We have waited to write about the GNG kit because it proved to be immediately popular (electricbike.com’s leader Eric stumbled across the new GNG site in Aug 2012, about six months ago), and the inevitable upgrades that endless-sphere builders added were rapidly defining the strengths, weaknesses, and limits of this handy kit. Bottom-Bracket (BB) drives have really taken off this last year, and for good reason. The prices for the basic parts have come down, and the selection has improved too.
Part of the reason is because of a type of BMX bike called a “trials” bike, which is used for performing stunts (shown here with rider Danny MacAskill). They frequently have pedal crank-arms that hold a freewheeling chainring onto the BB. By putting two chainrings on a freewheeling BB…a motor can drive one of them, while the other chainring drives the rear wheel. This arrangement allows the motor to power the bike without the pedals moving. The GNG kit does not require an extra-wide pedal-axle, like some other BB-drive kits.
The biggest benefit of a BB-drive is that it is an inexpensive way to give the motor the use of the bikes gears. Whether you are limited by a legal street E-bike power-cap, or you have an unlimited-power off-road E-bike, external gears allow the motor to stay in its higher RPMs, so it’s not necessarily drawing high amps, or generating excess heat in the first place.
The EGO is an Austrian BB-drive that is a powerful and high-quality system, and has been around for a while, but…it is expensive (EU 2400, $3000 USD…Yikes!). The Cyclone is from Taiwan (which also has lots of hills), and although it is somewhat more affordable than the EGO, it is still pricey for the entry-level quality of parts you get, along with many customer reports of how noisy the system is.
A Chinese exporter called GNG recently began packaging and selling a brushless 48V BB-drive that they advertised as a 450W system. To the inexperienced buyer, that sounds weak, and we suspect it is advertised this way to sell in the many countries that have a 500W power limit for E-bikes.
The controller has been verified to put out a stock unmodified max power of 22-Amps, and since watts are “volts times amps“, that makes this a continuous 1,000W system. Plus, since this system applies its 1000-watts to the BB, the fact that this motor is using the bikes gears, would give it the same performance as a much more powerful “one-speed” hubmotor built into the wheel.
The GNG primary reduction uses a toothed belt from the motor to a jackshaft, and this runs quieter than the Cyclones metal-geared primary reduction. The stock belt is much looser than needed, and has a tensioner wheel to take up the slack. It is not spring-loaded, and you simply adjust the proper tension and then bolt it down.
The stock design has way too much “wrap” around the 14-tooth motor-drive pulley, and experimenters have verified that the excessive wrap uses up around 100W, even when its just spinning unloaded. The excessive wrap is there to help keep the belt from slipping when applying the full 1000W to the small pulley. Here is a graphic to show what I mean.
A quick e-mail to the customer service department of the famous Gates drive systems company provided professional engineering advice. Experimenter “LightningRods” then ordered custom laser-cut sideplates for the GNG mounting bracket that would allow the jackshaft to have its position adjusted, so as to properly tension the primary belt/chain without any wrap at all (these plates also allow a larger diameter pulley than the stock 80-tooth, which helps the motor RPM reduction). This lowered the parasitic power loss of the primary belt down to a more reasonable 10W.
However, now you have fewer pulley-teeth engaged, and if you apply high power, the belt will slip, leading to an early belt death.
If you run this kit with the advertised 48V, the motor will be spinning so fast that you cannot add pedaling when you are at the top speed (bikes are often designed for a cadence of 80-RPMs at the pedals). Two solutions for this have been tried. You could run it at 36V with a different controller, which makes it a well-balanced USA street-legal 750W system. When doing this, the slower motor RPMs (due to the lower volts) will allow you to pedal along with the motor. Adding pedaling will greatly extend your battery range, and make your motor and controller run a little cooler. The other option is to swap-in larger diameter chainrings while keeping 48V.
