How to choose between a Mid-Drive, a Geared-Hub or a Direct-Drive Hub Motor for your ebike project.

There are three primary types of configurations when building an electric-assisted bicycle:

  • Direct-Drive Hub Motor: Brushless DC motor, low to high powered, installed in rim; the largest and quietest of this group.
  • Geared-Hub Motor: Brushless DC motor, smaller less-weighty than above, internally-geared.
  • Mid-Drive Motor: Brushless high-speed DC motor, smallest of the group, and requiring externally geared-reduction drivetrain.

For many first-time DIY ebike builders, choosing which one to build can be a daunting task as there are many factors to consider. Our aim is to make this choice a better informed decision by evaluating the advantages and merits of each system, as well as their drawbacks.

Images: Rendering of Golden Motor 250W Geared-Hub motor (top-right), Bafang Mid-Drive (lower-right)



The most influential factor for the first-time ebike builder is Cost of goods. To be clear, this article will focus on the cost of the motor and controller kit distinctly and separate from the BMS-battery-charging system as that is a discussion all unto itself.

eBay, Amazon and AliExpress all carry inexpensive direct-drive hub motors at impressively low prices. Many first time builders will choose one of these <$350 kits as their first project: They are for the most part modestly powered - delivering between 500W and 1000W, and paired with a 36 to 48V controller rated for 15A to 30A of power. To be frank, these types of kits are a common commodity and sold in Asia by the millions; with all due respect, as a first kit, they are a good place to start – with one strong warning: Beware of scams! Make sure to factor in shipping and transaction fees into the final cost as many sellers will lure in with low pricing on their website, only to commonly tack on additional charges adding up to $100 or more on checkout. We've seen it, we've said it, take heed – Caveat Emptor.

For the very best value, Kinaye has chosen to offer High-Powered Direct-Drive motor kits for our product line that are similar in design to the very inexpensive offerings of the lower powered kits. The difference is that our kits include bigger, more efficient, stronger motors and controllers having up to four times total performance power (96V-60A). Our kits are priced between $700 and $1000 without a battery, and all of our kits ship for free within the continental USA.

Geared-Hub motors are also relatively inexpensive kits generally priced in the $300-$500 range, very similar to the Commodity Direct-Drive hub motor kits above. The distinction between a geared-drive and a high-turn direct-drive motor is that geared-motors are smaller though are able to climb hills at the expense of speed and a bit of noise. Eventually maintenance is required as gears do wear out.

In addition, there are a few popular Mid-drive kits on the market including the Bafang BBS02 Kit (~$600) the Bafang BBSHD (~$750) the Cyclone 3000W Kit (~$450) and our favorite Mid-Drive kits from LightningRods Development ($885-$1085). Mid-Drives, like geared-drives require gearing to transform their inherit high-spin into powerful torque. One advantage of this speed is their snappy acceleration! The primary disadvantage is the expensive somewhat noisy albeit precision reduction gearing; to be very clear – anytime mechanical gearing is involved – there will be loss of efficiency.

Below is a summary of options presented in terms of Cost/Watt:

  • 1000W DD Hub Kit: $300/1000W = $0.30 per Watt
  • 3000W Kinaye "Basic" Kit: $700/3000W = $0.23 per Watt
  • 1000W Geared-Hub Kit: $450/1000W = $0.45 per Watt
  • Bafang BBS02: $600/750W = $0.80 per Watt
  • Bafang BBSHD: $750/1000W = $0.75 per Watt
  • LR Small Block: $885/1500W = $0.59 per Watt
  • LR Big Block: $1085/3000W = $0.36 per Watt
  • Cyclone: $450/3000W = $0.15 per Watt

Kinaye's MXUS 3K Turbo Direct Drive Motor

There is no way around it, adding a motor to the bike will make it heavier. Where that weight is put should be carefully considered. And not all locations are equal in terms of performance.

  • Having weight in the wheel is the least desirable.
  • Having weight in the center of the frame, especially when it is in a full-suspension frame, is ideal. This will allow the bike to perform its best when the terrain is anything other than a nice smooth roadway.

In this respect, Mid-Drives and lighter geared-hubs have an advantage over heavier direct-drive hub motors because the latter – depending on size can weigh anywhere from 15-30 lbs. The MXUS 3000W Direct-Drive hub motor Kinaye sells are actually fairly light compared to their wattage output, though still tip the scales at about 20 lbs. In contrast, a Bafang BBSHD motor weighs about 13 lbs. (sans reduction gearing), and a geared-hub motor like the 500W Mac weighs about 12 lbs.

If you will be riding your ebike on single-track or where there are lots of poorly maintained roadways a Mid-Drive will be your best option. If you will mainly be riding on smooth roads and easy trails, the poor weight distribution of a direct-drive hub motor should not be a major hindrance to overall ride quality. In some instances, the significant power advantages of direct-drive hub motors may still outweigh the better weight distribution of a mid-drive, even on rough terrain. Many ebike enthusiasts swear by the raw power of the DD hub motor, regardless of terrain.

