Direct drive hubmotors have remained popular for street E-bikes that are running hot rod amounts of power. Here is a list of some of the more popular rear hubs for building up a street-fighter E-bike that will embarrass an expensive sports car when the light turns green.
Last year was an exciting time for E-bike enthusiasts who wanted a mid drive system with many new component options. For commuters in a hilly region, the Bafang-BBS02 just keeps getting more and more popular, especially because it has been improved each year by the factory.
For off-road, the Lightning Rods mid drive remained a good seller, and they recently released their “big block” kit for riders who wanted even more power. Seeing all the buzz about mid drives might have gotten you started thinking that the simple hub motor is dying off, but…the truth is that large direct drive (DD) hubs have remained the best option for street E-bikes that are using very high amounts of power. So, let’s take a look at the popular Double-D’s!
If you are just a commuter that only needs 28-MPH (45 km/h) in a relatively flat region, you will most likely be best served by the most popular type of system, which is a geared rear hubmotor using about 50V X 25A = 1250W. They can freewheel very easily when not using any power. They also provide good torque by spinning a smaller (and lighter) motor much faster and then reducing the RPMs down to the wheel speeds with an internal set of gears.
However…they also have a smaller amount of copper mass than a DD hub, and they have a poor heat-shedding path. If you already know you will be frequently using the heat of 2500W (or more!), and will also spend a lot of time at speeds above 30-MPH…a large rear DD hub can be the ticket to a very fun ride.
Next up is a few things that you should know, so you will understand why I am recommending these particular hubmotors, and then…we will get on to the important part…the list!
We recently wrote about a new trend in large rear DD hubs, where builders were lacing their hubs to a moped rim instead of a bicycle rim. Due to the angled holes in the moped rims, a large DD hub can be laced with a one-cross pattern, which is much better than a radial pattern (which many builders had been previously forced to use to get a large hub in a smaller rim). Moped rims allow DOT-rated tires that are surprisingly affordable, and are also very flat resistant.
Because a large DD hub is heavy, we not only recommend using moped rims and tires, we also recommend using a bicycle frame with rear suspension (if you have a hardtail frame, please use a suspension seat-post). Although a rear suspension takes up some of the precious frame area with the shock absorber (limiting the size of a central frame-mounted battery), recent improvements in the battery cells that are available mean that even a small battery pack can have decent power and range. Read our article on the new high-performance cells.
A quality torque arm doesn’t wear out, and a cheap one can cost you much more than just needing to buy a stronger one if the first one fails at 40-MPH. Once you trace the shape of metal plate you’d need for whatever frame you have, the custom torque-plates can be water-jetted or laser-cut, and then shipped directly to your mailbox.
If 50V X 50A = 2500W is the starting point for the motors I’m describing here, you will need two very strong torque-arms, so don’t try to save a few dollars by going cheap on the quality or strength of the torque-arms, and don’t try to get away with only using one. Trust us on this!
Steel stator core vs aluminum
A couple of years ago, I noticed that the biggest and meanest hub motors were using a stator support system inside the motor that connected the axle to the stator-coils with thick aluminum spokes. This aluminum mass acted as a “heat sponge” that soaked up sudden and short heat spikes from using very high amps when accelerating…and then the aluminum mass would shed that stored heat over time when the motor was just cruising along. If you like the motor to stay sealed up against the rain, mud, and grit from the road…you should pay a little extra to get the aluminum stator support…it is well worth it.
[side note: of course, the cores of each coil on both styles are still made from laminated steel]
The old common style is a thin stamped-steel plate. It is lighter and costs less that the thick aluminum spoke style, but when you are shopping around, the Chinese websites rarely show the insides of the motor, because most customers don’t care about those types of details. We’ll tell you who has what.
Ventilation and oil-cooling
Large DD hubs respond well to ventilation or oil-cooling. These motors do not have a very good heat-shedding path. There is a thin layer of air between the hot (stationary) stator and the spinning aluminum side-plates. There have been many experiments that either added a little Automatic Transmission Fluid (ATF) to the motor, or… simply drilling some holes in the sideplate to let the excess heat out. Both methods have their benefits and drawbacks.
Oil-cooling keeps the motor sealed up against outside contaminants, but…no matter how well you use sealant to enclose the motor-case joints, there will likely be some occasional drops of leakage. You would drill a small hole near the axle, and with that hole positioned below the axle, you would add oil up to that point (you don’t fill up the motor completely). Since the oil will expand as it gets hot, the fill-hole should be covered by some type of vent to allow the inside and outside to equalize in pressure (this is important).
