Electric MTB

My electric MTB. I forgot to mount the battery pack for the photoshoot.

I’ve got a 6 km bike ride to work, the route is mostly flat but there is a high bridge I need to pass and the wind is not your friend when biking westward in Gothenburg. Despite this it is quicker for me to go by bike than public transport. The problem is that I do not want to be all sweaty when i arrive to work.

The solution: Put a motor on the bike

My bike is an Crescent Balder 24 speed mountain bike with front suspension and aluminum frame. Here in Sweden there are some regulations limiting an E-Bike to 250 W of power and a maximum speed of 25 km/h. That is not enough for me but I need a drive train that is possible to limit when driving on public roads.

After several hours of reading at the Endless Sphere forum I settled for a Nine Continents conversion kit including a 2809 rear wheel motor, 48 V 27 A motor controller, twist throttle, regeneration break handles and some other parts needed for the conversion. This kit has what is called an Infineon controller (based on the Infineon XC846 chip) and is programmable and possible to limit within legal limits.

E-Bike controller
E-Bike controller mounted behind the saddle

This is a 12 MOSFET version of the controller with a somewhat splash proof aluminum case that i mounted on a luggage rack from Biltema. One of the reasons that i choose the Nine Continents kit was that someone wrote on the web that it was one least bad china made E-bike kits regarding weather resistance.

E-Bike kit delivered
E-Bike kit delivered

The motor comes already laced in a 26″ rim and has mounts for disc brakes and a 7-speed freewheel, the only thing missing is a tire.

The 2809 means that the stator and magnets are 28 mm wide and each stator tooth have 9 turns of wire wound around it. There are different configurations of this motor, for example 2807 which have 7 turns wind resulting in a faster motor. The 2809 has a top speed of 35 km/h, on flat ground with 48 V battery voltage, which is enough for me. The slower speed means that the motor will be more efficient than the faster one at slow speed, for example uphill or with strong headwinds.

To power the motor I use Lithium Polymer (LiPo) batteries made for electric RC airplanes. At the moment I have 4 Turnigy 5000 mAh 6S 20C LiPo Pack batteries with 6 cells @ 5.0 Ah each. These are connected together as two 12s1p (12 cells in series and one paralell) packs like the one below.

Battery Module
One of my two 12S1P battery packs. These can be connected in parallel as well for a 12S2P pack. The serial wiring contains a 30A automotive fuse and a Anderson SB50 connector.

But the two packs can also be paralleled for a 12S2P pack with twice the range. The serial/paralell harness is made out of HXT 4mm Gold Connector for the battery, 10 AWG (~5,27 mm²) wire, a 30 A automotive fuse and a Andersson SB50 connector.

12 cells, at 3,7 V nominal voltage, in series results in a battery voltage of 44,4 V. 2 cells, of 5,0 Ah in parallel results in a total 44,4 V 10 Ah battery or 444 Wh of energy. My plan is to, at a later stage, extend this to 3 packs in parallel to 666 Wh of energy. Commuting to and from work with the bike has shown a energy usage of ~12-14 Wh/km. Mostly dependent on the amount of pedaling from me and wind speed/direction.

The battery specs say that they will deliver 20C continusly which means 20 times the capacity. My current battery with 10 Ah capacity will hence deliver a maximum of 200 A @ 44.4 V which is almost 9 kW of power.

There are a couple of different types of batteries used for electric bikes LiPo (which I use) has the highest power- and energy-density. The downside with LiPo is that they can be quite dangerous if you mistreat them, for example overcharge or puncture them in which case they will explode in a ball of fire. There are other lithium chemistrys for example LiFePo4 which are more stable but have lower energy density and much lower power density. There exists LiPo batteries that can deliver up to 90C while I haven’t seen LiFePo4 batteries capable of more than 10C. There are nickel and lead based batteries as well but they belong to the last century IMHO. Maybe I’ll write a more in-depth post about batteries, battery management and chargers in the future.

If you have any sort of experience repairing bicycles the install is easily made in a couple of hours, the most difficult thing was that the dropout has to be filed a little to suite the large axle on the hub motor. Since all torque from the motor is transfered through the dropouts the axle is 14 mm instead of the ordinary 10 mm bike wheel axle. The hub motor axle is flat on two sides making it fit in the 10 mm dropouts. Since the bend radius on the non-flat sides is 2 mm larger than on a regular bike wheel the frame dropouts has to be filed to a bend radius of 7 mm for a good fit.

Original dropout and axle to the left, hub motor dropout and axle to the right.

Since an aluminum frame is not as strong as a steel fram I decided to use what is called a torque arm to strengthen the dropouts. When using a hub motor that delivers a considerable amount of torque to the small flat sides of the wheel axle this is a must if the dropouts should survive. For example a powerful hub motor on an aluminum front fork without torque arm could end bad if the motor manage to twist itself out of the dropouts and the wheel comes loose. The rear wheel is probably less dangerous but I still don’t want the bike to break.

Torque arm
Torque arm. Note that the rear derailleur isn't used.

In the picture above you can see that there is no wire to the rear derailleur. The bike is originally a 24-speed bike but the hub motor only has room for 7 sprockets. Since the gear lever was mounted on the handbrake lever on this bike the rear 7 gears had to leave room for the regenerative brake lever supplied with the conversion kit making this a 3-speed bike. The left gear and handbrake lever is still intact controlling the front derailleur and brake.

I have used the bike to commute to work for a couple of months now and it works perfect. I have some future upgrades I want to make:

  • Move controller to water bottle mount
  • Replace phase wires to 12 gauge
  • Create fiberglass battery box
  • Tidy up cabling

This time of the year the weather in Gothenburg usually not invite to biking so I’ll have the winter to perform these upgrades on the bike.

2 thoughts on “Electric MTB”

  1. Hello
    Jag är mycket intresserad av ett kit. Vi har två bra 28″ Monarker min fru och jag men vill gärna se att vi kan använda dem lite bättre, (lättjigare) säger Lasp.

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