We think it is perfectly acceptable for an ebike enthusiast to have at least one bike of each kind (Mid-drive bike and a DD Hub Bike) to best-suit the ride planned for that given adventure. Given that, we believe that the best understanding of benefits and drawbacks comes though experience in riding all types and configurations.

An electric motor is a very simple machine: In its most basic form it is a stator, made with laminated steel and magnet wire (as Electromagnets), and a rotor (the spinning part), with permanent magnets to react to the changing magnetic field of the stator's electromagnets. How this rotational force gets converted into forward motion on a bicycle varies depending on the application. That said, there are three forms by which these motors are configured:

  • Direct-Drive, brushless: Wire-wound Stator is fixed to the axle, axle is fixed to the frame. Rotor contains the permanent magnets, and via bearings spins around the axle and is connected to the rim via spokes.
  • Geared-Hub Motor, brushless like above, except with internal gearing to give a smaller motor more torque.
  • Mid-Drive brushless DC motor is reversed in design whereby the Stator casing contains the wire-wound electromagnets is fixed to the frame, and the permanent magnet Rotor spins within, like a drill or starter motor at high-speed, though needs gear reduction to convert speed into torque.

Therefore, Direct-Drive hub motors are the SIMPLEST of ebike motor designs. The ONLY moving parts are the two bearings at the axle. Because of this simple design, Direct-Drive Hub motors are the most efficient, robust, and durable motors bar none. It is also partly for this reason that they can take the most power input.

Caveats: DD Hub motors will fail for the following reasons…

  • Motor interior gets wet and rusts/Seizes
  • Bearings fail
  • Windings overheat from excessive current and cause a short
  • Hall sensors/wires short or fail. (Can be remedied by using a sensorless controller)
  • Motor's Axle may break from impact

Geared-Hubs are similar to DD hubs, except that they use a planetary gear system to multiply torque and reduce speed at the wheel. In addition to the same failure points as the DD Hub, a geared-hub will have normal wear requiring eventual replacement on the planetary gears. This is a relatively straight forward repair that may need to be done more or less frequently depending on how much power is forced through the motor and total hours of use.

Mid-Drive systems are much more complex in nature, and as such these systems require more maintenance and repairs than either DD Hubs or Geared-hubs. The main motor typically turns at a much higher RPM and must be "reduced" via reduction gears or a set of sprockets and a chain, and on some set-ups this may involve two stages of reduction. Further, the ultimate method of power transfer must happen through the bicycle's standard chain and rear sprocket set, which to this day are still designed for Human power, NOT a machine that is up to 5 times as powerful as an athletic human. Chains and sprockets will wear out and break. And if the chain breaks, there's no way of pedaling the bike - anywhere!

And of course, with Mid-drive kits, quality makes a big difference:

  • The strength of mounting brackets and quality of the reduction gears is paramount to the longevity of the system.
  • Buying a poor quality mid-drive can quickly become a moment of regret.

Overall, we have been pretty impressed particularly with the Bafang BBS series mid-drives, as they are quiet, and the reduction gears are well-made, well-sealed and protected from the elements.

DD and Geared-Hubs are available in a variety of windings which produce varying amounts of torque per amp as well as varying rpm per volt. However, most hub motors are "single-speed": To change top speed or total torque output we need to change battery voltage and/or the controller's current output. The advantage with this, however is that there is no need to "shift" to come up to full power and speed. The torque provided by a DD or geared-hub motor is constant until reaching cruising speed, and is comparable to driving an "automatic" car vs. a "Manual Transmission" car.

In contrast, a Mid-drive (assuming it uses the bike's rear multi-speed cassette/freewheel) can use the advantage of gearing to produce massive torque at low speeds (Using the bike's Large Rear Sprocket), and then shift through gears to gain speed until the smallest sprocket is used to achieve maximum speed with relatively low torque. This range of gearing and torque output is one of the most desirable features of a mid-drive, and is especially useful on difficult and challenging terrain, where precise speed and torque control is paramount.

We must also point out, though that care must be taken when shifting gears with a mid-drive, as doing so under power will be very hard on the drivetrain components (Chain, derailleur & sprockets) of the bike (see Reliability and servicing). Some systems will automatically cut power when a gear shift is sensed, however many systems do not so it is up to the rider to learn the proper techniques for accelerating and shifting.



Kinaye Torque-Arms

Hub motors install into either the front or rear wheel position of the bike:

  • Most hub motor kits are sold "Pre-laced" where the motor is already laced into a rim
  • Where we just have to mount a tire onto the rim, and
  • Then mount the motorized wheel into the bike's drop-outs.

With 1000W and lower hub motors we can often mount the motors without the need for torque arms. Conversely, on more powerful hub motors, there is a lot of torsional force at the motors axle that can break a standard bicycle's drop-out; therefore it is strongly recommended to use some kind of torque arm (on both sides of the motor axle) with more powerful Hub motors. This reinforces the frame where the axle of the motor would otherwise want to twist itself out of place under high motor loads.