This small amount of oil also acts as an immediate heat-sponge, and then it will rapidly transfer the stator-coil heat onto the aluminum side-plates, where it can be shed much faster compared to not using the oil. ATF will not boil at the temperatures that are below the max temps we recommend for these motors, plus ATF is affordable, readily available, and already has anti-foaming agents. I have also heard of electrical transformer oil being used successfully.
Ventilation seems to be more popular option, and that is when you remove the side-plates and drill holes in them to ventilate the inside. It makes sense that a car alternator has a skeleton-ized frame to allow for free air-circulation, and they spin thousands of RPMs while producing 100A. If you want to ventilate a hubmotor, I advise you to spray a coating onto the stator to water-proof it against rust (see our article on hot rodding hub motors for more details). Also, if your motor is getting way too hot on a frequent basis, you might be using a motor that is too small for the job
Be aware, the Hall sensors are typically more fragile than the stator (including the fragile soldered joints), and Halls have frequently been found to die when abused with too much heat. If you like using tons of amps, you might be well-served by a sensorless controller (or at least have one handy as a spare). Just remember that; if you have three functioning Hall sensors, those sensors can make a slow ride on steep uphills more smooth and controllable.
I always recommend that you order a temp sensor to be installed from the factory, if they have that as an option. Even if you don’t add a read-out to it right away, you have it available without needing to take the motor apart to add one later.
An upper heat limit of 200F / 93C is a reasonable balance between performance and caution, and that temp limit will likely allow your components to last a very long time. Be aware that if you get the magnets on the rotor too hot, they will permanently lose some of their power, even if the motor still runs.
If you use a $2 10K Thermistor as your temp sensor, the Cycle-Analyst V3 has an awesome heat management function. You can program the amps from the controller to automatically be cut back when it gets very warm (perhaps at 160F / 71C?), and if the heat keeps rising, you can program the amps to be completely cut off at your designated set-point (like 200fF/ 93C?). Once the motor cools down, the CA-V3 will re-apply the amps from the controller.
If you determine that your motor is not big enough for the job (due to frequent heat issues), at that point it will still be running and sellable, so you can apply that money to purchasing the new parts. Many early E-bike hot-rodders still own several fried and un-sellable motors that they use as “wall art”.
If you wrap some copper wire around a chunk of steel and apply electricity to that wire, the steel will temporarily become an “electro-magnet”. However, as some of the electrons in the steel material move around due to the magnetic forces, their movement causes “eddy current” heat. Since the copper wire is already getting more heat than we want, the undesirable heat from eddy currents can be reduced by making the electromagnet cores out of a stack of thin steel plates, called laminations. The quality of the steel in the laminations (and also their thickness) can affect how much eddy current heat a motor will suffer.
Common E-bike motors use lams that are 0.50mm thick, but…for a small extra charge (less than $10 per hubmotor?), many motor manufacturers have begun using the thinner 0.35mm lams, which not only reduces eddy current heat, it improves the motors efficiency about 3%, and lowers the cogging by about 30%. Cogging is the magnetic drag that some motors experience because the permanent magnets in the rotor are attracted to the steel in the lams, even when the motor is turned off and just rolling along. Geared hubs have no cogging due to an integrated freewheel inside the motor-case. DD hubs, however…often have “some” cogging. Thinner lams make DD hubs easier to pedal when the motor is off.
Turn Count, Kv, and Wheel Diameter
Each stator tooth has a certain amount of airspace into which the manufacturer can wind copper wire around it. You can use many turns of thinner wire, or fewer turns of thicker wire. This is referred to as the motors “turn count” and a motor with 5 wraps of copper wire bundles around it is referred to as a 5T. The width of the stator also affects the RPMs the motor will spin at (due to the increased resistance to current flow from longer lengths of wire in the coils).
So…once all the factors are calculated, each model of motor has a resulting RPM-per-volt that is applied (more volts = more RPMs). This is referred to as the motors “Kv” (19th century German for Konstant per voltage). A 5T winding in a narrow stator will have a different Kv compared to a 5T winding in a wider stator.
To make it simple (as an example), if you apply 48V to a motor with a Kv of 10.0, then…48 X 10 = 480-RPMs at its top speed (when unloaded and just spinning in the air).
The wheel diameter is another factor that you have to calculate. For the street, a smaller diameter wheel is more efficient (due to the higher motor “magnet speed” per distance that the wheel traveled), but…switching from one wheel to a smaller one will also lower your top speed.
In this example, a motor spinning 480-RPMs, would be traveling 37-MPH (59-km/h) in a common 26-inch outer-diameter (OD) of tire, but…if you switched to a smaller 17-inch moped rim with the recommendad 2.5-inch tire, the OD would be roughly 21-inches, and then the 480-RPM would result in 31-MPH (49-km/h).
