Tuning an electric hub motor is a relatively straightforward thing. The biggest obstacle to overcome is excessive heat build-up in the hub motor.
Most hub motors are cheaply made in great volume. There is no specific eye in using high quality components, thus the efficiency factor of a given hub motor is relatively modest (70-80%) at medium to high-speed. At low-speed, efficiency drops considerably down to 30-50%. Dropping high power at low-speed (thus low efficiency) into the hub motor, results in converting 50-70% of that power in useless heat. That’s ok for a short period, but consecutive hard accelerations from a stop or climbing a long steep hill can ‘cook’ a hub motor.
While the motor coils are relatively robust up to 150°C or 180°C, the neodymium magnets are way more finicky in this regard. Depending on the quality and heat resistance of the neodymium magnets used, temporary loss of magnetisation can start at 80°C and permanent loss of magnetisation can start at 120°C.
Standard ball bearings have a maximum operating temperature of 120°C as well.
Direct Drive Hub Motors
Direct drive hub motors have a small air gap between stator (the motor coils) and the rotor (the magnet ring). This air gap prevents easy transfer of heat from the motor coils to reach the outer shell. To bridge this gap, liquid cooling of a hub motor is the most obvious solution. For a couple of years, the liquid of choice was ATF (automotive transmission oil).
ATF got all the right property’s and additives to handle the job, but the stuff can make a real mess if you fill too much of it into a hub motor.
Recent tests by Justin Lemore from Grin Tech showed a pretty smart alternative. Ferrofluid. His extensive tests showed, 4-5ml of ferrofluid has a similar cooling effect as 125ml of ATF. Actually his tests of ferrofluid where so succesful, Grin Tech decided to turn it into a product, call it ‘Statorade’ and sell it in their online shop.
If you got a direct drive hub motor and like to drop some serious power into it, go to the Endless Sphere forum and start reading about the motor tests.
I’ve already got a syringe with 10ml Statorade from Grin Tech, but didn’t fill it into my direct drive hub motor, yet.
Geared Hub Motors
Since geared hub motors have two air gaps between motor coil and outer shell (one air gap between motor coil and magnet ring, the second air gap between magnet ring and outer shell), they are even more prone to overheating.
Geared hub motors have an other structural weak point. The planetary gears are made from glass-fiber reinforced Nylon (probably PA6 or PA 6.6). While being a perfectly fine material choice at the nominal power levels of geared hub motors, at higher power levels they can’t endure the higher temperatures and higher mechanical forces (speed and torque) for extended periods. The overrunning clutch (that provides freewheeling) is prone to seize-up at high power levels as well.
While running geared hub motors at twice their nominal power level for extended times is ok, tripling the power will shorten its lifespan considerably. At power levels above that, the planetary gears in a stock geared hub motor will fail in relatively short order.
I’ve run several geared hub motors at elevated power levels till they broke down. The obvious solution to stripped nylon planetary gears are steel planetary gears.
Since the small sun gear and the ring gear are made from tempered steel, planetary gears made from mild steel work fine when lubricated with oil. ATF (automotive transmission oil) to be precise. Getting the oil into the motor can be a problem, though. The holes for the brake disk screws of my Bafang SWXH did go through and it was easy to fill in the ATF with a syringe. The holes of the Bafang BPM, BPM2 and CST needed some drilling.
A small amount (30-50ml) of ATF is sufficient to fill the air gaps between the components for better cooling and provide way better lubrication to the overrunning clutch than the stock grease filling. Since lubrication of planetary gears and overrunning clutch is at least as important as better cooling of the neodymium magnets, ferrofluid is not a preferable solution for geared hub motors as in direct drive hub motors.
The tale of an abused Bafang SWXH
Bafang SWXH planetary gears destroyed from using a 15s Lipo battery (54V) and 35A controller (KU123) for nearly 1,500km
new steel planetary gears for a ‘250W’ Bafang SWXH geared hub motor
With the steel planetary gears and ATF lubrication, the ‘250W’ Bafang SWXH was able to survive astonishing power levels. After the upgrade, the 15s (54V) battery and 35A controller didn’t pose any problems for the little hub motor anymore. It merely shrugged at dumping 2kW bursts into the little bugger and running for prolonged periods at 1kW continuous. In a torture test, I ran it with a 22s battery at 90V, where it propelled my trike to 68km/h on the flat at 2kW continuous and even that couldn’t kill it. The steel planetary gears are much louder, though. The first 100km or so, it sounded like a coffee grinder.
The SWXH was my ‘test bed’ and proved what these small geared hub motors are capable of, once the power restricting Nylon planetary gears are replaced.
It got flogged more than 3,500km since the planetary gear upgrade and still serves as my back-up motor, but after running 11,500km at any weather, the seals of the ball bearings started to leak oil and need replacement.
steel planetary gears for a ‘500W’ Bafang BPM geared hub motor
They are not easy to come by, though. Some ebike shops and might offer steel planetary gears.