Direct-Drive motor as FWD on a fatbike

Aside from the need for torque arms, installing a hub motor is much like installing a normal bicycle wheel. For rear wheel installations, we need to manage the chain and adjust the derailleur to suit. We also need to route the cable from the motor to the controller, and secure it in place which can generally be done with Zip ties. A person who is somewhat skilled at general bicycle maintenance should be able to install a hub motor within an hour or so, with the electrical connections taking another couple of hours, depending on the electrical skill of the installer. Most kits, including Kinaye kits come with detailed wiring instructions, plus all connectors pre-made for "Plug & Play".

Mid-drive kits will vary in complexity with respect to installation: The Bafang BBS series are fairly simple to install, and require only that you remove the existing pedals, cranks and bottom bracket, and install the motor unit through the bottom bracket opening of the bicycle. For those without the specialized tools for this operation, the easiest way is to take the bike to a local bike shop and have them remove those parts for you. At most, it should take an experienced bike mechanic with the proper tools about 10 minutes to remove the bottom bracket. Installation of the motor kit is then straight forward: Just mount the motor, route the chain onto the motor's chainring, and complete the wiring much the same as a hub motor kit.

Mid-Drive Kits such as the Cyclone and the LightningRods Kits are a bit more complex in that there are secondary reduction gears and chains that need to be installed and tensioned properly, as well as a more involved mounting process that requires a bit more skill to set-up securely.

Since adding a motor to your bicycle makes it more powerful and faster, braking is of the utmost importance, and upgrading your bicycle's brakes is of the utmost importance for safety.

Braking is where direct-drive hub motors have a distinct advantage over geared-hubs and mid-drive systems because only a direct-drive hub motor will allow the motor to also operate in reverse to actively brake the bicycle: This is called regenerative braking because when the motor is braking, it is also feeding energy back into the battery, thus recovering some of that energy. Regenerative braking is often not available on cheaper 1000W kits, so it is wise to check with the supplier beforehand to confirm if the kit has this feature. In our opinion, this feature is a MUST-HAVE for a direct-drive kit: All of Kinaye's kits have regenerative braking capability.

Geared-Hub motors and mid-drive systems do not generally have the ability to provide for motor braking and battery regeneration because freewheels in these systems prevent the motor from applying force in the opposite direction. Therefore it is imperative to be sure the candidate bike has a very good disc brake system. For these kits, we highly recommend that the front wheel have at least a 200mm rotor paired with the quality Avid BB7 brake caliper. Some may choose to go with a hydraulic type of disc brake, but if set-up properly, mechanical disc brakes like the Avid BB7 are generally durable enough for ebike use. Better yet, place a matching set on the rear as well.

Top Speed is ultimately a function of how much power a bike has, and how aerodynamic its shape. Upright ebikes have an exponential speed to power relationship. It takes about:

1000W to achieve a speed of 30 MPH
2250W to achieve a speed of 40 MPH
3200W to achieve a speed of 45 MPH
4300W to achieve a speed of 50 MPH
7000W to achieve a speed of 60 MPH
11,500W to achieve a speed of 70 MPH

One should not assume that a kit rated at a specific wattage (say "3000W") is able to attain a top speed in the low 40s MPH however, because the gearing or tire size must be properly engineered to optimize the power level at the desired speed. If not, then the bike's speed will "top out" below its maximum power output:

  • Insufficient battery voltage
  • Motor was designed for lower Kv (low RPM per volt)

Such a set-up would provide impressive acceleration, but not as impressive top speed.

Geared-Hubs are generally rated for up to 2000W of total power, depending on the motor quality. Beyond that power level, the gears wear excessively fast, and the motor will overheat.

High-performance Mid-Drive systems like the LightningRods kits are limited by the amount of power that can be safely run through the bicycle's drivetrain. Depending on component quality, this is generally in the 2500W – 3500W limit.

Direct-drive hub motors are a bit unique in this feature set because they have the mechanical ability to take brief spikes of power up to 4 times their rated power. Kinaye has raced a 3T (Fast winding) "3000W" MXUS motor at over 66 MPH (About 10,000W) in a ¼-mile drag without damage to the motor. Power levels this large can only be run briefly though because the motor WILL overheat at these extremes eventually. However we believe this illustrates the main reason that ebike enthusiasts like large direct-drive hub motors: Direct-Drive hub motors are the power and speed kings of the ebike world.


66 mph @ Silent Thunder, 9/20/2015
Pacific Raceways, Auburn, WA

Summary:

We are fans of all types of ebike systems: Each type has its special place on the road and in the wilderness. We think once you have tried all the designs, you will discover that one ebike is not enough; You will want a mid-drive for good weight distribution on the trails, and you will want a DD hub for those long road trips.

Happy venturing!