GNG Gen2 750W Mid-Drive Kit Review

May 1, 2013

This is an affordable and useful mid drive, if you think you would be happy with 750W…and live where there are hills. Quite a few buyers who have heard about the GNG kits opted for the 1,000W-2,100W  big-brother kit (article about the GNG 1,000W here), that has proven itself to be a very capable and versatile off-roader.

The GNG-Gen2 kit featured in this article is listed as the 350W/400W brushless kit depending on which voltage you choose.

The GNG 1,000W kit has gotten most of the recent attention (listed on their site as the brushless 450W), because most of the experimenters at really love the feel of higher powered systems, but…if you are someone who actually wants a street-legal system at 36V X 20A = 720W, then this kit has a lot going for it (when the battery is full, the slightly higher voltage will provide the full USA-legal 750W)

The kit + shipping price is listed as $402 for the 8-week sea+truck shipment, and $446 for the one-week air-freight. Price does not include the bicycle or battery/charger.




Two Gen2 choices, which to buy?

There are two Kv’s of motor available. The 36V version, and the 48V version. Both provide the same RPMs to the crank. This is because the 48V version was wound with more turns of thinner wire, so it will spin slower per each volt that is applied (it’s “Kv”). If you took the 36V version and applied 48V to it, the motor would spin roughly 30% faster. However, the controller in the 36V kit has its Low-Voltage-Cutoff (LVC) set for for a 36V battery, so using a 48V battery on the 36V motor would still need a 48V controller (or the battery would be allowed to run down way too low to about 33V or so, causing damage to the 48V battery).

Since the 48V motor-winding and controller are the same price as the 36V motor and controller, why would anyone choose the lower-powered 36V kit? Well, the battery pack on a 36V kit will be cheaper and smaller for the same Ah-size of pack (same Ah size = roughly the same amount of miles for both). And if you buy the same size 36V pack as a similarly-sized 48V pack, it will provide about 25% more miles worth of Ah’s. But if you live where the hills are unusually steep…I would definitely choose the higher volts on the 48V system. For mild hills, the less expensive (and smaller) 36V battery pack would be my choice (if both 48V and 36V packs were the same Ah size), with both systems being limited to 25A (The factory controller has a 20A limit).

This motor runs well at 25A, but if you want to run more amps than this, we recommend a temp sensor to save the motor and controller from overheating on an unusually steep and long uphill, although it should actually be fine at even 35A if it was run on flat land. More amps will equal more torque and acceleration, but more amps also results in more motor/controller heat…so exercise caution.

Because of it’s poor heat-shedding being this motor’s common-sense amp-limit, I haven’t been able to find the saturation limit of amps for this mass of copper, but I can guess that anything over 30A will have a lot of its energy converted into waste-heat, rather than work. By that I mean that if you raise the amp-limit from 20A to 25A, you will feel a noticeable increase in the acceleration, but not much extra motor-heat. If you then raise the amp-limit to 35A, this kit will definitely feel more powerful, but it will get very hot….very fast. Perhaps this is acceptable on flat land where hard acceleration only lasts a few seconds…and the motor has time to cool off during the cruising phase of the ride. But if you have a long and steep uphill? Get a temp probe and limit the amps to 25A or less.

It’s too bad this handy kit is not getting as much attention as it deserves. It’s just a hair too small to interest the hot-rodders, but it is perfect for someone who wants to comply with the USA federal power limit of 750W (one-horse-power). If there is another kit that catches your eye, and is using a common 1,000W hub-motor, you may wonder why someone would want 25% less power by selecting this kit. The key feature is that…this kit allows the motor to use the bikes gears, and comparable rear-wheel hub motors are in effect a “one-speed” motor system.

Another secondary feature of this kit over a common 1,000W rear hub is…this kit is lighter, and whatever weight it does have is more centrally located.

There has been a recent emergence of popularity for bottom-bracket (BB) kits that hook up the motor to the pedal-axle chainrings. By having a freewheeling BB with two chainrings, the motor can drive one chainring while the other chainring drives the rear wheel.

