Machine Design
Your first e-vehicle could be an e-bike

Your first e-vehicle could be an e-bike

Key points:
• Electric motorcycles are much easier to produce than electric four wheelers, a fact that has attracted numerous entrepreneurs.
• The tough part: Kinetic-energy recovery during braking because the front wheel loads much more than the rear.

Chip Yates Bonneville World’s Fastest Electric Motorcycle Video
Current Motor Current Motor interview
Lit Motors
Lit Motors C-1 on YouTube Pro Racing
(Chip Yates)

You can thank the Japanese for fostering the birth of the modern electric motorcycle, but not in the way you might think.

“The big four motorcycle companies — Honda, Kawasaki, Suzuki, and Yamaha — are all very conservative. Rather than come up with their own e-bikes, they sat back and watched events unfold. That let a bunch of guys in their garages on low budgets use creativity to demonstrate e-bikes that were at parity with gas bikes,” says retired electric motorcycle racer Chip Yates.

Yates is in a position to know what he’s talking about. In the first-ever organized race between an all-electric motorcycle and beefy, twin-cylinder, 1,150-cc gas bikes, Yates collected third-place honors on a bike carrying 102 ƒlithium-ion polymer pouch cells and a dc permanent-magnet motor. He and his team also set the world-speed record for electric motorcycles in 2011, hitting 190.6ƒmph during a one-mile, standing-start run in the California desert.

The accomplishments of Yates and his crew personify the role garage entrepreneurs have had in advancing e-bike technology.

E-bikes are now a hotbed of development dominated by independent inventors and small start-ups. One reason is that for someone set on producing an electric-powered vehicle, it’s far more manageable to bring a motorcycle to market than anything with four wheels. “In some ways bikes are a natural platform for innovation because they are easy to get your head around,” says Yates. “An e-bike is relatively inexpensive to build. It is a much simpler undertaking than a car. You can spend more of your time on new technology rather than on building a giant platform.”

In Yates’ case, some of that innovation went into coming up with a patent-pending way of pulling energy from a bike’s front wheel to help recharge the battery. That’s important, Yates explains, because a bike’s rear wheel is lightly loaded during fast stops and offers minimal opportunity for regenerative braking. So Yates and his crew equipped the front wheel with special one-way clutches. They sit in the front wheel hub and let the front tire transmit more than 500 lb-ft of braking torque to the electric motor, but they prevent the electric motor from driving the front wheel.

Specifically, the clutches mate to a ring and pinion gear set on the front axle. The pinion gears turn two counterrotating and telescoping driveshafts running along the outside of the front forks. (The driveshafts share the torque load and so can have a smaller diameter than if using a single big shaft.) Each drives a chain that respectively turns an inner and outer shaft rotating in opposite directions. The shafts go to a custom steering head in the bike frame. Inside the steering head, the shafts respectively turn a lower and upper bevel gear. Between these counterrotating bevel gears is an output bevel gear that exits the steering head and turns a driveshaft that runs inside the frame rail of the bike. The counterrotating driveshafts and bevel gears act to counteract torque steer felt through the handlebars.

The driveshaft runs from the steering head down to a gearbox inside the frame rail around the area of the rider’s knee. The gearbox drives a chain going to the electric motor shaft where it can be used to generate electricity for recharging the battery pack. For safety, proprietary control software limits the amount of front-wheel braking based on factors such as lean angle and the potential for overcharging.

Biking in the cloud

Another start-up that sees e-bikes as fertile ground is Current Motor in Ann Arbor, Mich. “The two-wheel space is a sweet spot for electric-power entrepreneurs, says Erik Kauppi, Current Motor cofounder and chief engineer. “It is just easier when the vehicle is small. You need less of a battery, it costs less and it needs less power. You can charge from a regular wall outlet and there are fewer regulatory requirements for producing a vehicle.”

Current Motor began shipping its Super Scooters in November, though they are officially classified as motorcycles. The bikes are said to have 60% more peak power than 150-cc gas scooters and hit 65 mph or more with a range of about 40 miles/charge or better. An onboard 3G connection lets a digital dash relay rider information back to Current Motor via the cloud.

Some of Current Motor’s best technology resides in the scooter’s 5-kW permanent-magnet dc-brushless motor. The motor is built into the hub of the rear wheel rather than sitting on the frame as in most bikes, electric powered or otherwise.

“We started with a commercially available hub motor and saw a lot of room for improvement,” says Kauppi. The problems, he explains, arise because the motor must supply a lot of torque at low speed to get the vehicle moving from a dead stop. But there’s also a need for appreciable power at high speeds to overcome aerodynamic drag. Highly efficient permanent-magnet motors commonly used for e-vehicles have trouble performing well in both operating regimes. The usual way of handling the problem is with field weakening, reducing the magnetic field at high speeds so the motor generates less electromotive force that opposes its rotation. “There are broad categories of permanent-magnet motors where field weakening doesn’t work,” explains Kauppi. “So we’ve been granted several patents on technology that applies to hub motors that address these sorts of problems.”

