Measuring the ride

01March2007

When this ride idea started, there was just one rider, and one bike. This certainly made life simple - all I needed to do was make sure that I had enough spare batteries, maybe a suit or two, and that was about all.

But then we had the great idea - "why not invite 20 or so people to join me?"...

Only trouble is, now there is a lot more at stake than just one guy’s battery running out at an annoying moment. Suddenly, the actual physics of an electric bike and its power consumption seem rather important.

Firstly I needed to do a reconnaissance mission rather urgently. Seeing as some very close friends had a car in Sydney that they needed moving to Melbourne, that seemed a good place to start. All I had to do was fly to Sydney, pick up the car, and drive it home to Melbourne. The vital piece of information I wanted was, "when will the batteries run out?" You know how people get tired when they ride up hills?

Well, not surprisingly, so do the batteries of electric bicycles. Since there are no maps with sufficient details to describe the extent of the hills, I needed to make one.

The equipment used was an aircraft EFIS – that’s geek for Electronic Flight Instrument System. This was something I just happened to have lying around (I guess that makes me a geek..) It would tell me two useful pieces of information – altitude, as in height above sea level, and attitude, as in the angle of pitch, or steepness. The EFIS actually does numerous other things, but none that matter much to someone wanting to suss out a bike ride.

Once I found the car, a lovely little silver Honda, I attached the EFIS to the dash (very Myth-Busters), and attached a knee board with a kids school pad to my knee and set off for Melbourne. At each peak and trough of each hill, I noted the altitude and odometer reading in my notepad, as well as any extremes of pitch.

Finally, when I arrived in Melbourne, after seeing so many beautiful sights on the way, I had a notebook with 1000 or so altitude readings, all in my very rough hand writing. These were carefully entered into a spread sheet for later processing.

Now, how does one figure out when a battery will get flat? There are many ways to so this, and one can get very clever about it if you like – power consumption due to wind resistance is roughly a cube law of speed, while rolling resistance is roughly proportional to speed, and motor efficiency is a function of torque etc. I was starting to get bogged in this, when a very intellegent Aeronautical Engineer friend advised, keep it simple.

All I did was measured the range of the bike on a flat road when pedalling gently (about 35-40km) and assumed that when going up hill additional energy would be taken from the battery to raise the potential energy of the bicycle and rider. From high school science:

Energy (joules) = mass (kg) x gravity x height (m)

It turns out that it takes about 120 watt hours to raise 110kg of rider and bike 1000 feet. (For some reason altitude in planes is always measured in feet rather than meters – don’t ask me why, its just what is done). So if the battery has a capacity of 360 watt hours, then the range will drop from about 35km to 24km if a 1000 foot hill is involved. This does not allow for the fact that no energy will be consumed going downhill, but my experience is most things get worse rather than better, and I would rather keep the estimate conservative.

So thats how I have estimated where I expect to need to change everyone’s batteries, and where to have the support van waiting.