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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.

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