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Building a Better Transmission

The Diametroid DT-CVT: An inside look in the new transmission concept by Australian inventor John Bisby.

The Diametroid DT-CVT: An inside look in the new transmission concept by Australian inventor John Bisby.

Inventor John Bisby’s quest for the ultimate CVT

By Brett Duesing and Alex Dickey

John Bisby was simply looking for an easier ride home.  On the way, he found himself entangled in a geometric problem that has puzzled mechanical thinkers since Leonardo da Vinci.  The improbable difference this time:  it looks as if Bisby may have come up with a new path to the solution.

The story begins when the 48-year-old automotive electronics technician wanted to modify his bicycle with an electric motor to assist in the steep climbs near his home in Jordan.  Having had some experience converting cars to electric vehicles in his native Australia, Bisby thought he could do it himself, but soon ran into a conspicuous gap in available technology.

“I searched the Internet for everything,” Bisby recalls.  “I could find suitable motors, controllers, and lithium batteries, but the transmission appeared to be an engineering impasse.”

Most electric assists make do by using the middle gear on the existing transmission and adding an electrical speed controller to the motor.  This did not satisfy Bisby.  He wondered, why the motor couldn’t output a constant rate – what an electric motor does best — and have mechanics transmit that power to gradually turn the rear wheel to higher and higher rotations?

That seemingly simple question de-railed Bisby from world of practicalities and plunged him into theory.  For 18 months, the idea that the input and output of a transmission could interact within a single dynamic system consumed Bisby.

“It’s not as if I was trying to break any laws of physics, so I kept at it,” he says.  Thinking about the puzzle kept him up at night.  New insights into the calculus came to him during long walks in the woods.

He spent several more months writing out the explanations and equations of his solution for a new type of continuously variable transmission (CVT).  He applied for a UK patent on his invention, the Diametroid, or what Bisby labels more generally as a DT-CVT.

Input and Output:  The device is designed to  receive a constant rotational speed of input, which continuously varying the speed of rotational output.  Once in operation, the device balances this ratio dynamically, and needs no manual or mechanical regulation to affect acceleration.

Input and Output: The device is designed to receive a constant rotational speed of input, which continuously varying the speed of rotational output. Once in operation, the device balances this ratio dynamically, and needs no manual or mechanical regulation to affect acceleration.

“In his research, my patent attorney could find nothing similar to the Diametroid,” says Bisby, who dubs the DT-CVT as the “holy grail of power transmissions.”  By all appearances, the mechanics of Diametroid approaches the problem in a fundamentally different way than contemporary CVT designs.

By his own admission, Bisby is not a transmissions expert, but the title might be fitting if the Diametroid proves to have viable commercial advantages.  Bisby claims that they are all there:  a smoother acceleration and a more compact size; less wear on engines and its own components; and perhaps most striking — a negligible loss in efficiency.

The Long, Strange Trip of the CVT

Transmissions most common in everyday life – our stick shifts and ten-speeds — operate on fixed ratios set by gears and controlled by clutches or derailleurs.  A shift in gears moves to rotational force onto a circle of wider or smaller radius, accompanied with the familiar lurch in acceleration or sag when downshifting that jerks the heads of drivers and passengers.

A continuously variable system, on the other hand, provides for a stepless acceleration. It gives not five or so intervals of fixed ratios, but an infinite number in between.   The simplest form of a CVT utilizes a belt between two rotating pulleys. Instead of 2D discs, the pulleys are actually cones.  A hydraulic regulator shifts the position of the cones perpendicular to the belt. The belt slips down and up the cones to a wide range of radii, smoothly varying the ratios between engine and wheel.

“Other CVT designs share a common characteristic,” explains Bisby.  “The input-output ratio of the transmission changes by varying the effective diameter of one or more components.”  Later variations of the CVT replace the spinning cones with some other 3D shapes to produce a similar variance.  Bisby’s Diametroid, however, does not employ this technique, instead relying solely on planetary gearing.

Since our conventional transmission is full of clunky shifts, each of which works engines harder and leaks efficiency, one might question why we have the transmission we have now, instead of a CVT.  One can only look back at the oddly sporadic evolution of the CVT and speculate.  Da Vinci sketched his version of a CVT back in 1490, way back before there were cars or bicycles to make use of it.  Four centuries would pass before a CVT device would be patented.

