The eTrek Bicycle: A practical eHPV design

Electric motor-assisted human power vehicles (eHPV) have the potential of providing an alternate to both overpowered internal combustion engine (ICE) driven vehicles and underpowered bicycles (when less than fit humans are pedaling them) for intra-city or short-trip personal transportation. The dream of human only powered vehicles replacing cars has long be held by those valuing efficient, sustainable, environmentally responsible, and just plain sensible transportation, but cars remain dominant.  Perhaps the HPV dream needs a boost.

Practical HPVs, velomobiles being an example, offer streamlined weather protection, cargo carrying capability, safety features, full suspension, and other amenities not often found on light weight bicycles. Bicycles tend to lack such features because adding them adds evermore weight requiring evermore muscle power to pedal. Vast numbers of people already feel that the effort needed to pedal even an ultra light bicycle is too great, so doubling or tripling the weight insures that only a few ultra fit humans can pedal them (except on flat land).

An aerodynamic fairing more than compensates for the extra weight when operated on flat ground, but performance rapidly declines as the slope increases. Apart from a few flat regions, such as the Netherlands, practical HPVs have proven to be impractical.

Most velomobiles are trikes, which have the considerable advantage of not falling over at a stop. The down side is that on roadways lacking a paved shoulder where cyclists must ride the white line or be run over, trikes stick out into passing traffic more, either obstructing the flow of traffic or increasing the risk of being clipped (or worse) by passing cars and trucks. Trikes weigh more than bicycles, and the scant sales of velomobiles makes them very expensive. While adding power-assist would make them practical for average humans, the added price makes them even more impractical from a financial perspective.

Part of being practical is being comfortable. Imagine taking the seat off a racing bike, mounting it on a stool, and putting it in among the other seating offered by a large furniture store. Just how many customers would look at the bike seat and say, “Yeah, that's what I want to sit on after a hard day at work.” More than likely no one would choose to sit on an upright bike seat no matter how much gel it had.

There are, however, bicycles without bicycle saddles. Such bicycles are called recumbent bikes or bents. They've been around a long time now, and absolutely every one can agree that they are more comfortable to sit on. Recumbents also have a safety advantage: if you run into something, would you rather it be feet first or face first? Like velomobiles, however, bents tend to be expensive due to low production.

The Cruzbike is a full suspension recumbent front-wheel drive bicycle that uses mass-produced mountain bike (or other) frames, and standard bike shop available parts. This allows a quality bent to be made at a comparatively good price. In a Cruzbike conversion the mountain bike's front wheel becomes the rear wheel, but if instead another geared rear wheel is added and an electric motor used to drive it, then the result is an all-wheel drive bicycle. By not trying to combine human and motor drive trains, the design of both can be much simpler as no freewheels are needed to allow pedaling without using the motor, or motoring without forced pedaling.

While the Cruzbike doesn't need an electric assist motor, turning it into a practical HPV by adding fairings, cargo carrying ability, trailer towing ability, lights, horn, fenders, etc., pretty much necessitates also turning it into an eHPV. The Cruzbike is ideally suited to being converted into an ebike as both motor and batteries can be mounted so as to keep the center of gravity low.

One use for the otherwise unused bottom bracket is as a mount for what might be called a “wheelstand” to replace the kickstand. The wheelstand acts as a landing gear, allowing the bike to remain upright at stops without the need to take a foot off the pedals. This design has the advantages of the trike without the disadvantages. With four wheels down it is stable at rest and at low speed, yet pop the wheelstand back and on two wheels the bike can lean into corners, go down narrow paths, and stay on or to the right of the white line.

For clarity the batteries are shown mounted as low as possible, good for lowering the center of gravity, but for improved aerodynamics it would probably be best to mount them slightly higher on either side of the motor.  The rod on the rear fairing would support the upper part of the soft fairing and would be fiberglass.  It could extend higher to carry a pennant for improved visibility.  Lights, horn, fenders, electronics would also be added.  Another addition would be a retractable head cover to provide shade when needed.  The cover could be a small solar panel to provide power for onboard electronics such as a meter, bike computer, lights, AA/AAA battery charger for GPS, iPod, cell phone, whatever.  Having a separate 12V system might be simpler that stepping down voltage from the motor battery pack.  While solar powering the ebike may not  be practical (unless solar panels are part of a recharging station), powering the bike's electrical system using solar would make sense.

A gearhead motor is shown, but there are two other approaches.  One is the use a rear hub motor mounted on a short fork.  The entire motor turns to drive the rear wheel.  These slow turning motors are heavy, but without gears, they make little noise, which avoids the major objection to gearhead motors.  To be small and efficient a motor has to turn at high rpms, about 2800 rpm or higher.  The problem is to gear the motor down efficiently without generating a lot of noise (think electric drill).  One way would be to go with a 5:1 belt drive reduction, then a 5:1 chain drive reduction.  Belt drives are not as efficient as chain drives, but are quiet--a chain drive directly off the motor would be very noisy.   

Wal-Mart has an Ultra-lite folder (Kent International) for $150. Removing just the front wheel of the folder and hitching it on to the eTrek turns it into a tandem bike. Striped down it would make a cheaper trailer than the B.O.B. trailers. Using a front fork trailer hitch allows old bikes to be easily turned into a variety of utility trailers on the cheap. Add a solar panel to the trailer and extra batteries (even motorize the trailer), and more power will be available for towing, or touring. The eTrek concept is starting to evolve into a practical EV. More heads are better than one, so anyone interested in developing ebikes along a practical line should join the Yahoo! group mentioned below. The ultimate goal and proof of concept is a cross-country ebike tour. Anyone want to go for a ride?

My wife thinks I should patent something or another and get rich, so for what it's worth consider the above to be a “disclosure statement” and any patents pending. And what the hell, let's just throw in Copyright © 2006 by R. Eric Lee for good measure. It's hard to say when design crosses the line and becomes “invention.”  What we want is to advance the technology so we can all benefit.


Related articles:

The Case for Power-Assisted HPVs

Ebike Design: How much power is enough?

Ebikes as Exercise Bikes

Power vs Slope


Related links:

Ebike Touring Association



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