I am not selling these kits, so I have nothing to gain by trying to “sell” you on a particular feature. It’s up to you to decide if I have made a persuasive argument, and if this kit suits your user-profile. The freewheeling 2-chainring BB means that the motor can drive the bikes gears, while your pedals are not moving. Pedelec-style systems and the Stokemonkey kit require the rider to pedal when using E-power, but some riders prefer to occasionally just power along with a throttle, while resting their legs. Or, they might pedal on the flats, while resting their legs when E-powering up a hill.

Compared to GNG’s more popular 450W kit (actually 1,000W in stock form), this Gen2 kit cannot be over-volted anywhere near as much. The so-called 450W kit has been run at up to 72V X 30A = 2,100W successfully, and I would recommend limiting this Gen2 kit to 48V X 25A = 1,200W (perhaps 30A on flattish terrain). I would list this kit as a 750W-1,200W system…and when you can put that kind of power very efficiently through the bikes gears, it can be a lot of fun!

Where this Gen2 kit really shines over the other GNG kit (the one that has been more popular), is that this one has proven to be fairly trouble-free. You can buy this easy-to-mount kit, simply works! The higher power potential of the other kit has drawn many experimenters to embrace that one instead…in spite of its need for various upgrades to make it reasonably reliable.

One of the big features of the other GNG kit is its ability to upgrade later to 72V without eddy-current losses due to its low pole-count, and a higher volume of copper-mass that allows it to use a high continuous 30A limit (with 45A peaks if you exercise caution and have a temp sensor), and that motor has excellent heat-shedding characteristics (when on an uphill). The featured Gen2 kit here has a lower copper mass, and since it is a converted geared-hub, it has weak heat-shedding since the stator is not connected directly to the aluminum housing in the same way as the so-called 450W kit.



How many gears to use with the Gen2?

When interested people are first introduced to E-bikes, they often have a hard time shifting mental gears to understand what real-world experience has proven to work well on non-hub E-bikes. For pedal-bikes…having more gears is almost always better, because having a lot of gear choices allows you to find the best gear to get the maximum effect when using a weak human 200W motor (your legs). A Tour de France competitor can put out 400W, but sadly, I am more of a 50W pedaler.

The freewheeling BB pretty much limits you to having one driving chainring, and occasionally someone asks if its possible to mount a 3-chainring set and derailleur to a bike with a BB-drive system. Well…yes, it’s “possible”. A three chainring set would mean the rider can select either one of the two undriven chainrings with a derailleur. For a non-powered pedal-bike, I highly recommend having two or three chainrings to choose from in order to provide a wide array of gear choices, but…everyone I have talked to who has had an E-bike for any length of time has ended up taking off the front derailleur and extra chainrings as an unnecessary complexity.

Do as you wish, but…if you are still reading, lets move on the the rear wheel gears. The fewest gears I would recommend is the Nexus heavy-duty version of their common 3-speed internally-geared-hub (IGH). Then select a front chainring tooth-count  that makes the 2nd gear the everyday starting gear. 3rd gear would be for high speed, and first-gear would be reserved for steep hills. The common gear spacing of 30% on the 3-speed IGH would mean that: although the 1:1 second gear would provide roughly the same performance as this size of motor if it was mounted as a hub in the rear-wheel axle, the 30% under-drive first gear would provide 30% higher RPMs on a steep uphill when in first gear.

So, if I have convinced you to have just one chainring upfront, let’s imagine that you have now decided you want to be clever and buy a money-is-no-object German Rohloff 14-speed IGH in the rear.  This is an AWEsome product for pedal-bikes, especially in the steep Swiss Alps with their constantly changing hill-grades, and it provides the perfect gear just a few clicks away at any given moment. But…shifting up and down through the gears while using a motor means that you will be shifting quite a bit, since the gears are closely spaced to each other. This isn’t a “problem”, but you need to understand this so you can make the right choice for you.

What do I recommend? the common, cheap, and available 7-speed external gear cluster. The external gears can actually take more power than most IGH gear-teeth. And 7-speed chain is stronger than 9-speed chain. Having 9-speeds sounds like it would be a great option, but the way that 9 speeds were squeezed into the space that would normally hold 7, is that they use a narrower chain (with more closely-spaced sprockets), and 9-speed chains are usually made to be as light as possible.

