2-speed hub motors, E-matic and Xiongda

June 29, 2014

This is an important development for E-bikes, but they have not yet evolved into their final form. These are relevant because hub motors remain the simplest, easiest, and most affordable way to electrify a bicycle. When we have discussed electric kits that can be affordably added to a bicycle, they generally fall into three categories. Direct-Drive (for silence, or high-performance), geared hubs (for general efficiency and stealth), or non-hub (for street-legal low power limits, or extra steep hill-climbing).

A 2-speed hub creates a completely NEW category. Currently (in 2014), there are two examples…and their performance shows that a 2-speed hub could someday take a solid share of the global market for E-bike kits…

We recently wrote an article about the spread of popular E-bike kits, found here. The results are that; the majority of people who are very happy with their E-bike?…are using 48V X 25A = 1,200W. Of course you can use more power if you want, but…more power is more expensive, so…1,200W seems to be the “sweet spot” between more performance and achieving affordable fun. (electricbike.com supports safe riding and speed limits on public streets, but…each consumer should be able to have as much power as they can afford)

This past year we have written quite a bit about mid-drive kits, since they are the exciting “new kid on the block”. However, mid drives can be noisy and expensive (noisy due to high-RPM chains), compared to hub motors. So…if you are most interested in a hub motor kit (because of affordability or quiet operation), you have to decide between a geared hub, or…a Direct-Drive (DD) hub. Geared hubs are more popular because they freewheel easily when just pedaling without E-power, and DD hubs remain the most affordable starter kit, plus…at power levels above 1,200W…they remain the best option for very high power.

Where do 2-speed hubs fit in? And…why are they a real milestone?

To really understand why a 2-speed hub could turn into a game changer, we would like to tell you the story of the original bicycle 2-speed “retro-direct” system.


The 2-speed Retro-Direct

First of all, I have to thank endless-sphere member “Miles” for freely giving out his expertise concerning the engineering principles that have benefited us all, and it was he who first told me about retro-direct systems.

The first modern-looking “safety” bicycles in the 1880’s  only had one speed. Since they were invented 30 years before an affordable car, bicycles were a milestone that were enthusiastically embraced by the general public in a way that is difficult to convey how important they were in the 1880’s.

The first multi-speed system for bicycles was called a 2-speed “retro-direct”.



This image shows a bicycle that is arranged so that when it is pedaled in the forward direction, the bike is in top gear. The two freewheels on the rear wheel always spin in opposite directions. When the bike is pedaled in reverse, the larger freewheel is driven, which provides low gear for hills.


The “retro-direct” system means that…the bicycle will provide ONE gear ratio when pedaling in the forward direction, and…it will provide a different gear ratio when pedaling in the reverse direction. The rear wheel has two single-speed freewheels threaded onto it, one facing the normal way, and another (with a different tooth-count) reversed. It would be typical for the larger freewheel to be on the inside with the smaller one outboard of it, but either way will work.

The retro-direct system was patented by Barberon and Meunier  in 1869. The original system used two complete and separate sets of components, with one on each side of the bike. In 1903, the Hirondelle Bicycle company patented a version with a single chain driving two freewheels on the right side.


Reversing Motors

I was quite surprised to find out that electric motors of the size and speed that are found in the common geared hubs (BPM, BMC, eZee, etc) can easily be stopped and reversed almost instantaneously. I had heard of E-bike controllers that had a reverse mode available (typically only used by a heavy cargo trike or Pedi-cab). But in this instance…the reversing of the motor must happen smoothly and rapidly when called upon to do so….and without the bike coming to a stop.



Here is the drawing from the 2010 patent filed by SRAM for their 2-speed E-matic system, and listing  Brian Jordan as the inventor.