Only in the last few decades have CVTs achieved some a commercial popularity.  Small CVTs are found in a wide range of power tools, tractors, and snowmobiles.  More complex – and much bulkier — CVTs have been adapted in the automotive world by a few companies looking to boost efficiency.  Automotive CVTs earned public scorn when mated with lower-performing engines as with the Subaru Justy or the Ford Fiesta 1.1 CTX, but found more enthusiasm in more muscular offerings by the European carmakers DAF and Audi.

CVTs in   Cars:   The Danish automaker DAF was one of the fi rst to employ a CVT in cars.   This TDL model concept from 2007 features its fourth generation Variom-  atic transmission, designed   to work with a V8 5-liter engine. Automotive  engineer Pablo Serrano, who worked on the TDL transmission, weighs in on the Diametroid invention.  Image credit (TDL):   Pablo Serrano

CVTs in Cars: The Danish automaker DAF was one of the first to employ a CVT in cars. This TDL model concept from 2007 features its fourth generation Variom- atic transmission, designed to work with a V8 5-liter engine. Automotive engineer Pablo Serrano, who worked on the TDL transmission, weighs in on the Diametroid invention. Image credit: Pablo Serrano

Balancing on the Dynamic Threshold

There are big differences between Bibsy’s concept and current CVT designs.  First, as mentioned, the Diametroid is a compact box of orbitals and gearing, and does not rely on a big spinning solid to vary diameters (which makes current CVTs bulky) nor anything like a friction driven belt (which is liable to wear out and break). There are no variable elements at all in the Diametroid.  Unlike the pulley-driven CVT example, the Diametroid needs no hydraulic or other powered mechanism to regulate the ratio of rotational velocities.

Sweet Spot:  This graphs illustrates the concept of the  dynamic threshold.  Power begins transferring  to the wheel once the input RPM reaches an  established cadence, the dynamic threshold.   After that speed is achieved, the transmission accelerates smoothly and continuously.  The

Sweet Spot: This graphs illustrates the concept of the dynamic threshold. Power begins transferring to the wheel once the input RPM reaches an established cadence, the dynamic threshold. After that speed is achieved, the transmission accelerates smoothly and continuously. The

“The DT-CVT does not require any electrical, magnetic, hydraulic, friction or active centrifugal devices,” Bisby explains.  “Its function depends on the ability to precisely balance differential rotating forces.”

The dynamic threshold is the “DT” in DT-CVT.  It is the point where the power input reaches an established RPM rate.  Once reached, the dynamic system kicks in.  The output RPM then increases continuously from zero to its maximum rate, while the input remains always at the same RPM.   Once within this dynamic system, more torque (at the same rotational speed) translates directly to output acceleration.

Bisby's Rhinoceros model of a hypothetical bicycle application of his dynamic threshold CVT, one of many vehicular applications on display at his website www.diametroid.com.  In this scenario, the bike transfers power through a drive shaft rather than a chain to the transmission attached to the rear hub. In far right, an added motor for an electric-assist bike.

Bisby's Rhinoceros model of a hypothetic bicycle application of his dynamic threshold CVT, one of many vehicular applications on display at his website www.diametroid.com. In this scenario, the bike transfers power through a drive shaft rather than a chain to the transmission attached to the rear hub. In far right, an added motor for an electric-assist bike.

For sake of example, apply a small version of the transmission to a hypothetical pedal-powered bike.  The rider would give the bike a push off and start peddling; no power will be transferred to the back wheel in the first few seconds. But once the feet pedal at the prescribed dynamic threshold, for this example, say, a typical cadence of 66 revolutions per minute, the bike begins to smoothly accelerate.

“If you’re peddling along at 66 rpm, it does not matter how much force you put on the pedals, they will always spin at 66 rpm,” explains Bisby.  “If you try and pedal faster, it will simply translate the power to the back wheel as more torque.”  Balancing on this dynamic system, the rider needs no manual controls or outside regulating devices.

But it’s when an engine replaces leg muscles that the full potential of the Diametroid is seen.  Internal combustion engines are happy to run at a constant rate.  Revving from low to high RPMs creates wear on the engine.  This is even truer said for electric motors, which drain heavily on battery power in order to get up to speed.  By design, electric motors operate at a maximum efficiency when they run at one constant rate of output.