Many E-bikers would like to add pedaling, in order to make life easier on their battery (more pedaling=lower amp-draw = lower heat), and also extend the range of their battery. The most popular cadence seems to be between 70-80 RPMs at the pedals. To achieve this easily and cheaply, you need to look at the tooth-counts on the front chainring and the smallest gear at the rear wheel gear cluster.


Here it is upside down, and this is a good pic of the tiny 9T drive sprocket.

Here it is upside down, and this is a good pic of the tiny 9T drive sprocket. You can also see how one chainring is driven by the motor, and the other drives the rear wheel.


What gears to choose?

First lets look at the motor-to-BB sprockets. Start by choosing the largest chainring that will fit your frame, and will still give you adequate ground clearance. This large chainring will be positioned so it is driven by the motor. A 48T-54T chainring will likely become a popular upgrade from the 38T stock chainring found on this kit. The stock 38T:9T on the motor provides a 4.2:1 reduction. A 54T:13T reduction would be about the same ratio (4.2:1), but will run quieter and will not put as much stress on each sprocket tooth, so these parts will last much longer.

Second, lets look at the rear wheel gears. If you get the absolute lowest tooth-count on the rear wheel, that means that the amount of power you are using is spread across fewer teeth, so each tooth becomes heavily loaded. A low tooth-count rear sprocket will wear out faster, and is more likely to break if you use higher power. It is possible to find a rear gear cluster that has an 11T as their smallest sprocket. I recommend you choose a cluster that has a 13T at a minimum, and a 15T is even better.

Once you have settled on the rear wheel gear-ratios, then you can choose a chainring that will put the bike’s top-speed where you want it. Remember, the lower the top speed you select, the better the hill-climbing will be.


This kit comes with tiny 18-Ga motor-phase wires. I have bought wire from China, and at most they might have saved about a dollar per kit by not using thicker 14-Ga wire (I would have used even larger 12-Ga). To be fair, the stock controller is limited to 20A, which would get 18-Ga wire very warm, but not dangerously hot. If you are handy with a soldering iron, swapping-in some 12-Ga motor phase wires would be a cheap way to allow you to use up to 30A without the wires being the limiting factor.

This kit does not have an E-brake switch, so that when applying brakes, the motor-power should also automatically get cut. This is an important safety feature for when a faulty throttle or controller gets stuck in the “ON” position. The natural reaction is to apply the brakes.

The 9-tooth drive sprocket is too small because it only has 4 teeth engaged with the chain at any given moment. I am certain that this is so the manufacturer can use a small enough chainring to ensure that this kit will fit on a wide variety of frames and still have adequate reduction (larger than average chainrings might rub against the front of the chainstay on some frames) .

I recommend to any buyers, that they immediately install the largest chainring on the motor-drive that will fit the frame and still provide adequate ground clearance. Having a larger chainring will then allow you to install a 13T drive-sprocket while still keeping a useful reduction ratio, which would have six teeth engaged instead of four. Also, a 13T would run quieter due to the shallower angle of mesh between the sprocket and chain. The factory could have specified a slower motor-winding to compensate for a better sprocket ratio, but…they used available motors rather than special-order.

The stock chain idler is very low quality and weak. I would elongate the two lower mounting holes of the mounting bracket in a way that turns them into pivot-adjustment slots, which would eliminate the need for the idler. You may have to add two or three “half link” chain links to be able to adjust the tension on the chain that runs from the motor to the crankset, however…that would be worth it to eliminate the poor-quality (and un-necessary) idler wheel.

DIY mods for the adventurous

If you want to be a pioneer with the GNG Gen2 kit, nobody has tried the oil-cooling mod on this motor yet, and you can be the first. It involves dis-assembling the motor (please take lots of pics!), and drilling a fill-hole/vent-hole into the housing side-plate, somewhere in the top half of the motor.

Then, when re-assembling the motor-housing, use high-temp silicone to prevent any leaks at the joints. You then fill 1/3rd of the interior of the motor with synthetic automatic-transmission-fluid (ATF). The stock heat shedding ability is the biggest weak point of this design. The restricted reduction limits this kit to 48V and the weak heat-shedding limits the amps to 25A. Beware, oil-cooling on this motor might be a disaster…nobody has tried it and written about it yet.

