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Friday, December 28, 2012

Battery Case Design and Construction

As mentioned in my previous post, the battery case consists primarily of a coroplast housing.  Materials used to construct the battery case are listed below:

  1. white coroplast
  2. UV stabilized cable ties
  3. 4" long bolts
  4. nylock nuts
  5. washers
  6. 3 1/2" long nails
  7. old rubber bicycle tube

The following video highlights the major design elements of the coroplast battery case design:

The following pictures show the battery case from various angles.  I still plan to add an upper and lower fairing to hide the cable ties and wiring that will be running from the handle bars to the controller mounted on the rear rack.
Front 1

Rear 1

Front 2

Right 1

Right 2

Top 1

Top 2

Sunday, December 23, 2012

Selecting the Battery Pack

Selecting the battery pack is dependent on my performance targets and the available space within the front frame triangle for the battery case.  I've used corrugated plastic, "coroplast", in the past for various automotive projects and it holds up surprisingly well to outdoor use.  I decided to construct the battery case out of corrugated plastic as it is:

  • stiff
  • light weight
  • tough
  • low cost
  • non-conductive.

Modeling the space available using the free Google Sketchup 3-D modeling tool, it is clear that the standard battery packs offered by Electric Rider do not fit within the bike frame.  Two 48V pack configurations and one 72V pack configuration are modeled and as you can see all resulted in interference with the bike frame:

I constructed the main spine of the bicycle case to obtain real world dimensions and sent the following dimensioned drawing to Electric Rider so they could design a custom triangular battery pack.  After looking at the drawing, Electric Rider advised me as to the largest sized 48V and 72V packs that could actually fit within the available frame space.

Electric Rider came back with two options both somewhat smaller than the packs I had previously modeled. The two packs that could fit within the frame were:
  • A 48V pack with 18.5Ah of battery capacity or
  • A 72V pack with 11Ah of battery capacity (not recommended since batteries would be operating close to their max current output)
Modeling the 48V 18.5Ah back performance to estimate range and top speed results in acceptable performance based on my design targets:

Flat Ground 48V, 18.5Ah Pack

3% Grade 48V, 18.5Ah Pack
As such, I have ordered the Crystalyte Phoenix II Kit from Electric Rider with a custom, triangular 48V, 18.5Ah pack.

Friday, December 21, 2012

Modeling Electric Bike Performance

An excellent website to model electric bicycle performance is the Hub Motor and Ebike Simulator site.  Alot of popular motors, controllers, and batteries are listed in drop down lists and custom hardware can also be specified before starting a simulation.  Key inputs to the simulation are:

  • Motor
  • Battery Voltage and AmpHrs
  • Controller current capability
  • % Throttle applied
  • Wheel Size
  • Bicycle Type
  • Terrain Grade %
  • Total Bike and Rider Weight
The key outputs given by the simulation are:
  • Maximum Speed
  • Range
My design targets are 
  1. To be able to carry out a round trip to work, a total of 10 miles with a 500 ft one way elevation change.  The elevation change is over 3 miles of the trip so the grade is steeper than would first appear.   I would also like to be able to do off road rides in the 8 to 16 mile range
  2. Target top speed is 25mph or greater. 
  3. .Battery pack must fit within the bicycle's main triangle (frame)

I simulated full throttle conditions for both flat ground and a worse case continuous 3% avg grade based on my route.  If the simulation at a 3% grade at full throttle meets my design requirements then I am confident I will have sufficient range at real world conditions since I will often be at part throttle, the entire route is not at 3% grade, and I will be constantly providing some pedal assist.

Looking at the TOP SPEED Estimates from the Electric Rider webpage, I feel I should target a 36V 40A kit or higher to meet my top speed goals.  The Cruiser motor, wound for higher speed, is better for my situation since I am a fairly light rider and am leaning towards a higher voltage kit that should have enough torque even with a high speed motor.  Simulation helps confirm the top speed and also helps to define the Ah capacity required to meet my range targets.  At higher voltages, less Ah capacity is required to meet the same range targets since a higher voltage motor draws less current to overcome a specific load.  A 72V kit is tempting for the incredible top speeds but I would need to ride with full motorcycle gear for added safety and the pack would be more difficult to fit within the bike frame.

Initial simulation of one of the Electric Rider standard kits at 48V, 40A, and 20Ah, shows it easily exceeds my design targets as shown in the image below:

Adding a constant 3% grade to the simulaton drops range and top speed somewhat but is still quite acceptable as shown below:

For kicks I also simulate several iterations of the fastest 72V kit and find that the 72V kit gives me the same range and top speed as the 48V kit simulated above but at 13Ah and 64% throttle application.  Full throttle in the 72V kit results in over 40mph top speed according to Electric Rider!  Scary on a bicycle that's not really designed for sustained speeds of that magnitude...The 72V, 40A, 13Ah simulation is shown below at only 64% throttle.

With these simulations as a starting point I can proceed to design the battery case that will sit in the main triangle.  I will then send the battery case dimensions to Electric Rider so they can determine if a custom shaped, triangular pack will fit within the battery case I design.

Wednesday, December 19, 2012

Selecting a Bicycle for Electric Conversion

In choosing the right bike to convert to electric, I first narrowed down what electric kit I was going to use.  Based on my research at and other sites, I knew I wanted a system that used a Crystalyte direct drive rear hub motor. Crystalyte motors seem to have a reputation for lasting thousands of miles of use.  I like the idea of a rear hub direct drive motor for several reasons:

  • low center of gravity
  • rear wheel drive
  • simple gearless mechanism for durability
  • simple installation
  • clean look

With the motor system selected I proceeded to select a bicycle with the following characteristics
  • Disc brakes for added stopping power given higher average speeds
  • Front suspension only to keep main frame triangle open for battery pack, again with the interest of low center of gravity and a clean look.
  • 7 or 8 speed rear cassette since Crystalyte motor will not accomodate 9spd or 10spd cassettes/freewheels
  • Additionally I wanted Shimano drivetrain and at least  4" travel fork from a recognized brand
Given the above requirements I purchased a K2 Zed 4.6 bike from the Sports Authority for $449 during a Black Friday sale.

The next step will be to select system voltage and battery capacity based on desired speed, performance, and range.