So is the GNG kit ALL good? to be honest, there are several weaknesses. It’s actually quite a nice kit at 750W (@ 36V / 20A). It has a reasonable top speed on the flats, and you can downshift the bike to tackle the hills. The kit is only $400 plus shipping…just add a battery, add a charger, and a bike (near $1000 total, plus a bike, when a significant 15-Ah battery is added). The mild steel mounting bracket has arrived bent for some buyers (due to rough handling by the shipper?), and it flexes when using 2,000W. Both of these points indicate that the bracket’s not quite as strong as it should be. Because of this, there are now custom sideplates available that are specifically designed to be stronger (found here at endless-sphere.com), and these plates also allow proper tensioning of the belt/chain without any idler needed.
If you add more power by raising the stock 48V or the 22A, the primary belt will begin skipping, and then quickly shred. Several off-roaders have upgraded the primary to a chain-drive, which is noisier, but then it can handle 3,000W without trouble. The example in this pic is using #25 chain (there are also examples using #219 chain). The large sprocket is bolted directly to the side of the stock pulley, and the small 12T drive-sprocket is from an 8mm shaft that is bored out to 10mm. Notice the idler wheel is barely deflecting the chain so drive losses will be low.
The stock belt is only 15mm wide, and builders immediately looked into a 20mm or 25mm wide belt-and-pulley set, so a fairly quiet belt could handle more power. But it’s not an easy swap-in. The stock 14T drive-pulley is a part of the motor-shaft, and must be ground off first. That is not easy to do yourself properly, and a machine shop can be expensive. One builder found that if he sanded the sharp edges off of the rough-cut stock 14T pulley, the belts lasted longer.
There are a wide variety of small sprockets that can be used on a GNG motor-shaft that has been turned down to 10mm, 12mm, or 0.500-inch. However, we have not yet found a good 20mm wide pulley with a small enough tooth-count, in order to make up for the removal of the deep idler belt-wrap. (the search continues, since a primary belt would be much quieter than a chain)
One major point in favor of the GNG kit (besides the affordable price), is that the custom Bottom-Bracket freewheel holder uses affordable and available off-the-shelf 16T freewheels, and this freewheel is the one part that is likely to need replacement about once a year with frequent use (almost forgot, the toothed belt is likely to need replacement every few months).
Also, the motor appears to handle 72V X 30A = 2,100W well, because it is an inrunner that has the hot stator coils attached to the aluminum motor shell, which helps it shed heat under heavy loads. Until the arrival of this affordable motor, most E-bike non-hubs were either too expensive, too small, too wide to fit between the pedals, or didn’t shed heat well. It’s just frustrating to find that this GNG motor-shaft and drive-pulley are one piece, when a 10mm diameter shaft stub would open up an entire world of opportunities for experimentation and using this motor in other applications. Even 12mm or 0.500-inch would be much more acceptable.
So…what’s the final verdict? This is not a plug-and-play system like the popular hub-kits. It’s more involved and also technically challenging to install it properly on a bike. But, in my humble opinion…it’s better and more affordable than the Cyclone, and it’s definitely much more affordable than the very expensive EGO.
Bosch and Panasonic have produced very high quality BB-drives for the power-restricted global markets. But I doubt North American customers will pay a high price for a low-power 250W system, no matter how nice it is.
If you are happy with 750W, you are mechanically handy, and you live around a hilly region, this might be the kit for you (you will save money by getting the lower voltage 36V battery, but you also have to buy a 36V controller, so the cost evens out). It “may” be possible to get GNG to swap the controller for one that already has a Low-Voltage-Cutoff (LVC) set for a 36V battery, so ask before ordering, and then check it when the kit arrives before plugging in the battery.
The stock kit is 1,000W (and 1,000W applied with the use of the bikes gears is a LOT of fun!), but it definitely wears out belts too quickly. It really needs a 20mm-25mm wide belt with de-burred pulleys and proper primary belt tensioning. If you perform a simple mod to the stock 9-FET controller, you can easily raise the max amps to 30A, which would provide around 1500W, but…if you do this…add a cheap temp probe to the controller so you can stay at least one step away from frying anything. And be aware at 1000W+ you will need to upgrade the primary drive to a chain, unless you don’t mind replacing belts often.
Link to buy the GNG BB-drive from this article.
endless-sphere.com forum discussion about the GNG.
Discussion about changing the primary drive to a chain.
Written by Ron/Spinningmagnets, January 2013