EBike Motors Middrive
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Middrives are located in the middle of the bike. They may drive the front chain sprocket by a chain (Cyclone, GNG), the final drive chain directly (Panasonic, a jackshaft design) or the bottom bracket shaft (Bosch, Sunstar). Middrive Motors that drive the bottom bracket shaft are further designated as crank drive motors. There are also motors that are located in the middle of the bike, but drive the rear wheel directly, without any connection to the pedals. These are designated as rear wheel drives to separate them from middrives. Middrives can use the derailleur gears for increased hill climbing capabilities compared to hubmotors with the same power. They are more complex than hubmotors and are usually used in bikes designed for hilly terrain and off-road bikes. Middrives are usually limited to about 1000 Watts of power, since the derailleur gears can not last more power reliably. If the rider wants to pedal without using the motor, it has to be disconnected. Usually a freewheel sprocket is used to implement this feature. If the rider wants to use the motor without having to pedal, the pedals have to be disconnected from the drive train. A freewheel crank set, that has a freewheel between the pedals and the chain ring can be used to implement this feature.
Advantages over hubmotors
- better hill climbing ability and stronger acceleration
- usually better efficiency at steep hills and acceleration
- no un-suspended motor weight at the axles
- more effective cooling
- higher power to weight ratio
- more complex
- limited power, limited by the bicycle drive train (usually to 1200W, using 9-speed chain and a 12T as the smallest sprocket). However, more power can be pushed through if using the higher tooth count sprockets.
- increased wear at the drive train
- usually less efficient at steady speed on flat roads due to drive train losses
OEM Middrive Motors
not a complete list yet
It drives the the chain wheel using one helical and one straight teeth gear set on a jackshaft. There is a 250Watt and a 350Watt version. It has a built-in pedal torque sensor. In 2013 a new version with built-in pedal overdrive was introduced, gear stages of this unit are yet unknown. The chain ring spins 2.5 times faster, which keeps torque on the output sprocket down. Forum members criticize that most bike manufacturers don't use this to reduce chain tension at the same time, as it is common sense that middrive chains wear out quickly. However, chain load and gear ratios of the chain drive can be modified using aftermarket parts. Active discussion: 
Yamaha Offers two systems: PWseries and PW-X.
PWseries The PW series is a center mount unit (bottom bracket motor). It supports up to 25km/h (15.5 mph). The motor is rated at 250 watts with a peak torque of 80 Nm. Continuous torque is rated at 70 Nm with direct transmission of the motorpower to the crankshaft. The system uses three sensors (like the Bosch) Torque, speed and crank. Weight is listed at 3.5 Kg (7.72 lbs). 
PW-x The PW-x series is an update to the PW series. Weight is down to 3.1 Kg. Power rating remains 250 watts. Max torque remains 80 Nm in EXPW mode with 70 Nm in High, STD, Eco and +Eco mode. Maximum cadence is listed as 120 in EXPW mode, with 110 rpm listed for High, STD, Eco and +Eco modes. The maximum speed remains 25 km/h (15.5 mpg). The percentage of assist for the five modes is listed at EXPW/320, High/280, STD/190, Eco/100, +Eco/50. 
It drives the final drive chain using a jackshaft. The jackshaft is driven by an inrunner BLDC motor via helical gears. There is a 250Watt and a 350Watt version. It has a built-in pedal torque sensor.
Frame intergrated Middrve similar to the Bosch system. Compatible to Shimano Di2 electronic shifting systems. Motor torque is reduced during shifting operation to enhance shifting performance and to enhance maintenance periods of drive components.
It drives the the chain wheel by straight teeth gears. Version 1.0 is designed for 250Watt cycles. The newer 2.0 version introduced in July 2013 has up to 500W. It uses magnetostrictive pedal torque sensor, which works similar as the Thun X-Cell RT bottom bracket cartridge. Unique features are coaster brakes and a shifter sensor, which can detect shifting operations and throttles motor power during these, to go easy on internal geared hubs. []
It drives the chain wheel via a jackshaft and an angle drive. There are very few different models available. Homepage: 
high power middrive using multiple chain drives. this drive is only available with two different models. Rumors tell that it uses the highly efficient "CPM" Compact Power Motors drive system that claims 95% efficiency and up to 4kW of power at the size of a coke can. Homepage: 
Active discussion: 
There are at least two versions. Both drive the chain wheel via co-axial freewheels. Both designs use a high motor speed reduction to the cranks. One drive uses a special planetary drive, the other one uses three chain drives using one motor-coaxial and one BB-shaft-coaxial hollow jackshaft, see Patent WO2011013109.