The ElectricBike.com Hot Rod hubmotor list for 2015:
Watch the 2-minute video of Luke/liveforphysics and his friends when they melt down a common Crystalyte H35 hubmotor to see how much side-plate ventilation helps it.
Leaf Bike 1500W 205/35H
This is my new favorite DD hub for the street. It has 130mm wide drop-out shoulders on the axle, so it fits into common bicycle frames without any spreading of the swingarm. There is room for a 6-speed freewheel, and a 7-speed will fit if you spread the drop-outs on the swingarm a few millimeters (a third of an inch?).
If the 35mm wide stator had been made wider, you’d have fewer gears, and also…the motor would be heavier and more expensive. The other selections in this article only have a single-speed freewheel, so if you want at least several gears to pedal with, this is the one for you.
Don’t let the 1500W catalog name fool you. This hub can eat 1500W while its sleeping without getting warm. The 205 in the name means the stator is 205mm in diameter, and the 35H means the stator is 35mm wide (“H” is for height, the Chinese motor catalogs list their internal construction this way, viewing the motor on its side). The vast majority of affordable DD hubs I see have a stator about 28mm wide, has a stamped steel stator core, and use the thicker laminations.
Speaking of watts, you could run this motor on 60V or 72V, but…it really shines at 48V. This fact would allow you to have a smaller (and easier to fit) battery compared to a higher-voltage pack, but still holds the same Amp-hours (Ah) worth of range. A few years ago, there were no practical high-amp batteries available for bicycles, so…high power meant your only choice was using higher voltages. The last few years, LiPo batteries have become popular for racing, but I do not recommend them to the average customer due to safety concerns.
Last year, we wrote an article on several new selections of safe high-amp cells. However, this year there has been a dramatic improvement in the selection of high-amp 18650 cells, which have proven to be the safest batteries available. Using 50V X 50A = 2,500W is now a plug-and-play option that you can use with confidence. Luna Cycle has begun carrying high-performance 25R and 30Q battery packs so that E-bike hot-rodders can have a US-based supplier without the risk and delays of buying from China.
This motor has thin 0.35mm laminations, so it runs cooler than an identical motor with thicker lams, with both using the same amps. Heat is the enemy when you are raising the power to find a motors’ limits.
I recommend ordering the factory-installed temp sensor, the upgraded thicker phase wires (3mm instead of standard 2mm), and run this motor with a 12-FET controller…if…the controller has authentic cool-running 3077-FETs, and adjusted for a max of 50A temporary peaks.
If you want to use a 60V nominal pack, you would need to move up to a controller with higher-voltage 4110 FETs, which are less efficient. That means you would have to use an 18-FET controller to safely run 50A peaks, due to more of the input watts being converted to waste-heat.
Rough idea of 48V speeds in a common 26″ OD diameter tire:
4T: 39-MPH (62 km/h)
5T: 31-MPH (49 km/h)
6T: 25-MPH (40 km/h)
7T: 21-MPH (33 km/h)
Even if you don’t need 39-MPH, you might order the “fast” 4T if you want to lace the hub to a smaller 17-inch moped rim (similar to a 20-inch bicycle rim). This would be the most efficient option due to the faster motor magnet speed. The “slow” 7T might be useful for someone who wanted to use 60V or 72V in a 26-inch OD tire (or 19-inch moped rim?), which would raise the top-speed back to normal when compared to using that particular Kv of hub at 48V.
[edit, July 2016…A new motor is now being retailed in North America. Its sold under the “Edge” brand at L.A. Ebikes in Harbor City, Southern California. It appears to be very similar to the Leafbike 1500W motor in every way, but it has the desirable aluminum stator support. I may be getting one of these motors soon from Alan Hu for testing, and I will report back]
From Crystalyte, The Crown / TC
The name Crystalyte is well known around E-bike hot rodders. Their H35 and H40 family of motors were affordable hubs that were used in a LOT of experiments. Sadly, the “H” axles have not proven to be as strong as I would have liked. They were adequate for their rated powers, but when using hot rod levels of power, they occasionally failed. The “H” hubmotor family doesn’t have the option of an aluminum stator-core, or thinner laminations. Also, the Crown has deeper stator slots (compared to the H-series), which means the H-family has less copper-fill, which equals fewer amps.
For builders who want all of these upgraded features, Crystalyte has “The Crown“, which can be had with a 30mm wide stator or a 40mm wide stator. The weaker 30mm version still has such a wide case that it still can only use a single-speed freewheel, so…I am only interested in talking here about the 40mm Crown.
