your conversion seriously — it's a low-cost safety component that prevents an expensive, hard-to-fix failure. If your kit is 500W or more, or if you have an aluminium frame at any power level, install it alongside the kit, not after you notice the first signs of wear.

For the full kit installation, including correctly protecting cables and connections, see our conversion kit installation guide. If your kit is already installed and you want to check other critical points, also see the 10 common conversion kit problems.

Have questions about torque arm compatibility with your bike? Contact us before ordering.