Clean Molbile was bankrupt in February 2012 and was acquired by T/Q-Systems 
Prototypes that did not reach mass market
There are many more drives that were announced but could never actually be bought. A more complete list: https://www.electricbike.com/2014-mid-drive/
Aftermarket middrive motors
not a complete list yet
Cyclone Kit 
Comes with a 9.33:1 planetary gearbox, a 14T freewheel sprocket, a long bottom bracket (153mm) to make the motor fit between the pedals and a freewheel crank set. It is available in various versions ranging from 24Volts to 48Volts and from 200Watts to 1200Watts, all of them use the same KV motor, the higher voltage versions are therefore geared very fast. It's known that the 1200W version quickly wears the planet gears is very noisy. Looks like the 2013 versions come with an improved mount. With these, the motor is fixed on both sides of the BB, this makes the mount stronger. The earlier mounts were more likely to bend.
AFT Kit 
Similar to cyclone motor with an improved mount. The motor is fixed on both sides of the BB, this makes the mount stronger. Homepage: 
ego Kit 
Similar to cyclone motor with an improved mount. The motor is fixed on both sides of the BB, this makes the mount stronger. Homepage: 
Comes with a 5.72:1 belt reduction, a 12T freewheel sprocket, a long bottom bracket (153mm) to make the motor fit between the pedals and a freewheel crank set with improved bearing. The motor is fixed on both sides of the BB, this makes the mount stronger.
Since 2013 this unit is sold by conhismotors, too.
Specs: GNG V1 Specs
Similar to the GNG V1, but comes with a 4.9:1 chain reduction and a 60v650w controller option in addition to the 48v450w controller option as on the V1.
LightningRods mid drive kit 
Similar to GNG, much stronger
GNG Kit Gen2 
Comes with unknown planetary reduction, a 9T sprocket, a build-in roller type freewheel and a freewheel crank set. Looks like a converted geared hubmotor (making the axle rotate). Sold with a 36V controller, but it has been said that it's geared too slow for 36V, should run better using 12s lipo battery (44.4V) though. It should also be possible to swap the tiny 9T output gear for a 12T to gain increased reliability and decrease chain wear if using a 36V battery.
Sunstar Kit 
Comes with an unknown 2-stage gear reduction to the BB shaft. Rated Power is 250W, Motor weight is 3.2kg, battery voltage: 24-36V The gear reduction to the crank shaft is very high compared to common aftermarket reductions to the crank shaft. Using a gear reduction to the cranks is made possible by integration of the bottom bracket cartridge into the motors housing. The right piece of the bottom bracket (BB) has there for no threads. They are not necessary. The motor is fixed on both sides of the BB, this makes the mount stronger.
Manufacturers website: http://www.szbaf.com/en/components/motor.html
This mid drive looks similar to the Sunstar and also uses a BB cartridge built into the drive unit for mounting. Similarities between the two have led to lawsuits between the two manufacturers relating to alleged IP infringements.
It is currently available in the following:
- BBS01: 250w, 350w*
- BBS02: 500w and 750w
- BBSHD: 1000w
It uses 2-stage gear reduction to the cranks. Bafang's website discloses the reduction 1:21 reduction. Previously disclosed on this website a total reduction ratio 30:1.
Bafang's website discloses it's noise to be less than 55 dB. It is relatively quiet during operation and undue noise is usually an indicator of mechanical failure. Such failure typically relates to mechanical failure (detailed further below).
Bafang's website discloses the weight of the unit at 4.3kgs. It is unclear whether this is the motor, or entire kit. Bare weight measured by Tiverion is around 3.6kg.
Of recently times, Bafang has released a second generation of BBS units. The following relates to BBS Gen 1 units:
It was designed to be fitted to a frame with a 68mm BB width. Upon slotting the unit directly into a frame's bare BB, a bracket and two locknuts are threaded onto the nondrive side of the unit, clamping it to either side of the frames BB. The inner nut secures the unit to the frame, whilst outer nut's sole purpose is to to lock against the first nut to prevent the unit from pivoting unintentionally or becoming loose. The unit can also be fitted to a frame with a 73mm width BB if the inner, thicker and stronger nut is applied solely and the outer locknut is discarded. Further, a fatbike kit is available from California EBike which has a wider crank spindle and extension to allow the unit to be fitted to frames with 100mm BB width.
BBS02 and BBSHD both have the same (different) BB sizing listed, being: 68 / 73 / 90 / 100 / 110 / 120mm. BBS01 is no longer listed on Bafangs Website and it is unclear as to whether it will continue to be produced. Again, presumably installation follows the same method described above.
