There is a particular kind of disappointment at the top of a long climb: the motor that pulled hard for the first kilometre is doing almost nothing by the last. Riders usually blame the battery. Most of the time it is heat.

The two failures feel nearly identical in the saddle — assist fades, the bike gets heavy, the display may or may not show anything — but they have completely different causes and completely different fixes. Learning to tell them apart is the difference between buying a battery you did not need and changing how you ride a hill.

Where the heat actually comes from

An electric motor converts electrical energy into motion, and not all of it arrives as motion. The losses become heat, mainly in two places: The windings, where current flowing through copper produces resistive loss. This loss rises with the square of current — double the current, four times the heating. The controller, which is doing the switching and is usually mounted somewhere with even less airflow than the motor.

The critical relationship for climbing: high torque at low speed means high current and low cooling. When the motor turns slowly, it generates less internal airflow, and the heat it produces has to conduct out through the housing. On a long grade, production outpaces dissipation, and temperature climbs.

That is why the same motor that is perfectly happy at 30 km/h on the flat can be in trouble at 9 km/h on a 12% grade.

Why hub and mid-drive motors behave differently

Hub motors drive the wheel directly, so motor speed equals wheel speed. On a steep climb at walking pace, the motor is turning very slowly while producing maximum torque — the worst combination for heat. Many hub motors also have no internal temperature sensor, so there is no warning: you get progressive power reduction, or a sudden cutoff, with nothing on the display to explain it.

The only levers a hub-motor rider has are speed, duration, and load. There is no gear to shift.

Mid-drive motors drive through the bicycle's drivetrain, which means you can shift down. In a lower gear, the motor spins faster for the same road speed, which moves it toward a more efficient operating point and reduces the current needed for the same torque. That is a genuine thermal advantage — but it only exists if you actually shift. A rider who leaves a mid-drive in a high gear and lets the motor lug up a hill has thrown the advantage away and is heating the motor exactly like a hub.

The counterintuitive rule: on a climb, a mid-drive motor runs cooler in a lower gear at higher pedal cadence, not in a higher gear at low cadence.

What makes it worse

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Hot motor or tired battery? How to tell them apart

This is the most useful diagnostic in the article, because the two feel the same and the fixes are opposite.

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If the bike recovers after a rest and the housing is hot, you have a thermal problem. If it fades at 40% charge on flat ground and recovers when you charge, you have a battery or sag problem — which is a different article, and a different purchase.

The seven rules for long climbs

1. Shift down early — before the grade, not on it. On a mid-drive, dropping two gears at the bottom keeps the motor in its comfortable RPM range for the whole climb. Shifting mid-climb under load is when you damage the chain and miss the gear you needed.

2. Keep the motor spinning. If your bike has a cadence display, use it: a faster, lighter pedal stroke means less current per unit of torque. Grinding in a big gear is the single most common cause of mid-drive overheating.

3. Contribute more, not less. On a climb, the cheapest watts are yours. Standing and pedalling harder reduces the current the motor must supply, which reduces heat roughly with the square of that reduction.

4. Avoid full-throttle starts on the grade. If you must stop, restart in the lowest gear and pedal first. A throttle launch on a 10% slope is the highest-stress moment the motor will see all day.

5. Break the climb if it is long. Two minutes stopped in shade at the midpoint can be worth more than any accessory. Heat that has somewhere to go will go; heat with nowhere to go keeps climbing.

6. Drop assist one level and keep the speed. A lower assist level reduces current draw, and on most bikes the speed penalty on a climb is smaller than riders expect. This is often the single most effective change.

7. Reduce what you are carrying. Two litres of water and a loaded pannier are real torque. On a known long climb, leave what you do not need.

What to do when the motor cuts out

A thermal cutoff is a protection feature, not a failure. The controller has decided that continuing would damage the hardware.

Stop, and let it cool in shade. Do not pour water on a hot motor — thermal shock can damage seals and the housing, and water near connectors is its own problem.

Do not keep restarting to "test" it. Each restart puts current back into a hot motor.

Let it cool before charging, and charge somewhere out of direct sun.

Note the conditions: grade, load, ambient temperature, how long it took. That record is what support will need if it turns out to be a fault rather than a limit.

Repeated overheating is not harmless, even when the bike recovers. Sustained high temperatures degrade magnet strength and winding insulation, and cook the grease out of bearings. A motor that has been overheated a dozen times will not fail dramatically — it will simply become less efficient and noisier over the following year.

When it is a fault, not a limit

Some overheating is a design or component problem. Escalate if:

• It overheats on flat ground at normal assist levels

• It cuts out within a few minutes of starting, from cold

• There is a burning smell, or visible discoloration on the housing

• The motor is noticeably hotter than the other end of the bike for no reason, or one side of the housing is hot while the other is not

• It throws an error code you can reproduce

• It began suddenly, after no change in route or load

In those cases, stop riding and contact support with the conditions you recorded. "It cuts out on my commute" is a hard ticket to resolve; "it cuts out after 6 minutes on a 6% grade at 22°C carrying 15 kg, three times, error 07" is a ticket an engineer can close.

Planning around heat

If your commute includes a climb you cannot avoid:

• Ride earlier. Ambient temperature matters more than most riders think, and a 7 am climb is a different thermal event from a 5 pm one.

• Choose the longer, shallower route if the alternative is short and steep. Sustained low-speed torque is the enemy; distance is not.

• Check tyre pressure before the climb, not after the ride.

• In summer, carry less and assist less. Both reduce current, and current is what makes heat.

FAQ

Is it normal for a motor housing to get hot?Warm is normal; too hot to hold your hand on for several seconds is a signal to reduce load and check the conditions. Most housings are designed to run hot, but sustained heat still shortens life.

Does overheating permanently damage the motor?A single cutoff usually does not. Repeated overheating degrades magnets, insulation, and bearing grease over time — which shows up as reduced efficiency and noise rather than a sudden failure.

Why does my mid-drive overheat when hub bikes do not?It may not be the motor type but the gearing. A mid-drive left in a high gear at low cadence is working harder than a hub motor at the same speed. Shift down and compare.

Can I add cooling?Not meaningfully on a sealed hub motor. On some mid-drives, aftermarket heatsinks exist, but the practical fix is riding style and load, not hardware.

Should I stop using throttle on hills?Throttle-only climbing is the highest-heat way to get up a hill. Pedal first, use throttle to supplement, and keep the motor spinning.

Will a bigger battery stop the overheating?No. A larger pack reduces sag, which helps slightly, but the heat is generated by current through the motor, not by the battery's size.