In North America, the most popular direct drive hub motor today is the Nine Continent (“9C”) series, a family of rare-earth radial flux motors produced in mainland China. The design and construction of the motor lends itself to many applications having modest power-to-weight ratios, good energy efficiency, and still relatively affordable. There is however more to the story and this article aims to illuminate the strengths and weaknesses which we hope will extend the utility and lifetime of a smart purchase.
A few years ago I returned to riding daily and considered myself in pretty good shape. However the hilly nature of the Pacific Northwest presented a real challenge and curtailed the range, not to mention the fatigue at the end of the ride. Soon I began picking up clients that were farther away and began to look at options, concentrating for weeks on research and reading reviews. Finally I settled on building a two-wheel drive electric mountain bike thinking this machine could serve two functions both for work and for recreation, plus it could get me through the rare snow events. Without forbearance the pregnancy of this decision would completely alter my lifestyle.
|Settling on the 9C, I made my purchase from EBikes.ca(now called “Grin Technologies”) located in beautiful Vancouver, B.C., Canada, and mail-ordered for two complete 36V 25A kits. Excited, the first thing I did was take the motor apart to see how the unit works: Forgive me – I’m an engineer, I’m hugely curious, and chock full of questions. Needless to say I never did get around to reassembling and mounting that particular rear hub motor; instead it’s a great reference for study. However, the front hub was mated to my old 19 year old Specialized Rockhopper that still had the original RockShox, and from that moment on I was uncaged… quite literally!My 5-mile rides soon became 20, then 40 miles in a day. Within 3 months the weekend expeditions exceeded a personal record at 75, then felled shortly by my first Century! Amazing feats were accomplished with this simple 9C 2806 model direct drive motor which I had attached to a modified 25A controller, and (for the Century) mated with a custom 42V/50Ah (10S10P) LiPo battery configuration. Eventually I got around to building the two-wheel drive ebike, but that’s another story. The point is that these simple motors are tough little beasts, and with proper care and awareness of their flaws, they are completely suitable for most common applications.|
On the Road (Klamath Falls -> Sacramento): July 2nd, 2010 @ A21/Mooney Road & Hwy 36 in Westwood, CA. 1991 Specialized Rockhopper, 9C (non-disc) FWD, 15S12P 60Ah LiPo; bike & rider = 350 lbs.
Typical electric bikes will have one motor mounted on either the front or the rear, and operate within the 36-48V range. Generally, the 25A controller can provide sufficient power to drive a 26”-wheel Cruiser or MtB up to 24 mph with ease and dispassionately melt away most hill challenges with room to spare. But what makes the motor tick; what’s inside this fantastic little gem, and how does it work?
The 9C a Radial Flux Brushless DC motor having two primary assemblies:
- A Rotor (which rotates) consisting of a circle of metal, called “back iron” because it’s made of iron alloy, and bonded to the inside are a series of magnets with alternating poles facing inward and outward.
- A Stator (that is static/fixed), fabricated from a series of stacked “electric steel” plates that are machined with notches which create “teeth”, and each notch holds loops of copper-stranded coils (also called winds, windings, and turns).
|Rotor: Cover with bearing, flange assembly with a ring of magnets bonded to back iron (highlighted in yellow). A rubber shaft seal (not shown) keeps debris out.|
|Stator assembly: axle, windings wrapped around electric steel plates, held together by riveted stamped steel plate.|
|Imagine a small coil of wire attached to a controller which smartly manages the power passing through it by way of throttle. Current flowing through that coil determines the amount of magnetic field energy (“flux”) that is generated. The flow of the current through the coil creates positive force on one side of the loop and negative force on the other side. More force is created by adding more loops of wire to the coil, by raising the current, or by raising the potential voltage.To create opposing force we place a magnet next to the coil; ideally the magnet will be attracted or repulsed to the coil depending upon which magnetic pole is facing what side of the loop. The strength of the magnet and the amount of force created by the coil determines the total amount of power (attraction/repulsion), and the effect is inversely proportional to the square of the distance between these objects. In other words: The closer they are to each other, the more dramatic and more intense these forces of attraction and repulsion become, exponentially.|
All motors have at least two coil circuits, and for 9C motors – they have three; it’s 3-Phase, and the current traveling through each phase is shifted 120* from each other which allows more power through the motor and increases efficiency.