Noted differences between Gen 1 and Gen 2:
- The nylon gear which meshes to the motor shaft is of a different design. It contains three needle bearings rather than one and they appear to be of a larger OD than the original. The internal diameter of the Gen 1 Nylon Gear is 16mm and Gen 2 is 14mm, and there is not immediately interchangeable between the two generations. The original Gen 1 replacement is required and is allegedly no longer being produced by Bafang. Gen 2 can be used if a Gen pinion is also used.
- Gen 2 controller is different in size and shape, ergo is not compatible/a replacement for a Gen 1 controller.
Tools/Items required for installation
- Crank puller to remove old cranks
- Allen key for crank bolt replacement
- Supporting bracket bolts
- 10 x 1mm Washers for between motor and supporting bracket if BB is 73mm
- Lockring spanner
- Pedal spanner
- Cable ties
[following information verified by forum member Lurkin, from a 250w Bafang Mid Drive]
The unit is constructed of the main metal chassis. The two sides act as cups, one housing the actual motor core and the other the controller.
The motor shaft consists of a metal core with 9 magnets which turns into a nine spline, metal worm gear. It has two o-rings on the worm gear side of the metal core and one on on the opposite side. The metal worm gear meshes with a nylon worm gear, which is on a separate shaft. The opposite end of this shaft protrudes into the controller area and has a metal gear with 11 teeth on it. This metal gear meshes into a large metal gear, with 68 teeth. The large metal gear is fitted with a free wheel and it's shaft is the crank, which is in turn connected to the pedal arms.
The remainder of the motor area consists of 18 stator poles and 3 hall sensors. 3 phase wires go through holes in the chassis to the opposite cup, which houses the controller. Accordingly, a replacing the controller is a relatively simple task.
Two common faults appear to occur with the Bafang units. They relate to either the failure of the bearings situated in the cartridge BB, or the destruction of an o-ring on the motor shaft and consequent binding on between the shaft and bearing holding the shaft. Both faults are detected through undue noise emitted from the unit.
The first of these faults occurs due to the poor waterproofing solution inherent in the Bafang BB design. Waterproofing the needle and thrust bearings situated centrally in the BB is provided on the non-motor side of the unit by a silicon washer. This washer is placed between a four notched nut and a flat washer, which can lead to the four notched nut piercing the silicon washer and leading to it's eventual destruction. Solutions discussed have included replacing the silicon washer and packing the crank area with waterproof grease periodically. Note neither is a complete, permanent waterproofing solution for the BB.
At worst, bearing and crank shaft replacement is required. It will depend on the amount of wear. A forum user has detailed the procedure to add a grease nipple to the bottom of his frame to access this area for grease replacement. This is permanent solution, however is likely to be unpopular given it requires drilling a hole in the bottom of the frame and Bafang BB. Alternatively, periodic greasing of these bearings is required.
The later has been attributed to the construction of early 750w units, however has been experienced recently with a 250w unit. The motor shaft it held at either end within the unit by two cassette style bearings. The shaft should spin with the internal part of the bearing, forcing the bearing to conduct the movement. This is achieved by an o-ring situated at the end of the shaft, to fasten it to the inside of the cassette bearing. Historic failures has been attributed to the lack of a groove for this o-ring in the motor shaft. Motor produced later by Bafang have included a groove on the motor shaft to house the o-ring. The failure occurs when the bearing does not spin, and the motor shaft spins within the internal area of the bearing (i.e. the o-ring is ineffective), leading to the destruction of the o-ring and eventually wear on the motor shaft and internal bearing.
A replacement motor shaft and corresponding bearing may be required, depending on the level of wear. Disclosure to the dealer that this area has not been accessed and therefore is a design/manufacturing failure may result in a more successful warranty replacement. Solutions discussed have included loctite on the motor shaft where it contacts this bearing to 'glue' the inside of the bearing to the motor shaft and force the bearing to spin instead.
Disposable items required for servicing
Bafang has advised the following greases are applied during assembly to Bafang units:-
Bearing grease - Unical Lithium grease No 2. This grease is applied to the thrust and needle bearings contained in the BB.
Plastic gear grease - Unical Aviation Synthetic Grease AG-11. A white lithium grease is used between the motor shaft and the nylon helical gear.
Metal gear grease (Black)- HCF Lubrications Technology Co Ltd's MG - 2A grease. This is situated on the large reduction gear and the small metal transfer gear. This should be periodically replaced, given grit can get into this area and grind on these gears.
Metal gear grease (white) - Unical Polybear Grease BLD 227B. From correspondence issued by Bafang, it is unclear where this lubricant is used.
