h a l f b a k e r yExpensive, difficult, slightly dangerous, not particularly effective... I'm on a roll.
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The rocker bogie is a simple mechanism that allows
wheel
articulation to smooth travel over obstacles. The best
examples of this technology can bee seen on the Spirit,
Opportunity and Curiosity Mars rovers.
To adapt this system to more Earth-bound ventures,
imagine a bicycle as one side
of the rover with a
standard
front wheel and two inline smaller rear wheels forming
the
rocker bogie. Power is delivered by conventional chain
drive with two possible configurations. 1. Drive one
wheel
in the standard manner and then drive the second
directly
from that. 2. Drive a sprocket between the wheels and
distribute power to each individually.
Unlike the Mars rovers, the bogie motion should be
constrained by a spring and damper, since it would be
better to prevent the front wheel of the bogie
descending
completely into any hollow at higher speeds. The two
smaller wheels effectively simulate a much larger single,
wheel while maintaining the strength, weight and
aerodynamic characteristics of smaller wheels*.
While the vehicle would technically be a tricycle, you'll
still fall over if you can't balance. Handily, with clever
folding jiggery-pokery, the whole thing should fit into a
smaller space. It's the wheels that are the space hogs.
Off
road, there should also be increased traction and the
pivot
point can be played with to stop the annoying wheelie
tendency you get with climbing steep hills.
*Raleigh conducted a study back in black & white days
and
the 20" wheel is the fastest on the track.
[link]
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It would be fairly easy to make a bolt-on accessory to a conventional bike to test this, but cornering might become interesting. |
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If you need to corner, you were going in the wrong direction
to begin with. |
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Replacing the front wheel with a similar arrangement would
be necessary to enable the smallest folding. |
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How steerable would such a pair of wheels be? (A
grammatically valid reading of that sentence: How would
such a pair of wheels be steerable?) |
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It will be effectively Configuration 2 anyway; even if you
drive front, which drives back, the drive is initially coming
in via the pivot (ie: not an axle) unless you're doing REALLY
complicated things with the chain tensioner.
Cornering will be interesting as 2 wheels mounted in a line
like to follow that line; perhaps the rearmost wheel could
be some-what flexibly mounted on it's axle (a la semi-trailer
self-steer wheels)? |
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Or the rocker-bogie could just have a vertical-axis bending
hinge at the same place it has its horizontal-axis rocking
axle, making the rear wheel a caster. That would make
driving the rear of the rear wheels
harder, but you can get away with driving the front one only.
More importantly, it might not actually allow them to follow
the same track. |
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No, the 2 wheels at the back need to be able to
turn/bend relative to one-another. Try going around a
corner on a normal bike without turning the handlebars...
If the pair are still fixed in a straight line, but can rotate
relative to the frame, they will keep going straight while
the rest of the bike turns, which would look funny and
provide endless "Fail" videos for YouTube.
Perhaps an offset, angled pivot (for just the rear wheel of
the pair) that will take advantage of gyroscopic effects
the same way the front wheel does (ie. you lean, it turns.
You straighten up, it re-aligns). |
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I would think you coulld have a mechanical linkage between front handlebars pivot and rear wheels, so that the three wheels of the bicycle are always automatically arrayed along the arc of a circle. Might not work for slalom courses though. |
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Alternatively have a second handlebar which controls the alignment of the rear pair. Perhaps attach the linkage to the saddle post, so that the rider can steer the rear pair by wriggling their arse. |
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"Rocker-bogie" is obviously some sort of 1950s slang, as used
by Teddy Boys. How is this not part of the idea? |
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// No, the 2 wheels at the back need to be able to
turn/bend relative to one-another. // |
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That's exactly what I was saying: //vertical-axis bending
hinge […] making the rear wheel a caster// |
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But the contact points between the three tires and the
ground must always be along a straight line, so that the bike
can lean to the side when turning. Could be tricky. |
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[notexactly]; sorry, I mis-interpreted your comment...
I think, because the rear pair can also move vertically
relative to one-another (the whole point of the idea) the 3
wheels don't need to be in a line, as any sideways offset
(when leaning/cornering) will be compensated by the
vertical change at the same time. The mechanics of the
steering (whether controlled or passive at the rear) will be
interesting... |
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It may be useful to consider the geometry of 6 x 6 vehicled, be they 2 - 2 - 2 or 2 - 4 in configuration, altho since the wheels are differenially driven and don't intentionally tilt it's not the same geometric problem. |
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The nearest thing might be a single-track version of the Kleines Kettenkraftrad. |
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Wikipedia says "The vehicle was designed to be delivered by
Junkers Ju 52 aircraft, though not by parachute. " That is
one tough motherfucker of a vehicle. |
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But that is by the by. Bogied bicycles (and tricycles) were
used in the 1970s and 80s by the Botswanan police, to help
them negotiate rutted and/or log-strewn tracks in going
about their duties. The most common versions (known as
nyoka, or snake) had a normal front wheel and a bogied
rear wheel. One variant had both front and rear wheels
replaced by bogies, and were affectionately known as
nyoka-nyoka. They were provided to all patrolling
policemen (no policewomen at that time), and were
universally reviled. Most were converted to "normal"
bicycles by their owners. The rear bogie assemblies were
often re-purposed for use in home-made air conditioners,
some of which are operating to this day. |
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//home-made air conditioners, some of which are operating to this day// I believe there is one in the houses of parliament, part of an "alternative" official gift from unafilliated members of the Botswanan counter-parliament. |
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There is, but unfortunately the Botswanan broke down after
two years and they haven't replaced him. |
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