Geared hubs already have one reduction built in. This allows the motor to typically spin five times for every wheel rotation. This increases the torque output from a smaller motor, and it also provides a small help to the bike systems’ efficiency. By adding a second set of gears and a clever arrangement of one-way roller-clutches, we can have a motor that provides 16-MPH in one direction, and when the motor is reversed “on the fly”, it provides a low gear for hill-climbing, perhaps 10-MPH as an example. This would allow a modestly-sized motor run cooler on the hills, while still maintaining a reasonable speed.



Only the battery is separate on the SRAM E-matic system…the motor, controller, and torque-sensor are all integrated inside this single housing. There is only room at the hub to contain a single freewheel, but the crankset can still hold two or three chainrings, if desired.



There are two companies making these that I know of. First is  SRAM’s “E-matic” system (found on the Electra Townie Go! and the Ohm XU450-E2), and a new company called Xiongda, who calls their motor a “double-speed”. Xiongda is based in the Chinese city of Suzhou, near Shanghai.

The SRAM E-matic is not sold to the public as a free-standing system that you can install on your bicycle, it can only be purchased as part of a factory E-bike. And although it is now possible to buy a Townie Go! in North America, the E-matic is designed to meet the European Union E-bike laws.

This means that it is a pedelec only, so there is no option to use a hand-throttle. The integrated torque-sensor understands when you are pedaling, and only adds power when the pedals are moving. The power is also limited to 250W, and the system will only add power up to 16-MPH (25-kph). You might be able to pedal faster than that, but the electric assistance fades above that speed.



The 2015 Electra Townie Go! which uses the SRAM E-matic 2-speed rear hubmotor.


I don’t know if Xiongda has licensed this technology from SRAMs patents, or if they have developed enough of a variation that it does not violate the patent laws. Either way, their “double speed” motor is clearly a retro-direct that changes speeds by reversing the motor. Here is a 2-speed hub patent from 1982, from our article on E-bike history, so perhaps SRAM and Xiongda are both using an expired patent that is available to everyone?

One huge performance benefit of the Xiongda motor is that the controller is separate from the motor housing.  The SRAM E-matic is a very sleek device, with all the components hidden inside the motor-shell, but having the heat of the motor and controller enclosed together means that you cannot “hot rod” the E-matic hub.


The Xiongda Double-Speed motor

If you want to experiment with a small 2-speed hub kit, the Xiongda is the only game in town right now. The stock controller provides a maximum of 15-Amps, but you can order the kit with controllers that will work with 24V, 36V, or 48V. Since the majority of the heat that can damage a motor and controller will come from the amps, I recommend the 48V controller to really see what this system is capable of.

48V X 15A =720W. That might not seem like a huge amount of power, but since the motor uses two widely-spaced gears, I am told it still performs quite well at that modest 720W.

endless-sphere.com member Dave (user name: d8veh) in the UK brought this motor to our attention in this discussion thread. When using this motor with a 36V controller, the top gear provided 25-kph (16-MPH) with NO pedaling (in a common 26-inch wheel), which is the EU maximum legal pedelec speed. Dave tried it with a 44V LiPo battery, and he felt that it performed much better at that voltage, and it managed to climb a hill that is nearby without overheating, which has a very steep 14% grade.



Here is the full Xiongda kit with a rear motor. A front motor is available by request.


The hubs unloaded speeds (with the wheels in the air) when using roughly 46V are reported to be 22-kph in low, and 36-kph in high gear (14-MPH and 22-MPH). The bikes computer uses a measurement of the bikes speed and current that is being drawn through the controller to determine when to shift up or down. The kit also has a handle-bar mounted control switch that allows the rider to select for the system to stay in either high, or low gear, or to allow it to shift by itself…automatically.

When loaded down with the weight of a rider, the top-speed using a 12S battery (44V nominal), should be below the US federal legal street-limit of 20-MPH (32 kp/h)



You can specify the Xiongda motor in any one of three finishes. Polished aluminum, black, and silver/gray.


Why not a mid drive?

