Thea and JJ model the passenger bike prototype.

Why I Invented an Electric Bike for Carrying Adult Passengers


“On June 4, 1896 in a tiny workshop behind his home on 58 Bagley Street, [Henry] Ford put the finishing touches on his pure ethanol-powered motor car. After more than two years of experimentation, Ford, at the age of 32, had completed his first experimental automobile…The two cylinder engine could produce 4 horsepower…achieving a top speed of 20 mph (32 km/h). Ford would later go on to found the Ford Motor Company and become one of the world’s richest men.”

I’m gradually reducing my car dependance. I enjoy traveling to meetings, hauling groceries, and taking my kids to their dental appointments all on my bike. But one of the few remaining compelling reasons for using my car is to carry adult passengers. For a variety of reasons my adult friends and family do not feel comfortable hopping onto my longtail cargo bike. Can bicycles ever fulfill the role of carrying adult passengers? I believe they can, and (like Henry Ford) I’ve built an experimental vehicle to test my conviction.

Here are the design goals I began with: build a bike that can safely and comfortably carry both a 200-pound driver and a 200-pound passenger at an average speed of 20mph. The bike should be able to go up and down a 20% grade. It should have a range of at least 20 miles. It should weigh less than 100 pounds, be less than 30 inches wide and less than ten feet long. And it should cost less than $2,000. I also have two significant design biases: I don’t think lightweight three-wheeled or four-wheeled vehicles are stable enough, especially on the steep descents in my town. And secondly I prefer that drivers and passengers be seated in-line rather than side-by-side, mainly because our autocentric society is none too kind to slow-moving vehicles that don’t fit on the shoulder.

What kinds of bicycles or tricycles have been built to carry adult passengers? The tandem, pedicab, and sidecar come to mind (see the photos below). Of these, the tandem comes closest to fitting my design criteria. I would argue, however, that the tandem has limited appeal for most adults. I can’t imagine my 80-year-old mom swinging her leg over the top tube and huffing up a hill pedaling in sync with me on the way to her doctor’s appointment. If not a tandem, pedicab, or sidecar, what then? What else is possible?

Well, the electric hub motor is forcing bicycle designers to reconsider what is possible. Hub motors are small, lightweight, and unobtrusive electric motors built into the hub of a bicycle wheel. They weigh between five and twenty pounds, yet have as much as thirty times the power of a human rider. Several advances have improved small electric motors recently, including better magnets and “brushless” microprocessor-driven motors. Battery technology has also improved dramatically. Electric bikes used to use heavy lead-acid batteries, the same kind used in cars. Now a six-pound lithium polymer battery (such as those found in laptop computers) can power an electric bike the same distance as a fifteen-pound lead-acid battery. These advances open bicycle design to many new possibilities.

Astute readers may object with a statement such as “If it has a motor, it’s a motorcycle, not a bicycle! Why don’t you just get a scooter instead?” This is a good question, not easily brushed off. It forces us to think about what we value in a vehicle, and to apply a name to that vehicle rather than try to fit an existing name. I seek a vehicle that is slow, narrow, and lightweight. It has to be useful–something more than an exercise machine designed to fit on a roof-rack. It has to be quiet and pleasant and pedestrian-friendly. Notwithstanding some attempts in the 1920’s to make motorcycles useful (see the gallery below) motorcycles and scooters tend to be unnecessarily fast and heavy, incredibly loud, and foul smelling. And for some reason (possibly having to do with their speed) they have pitifully small cargo space. For these reasons I say “No thanks” to scooters and motorcycles, and embrace the as-yet-unnamed class of vehicle I am building.

Over the past three years I’ve put many kinds of electric cargo bikes to the test (as have my blogging colleageages). My first bike has high torque. My second bike is capable of faster speeds. Lastly I’ve started building a third bike that has the best of both worlds. Allow me to indulge in some details. The first bike is a Surly Big Dummy with a Stokemonkey mid-frame motor. This bike has incredible torque. It’s like a winch on wheels. This is because the motor is geared down first with a chain to the crank and then through a second chain to the rear wheel. It can easily carry 400 pounds of people up steep hills. However, it suffers from what I call the “intimacy problem”: passengers must snuggle up behind the driver in a way that is just not socially acceptable to all but the most intimate of acquaintances. The Stoked BD is also somewhat slow. For me this is a safety issue–I dreaded making a left turn in busy traffic while going uphill with a load. I tried making the system more powerful but then ran into troubles such as broken chains and broken freewheels. The mid-frame motor is just not as elegant and trouble-free a solution as a hub motor.

So I built a second bike that is capable of carrying an adult and that uses a hub motor instead of the Stokemonkey mid-frame motor. I began by putting an Xtracycle extension on a mountain bike. The mountain bike has good quality shock absorbers and hydraulic disc brakes; I recommend both for any bicycle intended to carry adult passengers. I outfitted this bike with the largest hub motor I could find. This bike is fast–it’s capable of going over 40mph–but surprisingly it cannot carry a lot of weight up a steep hill. It is designed to have maximum power at a high speed.

I concluded that my ideal bike needs two hub motors: one fast and one high-torque. It’s kind of like a two-speed transmission for an electric bike. The fast motor will be most efficient on the flats, and the high-torque motor will shine on the hills. The fast motor should be a high voltage direct-drive hub motor in a 26″ wheel. The high-torque motor should be a “geared” hub motor in a 20″ wheel.

Another issue is pedals–does this vehicle even need them? They add very little power. The average human only produces about 100 watts of power, whereas a hub motor can produce 3,000 watts of power. Pedals seem superfluous. (In fact some Chinese electric scooters have removable vestigial pedals.) However, ideologically pedals add a lot. They make it possible for this vehicle to mix with other bicycles. They make it legally a bike. That’s worth a lot. Eventually our legal system will judge a vehicle by it’s weight and speed rather than its power source, and pedals won’t be necessary for larger ebikes. But for now I plan to keep the pedals.

Another design issue: how can I avoid carrying the extra weight of a seat if I’m not carrying an adult passenger? A tentative solution: the whole back end of my passenger bike is detachable. I can potentially replace it with back ends designed for other purposes: carrying cargo, carrying kids, farming implements, whatever. This design direction has been tried in the past–in the late 20s Dunelt motorcycles had detachable sidecars for various trades during the week which could be swapped out for carrying friends and families on the weekend.

So last month, inspired by Atomic Zombies’ Brad Graham, I set out to build a passenger bike in time for the Ithaca Festival Parade. It was surprisingly easy. I started with a bike that my kids outgrew. To this I added parts from a discarded “Mobo Triton” crummy adult trike. I welded together my own Xtracycle-like extension out of a length of two-inch square tubing. I bolted the extension to the bike at the bottom bracket and the dropouts. Then I added a 20-inch hub motor and the Triton seat to the backend. Lastly I replaced the bike’s front wheel with a 26-inch hub motor and presto, I was ready to go. The parade was a great success (see photos).

Did I meet my design goals? I haven’t quite finished the bike, so that remains to be seen. But so far so good: it weighs 84 pounds, it cost about $1,400 so far, and it has fair hill-climbing ability. I’ll keep you posted. It’s amazing what an amateur bike builder such as myself can accomplish. I think if cool bike companies like Yuba and Surly and Xtracycle embraced electric hub motors, they could create a new industry. Henry Ford had a similar humble start.