The in-hub gears used in the bicycles of the discerning are an astonishingly rare example of an American invention being perfected and made commercially successful by the canny Brits <link>. They are clever devices which provide a geared output from an input with reference to a fixed point. In a bicycle, the sprocket is the input, the frame the reference (usually via an antirotation axle) and the hub shell the output. The input:output ratio may be anything you like, several selectable ratios may also be crammed into such a hub*.

I recently improved the design of the wheel <link> by making it significantly more compact. I don't blame George Cayley for his shoddy design work, it was simply a "needs must" solution to a specific problem, like Australia. The idea was for a whole wheel to be replaced by two quarter-wheels. A bicycle rolls forward from one quarter- wheel to the other, as the weight is taken off the first quarter-wheel it races 'round to be in front of the second quarter wheel, then the whole cycle repeats. To my eternal shame I proposed a solution involving sensors, electric motors and all sorts of unnecessary gubbins. More sensibly, let's use hub gears.

The reference for one quarter-wheel is the left fork leg, the reference for the other quarter-wheel is the right fork leg. The input is the hub shell of the first quarter-wheel, this drives the output at a 3:1 ratio. The output is the second quarter-wheel. As you push the bicycle along, torque applied to the first quarter-wheel drives the second all the way over the top, moving 270 degrees compared to the 90 degrees rotation of the first. Then the second quarter-wheel drives the first over the top in the same manner. Since the drive from one quarter-wheel to the other is only ever unidirectional, the two can be separated by simple freewheel arrangement. I suspect a bit of a cush- drive arrangement might be necessary to absorb the opposing torque at the segment to segment transfer.

I haven't however, worked out how you apply drive to this wheel, so it might have to just be a scooter. Or possibly jet-propelled.

*Although bicycles with more than 3 ratios have been linked to indecision, unhappiness, hysteria and ungentlemanly conduct. Such problems are at epidemic proportions in continental Europe.
-- bs0u0155, Sep 13 2016

Hub Gears https://en.wikipedia.org/wiki/Hub_gear
[bs0u0155, Sep 13 2016]

Needlessly complex predecessor. Electrically_20Actu...l_20Quarter_20Wheel
[bs0u0155, Sep 13 2016]

For a smoother ride, you could build a mechanically actuated pair of quarter wheels using elliptical or non-round gears. You might have to have two drive gears on the same shaft but out of phase, and the driven gears then connected to the wheel segments. It should be fairly simple to devise a gear profile with the desired variation in speed.
-- mitxela, Sep 18 2016


I have been pondering this sort of thing, in light of [bs]'s wise observations.

The logical limit, of course, is not to have any wheel-segments at all, but simply two pointy- ended spokes which "walk" in tiny steps in a rapid blur, creating the same effect as a very-many- sided polygonal wheel (ie, a circle).

But having the two spokes swing back and forth (like legs) involves lots of reciprocating mass; and having the rearmost leg swing back-and-up-and- over each time involves lots of unnecessary movement, as well as large ac- and de- celerations.

Better, surely, to have the two spokes welded together to form a very narrow inverted V (both points on the ground). Then tilt the V very slightly sideways (say, towards the midline of the vehicle). Now, if you just spin the entire inverted V along its long axis (that is, along the midline between the two spokes), you will have achieved the required result, with no reciprocating masses and no large changes in velocity.

This, of course, is exactly the way that one marks out multiples of a given distance on a chart, using a pair of compasses: the compasses remain at a fixed spacing, and simply tilt-and-rotate to make successive steps. A skilled user can "walk" thirty of these steps faster than you can count.
-- MaxwellBuchanan, Sep 18 2016


//Then tilt the V very slightly sideways (say, towards the midline of the vehicle).//

After brief experimentation with a pair of straight forceps I happen to have lying around there's a problem. My forceps are about 11 cm total length with about 1 cm of separation at the tip. In order to gain 5 mm of ground clearance for the non-active tip, I have to deflect the top 6 cm. That's quite a lot. If you scale it up to vehicle sizes you are faced with either a very wide V, or a very very tall V just to clear a medium pebble.
-- bs0u0155, Sep 19 2016


You lost me at // More sensibly//
-- Voice, Sep 19 2016


//In order to gain 5 mm of ground clearance for the non-active tip, I have to deflect the top 6 cm.//

Ah, but why aim for 5mm of ground clearance? Scale that up to a man-sized vehicle and it would become several centimetres - far more than the micro-gap which is fashionable for today's high- performance vehicles.

A greater problem, I suspect, is that the compasses will have to be spun at an astonishing rate to achieve significant forward speed. Howevertheless, this simply means that we have found the ideal automotive application for the jet turbine, without the need for immense gearing ratios.
-- MaxwellBuchanan, Sep 19 2016



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