Mid drives are a popular subject this year (in 2014). By having a motor drive a freewheeling Bottom-Bracket (BB), the motor has the use of the bikes gears on the rear wheel. A 7-speed derailleur system is common and cheap, so that gives the motor an impressive 7 gears. And the upscale Rohloff IGH would provide 14 gears!

The reason I feel these 2-speed hubs will eventually do well is that they represent a huge improvement over the popular “one-speed” geared hubs, and they cost less (and are simpler to install) than the popular mid drive kits. The GNG mid drive kit has many faults in it’s design that need upgrading to be reliable, and even so it is still popular at a price of $400 (without a battery or charger). The Xiongda 2-speed hub is reported to be available at various prices around $250.

I can see the Xiongda becoming popular as a street-legal hub kit in places that have a 250W power limit. If you have extra-steep hills, I still believe a mid-drive that gives the motor the use of the bikes gears is the still best option (typically seven gears), and it’s worth the extra cost and effort. However, if you only have medium hills, the Xiongda at 48V is a simpler install for the do-it-yourselfer, and would likely perform better than a one-speed hub (whether geared or direct-drive).

If the price is similar between a common geared 250W hub, and the Xiongda with 2-speed?…I can imagine quite a few people choosing the Xiongda. Make note that the 48V version actually supplies 740W, but…it looks exactly like the EU legal 250W version!

Dave reports that the auto-shifting between gears works well, and this hub costs less than many of the available mid drives.



The phase wire cable exiting the hollow axle indicates that this is the disc brake side (left).


The motor is a radial outrunner with 18 stator-teeth and 20 neodymium magnets, 3-phase, with hall sensors, The stator and magnets are both 20mm wide, and the stator diameter is 90mm (airgap radius is 45mm). The laminations are the common 0.50mm thick, so 48V is probably it’s voltage limit, to avoid eddy current heat. (Xiongda! please upgrade to the thinner 0.35mm laminations!)



This is the same assembly rotated around to show the reduction gears. The first row of gears merely reduces the motor RPMs to the second stage. This is the right side of the motor.



This is the power-output mounting-plate that encompasses the two way clutches. The small planet gears are always meshing with the floating inner sun gear, and also the floating outer ring gear. The ring gear has more teeth, so when it is driven, that forms the low gear. When the motor reverses and engages the sun gear, that forms the high gear. This plate bolts to the right side of the hub shell.



Here is a pic of the outer side of the power output plate, showing the inner and outer clutches. The oblong holes that the rollers operate in look like the long sides are parallel, but…they are actually tapered, wider at one end and narrower at the other.


As you can see in the pic above, if a more powerful version is designed, there is plenty of “meat” in the mounting plate to use five or six ramped clutch rollers instead of three (per each clutch-set). Three rollers is the minimum number for self-centering and affordability, but…not for performance.

The position of the roller-springs clearly shows that both clutches “grab” in the same direction, and also freewheel in the same direction. The rotation direction of the motor determines which gear-set engages its respective clutch.

Since these pics shows the internals from the right side (the drive sprockets’ side), we can see that if the motor is spinning counter-clockwise (CCW, the opposite direction as the wheel, when the bike is in forward motion), the drive planets will be spinning ClockWise (CW), and thus the outer ring gear will be spinning CW…which drives the low gear.

When the motor spins CW (the same direction as the wheel in forward), the planets spin CCW (reverse), and the inner sun gear is driven in the CW direction, providing the high-speed gear.

Dave disassembled this motor to check the internals. He found that there was not enough grease from the factory, and the included grease was of a poor quality (waxy). He took these pictures, and then packed it with high quality synthetic grease, and the motor ran much quieter after that. He mentioned that the dis-assembly was quite easy and quickly done.



A Sprag clutch is shown on the left, and a ramped roller clutch on the right


I’m including this group of pics just above to help answer questions I have been sent about the Xiongda clutches. The two common industrial radial-roller clutch styles are called a sprag and a ramped roller. The sprag style of clutch has more holding power for a given size because it has more contact points holding the inner and outer races together. This also makes them more expensive.

