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Magnetic Shaft Drive Bike

Already 3/4 baked but I thought of it first
  [vote for,

I really thought this up back around 1980, but never got past drawing the basic geometry of the shaft and rear wheel. Someone else has recently built a similar layout shaft drive bike but using bearings in place of gears or my idea of magnets.

See link for Ceramic Speed "Driven" bicycle. My idea is to replace their bearings and that weird cheese grater thing with magnets on the drive shaft, with appropriate magnets arranged in rings on the rear disk and a single ring/disk at the pedals.

They got the geometry down to almost exactly what I had in mind forty years ago. But they built it as a technology demonstrator for their ceramic bearings. Magnets will be cheaper, lighter, quieter, more durable, easier to shift between ratios, not need lubrication ... better all around. As an added bonus: a bike with a magnetic shaft drive won't need a freewheel. Just have "coasting" trigger the shifting mechanism to move the shaft to a section of the rear disk that DOESN'T have any magnets on it. And conversely, trying to back-pedal with the magnets still lined up could provided braking force ... kinda like a fixed gear ... eliminating need for a rear brake.

kdf, May 15 2020

Ceramic Speed "Driven" bicycle https://www.ceramicspeed.com/en/driven/
[kdf, May 15 2020]

Magnetic gear https://youtube com/watch?v=jgff2t06I6I
[kdf, May 15 2020]

Magnetic gear, part 2 https://youtube.com/watch?v=aeJVIEFv8QE
[kdf, May 15 2020]

Torque transfer efficiency in magnetic gears https://www.jmag-in...orqueandefficiency/
[kdf, May 15 2020]

More about magnetic gears http://www.magnomat...-magnetic-gears.htm
"Magnetic gears inherently protect against overloads by harmlessly slipping if an overload torque is applied and automatically and safely re-engaging when the fault torque is removed." [kdf, May 15 2020]


       What are the eddy-current losses in the coupling ?   

       Direct mechanical drives (sprocket-sprocket or sprocket-chain) are pretty efficient, maybe 3 to 5% loss; the bicycle is one of the most efficient energy transmission systems humans have created, though it has the advantage that it only has to handle a few hundred watts at most.
8th of 7, May 15 2020

       I think eddy current losses can be mitigated, a lot of work has been done in this area and magnetic gearing systems can be up to 99% efficient. But other than a few rough looking youtube videos, I haven't seen any magnetic gear systems with this geometry ... so more research would be needed on this implementation.   

       I don't dispute that roller chain is very efficient, that's often used as an argument against alternatives. But a few % lower efficiency may be acceptable trade off for less maintenance. Especially as the efficiency of the magnetic drive won't diminish with use as chain does over its service life from wear, dirt, lack of lubrication, etc.
kdf, May 15 2020

       What about slip at high torque ? The torque at the wheel - as compared to the bottom bracket - can be quite high, particularly on an application like a mountain bike. A mis-tensioned chain can "jump" ... presumably a magnetic coupling would just slip.
8th of 7, May 15 2020

       "slip at high torque" ... Probable, and desirable as long as it doesn't happen until torque is so high that a chain would skip or break. Slippage would be overload protection, preferable to a complete failure that could happen with a chain.
kdf, May 15 2020

       It might be desireable to design for high shaft RPM and thus lower torque for the same power transfer, but the faster it spins, the more the gyroscopic effects. A very lightweight torque tube - maybe a composite - would compensate, but might then mean it has to be enclosed to protect it from damage.
8th of 7, May 15 2020


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