Beginner motor power and battery needs

kvt

New Member
I am just bought a 250 watt front hub bike kit a month ago. When I purchased the wheel, I limited my choice based on cost and dealer estimates of speed and range. I have been pleased with my choice. I could not find a source for choosing the 'right' capability for me. I put together a spreadsheet that helps me understand what different motor and battery combinations might do. It is very basic and limited to my brief experimentation but it does follow the laws of physics.
https://1drv.ms/x/s!AgwNjrgc6Iuthukr1DVSDrZqUrQ4hg
 

George S.

Well-Known Member
It's a great overview of how power relates to slopes and wind, and you include the all important rider input as a given. You could say that a mid-drive can be geared for most hills, of course.

There's a lot of nuanced information in this sheet. Basically, winds are the killer. Most people could get by with a fairly small battery going speeds that a regular bike goes, maybe 18 mph. But you add ten miles of riding into a 15 mph headwind, and the battery has to get a lot bigger to handle it.

Really nice job. I wish things like this could be permanently linked somewhere. It would be nice if every motor on every bike had a chart something like this.
 

harryS

Well-Known Member
Thanks for sharing. Nice job. I believe your numbers, although 12 watt-hour.mile is a bit lower than what I would expect at 18 mph. You must pedal hard.

You might want pick up an inexpensive watt meter and see what it says. I generally ride a hub motor bike at bike path speeds (12 mph), and the meter claims I use 7-8 watt-hour/mile. I don't have any numbers for higher speed for that bike.

I do have numbers for my BBS02 Bafang mid drive kit. A 55 pound bike with a 190 pound rider.
16 mph - 15 watt-hr/mile with pedal assist (4 mile run)
20 mph - 20 watt-hr/mile with pedal assist (4 mile run)
17 mph - 15 watt-hr/mile with throttle only (3 mile run and the motor got to 120F)

By the way, I used get a kick out of active motorsports having taken my cars on the drag strip, and high speed drives on race tracks, but 20 mph on a bike on the street with cars around me is too scary.
 
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kvt

New Member
Thanks for sharing. Nice. I believe your numbers, although 12 watt-hour.mile is a bit lower than what I would expect at 18 mph. You must pedal hard.

You might want pick up an inexpensive watt meter and see what it says. I generally ride a hub motor bike at bike path speeds (12 mph), and the meter claims I use 7-8 watt-hour/mile. I don't have any numbers for higher speed for that bike.

I do have numbers for my BBS02 Bafang mid drive kit. A 55 pound bike with a 190 pound rider.
16 mph - 15 watt-hr/mile with pedal assist (4 mile run)
20 mph - 20 watt-hr/mile with pedal assist (4 mile run)
17 mph - 15 watt-hr/mile with throttle only (3 mile run and the motor got to 120F)

By the way, I used get a kick out of active motorsports having taken my cars on the drag strip, and high speed drives on race tracks, but 20 mph on a bike on the street with cars around me is too scary.

HarryS
I did not want to get into wiring but I did some more riding and testing on different slopes and modified the rolling resistance to better match the lower speed results.
This did increase the power and battery needs a bit.
https://1drv.ms/x/s!AgwNjrgc6IuthuktYSTVrZEfIPjyqw