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greentechZONE Products for the week of August 25, 2008
Pi Mobility Unique Electric Bike Delivers Practical Transportation in a Sexy Package
Love it or hate it, it’s tough to not have an opinion about Pi Mobility’s unorthodox electric bicycle. For some, the single swooping arc of aluminum tubing that constitutes its frame is just too weird (even Marcus Hayes, founder/CEO of Pi Mobility, says that they had to design a headset that would help people know which end of the bike was the front) while others swoon over its simple rugged lines. Likewise, bicycle tech-freaks often find fault with the Pi 50-lb weight and cruiser-style upright riding position while people who tend to use their bikes for grocery runs and casual outings usually fall in love with its 30+ mile range, strong hill climbing ability, and design features that emphasize comfort and reliability.
Hayes, and his compatriots at the Sausalito, CA, headquarters are counting on these strong reactions to help energize their campaign to drive electric bikes into the mainstream of North America’s transportation mix. A look at the design and construction of this unique bike reveals much about how it delivers the performance and utility that justify its $5500 price tag. Many of those same details also provide clues as to how its designers intend to drive future model pricing down towards $1200 - $1500 where it can effectively compete with the small (50cc) fossil-powered scooters that dominate the light transport market today.
Pi's minimalist single-tube frame is its signature element and the clearest example of Hayes’ design philosophy that stresses elegance, simplicity, and reliability. Formed at the factory from 4-inch 6061-T6 aluminum tubing, the frame requires only a handful of heliarc beads to attach the head tube/fork assembly – a feature that reduces both labor costs and production time. Since Pi is intended as an everyday hauler, the frame also includes ample attach points for a cargo racks, panniers, and even a trailer.
In addition to providing a place to attach the wheels and seat, the frame’s main tube serves as both a sturdy enclosure and heat sink for the main battery and its associated electronics. A small fan assists the induction of cooling air through two machined intakes to improve the lifespan of the battery and electronics.
The arched frame also accommodates a much wider range of body sizes and riding styles than a traditional double-diamond chassis. A gas strut in the bike’s sliding seat mount allows quick adjustments that lets riders from 5 ft 4 in to 6 ft 8 in ride the Pi comfortably and still be flat-footed at rest. This one-size-fits-all frame dramatically lowers inventory costs and frees the manufacturer from having to forecast how many of each size bike they would need to build.
About the only downside is that riders on the far ends of the fitting range might not enjoy the perfect ergonomics of a custom-fitted racing frame. Pi Mobility says that any mechanical disadvantage a rider might encounter is offset to the point of insignificance by the electric assist system. While this sounds reasonable, I’ll look forward to a test drive (hopefully a bit later this year, 2008) to verify Pi Mobility claims.
The Pi Mobility design team has taken an equally unconventional approach to the bike drivetrain. The 750 W electric motor is run in series with the pedals so they both enjoy the mechanical advantage of the 8-speed Shimano internal transmission hub that’s laced into the rear wheel. The hub is a sophisticated cousin of the rugged, maintenance-free Sturmey-Archer planetary transmissions commonly found in English Racer-style bikes before derailleur systems became popular a few decades back. This gives Pi a much wider speed range and much more hill climbing ability than most other e-bikes that use a hub-mounted direct-drive motor. Pi standard drivetrain can pull a 150 lb passenger up a 25% grade without any pedal power, or zip along at 20 mph on flat terrain. Riders who opt for the 1 kW motor and a companion Rohloff 14 speed hub enjoy a 40 mph top speed and can take 35% grades.
The bike motor, transmission and pedal system are all mounted in a forged aluminum housing to form a self-contained undercarriage module. This module is assembled on a separate manufacturing line and only bolted to the frame at the end of the production cycle. Besides allowing for faster, more flexible assembly, splitting the frame and undercarriage production allows for easy addition of optional drive components or custom-painted frames. Pi Mobility says that while their basic Ferrari Red paint scheme is very popular, they are happy to match any Pantone color (for an added charge) or even send you an unpainted bike that you can customize yourself.
Pi’s tidy power module would seem to be a very sensible approach to manufacturing a rugged, reliable bike but, to my untrained eye (I’m an electrical engineer, not a mechanical type), its cantilever-style attachment to the frame would create a huge amount of torsion load for the frame end caps to support, even before a rider stands on the pedals. I did not notice this until after my briefing so I did not get a chance to learn if there is anything else that braces the drivetrain module besides its attachment to the end of the frame. A close look at a photograph of the Pi reveals a small piece of material where the rear fender would be but it's impossible to tell if that's a diagonal brace of some sort or not. I’m hoping that the test drive I’ve got tentatively planned will help resolve this one big concern I have about the Pi design.
For all its sexiness, the driving factors behind the Pi design are efficiency, reliability and ease of service. Unlike some of its higher-priced carbon-framed brethren, its mechanical elements have all been designed to be serviced by the average bike mechanic, and use as many COTS parts as possible to ensure that your machine will remain on the road for many years.
The current drivetrain uses a basic brushless dc motor coupled to a simple manually-throttled 35 A controller instead of the more sophisticated systems that have regenerative braking or automatically mix pedal and electric power. The Pi design team has opted out of these complex drive systems (at least for the moment) both to keep costs down and to boost reliability. In keeping with a design that is intended, among other things, to do rough service in bike rental fleets, the exposed controls have been minimized to protect the electronics against damage from falls, drops, and accidental lay-downs in the dirt.
Even without an exotic controller or regenerative braking, its standard 0.5 kW-h (36 V, 15 A-h) internal LiIon battery pack gives it a 25 – 30 mile range with only light pedaling. Depending on how you figure conversion losses, this translates to between the equivalent of 1000 - 2000 mpg of gasoline. Adding a second clip-on battery effectively doubles the range.
The component choices and design trade-offs that have gone into the Pi e-bike all seem to have positioned it nicely to enjoy rapid cost reduction as production volumes grow. Marcus Hayes already has solid plans in place for the day when demand for his bikes is sufficient to justify some level of manufacturing automation and other economies of scale. Part of his plan includes streamlining the bike assembly process to the point where it can be affordably manufactured in the US. Hayes notes that one of the few good things to come of the spike in energy prices is that it has boosted the cost of transporting bicycles from Asia enough that it could come close to offsetting the higher cost of North American labor once high-volume production begins.
Of course, some factors remain out of his control, such as whether or not the price of the AA-sized LiIon batteries that go into the Pi power pack will continue to follow the downward trend that he’s counting on. But if Pi Mobility can achieve its goal of bringing down the cost of its subsequent products to under $1500, it’s easy to see how getting a few hundred thousand of these vehicles onto the road each year could help keep an equal number of fossil-burning scooters and commuter cars in the garage.
The base version of the Pi Mobility electric bicycle is priced at $5500, and is available now.
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