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John B. Slocomb[_3_] July 30th 15 01:27 PM

OT My simple electric bike
On Thu, 30 Jul 2015 00:46:26 -0400, Ed Huntress

On Wed, 29 Jul 2015 23:24:58 +0000 (UTC), John Doe

Took it out in sunny afternoon 98F weather for a 6 mile trip. The 20V
5AH battery shows one out of three bars remaining. Drill got hot but
there was no performance change.

This is really interesting, but I'm having trouble with some of the
numbers. Say the "two bars" consumption equals, roughly, 3 A-hr. At 20
volts, we have 60 W-hr. Just guessing here, but say you travelled that
6 miles at 12 mph.

That means the drill was drawing an average of 120 W for a half-hour.
Let's be generous and say that the system efficiency is 80% from
storage to power at the drill shaft. So, given chain and friction
losses at the tire, you have 1/8 hp driving the bike.

Will that be enough to move you at 12 mph? I'm not familiar with the
actual power consumed by these bikes, but that sounds low. Am I
missing something?

1/8th H.P. is about 90 watts. The average person can do about 3 MPH
using 60 watts. Wind resistance goes up at a square of the speed. The
usual calculations show that an average person riding at a steady 16
MPH on level ground requires about 116 watts. 22 watts to move the
bike and 94 watts to overcome wind resistance.

And of course, weight of bike and rider, tire pressure, type of
pavement, any variation from perfectly level all change the power


John B.

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