Finite Element Analysis Diagram: Magnetic Field intensity varies with current passing through the windings. Similar to how the copper coils rely upon the electric steel plates to create a path for the magnetic circuit, the Back iron works in conjunction with the rare-earth magnets to complete the entire loop:
|Rotor (as Magnets & Back iron) +|
Stator (as copper coils & electric steel) +
Power (as Electric Current) = Torque !!
Real-world image of the Diagram above, with Back iron highlighted in yellow.
The ratio of magnets (poles) to coils (teeth) is called “gearing”. Direct Drive motors use this fixed gearing to apply torque, to rotate the rotor relative to the stator (static) shaft. For the 9C, this gearing ratio is 46 poles to 51 teeth, or 23 pole-pairs (pp) to 17 teeth (t) per phase. It’s not precisely 1:1; however the ratio has to be unequal so that the rotor (and wheel) will turn. In comparison, RC motors and middrives have high gearing which is why they spin so incredibly fast, and correspondingly require external reduction gearing to lower the speed and multiply torque, and linked by chain to the rear tire. Direct Drive hub motors eliminate the extra mechanical links between the motor and wheel; it’s accomplished in one stroke, and therefore – if designed correctly, can be more efficient, if not simplifying.
Returning to the assembly, the stator of the motor is welded to the axle and the magnets are bonded to the rotor. The axle is fixed to the bike frame by way of the torque arm which prevent it from spinning. The rotor is affixed to the axle via the hub covers, and the wheel bearings provide nearly frictionless concentric alignment with the stator. The rotor has raised flanges on each side and drilled with a total of 36 holes which allows for spokes to mate the motor with the rim. The 9C motors accept 12-14 AWG spokes easily enough and are commonly paired to 20-27 inch bicycle rims that are 1.75-2.75 inches in width. Three phase wires exit the motor through the hollow axle. The winds of the coils are multi-stranded, lightly twisted, and roughly equal 10-12 AWG conductor. However the phase wires that exit the hub motor are closer to 14-15 AWG; they have to be smaller to fit through the hollow axle.
There are three Hall Effect (HE) sensors integrated into the stator and they provide accurate “sensored” rotor positioning to the controller. The HE sensors require +5V and GND to operate, and remit the signal back to the controller through a third wire, therefore the entire sensor harness assembly requires 5 wires, and these can be 28-36 AWG, although 33 AWG is common. There’s not a lot of room in the axle, and it’s a tight fit to be sure, but the sensor and phase wires are unified by a durable cabling sheath that resists tearing and cuts very well.
Hall Effect Sensors up close: Embedded into the teeth of the electric steel plates.
The 9C Model Family:
|Of late, the motors are available in several versions. Note that the motor finish is black paint. However some units are available as a sweet-looking buffed-out aluminum finish with protective clear coat; definitely magpie-attractive. Check with your supplier for availability.|
To the right are a set of front and rear 9C motors with clear coat finish mounted to Mavic rims ready to go home. REI did the work although any good bike shop should be able to handle it.
- Front and Rear; Front is less wide, and Rear supports typical freewheel cog mounting from single-speed to 7-speeds with some exceptions.
- Disc and Rim Brake; Motors for disc brakes have hub covers that sport a mounting boss for typical 6-bolt ISO bicycle disc rotors measuring between 5.5-8 inches (140mm-203mm). Motors for rim brakes do not have this feature.
- The Model Number loosely indicates the motor physicality. The most commonly stocked 9C models coming out of Canada are 2805, 2806, and 2807. The first two numbers indicated the width of the magnet – 28mm. The magnet thickness is about 1/8th inch/3mm, is rare-earth (RE), and likely between N35 to N42M in field strength. The second pair indicates the number of turns (loops) for each individual coil, thus a 2805 unit will have 5 turns, and a 2807 will have 7 turns. The number of turns affects two factors: Rotation speed and torque. More turns creates stronger flux density and greater energy conversion to torque, whereas fewer turns equates to faster spinning rotors. The implications are that more turns means more copper and more resistance. The bottom line though is that we want most copper possible to achieve the greatest efficiency. Therefore a 2808 motor is best for hill climbing, and a 2805 is better suited for more level road racing. Occasionally other versions will pop up, although these are likely custom-ordered and purpose-built for specific applications. I own both the 2806 and the 2807 flavors in disc and rim, front and rear.
