EBike Beginners info
Unfortunately, there is no magic formula for determining the best e-bike setup for you. Choosing the ingredients for your e-bike is an iterative process, however the steps below should be a good starting point.
Voltage (for cruising speed)
Firstly, decide on the top speed you would like. It is always tempting to aim to be the fastest bike on the block, but there are implications such as safety, cost and weight to be considered. So it is important to be realistic about the speed you require.
The required voltage can be determined using the Volts per RPM specification of the motor, however at this early stage you probably won't know what motor you will be using.
As a rough guide to unassisted top speeds on flat ground:
- 36 volts should do 20 mph (30 km/h)
- 48 volts should do 25 mph (40 km/h)
- 72 volts should do 37 mph (60 km/h), which is probably a dangerous speed for a bicycle on a public road.
(Volts x Amps = Watts)
Volts or Amps, by themselves, are relatively meaningless.
A motors "watt rating" is the peak output watts possible, from the specified voltage-amperage.
746 Watts = 1HP (1 horsepower)
Peak output watts are typically 50% 0f the input watts. (Exception is severely current regulated systems)
For Cruising Speed
Example: ~340 watts are required to motivate a Mountain Bike at 20mph. (350W motor)
- That would require:
- 24V x 28.3A = as high as 680W input, or
- 36V x 18.8A = as high as 680W input, etc.
Motor output watts required to sustain mph
- 15mph = 160w
- 20mph = 340w
- 25mph = 600w
- 30mph = 1000w
- 35mph = 1500w
- Note: In order for peak motor output Watts to provide graphed speeds, "peak output" must occur near specified speed.
Acceleration is motor applied torque ... beyond the torque required to overcome road and air resistance.
While your motors wattage output is the most important factor, acceleration can be "tuned" by judicious gearing.
24V motor geared with 16T freewheel, (vs oem 20T), has only 80% the acceleration torque at 0mph, it has 200% the acceleration torque at 15mph!
- Note - Speed-Torque lines are for gearing ratio comparisons and do not represent "torque curves".
36V 16T has 80% torque at 0mph but acceleration torque is 150% at 20mph and 200% at 22mph.
Speed vs Torque
Perhaps, best illustrated by a graph of a multi-geared mid-drive, speed and torque are in direct apposition to each other.
Gearing for higher speed reduces "to the road" acceleration-torque, and vice versa!
Yeah ... crank drives have a tremendous advantage!
- 300% the torque "off the line", in low gear, and ...
- Excellent top speed, with ...
- Optimal shifting allowing motor to:
- continuously maintain maximum wattage output, or ...
- maintain maximum economy-efficiency
Current (for acceleration)
The current (amps) is a key factor in the acceleration of an e-bike. Since people have different expectations for the amount of acceleration, it is not possible to recommend the current needed unfortunately. Also, if the bike doesn't have enough current, it won't reach the top speed potential of your voltage. Therefore it is best to err on the high side when buying components, and use the controller to restrict the current.
Some rough guesses of minimum current recommended for each voltage are:
- 36 volts = 15 amps
- 48 volts = 25 amps
The formula for power (watts) is: Watts = volts * current
How far do you want to go between charges?
You need to decide this and estimate your power usage (in Whr/km or Whr/mi). Then you can determine the Watt-hours (Wh) required for your battery using the formula: Watt-hours = Watt-hours per km (or mile) * distance
Now that you know the power and capacity requirements for your battery, you can start to choose which battery type (LiPo, LiFePo4, A123, etc) to use. Keep in mind that the battery's size and weight are important considerations when deciding where to mount the battery on the bike.
At this stage, you can also choose your controller.
See - EBike Efficiency
Also see - Speed vs Range
Almost any type of bike can, and has been, convert to an electric bike. Road, mountain, downhill cruiser, recumbant- just as people's use of their e-bike varies, so does the type of bike.
The main things to consider are:
- strength of the frame (and especially the dropouts for hub motors)
- where the battery can be located
- gearing to match cruising speed
- brakes and tyres, people!! E-bikes are heavier and faster than a pushbike, so don't build yourself a death-trap!
Now all that is left is to decide how to charge the battery, which type of throttle (or Pedal Assist Sensor), how to monitor the bike's performance (ie battery meter or an electric bike computer) and which friends to show it off to first!
Some small tips:
- While many e-bikes can be put together without any soldering (except LiPo's, where multiple battery packs need to be combined), it is still handy if you know how to solder in case urgent repairs are needed.
- Buy various sizes of cable ties in colour(s) which match your bike frame. Cable ties are fundamental to a good e-bike!
- Make sure the wiring is waterproof. Keeping the connectors, controller and battery dry is important for avoiding fireworks if you get caught in the rain.