Model_____Kv (RPMs per volt)
The 8.8-Kv seems to be more popular. At 60V X 8.8 = 528 unloaded RPMs. That RPM in a 26-inch diameter tire would be 41-MPH (66 km/h)…AND!…60V X 60A = 3,600W
E-Motorcycle Hubs with Bicycle Axles
The next three motors are very heavy and can take a LOT of power. They are essentially an electric motorcycle hubmotor that has had its axle adapted to allow it to be mounted on a bicycle. Because of their width, you can only use a single-speed freewheel, which only exists to legally make it a “bicycle”, because…how much pedaling can you really add, when you are going 50-MPH (80-km/h)?
I can’t imagine going to the effort of buying a motor as heavy and capable as these, and then running it off of only 48V, so…these motors are most frequently seen using something between 72V-100V.
72V is the limit to get what I consider to be reasonably affordable controllers. Any voltage above that will cause a sudden increase in the cost of a viable controller. At 72V and also above 72V, three names I read about quite often are Kelly, Sabvoton, and Adaptto, but…I have no experience with any of them.
MXUS 3000W V2
This motor made a big splash on the E-bike scene last year. It is the “value” leader in this category because it costs less than the other two, and it is also a little lighter with its 45mm wide stator. Of course that lighter weight means it can’t take quite as much power, but it can take “enough” that it is being seen very frequently these days. They can currently be purchased in North America at Kinaye Motorsports.
If you see a “V1” version on the MXUS AliExpress web-page, it has the steel stamped stator support and thicker laminations. You want the “V2” with the thick aluminum in the stator support, and thinner laminations.
The popular 5T at 72V would provide 518 unloaded RPMs, and when mounted in a 26-inch tire, that would be 40-MPH (64 km/h), and…72V X 70A = 5,000W
This is the motor that started this entire category. The Croatian company Zelena Vozila sounded crazy a few years ago, when they decided to order a large lot of these motors in order to get the factory to modify the axles to fit in a bicycle frame according to their specs (and add a freewheel mount on the right side), but there were enough crazy E-bike builders that this high-quality hubmotor is still selling to this day.
This model is listed as a 50mm wide stator. It only has a single-speed freewheel (of course!), but the drop-out shoulders on the axle are 150mm wide. This means you will need custom torque-plates to allow for the added width, in addition to needing the two torque plates for the high power.
This is the hubmotor that got bystanders to start yelling “That’s not an E-bike, it’s a motorcycle!”.
72V X 85A = 6100W
QS / Quan Shun, 205/50H V3 “Extra Type”
The QS motor company has also decided to begin selling their E-motorcycle hubs with axles that that have been modified to fit a bicycle. Their motor phase wires are thicker than the other motors listed, and their axle design is stronger. The stator width is 50mm, but also…the stator-teeth slots are deeper for more copper-fill, and all of this adds up to a motor that can take more watts.
Another way in which this motor is very interesting is the many different Kv’s that the factory will wind the stator coils. This provides the builder with a LOT of options when trying to balance which voltage, tire-diameter, and top-speed you want. 72V X 100A = 7200W
This broad selection is a result of QS supplying the enormous electric motorcycle market in China, which has been booming due to a crackdown on pollution. Concern about poor air quality in the cities has resulted in 2-stroke gasoline engines being banned in many places
There are actually other QS motors that are larger than this (with a 273mm diameter stator), but…this 205mm-dia stator provides a level of power that stretches the already abused electric “bicycle” label. Even if you don’t need more power than the other two options above this, the QS 205 / 50H V3 will run cooler than the others at all power levels.
The extra copper mass in this motor makes this hubmotor the heaviest of these options, and the biggest problem with it will be finding the space on your frame to mount a big enough battery to keep it fed. And make no mistake, this hub is hungry!
How much power can they really take?
I’ve listed my recommended power levels for each motor option, but…the real limit to how much power you can feed them is…how much heat they can survive?
If you live where it is fairly flat, you can dial up more amps, because the motor will temporarily draw a short peak of amps when the E-bike is accelerating, but once you attain a stable cruise speed…the amps (and heat) slide down into a milder level, and the motor (plus controller) will then have some time to cool off before the next acceleration.
The worst-case scenario is when you are climbing a very long and steep uphill, with no cooling-off phase. The aluminum stator-cores I promoted here will only help for a short while. Once the motor gets heat-soaked, you have to lower the amps, or risk a melt-down. If you notice that your temperature sensor is indicating your motor is getting too hot, and doing that too often? You must either lower the amps, oil-cool the insides, ventilate the side-plates, or move up to a larger motor.
However, if your commute leaves your motor and controller barely warm, then riding on your particular commute will allow you to raise the amps a little more. There is nothing quite like the feeling when you out-accelerate a $80,000 sports car with a $3000 hot rod E-bike…
Written by Ron/spinningmagnets, August 2015