The ramped roller style is shown on the right side (like the Xiongda). In both of the ramped roller graphics on the right half, the inner part can rotate in the clockwise direction quite freely, but when it begins turning in the counter-clockwise direction, the angled spaces clamp down on the rollers. The Xiongda uses the ramped roller style, and I believe that in this application, it will perform its duties quite well (however, more rollers in the same space would reduce the load per each roller, so…they should cram as many rollers as possible into the clutches).


What would we recommend for the future?

Nobody is asking our opinion of what features a new 2-speed hub should have, but here are our thoughts on this:

These two motors are too fat because the clutches, gears, and motor are side by side in both the Xiongda and the SRAM 2-speeds. This limits the size of the magnets and stator-teeth. For example, the Xiongda magnets are only 20mm wide, and the common 9C magnets are 28mm wide (while still allowing a 7-speed freewheel). If the rotor and stator were increased to just large enough in diameter to to allow the two-speed gears and clutches to be nestled inside the stators’ inner diameter, the magnets could easily be any chosen width up to 30mm wide, to provide more copper mass.

Such a hub could easily be configured to provide 20-MPH when using 36V X 21A =750W, in order to be street-legal in the USA. Also…the power could be limited to 36V X 14A if you wanted a 500W street-legal version for Canada. Then, in those areas that are unconcerned about power-limits (or off-road), that same motor could provide approximately 26-MPH (42-kph) at 48V X 25A = 1,200W.

A torquey large-diameter hub that in low gear will provide 13-MPH using 1,200W will be an exceptional hill-climber, and with a top speed of 26-MPH on the flats…I think that is something that would sell…

Without the stator and clutches being mounted side-by-side, the motor case should be narrow enough to allow a disk brake on the left, and also a 7-speed freewheel on the right side, while still easily fitting into the common 135mm width of drop-outs.

And last…if you are making a brand new design of hub that can’t share any existing parts to save on production costs, please…please…do NOT run the phase wires out of a hollow axle. Machine a large bushing into the non-gear side of the axle where it exits the side case, and use a larger diameter side-plate bearing on that side.


The stator from the Tidalforce motor from Wavecrest (no longer in business). Notice the phase wires pass through a fattened section of the axle center , instead of through a hollow axle.

The stator from the Tidalforce motor from Wavecrest (no longer in business). Notice the phase wires pass through a fattened section of a central bushing, instead of through a hollow axle.


A Warning

The NON disk brake Xiongda hubs (using V-brakes at the rim) will fit the common 100mm front / 135mm rear drop-outs. However, the disk brake hubs are about 10mm wider, so several builders who wanted to use disk brakes had to spread the drop-outs, which is only recommended on steel frames.

If you want to use one of these hubs (or two of them?) on a fat bike, order the hub with a drum brake, which has 166mm wide shoulders on the axles to fit the 170mm drop-outs on fat bikes.


How to order one?

It is possible to order a kit from Aliexpress (the Chinese “EBAY”), but several purchasers felt the best buying experience was from dealing directly with Xiongda’s customer service. The shipping distance and type of shipping chosen has a very big impact on the price, so check with Xiongda to see what it would cost to get one of these kits to where you are.

You can email their customer service here, at yona@xiongdamotor.com.cn

From Dave: “… it would probably be best to deal directly with the factory. They’ll send you a pro-forma invoice that you can pay with Paypal. Try and keep communications short and to the point. Use links where you can. Their English isn’t fantastic…”

Here is the endless-sphere.com chat thread on these.


Written by Ron/Spinningmagnets, June 2014

Grew up in Los Angeles California, US Navy submarine mechanic from 1977-81/SanDiego. Hydraulic mechanic in the 1980's/Los Angeles. Heavy equipment operator in the 1990's/traveled to various locations. Dump truck driver in the 2000's/SW Utah. Currently a water plant operator since 2010/NW Kansas


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