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Drivetrains for Rear hub-motor e-Bike: Gear system options

20.10.2021

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While developing my latest prototype Avial Commuter e-Bike [1] with 250 to 1000W rear hub-drive, whose frame design also allows for chain and belt use in the drivetrain, I have decided to investigate the capabilities of different gear systems for bicycles. Why?

The first factor is that common rear-wheel drive multipurpose bicycles for city and country use usually include the installation of a standard kit, for example, the Shimano Alivio 8-speed chain transmission system. It’s inexpensive, reliable, easy-to-use as well as allows the implementation of all the expected features of an electric bike. However, if the owner wants to install a belt drive instead of a chain, which is possible on Avial Bikes, then, as a result, he will only be able to get a single-speed bike.

On the Avial Mid-Drive e-Bike [2], the chain-to-belt replacement can be solved quite simply, for example, by changing the sprocket cartridge to an internal-gear hub, the first designs of which appeared as far back as the mid 1880s [3]. Such systems are now offered by Shimano and many other manufacturers, for example, Sturmey-Archer. Therefore, equipping an mid-drive e-bike with a rear planetary hub would be logical – the only question is the price.

The second factor may not seem so obvious to someone, although it has been noted by a number of experts over the past few years [4]. For me, it became tangible precisely when using an electrically-powered bike in the conditions of city streets and country roads with slight uphill climbs.
I have noticed that I mostly use only 3-4 gears when moving, often switching through one at a time. In other words, one gear is used for a quick acceleration from the traffic lights and uphill, the next one is used to pick up the speed, and another 1-2 to keep the chosen pace of travel. The standard chain and chain sprocket system, in my view, was designed for a classic bike without an electrical drive and, therefore, gears are close to each other. The manufacturers wish to bring the gears nearer is understandable, as the cyclist on the classic bike may find one gear too low and gear next to it too high.

What does it work out to be? When riding a rear-wheel drive Avial Commuter e-Bike with a chain transmission system, I only use 3-4 gears. At the same time, having the option to replace the chain with a belt, I can only get a single-speed bike. So, what are the solutions that are available here?

One option that comes to mind first is to install an electric gear-shifting system with three chain sprockets and a front derailleur [5], since the necessary power is already available on the electric bike.
But this system is still being designed to work together with the bicycle drivetrain with front and rear derailleurs. Therefore, if you keep only one rear sprocket and install three front sprockets, you will still need a chain tensioner to compensate for chain slack when downshifting. Moreover, the combinations of this system with the belt-driven system are impossible.

Interesting solution is a development of the German company Pinion, centrally integrated in a bicycle frame [6]. Pinion gearbox is a completely hermetically sealed oil bath with the teeth located on the crank, offering 6, 9, 12 or 18 different gears. The system is leak-proof and robust and can work together with a belt as well as with a chain. Of the drawbacks of this solution, the cost as well as the weight can be mentioned in the first place. The most compact 6 gears box to fit my e-bike model weighs in the range of 1800g, that is at least an extra 10% of the total weight of the electric bicycle along with the battery. In addition, the Pinion box is directly bolted to frame, which means some difficulties and limitations in terms of possible frame or bike upgrades.

One of the novelties of Eurobike 2021 is the 9-speed Mimic gearbox from the French company Effigear, designed for central installation on the frame [7]. Just like the Pinion, the French design is rated at a maximum torque of 250Nm and has a weight around 2kg [8]. It is quite clear from the performance list that the Effigear Mimic can work fine together with most models of rear drives with a different power, a chain or belt-driven transmission. However, in terms of criteria such as weight and price, this is not the best solution for mounting it on an affordably priced electric bike.

At the end of the 1990s, Swiss inventor Florian Schlumpf presented three innovations: Speed-Drive, Mountain-Drive and Highspeed-Drive [9]. Each of these systems integrates a crankset and planetary gear so that cyclists can shift between gears without the need for conventional front derailleurs and multiple sprockets. The Schlumpf gearbox instantly doubles your ratios and increases the range of gears as long as the remainder of your gear is the same. Schlumpf bottom bracket gearboxes have a super-flat planetary gear mechanism with a width of only ten millimeters and a wireless shifting system that can work in conjunction with both belt and chain drive. The weight is 790g, excluding chainring and cranks. Two speeds, in my opinion, is less than objectively needed, although for a single-speed city bike this could definitely be the best option. Plus, such a technical solution will perfectly show itself on usual bicycles when cooperating with standard internal-gear hub, expanding a range of gears. Well, again, this kind of gear shifting system is expensive enough that it doesn’t quite fit the affordable electric bike I’m developing.

