Your ebike pulls hard on the flat, then fades halfway up a long climb. The display throws a temperature warning, the power drops back, or the motor shell is too hot to touch when you stop. That is your hub motor overheating, and it is the most common limit on any high power hub motor ebike. Here is why it happens, how hot is too hot, and how fixing it lets the motor you already have handle far more power.
Signs your ebike hub motor is overheating
- Power cuts out or fades on long hills, then comes back after you rest
- Your controller or display shows a motor temperature warning, or limits power (thermal rollback)
- The motor shell is too hot to keep your hand on after a climb
- A hot varnish or burning smell from the wheel
- Over time, less torque and top speed than the motor had when new
Why hub motors overheat
1. Heat comes from current, and it climbs fast. Most of the heat in a direct drive hub motor is made in the copper windings of the stator. That copper loss rises with the square of the current, so doubling the amps makes roughly four times the heat. A bigger controller, higher voltage or a heavier load all push more current through the same copper.
2. Slow and heavy is the worst case. A hub motor is least efficient at low speed under high load. Steep climbs, technical trail at walking pace, a heavy rider, cargo or a trailer keep the motor in exactly that zone, where most of the battery power turns into heat instead of motion.
3. The heat is trapped. The stator sits in the middle of a sealed shell, surrounded by an air gap. Grin Technologies, who have measured this in detail, put it simply: a direct drive hub motor generates heat internally at the stator, but this heat has no easy path to escape since it is surrounded by trapped air (Grin Statorade research). Air is a poor conductor, so the windings keep heating while the shell stays comparatively cool.
4. The shell has to dump it all to the air. Whatever heat does reach the shell can only leave through its outer surface. A smooth round shell has limited surface area, so even once the heat gets out of the stator, the motor can only shed it slowly.
So there are two bottlenecks: getting heat from the windings to the shell, and getting heat from the shell into the air. Fixing only one of them leaves you limited by the other.
How hot is too hot for a hub motor?
There is no single number for every motor, because magnet grade and winding insulation vary. As a guide, the long running rule of thumb among builders on Endless Sphere is to never let the hottest part of the motor pass about 93C, with around 60C being a comfortable continuous temperature (Heat Damage to Motors (Magnets), Endless Sphere).
The damage you need to worry about first is to the magnets. They can lose strength permanently at temperatures well below what it takes to burn the winding insulation, so a motor that is regularly cooked will slowly get weaker even if it never fails outright. Burnt windings are the final, expensive stage.
If you run a big hub motor, fit a temperature sensor in the stator and display it on your controller or Cycle Analyst. You cannot manage what you cannot see.
How to stop your hub motor overheating
Ride and set up smarter
- Lower the controller current limit on long climbs, or set a thermal rollback so the controller protects the motor
- Avoid crawling up steep hills at very low speed under full throttle
- Match the motor winding and wheel size to your terrain
These help, but they work by giving up power. The two fixes below let you keep it.
Fix bottleneck one: Statorade ferrofluid
Statorade is a magnetic fluid that sits in the air gap between the stator and the magnets, bridging the gap with a conductor instead of insulating air. In Grin's testing on one hub motor, adding 6mL raised the heat conduction from windings to shell from 1.8 to 4.3 W/°C, and the steady state core temperature fell from nearly 90C to under 60C. Grin estimate that doubling the heat a motor can shed lets you run roughly 40% more sustained power without overheating (source).
Shop genuine Statorade ferrofluid
Fix bottleneck two: HubSink heat sink fins
Once Statorade moves the heat into the shell, the shell becomes the limit. HubSink is a set of six anodised aluminium fin segments that slot between the spokes and bolt around the magnet ring, with thermal paste for full contact. The fins multiply the surface area of the shell, and the wheel's own rotation and your forward speed push air straight through them.
Cooling is how you unlock more power
Most riders don't come to HubSink because their motor is overheating at stock power. They come because they want more: a bigger controller, more amps, higher voltage, harder climbs. The magnets and copper can take far more power than most motors are run at. Heat is what stops you.
To be clear, HubSink does not make power. Your controller and battery do. What HubSink does is let your motor handle far more of that power without overheating, so you can turn your controller up and keep it there.
Every hub motor has a temperature ceiling. How much power you can push before you hit it depends on how fast the motor sheds heat. Shed heat faster and you can run more amps for longer before reaching the same temperature.
Grin's lab figure of around 40% more power from doubling heat dissipation is for a motor at a steady, continuous load. Real riding is bursts, climbs and recovery. Better cooling slows how fast the motor heats up and speeds up how fast it cools down between efforts. That is why HubSink customers commonly push more than double the power through the same motor without it overheating or hitting thermal cutouts.
The riders below are good examples: an MXUS at 8kW, a fat bike at 7500W, an Adaptto at 120 amps, and race prototypes at 20kW, all on hub motors that would cook at those power levels without help.
