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 incentive for the manufacturers in using high quality components, thus the efficiency factor of a given hub motor is relatively modest (70-80%) at medium to high-speed. The best available direct drive hub motors might reach up to 84% efficiency.
At low-speed, efficiency drops down considerably to a mere 40-60%. Dumping high power at low-speed (thus low efficiency) into the hub motor, results in converting 1/2 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 pretty fast.
While the motor coils are relatively robust up to 150°C or 180°C, the neodymium magnets are way more delicate in this regard. Depending on the quality and heat resistance of the neodymium magnets used, temporary loss of magnetization can start at 80°C and permanent loss of magnetization can start at 120°C.
Standard ball bearings have a maximum operating temperature of 120°C as well.
Word of advise
Choose a hub motor with temperature sensor and use a Cycle Analyst v3 to prevent overheating. Touching the outer shell by hand will give you no idea how hot the stator already is! Even if the motor coils are simmering at 130°C, the outer shell is just a tiny bit too warm for comfort, to leave your hand on it for an extended time.
If your hub motor is too hot to touch, chances are you just ‘cooked’ your neodymium magnets already and they permanently lost part of their magnetization.
I usually set the Cycle Analyst to lower the power of the motor controller if the motor temperature reaches 120°C.
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. You really don’t want any oil near your disk brake, as well.
Recent tests by Justin Lemore from Grin Tech showed a pretty smart alternative:
His extensive tests showed, 4-5ml of ferrofluid has a similar cooling effect as 125ml of ATF. Actually his tests of ferrofluid where so successful, Grin Tech decided to turn it into a product, call it ‘Statorade’ and sell it in their online shop.
The ferrofluid sticks to sides of the magnets, forming little bulges, that bridge the air gap and ‘swipe’ slightly over the stator. While this ‘swiping’ adds only little additional drag, it enhances the heat transfer from the stator to the motor shell by a great deal.
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.
Since Statorade enhances the heat transmission from stator to the magnets (Remember? Neodymium magnets don’t like excessive heat), heat transfer to the surrounding area becomes even more important. To help shedding the heat, an ingenious Australian ebiker with CNC skills started to offer segmented heat sinks, that fit between the spokes and are held together by a couple of screws. HubSinks where born. Check his videos!
Testing Statorade and HubSinks
Initially, I’ve added 7ml of Statorade to my MXUS 3006RC with HubSinks, since the graphs from Grin Tech indicated that for this size of hub motor, 7ml of Statorade offers the best balance of cooling effect to drag increase.
At higher RPM’s however, the centrifugal force showed its effect on the Statorade. It needs a bit more Statorade at higher RPM for a similar cooling effect. Hence the optimal amount of Statorade depends on your usual speed.
For a week or two, I’ve flogged the trike to work every day while keeping an eye on the temperature displayed by the Cycle Analyst. When I though I got a feeling how the motor behaves when dumping up to 4kW into it, I’ve added the remaining 3ml of the Statorade syringe to the motor.
This tiny added amount lowered the mean temperature of my hub motor by a full 10°C without increasing drag in a discernible way.The time the motor needed to cool down also got a great deal shorter. Amazing stuff!
Now, I might stop at a red light with a motor temperature of 120°C and the short while I have to wait till its ‘green’, will be enough to let the motor cool down to 80°C or less.
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. Being much smaller and lighter they don’t have sufficient mass to suck-up heat spikes from consecutive hard accelerations for any length of time.
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 the 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
My first Bafang SWXH came with a conversion kit I’ve ordered for my ICE Trike.
It already served me well for about 6,500km in its 20″ wheel, but was prone to a stuck free wheel clutch. The free wheel clutch usually broke free after the motor had time to cool down, but the situation was annoying at best.
Hence, the SWXH was a good test subject to start an oil cooling experiment. Filling-in 40ml ATF through the brake disc screw holes cured the clutch problem immediately.
Great! Now, it was time to figure-out how much abuse the little bugger was able to survive.
I’ve relaced the SWXH into a 26″ rim and put it back into the ICE Sprint. It got used with a 54V (15s) battery and 35A controller, till it broke down with shredded planetary gears after nearly 1,500km.
destroyed planetary gears of a Bafang SWXH
Luckily for me, my Ebike parts vendor had three steel planetary gears in the right size for my early version of the SWXH left. As far as I can tell, they never ordered a new production run of steel spur gears, though.
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 tiny hub motor anymore. The steel planetary gears are much louder, though. For the first 100km or so, it sounded like a coffee grinder.
The little bugger merely shrugged when dumping 2kW bursts into it, or running for prolonged periods at 1kW continuous. In a torture test, I fed a 35A Lyen Mini Monster controller from a 22s battery at 90V, that propelled my trike to 68km/h on the flat at 2kW continuous and even that couldn’t kill it. It got pretty warm, though.
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.
The Bafang BPM
With a curb weight of 4.6kg, the ‘500W’ Bafang BPM is a slightly bigger version of the SWXH (2.9kg). It got way more torque and can withstand much higher power pumped into it by the motor controller. In stock configuration, a BPM will do fine up to a 15s (55V) battery and 45A controller (thats around 2.5kW), on the flat.
It will definitely not be able to climb long and steep hills at these power levels for any length of time without overheating, though.
If you use a 72V battery and 45A controller (3.5kW) however, the fiber reinforced planetary gears will fail in short order. You either use a smaller controller, or lower the controller current to about 30A with a Cycle Analyst.
Next option: Steel planetary gears and oil cooling (you kind of predicted that, right?).
Steel planetary gears are not easy to come by, though. Some ebike shops (unlikely), or maybe someone in the Endless Sphere forums might offer them. Chances are, you have to order them from a company like Mädler (in Germany) and have them machine a ball bearing seat to their spur gears. Expect to pay around €160 for three spur gears with ball bearings.
Anyway, the correct size of the needed spur gear is 42 teeth, Modul 1.25 and 10mm width. Choose spur gears made of mild steel (C45 or equivalent). The ball bearing seat needs to have a 28mm diameter and 8mm width for 6001 size ball bearings (12x28x8mm).
steel planetary gears for the ‘500W’ Bafang BPM geared hub motor
Same goes for the Bafang CST, since the planetary gears are literally the same.
Bafang CST ready for assembling with steel planetary gears
With steel planetary gears and 50ml ATF lubrication, a ‘500W’ Bafang BPM (screw-on freewheel) or Bafang CST (cassette adpter) will run just fine with a 72V battery and 50A controller. Better have a temperature sensor in your hub motor at these power levels, though.