"Transmission Impossible," Input/Output, ME Departments, August 2007

input/output

Transmission Impossible

by Michael Abrams

Most avid bicyclists, at one time or another, have found themselves climbing a hill, wishing they could find a gear somewhere between pedaling a little too hard and pedaling a little too fast. And even if they manage to find that sweet spot, on a tough slope, the hiccup that comes when the chain slips off one set of teeth and onto another can be just as frustrating.

A company in San Diego claims it has a new transmission that will make that ride easier. The company, Fallbrook Technologies Inc., says it has a continually variable transmission, which it calls NuVinci, that has already smoothed the ride for a handful of cyclists. The company adds that the transmission can work in cars, scooters, and riding lawn mowers, thanks to its scalability.

The idea first occurred to inventor Don Miller in the early '90s. "He was not an engineer, he was this surfer dude," said Emile Barrios, Fallbrook's spokesman.

Miller was also a bike nut, and he had set out to make the fastest bike in the world. He soon discovered that shifting gears caused a significant loss of efficiency, so he started looking into a way to remove that need.

According to Miller, "On a bicycle, there are two torque spikes when you're pedaling. Those torque spikes are drive chain killers. Climbing a hill, standing up, pulling the handlebars, your torque skyrockets. And that's what kills CVTs in a bike."

With the NuVinci CVT, as the ball axle tilts, the radius of the sphere section that does the separating changes, allowing an infinite number of settings for the pumping pedaler, and someday for the driver.

Although other continuously variable transmissions have existed for some time, their complex geometries have made them delicate and heavy. So heavy, in fact, that engineers told Miller that a CVT for a bike was impossible. The textbooks agreed.

"Mathematically speaking, the transmission shouldn't work in a bicycle," said William Klehm, Fallbrook's CEO, "If you use what you're taught, the numbers don't add up."

Miller learned that a CVT is inherently hard to shift and, with its low torque density, hard to control. "Once I understood why CVTs were hard to do, I set about attacking those problems," Miller said.

Miller's design uses spheres to separate the input and the output discs in an enclosed hub. The key to the whole design is that the diameter of the part of the ball that makes contact with the discs changes when a frame that holds the balls tilts. "The ball is the only geometric shape that you can tilt and spin without changing its special position in space. From that standpoint alone, it's the most compact shape to produce a CVT," said Miller. "And balls are the least expensive shape. You can get them anywhere, and they're incredibly precise."

Another essential ingredient is the elastohydrodynamic lubrication that passes between ball and disc. This fluid, made by Valvoline, acts like a solid when the pressure is great enough. So at the point where ball meets disc, the lube behaves like steel and torque is efficiently transferred.

The transmission made its debut in a bicycle called The Ride. "My goal was to create a bike that would invite you to come ride it. We want people to want to explore that magic machine a bicycle can be," said Anthony Ellsworth, founder of Ellsworth Bikes, the company that makes The Ride. "This is going to change the way bikes are interfacing with people." The Ride uses a belt instead of a chain. Owners change the NuVinci's fluid every few years.

Already the transmission has made its way into vehicles that are powered by more than just legs. Currie Technologies of Chatsworth, Calif., has incorporated the NuVinci into its Fusion 1000 scooter and its IZIP electric bike.

So what's it actually like to ride without gears? "Everything you've ever operated has gears," said Ellsworth, "your car, your bike, your blender. So when you first get on, you find yourself a little gear-obsessed, looking at the shifter—'Is it in the right place, should I be working a little harder?' But then you find that spot and just work it. You're operating at peak efficiency."