Note: Motor weight depends upon the number of turns, type of cover (disc, non-disc), and size of the rim if premounted, though essentially the numbers fall between 14-15.5 lbs./6.4-7 kg each.
9C Rear hub with 7-speed freewheel.
9C Front and Rear Disc mated to 26” and 24” rims with phatty Hookworms for conquering der urban-Welt, ja!
The 2806 unit is a good middle of the road choice: It’s fast and yet strong enough to take on locally organized bike events – and win! There’s just something magical about passing a group of lycras struggling to get up the hill on non-assisted bikes. Personally, I play-nice and pass with modesty and safety in mind, though I’m definitely snickering beneath my breath.
The motor operates well between 36V and on up to 100V. They are rated out of the box at 500-750W, though can easily accept 1000W all day long. For cross-country, I run mine to 1500W and only have to watch it during the heat of the day on a long aggressive hill climb. The motors have been known to take 5000W momentarily; however these have been modified for race and won’t last a minute if prolonged. Still, there’s lots of room for adaptation to suit most needs; a natural fit for cargo bikes and recumbent trikes.
For all the wonder and greatness these units provide, they suffer from short-sighted thrift engaged in manufacturing. There are two basic problems that can affect both 9C – and just about any other motor design: Heat and Moisture.
- Heat will destabilize magnets and cause them to lose their magnetic properties if the temperature climbs too high, and it can also cause other materials – such as insulation to degrade and melt.
- Moisture is a double-edged sword because it leads to oxidation, meaning rust and corrosion, as well as causing coil short circuit. Combined with heat, moisture will be insidiously quick to destroy material integrity, leading to cascading failure. Therefore when we design a motor, it’s best to create ways to eliminate build-up of heat and resist attack by moisture.
|Heat is easily checked by limiting the amount of power from the controller (through firmware & current shunts), or by venting the hub covers through modification. For example, in Arizona venting is pretty much required three seasons out of four, whereas the Pacific Northwest – well, we’re just plain lucky if we get three consecutive days of sunshine so what does it matter? Seriously, venting – depending on size allows for air to circulate freely with the windings, and a cool-running motor is a healthy motor that will provide years of smart good service.|
|Moisture is a problem for areas of high humidity such as in the South or typical rainy Pacific Northwest. The motors are not watertight; they are sealed against inclement weather and are water-repellant, although by no means are they water-resistant. The motors breathe either through the hollow axle or through the hub covers. The motor becomes warm due to operation which causes pressure to build up inside as the volume of air expands and vents out. The opposite is true when we finish riding; the motor cools, it creates a vacuum and sucks air back in. The moisture is trapped inside and collects. Where it rains (daily in my case) the water also follows down the harness to the hollow axle and is pulled in to where it pools. The renewed heat of operation creates steamy Amazon-like rainforest conditions and will corrode all unprotected surfaces, including the magnets, the iron stator, and the hub covers.|
The motor in the image above was purchased new in May 2011 for a road trip into dry fair weather. Upon return in August I was commuting 45 miles/day into Seattle, heart of the Pacific Northwest where it rains nearly every day of the year. Both front and rear hubs on my 2WD ebike were rusted up by November. Neither hub was vented.
Once the hub motor rusts and seizes, it is pretty much ruined. The way to solve the problem before it begins is to vent the hub covers with at least one small hole at the periphery of each side which would be enough to cause any collected moisture to spin out. To properly arrest rust and corrosion, the stator, covers, and magnets require coatings of industrial-grade varnish in order to protect them at the same high-quality level of typical automobile starters and alternators. The good news is that distributors are listening: Recently attention has been given to provide better customer support, and models are just now appearing with overt modification options that includes venting and conformal coating.
