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buying an eBike – doing the research first

I (Joe) recently became interested in ebikes, have done some extensive research, and finally bought a commercial model on which I’ve put a few hundred miles in the last two weeks. I’ll provide a review of that bike shortly, but first wanted to collect some more general thoughts on ebikes which I’ve consolidated from my research, personal experience, and a good dose of engineering basics. I’m certain almost everything I discuss here has been stated better and more thoughtfully elsewhere, so please forgive my repetition and perhaps naive thoughts. I will say, I think I would have found this type of write-up helpful to me early-on, and hope this discussion may help other potential new ebike riders as well.

So, if you’re like me, the idea of an ebike is appealing. Gas prices are up, “green” transportation is in, and bike commuting has a number of distinct advantages over other forms. Personally, I’ve been a bike commuter for about 3 years now. My regular job is outside Santa Fe, NM, and I have a decent enough means to bike commute about 10 months out of the year, with snow and weather leaving a few weeks for the car. Of course, bike commuting entails other compromises, such as the difficulty in accommodating “business” clothes, “helmet” hair, inclement weather, and of course, safety. As experienced bike commuters known, there’s pretty good solutions to most of these issues, with equipment, planning, and setting realistic expectations all playing a role.

eBike, bicycle with electric assist motor

The appeal of an ebike to me is the ability to commute a bit farther, to eliminate the more difficult portions of my commute and arrive “sweat free”, to perhaps save a bit of time, and to allow farther in-day trips from my main work site to various other locales at my employer (which is spread over 43 square miles, I often need to travel to more remote sites where I work). As important, is what an ebike for me wasn’t. It’s not a replacement for a motorcycle. I didn’t want to travel long distances just punching a throttle. I wanted this to be mostly bike with the added benefit of power assist. Taking my bike on public transport, into my office via an elevator, parking with other bikes, etc. are all important.

So, how does this lengthy preamble relate to choosing an ebike? If you read the ebike forums, there’s a near-obsession with specific characteristics of these bikes such as top speed, weight, and perhaps most prominent, range and battery capacity. Naturally, everyone has a list of requirements and desires that they want their bike to meet. At the top of many lists are reliability, range, light weight, top speed, and of course, low cost and maintenance. This wish list quickly runs head-on into the reality of engineering and design. You quickly discover there’s no one perfect bike that does it all. Some of the better ebike retailers start with a very good list of questions to help determine a customers desires. Given this, they can direct you toward a particular system, design, or model.

I stated many of my desires above. Considering my criteria, it was clear that a pedal-assist application is the way to go. The engineering analysis of pedal assist is itself a fascinating topic. In order to appreciate the benefits of pedal-assist, and of ebikes more generally, I found it enormously helpful to discuss and understand some specific characteristics of the most important motor on the ebike. No, not the electric hub, but YOU. Your power output and contribution to propulsion, and understanding the highly non-linear nature of that power output, is actually the key to understanding why pedal-assist is such an attractive option. So, if you’ll forgive my diversion here, I’d like to discuss a few elements of human power output.

You and I consume energy just by living. A resting person weighing 150 pounds has a metabolic rate of about 70 Watts (W). Light activity such as walking and just doing your day to day thing without exertion bumps this up to about 100-125 W. That’s just how our bodies have evolved in our environment. As you start to exert yourself, you spend more energy. This relationship is highly nonlinear. A moderate workout, in which you get your heart rate up and sweat quite a bit, has a total power output of about 200 to 250 W. Tour de France riders with years of special training, nutrition, and conditioning can sustain power outputs of 400 to 500 W for hours on end. You and I, not so much. Over about 300 W, and we’ll tire pretty quickly. So, 100 W is literally “no sweat”, while that next 100 W isn’t nearly as easy. It’s this nonlinear relation between exertion and power output that makes a pedal-assist ebike so attractive.