Current Motor’s use of hub motors in any capacity is controversial because locating the motor in the wheel hub boosts the amount of unsprung weight in the bike, the weight which is unsupported by the suspension. High unsprung weight has a tendency to make the bike harder to control under hard acceleration or braking. All in all, some chassis design purists look on hub motors with disdain because they exacerbate such handling issues.

But chassis idealists have a lesspersuasive argument on this point when it comes to scooter-type motorcycles. Even ordinary gas versions have an engine-gearbox-final drive system that pivots as part of the rear suspension and hence is partly unsprung. “A lot of people who have tried both say the ride and handling of our e-bike is better than that of comparable gas bikes,” says Kauppi.

Running around regulations

Current Motor’s first customer was riding his scooter 18‡months after the company opened its doors. One reason for the short time-to-market is that companies producing motorcycles have lower regulatory hurdles than those making vehicles with four wheels. “It’s much easier for motorcycles to pass federal safety mandates because the standards are simpler and there are no crash test requirements.” says Kauppi. “And e-bikes have an easier time passing EPA emissions than gas bikes.”

Confirming this view is Daniel Kim, founder of Lit Motors in San Francisco. Kim’s firm is working on an e-bike called the C-1 which is billed as the world’s first gyroscopically stabilized “rolling smartphone.” The C-1 balances on two wheels using two gyroscopes so it stays upright when stopped. “Landing gear” deploy when the C-1 parks to keep it standing up. The gyros also let the C-1 lean itself into and out of turns while staying stable.

“It is about $11 million cheaper to bring a vehicle like this to market than to do the same thing with an electric car,” says Kim. “A car, for one thing, would have to pass crash tests. If its battery chemistry was new, it would take about 18‡months to go through National Highway Traffic Safety Administration testing. But a motorcycle has none of this. You can be quick to market.”

Though one attraction of developing e-bikes is their potentially simpler technology, you wouldn’t know that from the C-1. It uses a balancing scheme borrowed from spacecraft attitude-control systems. Onboard are two control-momentum gyroscopes (CMGs) consisting of a spinning rotor and motorized gimbals that tilt the rotor’s angular momentum. As the rotor tilts, the changing angular momentum causes a gyroscopic torque. In space, that torque rotates a spacecraft. In the C-1, it keeps the bike upright regardless of the terrain or sharp turns executed by the driver, effectively making the C-1 idiot-driverproof.

“What is exciting to many people is that the CMG controls not only balance but also the amount of lean in a turn so the driver doesn’t have to,” says Kim. “There have been many mechanically controlled CMGs, but this is the first time a vehicle company has used an electronic CMG. Our patents are in the actual CMG control system.”

The C-1 carries two gyros to provide redundancy and to assure stability in real-world scenarios. “It is one thing to create a self-balancing vehicle in the laboratory. But inclines, declines, and variations in the road are all tough to handle with a single gyro,” says Kim.

Lit Motors designed its flywheels internally. The two gyros in the C-1 rotate in opposite directions for balancing, and each spin at somewhere between 6,000 and 12,000‡rpm, rotational speeds high enough to provide stability but low enough to avoid concerns about wheels disintegrating from high centrifugal forces. The flywheels are made of chrome-moly 4140, a grade of steel known for its high strength-to-weight ratio and often used in such applications as gun barrels.

Unfortunately, Lit Motors isn’t discussing a lot of other details on the C-1. Kim says the whole vehicle will weigh less than 800‡lb and will be powered by a proprietary electric motor providing 20-kW continuously and 40-kW peak. Preliminary specs call for the C-1 to hit 100+ mph (160+ kph), do 0 to 60 mph in under 6 sec, and have a range of 200‡miles (320 km) per charge. The bike will also have a unibody construction like that of a conventional car. Lit Motor is now building two driving prototypes and hopes to be in production next year. “It’s still just a motorcycle. Once you have prototyped it a couple times, it’s not much more complicated to build a production version and sell it,” Kim claims.

Saving the planet one bike at a time

It’s likely that technology pioneered for e-bikes will advance the state of the art in a variety of fields where electrical motors can be more sustainable than petroleum burners. The first signs of the trend are already emerging.

E-motorcycle racer Chip Yates, for example, has raced his last ebike and is moving the technology his team devised into more promising areas. “For me, the ebike was a platform to push the limits and have some fun. I was after low-hanging fruit and I wanted to make a difference,” he explains. “But gas motorcycles are already incredibly efficient — they get 50 mpg. So if you convert every gas bike to electric you won’t move the needle for smog or pollution.”

Yates thinks aviation might be the next area primed for a shake-up based on technology originally created for e-bikes. “Airplanes are incredibly filthy and they fly missions where electric planes could do better,” he says. Yates and his crew have already come up with a way to recharge an electric airplane in flight and have been granted patents on the idea. Yates has set a speed record for an electric plane (200 mph) and eventually plans to duplicate Lindberg’s solo flight from New York to Paris electrically.

But the quest for that goal has been interrupted by a more pressing project. “The Navy wants to build an electric version of the Predator UAV,” he says. “They noticed that electric planes are tough to see with infrared. So we are building a UAV for them called the Silent Arrow in which we bury what little heat we generate in the airframe using a strategy involving compressed nitrogen.”

© 2013 Penton Media, Inc.

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