The Polish company Efneo has developed a 3-speed planetary gearbox with the same concept [10]. Like the rear planetary gear hubs, the Efneo can shift gears even if the cyclist is not cogging the pedals. According to Efneo CEO Franciszek Migaszewski, “One more gear is much more important than it might seem at first glance,” he said. “All two-speed systems have a gear ratio of at least 66 percent. That’s almost the same as between 1st and 3rd gear in the front three-sprocket crank arm … When you upshift, you feel too much resistance. When you downshift, it feels like your foot is falling into a hole. ”

The Efneo system has an overall gear ratio of 179 percent. The second gear is intended to be used as a “golden mean,” which most riders will use on regular roadways, the first gear is necessary for quick starts or up hills, and the third is necessary for downhills and tailwinds. The Gearbox from Efneo weighs about 1,000g, is operated by a shifter on the handlebar connected to a cable and completely protected from water, dust, salt and sand. No additional greasing is required after mounting. The developers offer models for both the chain and belt drive transmissions.

The Australian company Treadlie Engineering is engaged in the development of Continuously Variable Transmission (CVT) technology for bicycles [11]. This technical solution uses a V-belt driven Variable Diameter Pulley (VDP) system, which allows smooth shifting through an infinite number of effective gear ratios between maximum and minimum values. The V-shaped belt cross-section forces it to move higher on one pulley and lower on the other, which changes the effective diameter of the pulleys, allowing the overall gear ratio to vary. This development, which is not yet serially produced, in its concept compares favorably with other mechanical transmissions, which allow you to select only a few different preset gear ratios.

One of the first on the market, the Canadian company Bionx began to develop an electric Hub Motor with Integrated 3-speed [12]. In 2009, the company’s engineers demonstrated their new IGH3 technology based on a 250W BionX motor integrated in the rear hub, with 3 gears, overrunning clutch and disc brakes. The weight of the current prototype at the time was 5.5 kg. After that, already in 2017, the French company Accel Group made an effort to develop an SA IHG electric rear-hub motor a torque of 40 Nm and a built-in 5-speed gear system [13], which we could not see in mass production. That still leaves hope for Chinese manufacturer Bafang, which just lately introduced its new H700 250W electric bicycle rear-drive system of 32Nm torque and 3.2kg, complete with a 2-speed auto-shift system [14]. Although the two speeds don’t offer many features, it’s a pretty interesting model in my opinion and hopefully the price will be affordable.

Overall Thoughts
As you may know from reading my brief review, there are not many on the market for commercially available, low-cost drivetrains solution for e-bikes with a front 3 or more gears systems that allows both belt and chain mounting. There is the intriguing 3-speed planetary gearbox from Efneo and the 2-speed rear hub drive motor from Bafang.

Everyone who is familiar with the basic working principles of the Avial Bikes team already knows that before choosing any solution for their electric bikes, we should first test it. So, I set out to give the gearbox from Efneo a try for myself. Before installing the system on our latest Commuter e-Bike prototype, I primarily plan to evaluate how much the bike’s weight will vary after the 8-speed chain transfer system is stripped from the bike frame and the Efneo system is mounted. It will be also interesting to try to evaluate the capabilities of the Efneo working in conjunction with the Bafang H700 drive system to widen the number of gears.

Baruch Dorfman

In detail:
[1] Avial Commuter e-Bike – https://avialbikes.com/pre-order/commuter-e-bike/
[2] Avial Mid Drive e-Bike – https://avialbikes.com/pre-order/mid-drive-e-bike/
[3] Internal-gear hub – https://en.wikipedia.org/wiki/Hub_gear
[4] Derailleurs, internal gear hubs and the transport cyclist – http://www.bootiebike.com/igh/igh.htm
[5] Electronic gear-shifting system – https://en.wikipedia.org/wiki/Electronic_gear-shifting_system
[6] 11 Reasons to Tour with A Pinion Gearbox (And 8 Reasons to Not) – https://www.cyclingabout.com/tour-with-a-pinion-gearbox/
[7] Eurobike 2021: Gravel Bikes, Bikepacking Bags, A New 9-Speed Gearbox & More – https://www.cyclingabout.com/eurobike-2021-gravel-bikes-bikepacking-bags-new-gearbox/
[8] The New Effigear Mimic 9-Speed Gearbox: Better Than Pinion? – https://www.cyclingabout.com/effigear-mimic-9-speed-gearbox/
[9] Internally Geared Cranksets – https://www.alphabent.com/internally-geared-cranksets
[10] Efneo gearbox is made to replace a bike’s front derailleur – https://newatlas.com/efneo-front-gearbox/34155/
[11] Continuously Variable Transmission Technology for Bicycles – https://siamagazin.com/continuously-variable-transmission-technology-for-bicycles/
[12] BionX Launches Hub Motor with Integrated 3-speed – https://www.bike-eu.com/home/nieuws/2009/11/bionx-launches-hub-motor-with-integrated-3-speed-2-1014144
[13] Vélo électrique: deux technologies prometteuses – https://www.breezcar.com/actualites/article/innovations-technologiques-velos-assistance-electrique-0217
[14] Bafang unveils electric bike motor with 2-speed automatic shifting and torque sensor – https://electrek.co/2021/09/07/bafang-unveils-electric-bike-motor-with-2-speed-automatic-shifting-and-torque-sensor/