The goal is to transmit efficient power into the bike. As a rider, you can contribute a nice, healthy 50-100 W (on top of your metabolic 70-90 W) for a very long time, and you won’t notice this as much exertion. These should be the “first Watts” that the bike sees. It just so happens, that pedaling a bike on level ground in no wind requires roughly 100 W to sustain a speed of roughly 25 km/hr. This is the level that most folks would see as “easy” without much exertion. Of course, add a headwind, or hills, or a higher speed, and things quickly turn against you. Further, starts and stops require quite a bit of extra energy, too. Nearly every rider knows this intuitively without the engineering degree, and this helps to explain why bikers don’t particularly like to stop if they can help it, safety be damned.

Now, let’s consider a throttle-only, no pedal ebike versus a pedal assist. If your goal is to get long range with much less exertion on your part, the numbers quickly tell the story of why pedal-assist is so attractive. Remember that 100W figure for sustained 25 km/hr on level ground? At the theoretical limit, a throttle-only vehicle would last about 3 hours with a typical 36V, 10 Amp-hr battery pack (360 W-Hr), whereas a pedal-assist wouldn’t be contributing at all, and would have infinite range, since the rider is providing that first 100 W. Now, this is all theoretical. In practice, hills, starts/stops, the efficiency of your bike and motor, etc. quickly degrade these numbers. More typically, that throttle-only bike with a 360 W-hr battery may have a range of 30-40 km in total. Conversely, a good pedal-assist system in practical use on level ground driven in a way so that you don’t exert can have a range on order 80-90 km or even more.

And it’s the deviations from this ideal ride where pedal assist becomes so appealing. Take hills, for example. The added power to crest that hill can be contributed by the battery, so you don’t have to exert. You’re not constantly climbing during your ride (well, for most rides that is), and the addition of power assist as the last Watts added is what provides that marvelous ability to extend range, open up a whole-new way of biking, and achieve the near-nirvana of long-range, reliability, ease, and (relatively) low cost for an ebike. (And BTW, some studies have shown that the more constant exertion of steady, light exercise may be as beneficial as other forms. Pedal assist hybrid on ebike indeed may be a have-your-cake and eat-it-too proposition when it comes to fitness.)
Ok, so the previous is basically an engineering discussion of ebike propulsion. How does it work in practice and how can you use this understanding to maximize your ebike experience? A couple of things are important right up front, namely, speed and acceleration. Most folks want more speed, its only natural. Unfortunately, things quickly turn against you as you go faster from an energy management standpoint. Top of the list is drag. Drag increases as the square of your speed. Given that most of your energy (on flat ground) goes to moving the air out of the way, this gets ugly quick. If you’re willing to go a little slower, you have the enormous payoff of greatly extending your range. From a practical standpoint, 30 km/hr or so (19 mph) feels about right to me as a nice compromise between speed and efficiency. Others will balance this differently. It just so happens that 32 km/hr is codified in US law as well. At first, I wanted that super-fast ebike that did it all. After spending some time on my current bike, I’ve come to the conclusion that a moderate speed really is best, not only for efficiency, but for safety as well.

Ditto acceleration. My ebike (Trek Valencia+ with the Bionx system) has a 27-speed drive-train which I thought was ridiculous at first, since I could easily keep this in the lowest gear, and just let the motor pull me away from stops. It turns out, this is really bad energy management practice. Hard acceleration is inefficient and sucks tons of precious amps from your battery (just look at the meter on a Bionx system). I’ve come to realize that those extra low gears on the Valencia are actually very useful. Now, I down-shift at stops most of the time, and take just a bit longer to get up to speed, with the Bionx system providing more of a gentle nudge than full-on zippiness. Of course, that zippiness is there is you want it, or when time or safety require. The payoff in backing away from jack-rabbit starts is greatly extended range.

I rode the Critical Mass ride this Friday in San Francisco on the ebike, including some substantial distance to/from the event (yes, I mentioned my job was in New Mexico; I’m spending the year in the Bay Area on another assignment). I got about 50 miles out of the Trek/Bionx system, and had a grand time. Trek worked with Bionx and changed a number of factors in the Bionx system, some of which I think are not well-understood by the ebike crowd, including a change in battery voltage and capacity.

So, I’ve written a tome as a first post. Please forgive this. My goal was to pass along some of the things I learned in my research leading up to my first ebike kit purchase. More experienced folks here will no-doubt correct errors or offer additional points, which I look forward to.

Joe M.