Lesser Issues and Modifications:
Phase wiring through the axle is too light-duty for high-current situations and can lead to voltage sag (read: loss of power). Damage to the wires can occur when bogged down in the mud or crawling up a long hill. If the motor is too hot to touch, it’s been pushed too far. If you can smell burnt plastic (phenolic), the motor has been toasted and possibly lost some capacity. The phase wires will become hot before this occurs, and checking the insulation for flaws or melting will provide early clues to potential future failure. One way to resolve this is to upgrade the wiring; however, this is a very challenging mod and should only be applied as a last resort, or perhaps taken to a professional that has the setup to provide the upgrade. In comparison, venting hub covers is much easier.
All about power: This motor was modified with 10 AWG Mil-Spec silver-plated multi-strand aircraft wire. Trust me, it’s far easier to pull 12 AWG through the axle!
|Mounting Disc Brakeson the 9C is problematic, troublesome, and non-conforming with North America and Europe components. Electing to use these features means that we have to employ Chinese components to make it work or modify our own. At issue, the spacing between the inside of the mounted disc rotor and the hub cover is about 5mm too narrow. However this can be patched by:|
Once that’s accomplished, keeping the rotor trued and dealing with warping under operation present new challenges, and there isn’t much escape from that. It’s yet another fundamental design flaw that is easy to resolve if they made the part right instead of skimping. My disc rotors when attached to hub motors always rub!
Unfortunately the physicality of the hub motor will prevent complete truing, but you’ll be able to adjust most of the warp away. Warping is caused by two factors:
The last item is normal, at least from braking, and when the rotor cools, it rubs less. C’est la vie.
|Harness Connectors are only problem for mismatched controllers, although I will say that earlier models had clumsy torque arms and extra washers attached and I ended up cutting the sensor connectors off to remove the unneeded hardware. Some controllers use big mombo Molex connectors which are clunky and in my mind not very practical. But if the phase wiring mod is at hand, well… this is begrudgingly once more mod that can be had.|
First thing I do is cut the connectors off and remove the useless hardware hanging off the axle.
|Audible noise, or singing during normal operation is a patent trait of all BLDC motors, including the 9C; they have a low bell-like ringing noise due to the shape of the hub covers when combined with the relative frequency of a trapezoidal-shaped waveform during controller phase communication. The waveform causes audible resonance that is amplified by the hub covers like a large metallic speaker diaphragm, except we have one on each side!|
It’s possible to open up the covers and place sound-absorbing material on the inside… however this could interfere with heat dissipation, not to mention that it would throw the wheel off-balance. A better solution is to find a controller that communicated above human hearing, at frequencies greater than 22 kHz; good luck with that! If one could take away a positive thought: People and critters will curiously hear the motor as you approach in an otherwise stealthy manner. Perhaps it’s for the best.
Clones and Last Words:
This article is about Nine Continent motors, however there’s possibly more than one other motor on the market that looks remarkably identical except for the branding; less is known about the model specifics because the distributor does not go into details as well as EBikes.ca, though we feel compelled to list them anyway.
Last year I purchased front and rear disc motors from E-BikeKit.com and I thought I was getting 2806-equivalents, however when upgrading the phase wiring I noted “8×7” was broadly stroked on the stator which lead me to believe they were 2807 motors. Interestingly, there was about a 5-10% performance improvement over the original 2806. Was it manufacturing variance or mismatch? The jury is still out, though at this point – I’m not terribly concerned: Those motors got me to California and back!
Nine Continent motors are sold as 9C in other parts of the country and around the world, and are pretty much synonymous with Crystalyte in terms of brand recognition and being commonly available. It’s a good strong family of motors that is suitable for most needs, though certainly there are some issues to be aware of; if heat and moisture are problematic with your region then you will definitely want to modify the units to suit, otherwise I fear to suggest that failure is likely inevitable – and nearly always at the wrong moment.
For myself, except for pesky rust, it’s the best unit out there for the money and especially for what I’ve been using it for… which is urban assault and wicked cross-country adventures. With a departing caveat, I strongly wish that the manufacturer would get a clue and spend $5 or $10 more on quality during fabrication, making them suitable for American and European expectations instead of turning an apparent cheap blind eye and leaving it to the customer to make it right. Until something better rolls along, ebikes will be rolling on 9C motors.
On the Road (Washington -> California & back): July 27th, 2011 @ Spanaway, WA. 2009 Felt Compulsion 1 F/S 9C Disc 2WD (F & R), 15S26P 130Ah LiPo; bike & rider = 450 lbs.
Safe travels, KF