Written by Ron/Spinningmagnets
There haven’t been as many articles written here recently at electricbike.com, but we haven’t been just sitting around. There are lots of exciting developments these days in the “non-hub” world.
If you have an easy commute, with fairly flat terrain and modest hills, the simplicity and affordable price of a common hub-motor kit may still be the best choice for you. But…if you have significant hills (I’m lookin at you, San Francisco) or you want high performance for an off-road E-bike, switching to a non-hub drive with a system that gives the motor at least 3 gears to choose from…well, THAT might be the best choice for you.
We’ve published articles that documented the wider range of performance and the battery-range efficiency of giving a motor some gears, and there’s a lot of activity over at endless-sphere.com (the webs biggest e-bike builder and experimenter forum) about adapting and improving bottom-bracket non-hub drives.
See our story on Mid Drives, Creamy Alternatives to Hub Powered Bikes
See our story on 10 Mid Drive kits available now
See our story on Mid Drive Cargo Bikes
See our story on Mid Drive performance videos
See our story on Mid drives appearing at Eurobike 2012
See our review on the Ego Kit Mid Drive
The $400 GNG 1,000W kit (price is minus battery and charger, also this kit is advertised as the 450W version) has proven to be fairly adequate for the majority of street commuters. When it’s geared for the federal legal speed limit of 20-MPH, the bikes lower gears have incredible hill-climbing ability. But when experimenters upped the power to 2,000W it was found that the primary drive belt needed to be upgraded to a chain (which is noisier), and the motor was nearing its heat-shedding limit (but not quite yet, and this is before any active cooling is added).
So…what could an off-roader do, if they wanted the type of performance that you could only get with 3,000W or more? A German builder calling himself “crossbreak” posted that he thought that the common largest geared hubs could be easily converted so that instead of driving the motor shell, it would have a stationary shell with a driven axle. Several brands of geared hubs were broken open and modified to see how it would turn out. The results are good!
Big manufacturers don’t want to waste time on temporary fads, so they wait to see what customers will actually spend money on before committing to production of a new product. This geared-hub conversion to a shaft-drive uses a motor that is the right shape and size to use as a bottom-bracket drive (which cheaply lets the motor use of the rear wheel gears), and has enough copper mass to handle a higher level of watts, compared to the now-popular GNG kit.
This may sound confusing at first, since many geared hubs are advertised as only being capable of a continuous 500W. This is because the curve of amp-draw from a motor is much worse in the lower two-thirds of the motors RPM range. A hub-motor can be considered a “one speed” in this example, and this is usually not a problem for most applications. But…on extra-steep hills, a one-speed system can be trouble. If a hub bike is externally geared for low-RPM use, it will also have a low top-speed on the streets…and the same bike geared for a high top-speed will bog down badly at the low RPMs on the offroad trails. Giving a motor some gears will allow it to stay in the higher motor-RPMs regardless of the speed the bike at.
By keeping the RPMs up, you can dramatically increase the bikes power while keeping the amps fairly low, and it is the amps that create the most heat in the motor and controller. Low amps are also desirable to make life easier on the battery, regardless of the type of battery chemistry you have chosen.
Endless-Sphere experimenter crossbreak has proven that the popular MAC 500W geared-hubmotors can be converted from a shell-drive to a shaft-drive. This allows the garage-builder to give this affordable motor some external gears (most affordably by making it a BB-drive, like the GNG kit). This effectively triples its continuous wattage capability, since shifting gears allows the motor to stay in the upper RPMs at all times. The raw info on the geared-hub conversion can be found here
The converted motor is roughly 6-1/2 inches in diameter (after the spoke flange is cut off, if needed), and its narrow width easily fits between the stock pedals of most bikes. Since the shell is now stationary, it is easy to relocate the phase and hall-sensor wires from inside the tight hollow axle, to a simple large hole in the side. The wires are then easy to upgrade to thicker wire, and with high-temp teflon insulation. The stock primary gear reduction provides a 5:1 lowering of motor RPMs, but the conversion oddly makes it a 6:1 reduction (even better!). The internal gear-reduction eliminates the need for adding an external reduction.
Here you can see the wires exit the hub through the case instead of the tight hollow axle.
I was also happy to see that a converted MAC can easily and cheaply have active cooling added. Most industrial motors have an air-fan to keep it cool under heavy loads, and active cooling can raise the continuous and peak watt limit a motor is capable of. Another thing that I am happy about is that the MAC is available in 5 different windings. Most hubs are stocked with only two choices (typically called a “fast” or a “slow” winding).
The sudden popularity of the GNG kit has opened the eyes of many E-bike enthusiasts to the peculiar benefits of a BB-drive. Before the availability of the GNG, BB-drives were typically very expensive, and hub-motors required large batteries to supply the high C-rate that the amp-hungry hub-motors needed in order to provide high performance.
The GNG kit appears to be adequate for its stock power level of 1,000 watts. And regardless of its sales label of 450W, the stock system uses 48V while the controller maxes-out at 22A, and by our methods, we will call that 1,000-Watt system, so we can fairly compare it to other kits. Several off-road builders have upped the voltage to 72V, and the stock primary belt-drive has proven to be the next weakest link, leading some builders to upgrading the belt to a chain-primary. The chain is noisier, but it can easily handle much more power. The GNG thread found here
Here’s a BB cartridge and mid-drive bracket from the GNG to show the method that’s becoming common.
So…whats an enthusiast to do when they want an off-road system capable of 2,000W of continuous power (and 3,000W temporary peak)? The clever BB-mounted GNG bracket is certain to be copied, and the converted MAC looks like it will be the motor of choice for the upgraded affordable DIY off-road crowd…only time will tell, but like we said…lots of exciting developments right now!
