Regenerative Braking Gives Back Less Than People Expect

What follows is an argument about regenerative braking, and about where the received version of it stops being true.

How the motor becomes a generator

An electric motor and a generator are the same machine operated in opposite directions, so driving the motor mechanically produces current. During regenerative braking the controller uses the motor to resist wheel rotation, and the resulting current is fed back towards the battery.

The braking force you feel is the mechanical consequence of generating that current, which is why stronger regeneration slows you harder. Whether this is possible depends on the motor type, since a mid-drive is separated from the wheel by a freewheel that cannot transmit force backwards. Direct-drive hub motors are the usual candidates, and geared hub motors typically contain a clutch that prevents it.

Why the amount recovered is small

The energy available to recover is the vehicle's kinetic energy, which depends on total mass and on the square of speed. A bicycle and rider at city speed simply do not carry much kinetic energy compared with a car, so the pot to recover from is small.

With a loaded rack, converting motion into electricity and back into motion involves losses at every stage, in the motor, the controller and the battery itself. Batteries also accept charge less efficiently at high rates, which is exactly the condition a hard stop creates. The commonly reported outcome is a modest single-digit percentage improvement in range in hilly stop-start riding, and close to nothing on a flat continuous route.

Where it genuinely helps

Long descents are the best case, because the energy would otherwise be dissipated as heat in the brakes over a sustained period. On a steep hill regeneration also provides continuous speed control without the brakes heating up, which is a real practical benefit.

Frequent stops in dense traffic offer many small opportunities, and they accumulate over a long commute. Riders in flat cities with few stops will see very little, regardless of what the system claims. The brake wear saving is often more valuable than the energy saving, particularly on a heavy cargo bike.

What it cannot replace

Regeneration cannot bring a vehicle to a complete stop, because the braking force falls away as the wheel slows. It also stops working when the battery is full or too cold to accept charge, which is precisely the top of a hill on a winter morning.

By the second week of riding, that makes it a supplement to mechanical brakes rather than a substitute, and any machine relying on it alone is badly designed. Mechanical brakes still need maintaining even if regeneration means the pads wear slowly, since cables seize and fluid degrades regardless.

Check that both mechanical brakes work properly before every ride, and do not let light pad wear become an excuse to ignore them.

The drag question

Direct-drive hub motors produce a small amount of magnetic drag even when no power is being drawn, sometimes called cogging. That drag is present all the time, including on the flat, so it quietly costs energy on every kilometre you ride. Whether the energy recovered in braking exceeds the energy lost to drag depends heavily on how stop-start the route is.

With a loaded rack, on a smooth flat commute with few stops, a direct-drive hub with regeneration can plausibly be a net loss. This is the part of the argument that marketing material tends not to raise.

Frames, sizes and riding positions vary enormously; borrow before you buy wherever you can.

Judging a claim honestly

Any percentage quoted for regeneration should come with a description of the route, because the figure is meaningless without one. A hilly, congested route with a heavy load is where the best numbers come from, and few riders have exactly that route. If you are choosing between systems, treat regeneration as a small bonus rather than a reason to accept a worse motor.

Where it is offered as adjustable, a moderate setting usually gives most of the brake-saving benefit without unpredictable deceleration. The honest framing is that regeneration is a nice feature on a hilly commute and a rounding error on a flat one.

The takeaway

Treat regeneration as a brake that saves pads, and be pleasantly surprised if it saves any charge.

The bike you ride daily beats the better bike you keep indoors.