Any discussion of the next generation of fuel-efficient personal transportation seems to revolve around the prospect of electric motors, or at least some hybrid combination.

Not only does the Diametroid seem to answer this call because of its easy mating with electric motors, but also because the transmission itself is extremely efficient.  Bisby says conventional automotive torque converters are relatively inefficient.  Both stick-shift and automatic transmissions increase in friction while increasing rpm, while the Diametroid does exactly the opposite, approaching 100 percent efficiency at top speed.

Potentialities

For the mechanically and mathematically inclined, a detailed technical explanation of Bisby’s invention can be found at his website, www.diametroid.com.  In the 3D surface modeler Rhinoceros, Bisby has created a virtual prototype that simulates the rotational motion.  The site shows several exploded assemblies of the 3D model as well as a few hypothetical auto and bicycle application examples employing the Diametroid.

One mechanically inclined person who has reviewed Bisby’s materials is automotive engineer Pablo Serrano, who performed calculations for the original Variometric CVT on the 1985 SDL and TDL models from DAF, and has also worked on a more updated CVT concept for the Danish automaker last year.

Serrano says further analysis would be needed to come up with a full verdict on the Diametroid, but says that his first impression is that the invention certainly does what its owner claims.  “I believe all Bisby says in this explanation. I do think the transmission can be the substitutes for the current conventions, sharing a future with CVTs, including my own — the Variomatic CVT of DAF.”

One of the appealing features for Serrano is the size and simplicity of the device.  “It has less mobile pieces than current CVTs,” he says, explaining that Audi’s first-generation multitronic CVTs were about the same size as the engines.  A DT-CVT would very likely be more compact.

Although Serrano sees a “promising future” for the Diametroid in many applications, for automotive use he suggests that it may run into the same issue as other car-based CVTs:  the handling of high levels of torque.  “I see the same problem with this transmission,” says Serrano.  “It may not be able to cope with high RPMs because of torque. I would need to calculate it, but my guess is that we would need to have about three of these transmissions, one after the other, for powers of 100 horsepower and 80 pounds per feet, at least.”

Bisby confidently disagrees on the issues of high RPM and torque.  “Power transfer operates through a dynamic process which is completely immune to power levels,” he says. He also points out that the internal components spin much slower than a conventional automotive transmission, typically a maximum of 850 RPM, reducing to near zero RPM at vehicle top speed.

Also, unlike current CVTs, the transmission is not required to attach to a stable structure to brace against high levels of torque. This feature is visible in the Diametroid bicycle adaptation that Bisby modeled in Rhino.  The device attaches to the wheel itself, without any torque connection to the frame.  Bisby is sure that the concept can be scaled for nearly any power or torque application – conceivably even to a Formula One racecar, running at 20,000 RPM.

Whether or not the Diametroid can reach this vision may be answered in the next stage of Bisby’s journey:  testing.  The work continues for the inventor in a far less theoretical mode.  As he is starting to build out real components, Bisby is also shopping for interest among manufacturers, who would devote some R&D resources to development.  “I’ve just done the math,” says Bisby.  “There are other people — experts in materials and production — who much more qualified than me to make those types of decisions.”

And perhaps after his theory becomes a bit closer to reality, Bisby can get to work on putting that motor on his bike.

For more information and more models of the DT-CVT, please visit: www.diametroid.com.

About Rhinoceros

Rhinoceros provides the tools to accurately model your designs ready for rendering, animation, drafting, engineering, analysis, and manufacturing. Rhino can create, edit, analyze, and translate NURBS curves, surfaces, and solids in Windows, without limits on complexity, degree, or size. Rhino gives the accuracy needed to design, prototype, engineer, analyze, and manufacture anything from an airplane to jewelry. To see the many diverse products designed with this affordable 3D tool, and to download a free evaluation version, please visit: www.rhino3D.com.

2 Responses to “Building a Better Transmission”

  1. Where in the world is John Bisby? I want to build some prototypes of his transmission.

  2. I’ve been following this topic for a while now. I totally agree with what you’re saying. Nice site design by the way…

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