This is the most powerful production mid drive kit that you can run through the bikes gears, and the big surprise is how quiet it is. (edit: as of the spring of 2015, Lightning Rods now has his even MORE powerful “big block” mid drive, which can use twice as many amps as his basic kit, which is what is shown in this article)

We bought the first two production kits before they were even produced. I say that up front to give you an idea of how we feel about this particular mid drive kit. There is quite simply no other kit that you can add to your bike that comes close to what this kit can offer. There are many more mid drive kits available this year, but none are made with the quality or power level that this one will provide.

If you need a simple commuter, this is probably more complex and has a higher price than a 1,200W kit that would satisfy most E-bikers. And, if you want more power than 2,800W…you might be better served by a large hub motor, or a non-hub that drives a left-side chain to the rear wheel. This kit runs through the bicycle gears, and as a result it has all the benefits of letting the motor have more than one speed.

The two user-profiles where this kit will really shine is a heavy cargobike where there are very steep and long uphills…and an off-roader who likes a lot of power, but still wants to be able to crawl slowly around the technical obstacles without overheating the motor….and also taking big jumps.

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Back in August of 2012, electricbike.com was compiling a list of available mid-drive kits, and the new GNG company showed up on a Google search. The drive that they called their 450W unit looked interesting and actually used an impressive 1,000W. Several builders at the endless-sphere.com forum purchased the kit and tried it out.

The reports that were posted showed that it had many weaknesses that needed upgrading, however…it actually was a very powerful kit that could be installed on the majority of available bicycle frames (rather than shopping for an expensive factory-built mid-drive).

A new endless-sphere.com member named Mike Backus (username: LightningRods) began asking about what were the most desirable upgrades, and the following discussions identified those parts that seemed to be the most needed. One by one…he began designing and producing upgraded brackets…and other parts that would make the GNG kit reliable and more powerful.

The general configuration of the drive proved to be useful, and the motor that came with the kit turned out to be unusually well-suited to this job. The motor is just narrow enough to fit between the pedals, and since it is only 4-1/2 inches in diameter (115mm), it can fit onto most bicycle frames. It is an inrunner, so the hot stator coils are bonded to the aluminum housing, which allows this motor to shed heat very well.

And since it is being used as a mid-drive…you can shift the bikes gears when you encounter hills, so the motor can stay up in the higher RPMs. This prevents the motor from producing waste-heat in the first place, due to bogging down and running at low RPM’s, like a hub motor might do (a direct-drive hub spins at the RPMs of the wheel, and geared hubs have a poor heat-shedding path). Also, this motors’ low pole-count is a design feature that allows it to run at high RPMs without experiencing the waste-heat that is usually produced by eddy-currents.

If you raise the systems voltage without changing the motors’ Kv, it will make the motor spin faster. This motor has been verified to work extremely well at voltages between 48V and 72V, although…at 72V and full throttle…the motor will be spinning fast enough that the rider will not be able to pedal along.

This drive provides an impressive mechanical reduction of 33:1 between the motor and the crankset, and having such a high reduction allows the motor to be spinning very fast (with a high tangential magnet speed) compared to the wheel-RPM’s. Lukes “deathbike” racer has very large motor with a small diameter wheel and only needs a modest reduction of 4:1, but that system has very high power that is drawing very high amps from the battery. And don’t forget that Luke’s motor is large and heavy, which is what you would need with a low reduction.

This drive addresses these questions…if we want to design a mid drive system that allows the motor to use the bikes gears, what is the largest reduction that will fit? and then…once the best reduction components are identified, what motor has the most possible copper mass, but is still “just” small enough to fit on a common bicycle frame?

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The Motor

Many cities in China are banning the common 2-stroke gasoline-powered cargo trikes due to smog issues. There are several companies that have put together electric conversion kits that will swap right in to the differential. The motor design needed to be capable of high power, but small enough to fit a variety of trike models.