The ATF will absorb the heat from the stator, and spread it to the aluminum housing to be shed much faster than the stock arrangement. The bearings will be lubricated more than they would ever need, and the motor will run smoother and quieter than stock.

Here’s two discussion-threads on the theory and application of oil-filled E-bike motors.

“Someone finally oil cooled a hubbie!”  HS3540, 9 pages

“Oil cooling your hub- NOT snake oil!” 25 pages

Here’s another DIY mod: For a relatively flat street commute, I would be tempted to try a slow 48V-wound motor with 36V. Then the motor would spin about 25% slower than what the system was designed to spin at. Doing that would allow a larger drive sprocket by using existing kit parts that are available right now, and the rest of the gearing could be compensated for by selecting complementary rear sprockets.

From an engineering design standpoint, the ideal factory upgrade would have been a special-order winding that was so slow that you could use 48V battery together with a 13T drive sprocket. Some motors are wound with a coil-set termination in the “Delta” arrangement, or sometimes they use “Wye”. Most are Delta so that the motor will provide faster RPMs from lower volts.

If you take a Delta-terminated motor (and I am not yet certain that this one is Delta) and re-terminate it in Wye, it will cut the RPMs by about 1/3rd. It might be a worthwhile experiment to take a fast-winding 36V motor, re-terminate it in Wye, and then run it at 48V. I calculate that the resulting motor would spin at 50% of the factory-designed RPMs. That would require roughly 50% more teeth on the small motor drive-sprocket to maintain the same pedal-cadence, so the 9T sprocket could be replaced by a 13T without needing to increase the size of the motor-drive chainring. This would also allow you to try around 60V as an option if you still used a bigger chainring (who knows? it might be better).

A sine-wave controller might run this motor a little quieter, and a larger drive-sprocket (the 13T would definitely reduce the drivetrain noise).




This design is a modified geared hub-motor. That means that in stock form, it has no good path to shed heat. The stator coils get hot under normal conditions, and the center of the stator is connected to a small-diameter steel motor shaft as its only connection to the aluminum shell through a set of bearings that also do not conduct heat well. Its almost as if the motor layout was designed to insulate the heat, instead of shedding it.

There is no word yet on how thick the laminations are, or how many pole-pairs this motor has. Inductance has not been measured and posted yet. We don’t yet know the permanent-magnet strength.

Sadly, the build quality is poor, which is disappointing in a kit with as much basic design promise as this one. Although…it is very affordable. So, when we realize that some of the corners are being cut,  it’s to be expected…so they can keep the price easy.

To be honest, if I owned an electric bike business, I would actually keep a few of these in the shop for sale, but…ONLY after I had upgraded the issues I mentioned, and polished up the rough edges.

Whats the verdict?

My “go to” suggestion for the average builder who has mild hills, is to buy a MAC-10T rear-wheel kit from, and run it at 48V / 30A with a 12-FET. It’s  pretty much a plug and play system that suits the majority of new E-bike riders. It will provide a top speed of approximately 26-MPH, with better-than-average hill-climbing. It is roughly the same price as the featured Gen2 kit.

But…if your hills are super-steep and extra long, the MAC-10T kit might overheat, and like all hub-kits…the motor is a one-speed. Its obvious you can vary the throttle to change the speed of the bike, but by calling it a one-speed I mean that when you slow down, the hub-motor is spinning slower. With a system that gives the motor some gears, you can downshift, so that…even as you are forced to slow down on a hill, the motor is still spinning away at its max RPMs.

This is not an off-road kit. It simply doesn’t have the copper mass or heat-shedding that its big brother has (the GNG 450W brushless). The benefits are that; this kit has better hill-climbing than the well-regarded MAC 10T hub-motor, and its weight is centralized instead of located in the rear wheel for better overall balance.

Also, if you ever get a flat on the rear tire, a rear hubs is somewhat awkward to fix a flat on it, but the Gen2 is as easy to fix a flat on, as any non-electric bike.

Here is the discussion thread on this kit.

Here is a video that shows how loud the motor is when you’re riding.




Written by Ron/Spinningmagnets, May 2013

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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