CVT – Continuous Variable Transmission

Inspiration during a hydraulics lecture –
I came up with the idea after being introduced to the common power vs. speed curve during one of Mr. Andic’s hydraulic lectures that were part of BCIT’s curriculum for their mechanical design program in 1999.

 

 

CVT283x290.jpg

The purpose of showing us that curve was to compare the power vs. speed curve of geared transmissions in cars with that of most hydraulic piston pumps. Because of the stepped nature of geared transmissions not all the power available from the motor is used at different speeds. Hydraulic motors are designed so they can take advantage of all available power at any given speed.
Mr. Andic commented that ideally cars should have continuously variable transmissions. Indeed, there are some Honda vehicles with such transmissions but their used is limited to engines with a maximum power output of about 115hp. Most snowmobiles use a similar variable diameter sheave system on their transmission.
It was then that I thought it would be great to design a similar device for bicycles. I researched a lot and never found such a system in existence so I resolved to become first inventor to device a fully continuously variable transmission for bicycles.

How it works
Five laser cut sprockets slide inside curved slots cut into two stainless steel discs. The overall diameter of the pentagon created by the sprockets is regulated by the relative angle between the two discs. By default the overall diameter is hold at maximum by an adjustable spring (not shown in picture).
As the rider applies force to the pedals the spring yields and the discs rotate with respect of each other forcing the sprockets to slide closer to each other thus reducing the overall diameter. The diameter change is equivalent to the difference in diameter of the largest and smallest sprocket found in most mountain bikes.
I hope this half-ass explanation and the pic is enough for you to understand how it works. E-mail me for any questions you may have and I will try my best to confuse you even more.
Remember, this invention is not patented and anybody that has the resources and motivation is welcome to copy and improve it to make it work. Good luck!

How it was built
After two months sketching many different concepts I came up with the idea of having sprockets sliding inside slotted discs. Back then I just started as a laser and plasma cutter programmer for a company called Brenco. I got one of my friends there to cut the discs and sprockets using a scrap sheet of 10ga stainless steel. All the other parts, nuts and bolts were purchases at the local hardware store. It took me another month of grinding, cutting and testing before the final prototype looked like in the picture.

How it performed
The prototype worked as expected, adjusting its diameter automatically to pedaling force. However, the choice and thickness of materials made it very heavy (over 5 lbs) for practical use. Also, chain derailment was too frequent and diameter variation was too rough. Later, I realized that the adjustable spring made the design considerably less efficient since part of the rider’s effort was required to stretch it.

Conclusions
Most of the drawbacks mentioned could be eliminated after some redesigning, use of better materials and adequate tools.
To smoothen diameter change and reduce chain derailment the use of a belt instead of chain and sheaves instead of sprockets could help considerably.

Unfortunately, even with all this potential improvements, I believe that the CVT would be still very heavy and too cumbersome for practical use. In addition, the diameter change wouldn’t be big enough to eliminate sprockets on the real wheel.
In other words, it seems to me that we will be changing gears in our bicycles for a while to come.

Check these sites with more designs and info on CVTs:
http://cvt.com.sapo.pt/
http://www.gizmology.net/cvt.htm
http://www.swri.edu/3pubs/brochure/d03/cvt/cvt.htm

You can also check my CVT at Zed.bc.ca here:
Zed Open Source Television

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