This need spawned a family of motors with a low pole-count so they have the option of running at high RPMs without excessive eddy-current heat, and it would best for it to be an in-runner so it could shed heat well. Since it is a radial-flux design, a variety of powers can be easily achieved by adding more stator-laminations. This results in motor models with the same diameter, but with a selection of several lengths.

Mike is now ordering these wonderful motors direct from the motor manufacturer, and after they arrive, he swaps-in shafts that have been modded in the USA to his specs. The working end of the shaft has been shaped to a cylindrical 12mm diameter (1/2-inch is equal to 12.7mm), because 12mm still exceeds the strength needed, but is small enough to allow a very low tooth-count on the pulleys and sprockets. This provides the option of very high reductions.

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The Primary Belted Reduction

Belts are quieter than chains, especially at the very high RPMs of a primary reduction. Mike uses a 25mm wide GT2 belt, and the pulley tooth-counts are 90T/18T for a 5.0:1 ratio.

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The Secondary #219 Chain Reduction

Mike chose #219 Kart chain and sprockets for the secondary reduction. These affordable parts are designed for 2-stroke gasoline racers using 20-HP at 10,000-RPMs, and due to the quality of the steel, I suspect that this part of the drive will last the longest, when it comes to replacing worn parts. Also, the small pitch (distance between the pins), means that an 11T sprocket for #219 will be much smaller than an 11T sprocket for bicycle chain (11T is the smallest tooth-count to avoid the high noise of the “polygonal effect”).

It is not recommended make to any changes to the primary belted side (in order to dial in the drive to suit a particular application). The majority of any adjustment should be made with the sprocket tooth-counts of the secondary reduction. The stock reduction is a 12T drive sprocket and an 80T driven chainring, both of which are easily changed to a different size, or for replacing a worn part.

There is no need to shop for expensive custom-cut parts. The stock secondary is 80T/12T = 6.66:1, but by using only off-the-shelf parts, the tooth-counts can be changed from a high of 11T/93T (8.45:1 ratio) all the way down to 18T/64T (3.55:1 ratio). Since the primary reduction is fixed at 5.00:1, the options are:

17.7:1 Minimum reduction (for highest top speed)

33.0:1 Stock reduction

42.2:1 Max possible reduction (lowest speed, highest torque)

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Freewheeling Crankset with Three Chainrings

This kit has a freewheeling crankset that uses an interface that will accept two well-known replacement freewheels. You can use the stock ACS-Crossfire flanged freewheel, or the White Industries ENO flanged freewheel.

Also, rather than using proprietary drive sprockets, the 48T and 32T bicycle chainrings on this kit use the common 4-arm 104-BCD interface. A gear-change or replacing a worn chainring is easy and affordable.

The #219 80T chainring shown on the right (made for 20-HP and 10,000-RPMs) is also an off-the-shelf item.

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Recommended Controller?

This motor really shines at 30A, and at that power level…it can take a frequent pounding without much concern for heat build-up. However, if you give the motor an occasional cruise-phase to let it cool down some…several builders have dialed-in 40A, which is exceptional for a mid drive.

Using between 30A and 40A leads us to the programmable Lyen 12-FET as our top choice for a controller. If you are certain you will only use 65V or less, you can specify the efficient and cool-running 3077 FET. If you think you might want to try 72V someday, make sure to specify the 4110 FET, which can be run up to 100V (although the 4110 FET does run slightly warmer than the 3077 at all voltages).

Lyen personally hand-assembles each 12-FET controller. He uses only genuine name-brand FETs and ensures that each FET is properly bonded to the robust heat sink, so the controller will shed heat as designed. The no-name controllers use cheaper generic FETs, which run hotter, and if even ONE of the FETs is not properly mounted…it will fail from too much localized heat at the poor connection, and then the cascading effect will cause the entire controller to fail.

48V X 30A = 1,440W

72V X 40A = 2,880W

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140mm wide ISIS Crankset

To install the extra-wide bottom-bracket (pedal-axle), you might not be able to tighten down the cartridge end-cap with a conventional socket. Tightening down the BB cartridge shown above may require a tool similar to the “Pedros” brand wrench.