Monthly Archives: October 2014

E-bike Power

How much power does an e-bike(or any vehicle) need?

In Australia the limit for e-bikes is now set at 250Watts for pedalec and only 200Watts for electric-assisted bikes, previously it was only 200Watts. This brings Australia into line with Europe and Singapore. Some countries allow 500W and in the US some states allow 750W. Part of the Pedalec standards state that the electric assist can only work while the rider is pedaling, which is a good idea, and also state that the electric assist must cutout above 25kph or sooner if the rider stops pedaling. Both are good conditions to keep them as bicycles and not crossing over into the Moped/Motorcycle class of vehicle although I would argue that 25kph is a bit low if we want to see more people using them as an alternative mode of transport. In Australia bicycles are still seen as just for leisure or hobby.

Now back to the question, “How much power does an ebike need?”
For my formula I will use the speed of 10kph, just above walking speed, low enough to disregard most wind resistance and still well within the Pedalec category. The weight is 80kg for the rider and 20kg for the bike to give a nice round 100kg. And gravity as 9.8ms-2 .

Speed = 10kph           = 2.8ms-1

Power = Work / Time

Work = Force x Distance

so Power = Force x Distance / Time

Force = 100kg x 9.8ms-2 (gravity)
= 980N

Then to work out the power required, if I ignore friction and wind resistance, use the vertical velocity to get the distance traveled times the force(due to gravity) over the time period of 1 second.

Vertical velocity = speed x sinα
α = angle of incline

Angle of Incline 10° 15°
Vertical velocity 0.244 0.486 0.725
Horizontal velocity 2.789 2.757 2.705
Power Required (Watts) 239 476 710

As can be seen from the table 250 Watts isn’t that much. In a hilly place like Sydney more power at lower speeds would be a big help. Maybe the laws could allow more power but have it reduce at 25kph and cutout at 30kph. That way if you could keep your average speed up a lot more people would use it to get to work.


 Links to Pedelec laws for different states


South Australia

New South Wales

Western Australia – Has not yet adopted the Pedalec laws [Oct 2014] Coming in early 2015, hopefully.


3D-Printed Tooling

When working on a Honda CRM 250 I needed to disassemble the engines water pump from the casing to replace the seal.

To do this the impeller needs to be unscrewed from the shaft. There is a hex on the nose of the impeller which can be gripped with a spanner or socket, the other side is not so easy. A lot of people simply jam a screwdriver in between the teeth of the plastic gear and the aluminium casing but I wasn’t happy that this would allow me to unscrew the impeller without damaging the gear. If you look closely in the photos you can see some cracks where the gear is pressed onto the shaft (apparently quite common on these engines). Continue reading 3D-Printed Tooling

Quadcopter Still a Work In Progress

The quadcopter has been frustrating me a little. The ESCs still shutdown with random faults and the motors stall on start-up. One of the solutions I found was to disarm the controller, arm the controller and then give the transmitter a big hit of throttle whilst holding the quadcopter down with one hand. This sometimes had enough torque to get all motors spinning but wouldn’t always work and was quite hazardous having four blades with ~650Watts of power behind them. The last occasion I tried this a propeller blade flexed, the motor wires on the boom moved a enough to catch and a broken prop was the result. The broken blade was later found several meters away. Continue reading Quadcopter Still a Work In Progress