Guaranteed: double the power or no more overheating
We back this with a guarantee. Fit HubSink with Statorade to a compatible motor and your motor will either handle at least double the power it could before without overheating, or stop overheating at the power you already run. HubSink does not add power itself, it lets your motor take more from your controller. Most riders get both, turning their controllers up to more than twice the power while riding harder, longer and more efficiently. If it does not deliver, you get a full refund within 30 days. Read the guarantee and conditions.
The wind tunnel testing
HubSink was developed over more than a year of real world, computer modelled and wind tunnel testing, documented openly on Endless Sphere (HubSink official thread).
The independent numbers come from Grin Technologies in Vancouver, who built a wind tunnel test rig to measure hub motor cooling and documented the work in the long running Definitive Tests on the Heating and Cooling of Hub Motors thread on Endless Sphere. They split cooling into two paths, core to shell and shell to air, and tested a Cromotor with Statorade and HubSinks (results):
- Shell to air: Statorade made virtually no difference, while HubSinks gave a 50 to 60% improvement over the bare motor shell.
- Total heat flow: Statorade alone improved the motor's overall cooling substantially, and HubSinks added up to 20% more on top of Statorade.
That is why the two work as a pair. Statorade fixes the inside, HubSink fixes the outside. Grin also sell HubSink in Canada (ebikes.ca HubSink set).
Real world results from riders
2km hillclimb, before and after. On a 2km climb with around 400m of elevation at an average 15% grade, on a 25 degree day, the stock motor hit its thermal cutoff at 124C. With HubSinks fitted, the same motor was at 91C at the same point on the climb (Endless Sphere).
Racing at 20kW. HubSink prototypes were raced, and won, at the TT1500 at around 20kW.
From the forums: reports from riders on Endless Sphere.
- Quokka, running an Adaptto controller at 120 amps with HubSinks and ferrofluid, reported the motor running around 30C cooler on the same hills while going faster, over 80km of dirt and trail in mid 30s heat.
- Jesse Haubner, running an MXUS at 8kW, no longer had to stop to cool down on the hills that used to thermal limit him.
- Hugh Pomeroy, on a 7500W fat bike, found the motor not warm at all after a full speed mountain climb.
- Other riders running HubSinks with ferrofluid report peaks that used to reach 175 degrees staying in the 150s, and the motor cooling down in half to three quarters of the usual time (Endless Sphere).
- rberger, Crystalyte 3540 on a PhaseRunner controller with Statorade and HubSinks, carrying panniers at about 150kg total: hill peak motor temperature dropped from around 110C to under 80C (thread).
- Tats, HS3540, 1800W, about 125kg total, 37C ambient, hilly route with no pedalling: peak motor temperature stayed around 87 to 88C (thread).
- ridethelightning, QS205: reported the HubSinks "worked a treat", with motor temperature falling quickly whenever he eased off or stopped (thread).
- RollerBoy, a commuter, said the HubSinks were very well made and fixed his motor's cooling problems with Statorade after about nine months of use (thread).
- ebike11, QS273: found the QS273 HubSinks very effective on a high power hub motor, needing only a little massaging for full contact and one tube of Statorade (thread).
- Andy Braidford installed his in 10 minutes and reported better heat shedding, and Dr Bass said they work well and plans to test them in summer heat (thread).
Read the full reports in the HubSink official thread.
Will HubSink fit my motor?
HubSink V2 fits most direct drive hub motors, including QS205, MXUS, Leaf, Crystalyte 35XX and 54XX, 9C and clones, Cromotor, and generic 222 to 238mm shells, with larger options for the TC Crown and QS273. See the full list in our guide to running more power through a hub motor, or go straight to the QS205 and MXUS pages. HubSink does not suit geared hub motors or mid drives, which have a different construction.
Common questions
Is it normal for an ebike hub motor to get hot?
Warm is normal. Too hot to touch, power cutting out, or a temperature warning means the motor is at or past its limit for that load, and repeated cooking will weaken the magnets over time.
Can I just drill vent holes in my hub motor?
Vented side covers do move air through the motor, but they also let in water, dust and grit, which is a problem for trail and wet weather riders. Statorade and HubSink keep the motor sealed.
Does ferrofluid slow the motor down?
Grin found that filling the whole air gap adds drag, but a small dose gives nearly the full cooling benefit. Use the recommended amount for your motor.
Can I run more power with better cooling?
Yes, and for most of our customers that is the whole point. Heat is usually what limits sustained power from a hub motor, not the magnets or the controller. With Statorade and HubSink fitted, customers' motors commonly handle more than double the power they could before overheating, and we guarantee it.
Stop the overheating
Whether your hub motor is cutting out on climbs or you want to turn the power up, fix both bottlenecks: Statorade inside, and HubSink outside. Guaranteed to let your motor handle double the power, or stop the overheating, or your money back. Shipped worldwide from Australia.