Bafang have acknowledged that these greases are unlikely to be available locally and have advised to use Shimano substitutes where possible.
Other disposables used in construction:-
Blue loctite - application to various bolts and the end of the motor shaft.
White sealant - This is used where wiring crossing between the two sides of the motor. This only requires servicing where the motor has been disassembled.
Manufacturers website: http://www.tjbewo.com/En/
Only single reduction middrive that made it to market yet. This makes it more simple in construction, but the motor also sees more torque compared a with a 2 stage drive, as the possible reduction ratio to the cranks is smaller.
It fits 68mm and 73mm wide bottom brackets. There is a 250W and a 350W variant. Rumors tell that there will be a 450W version and a special 100mm version that fits the BB width of most fatbikes.
Small brushless motor. Not much info about this yet, looks like a gear drive similar to the SunStar. Weight is somewhere between 2.2 and 2.8 kg. Homepage: 
Custom Middrive Motors
There are some high power custom middrives using RC-motors on the sphere. Hubmotors can be converted to middrive motors by attaching a sprocket to them  or making the axle rotate on outrunner hubmotors .
In a Middrive, there are always two | freewheels:
The 1st disconnects the pedals from the drivetrain ( = the chain to the rear wheel) while not pedaling. The motor can be used independently when the driver is too lazy to pedal or when driving around corners, since the pedals limit inclination when they are in vertical position.
The 2nd one disconnects the motor from the drivetrain to reduce drag when only pedaling.
A 3rd one on the rear wheel can be found in every bicycle, it is not Middrive specific.
Single Freewheel-Crankset Config
A freewheel in the crankset implements the 1st function. Usually this freewheel is screwed on a special right side crank arm, using common BSA Threads. The chainrings are then bolted onto this freewheel.
The 2nd freewheel usually is a smaller threaded 30x1mm BMX sprocket directly attached to the motor.
This pic shows a cyclone motor and the common cyclone freewheel crankset:
Mostly aftermarket Middrive Kits use this config, since it can be added to almost every bicycle:
There are also aftermarket freewheel crank sets available . The Hammerschmidt planetary overdrive crank set also has a freewheel function built-in, that could potentially be used in a middrive.
Dual Freewheel-Crankset Config
It is possible to use a dual freewheel crankset, having both freewheels on the same shaft. This way the chain, belt or gears from the motor to the chain wheel do not turn if the driver uses only the pedals. This further reduces drag when pedaling only 
Many commercially available Middrives use this config (not yet a complete list):
- Bosch Pedelec System 
- Harmonic Drive System designed by Clean Mobile 
- Chinese made bikes sold by R-Martin and Evelo.
- Contec E-rider 
The only aftermarket Kit yet available that uses this config are the Sunstar Kit and the Bafang BBS01 and BBS02
Jackshaft Dual Freewheel Config
Both freewheels are on one Jackshaft. The 1st freewheel sprocket is driven by the pedal chain wheel. This can be located at the left side of the bike, to gain some space for a motor. The motor and motor reduction gears can be integrated onto the Jackshaft, as well as the 2nd freewheel that disconnects the motor. The jackshaft combines both power inputs and sends it to the wheel via a single output sprocket. This additional overdrive keeps the torque on the output sprocket down (same manner as the 2013 Bosch Middrive). This config was introduced by the user crossbreak in 2012 . In November 2013, user spinningmagnets discovered patent US4280581 by Lawrence A. Rudwick from 1981, which describes the same configuration.
Advantages over Freewheel-Cranksets:
- less torque at the pedal freewheel, no bending/twisting loads here
- additional overdrive, larger wheel sprockets can be used for more power
- increased ground clearance due smaller chain wheel
- the motor drives the chain to the wheel directly, increased efficiency, less wear
- Any bottom brakcet can be use as long as it gives enough space for two chains.
Drawbacks over the Freewheel Cranksets:
- does not fit every frame. Especially on full suspension bikes, the rear suspension can be influenced if the output sprocket is located on a bad spot.
- usage of a front derailleur is difficult
- the overdrive makes it impossible to climb hills without motor support
General Discussion about gearing in this thread (pros/cons): []
Below there is a picture of a dual freewheel jackshaft config, the motor freewheel is build into the motors' housing, it is not visible.
When HollowTech2 bottom brackets are used, a motor can be mount on both sides of the BB 
Ligthning Rods Dual Freewheel Jackshaft Drive: The Lunar Drive []
Panasonic uses a the tiny motor sprocket, which directly drives the final drive chain rather than the chain wheel.
- one less chain
- almost impossible to integrate into existing bikes
- limited chain wheel size
Explanation on Kalkhoff USA homepage