

A bike with a reverse-spinning wheel - RiderOfGiraffes
http://www2.eng.cam.ac.uk/~hemh/gyrobike.htm

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jrockway
Why would spinning the wheel backwards cancel out the gyroscopic effect? The
gyroscopic effect makes it hard to tilt the wheel left or right (from the
perspective of the rider). Spinning the wheel backwards makes it hard to turn
the wheel right or left, which has the same stabilizing effect.

I get that there is some effect other than the gyroscopic action of the wheel
(how else could you track-stand, after all), but I don't feel like this device
proves or disproves anything.

Also, the "begs the question" and the footnote explaining why it's OK to misue
that expression was annoying. (If you have to write a paragraph about why you
want to use incorrect English, wouldn't it be easier to just edit the misused
construction. "This leads us to ask, ..." Write that, and you save yourself a
footnote, and people will think you're not bad at writing. </rant>.)

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hristov
You are exactly right. This article suffers the common misconception that two
wheels spinning in opposite directions and same speed cancel out the
gyroscopic stability. And this is simply not true. In fact two wheels spinning
in opposite directions and same speeds will actually double the gyroscopic
stability (as opposed to one wheel of the same speed).

However, two wheels spinning in opposite directions may cancel out other
spinning wheel effects.

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jules
Which other spinning wheel effects?

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hristov
Another effect is the tendency to tilt to the side if you accelerate or
decelerate the rate of spin. This effect will be canceled by counter spinning
wheels.

~~~
jules
Can you elaborate on this? I have never seen a wheel tilt when you accelerate
or decelerate it.

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pge
a simple experiment can demonstrate what really keeps a bike up. Hold your
favorite bike by the seat and give it a hard push forward (even better run a
few steps with it to get it going and then let go). Then repeat, pushing it
backwards.

A bike moving forward will self correct to stay up until it loses speed. A
bike moving backward will quickly go into a hard turn right or left and fall
over. Gyroscopic moments for wheels in both scenarios are the same. The
difference is that as a forward-moving bike falls to one side, the front wheel
straightens out underneath it. As a backward moving bike (or any bike with a
turning rear wheel), starts to fall, the front (now rear) wheel turns further
to the side as it falls rather than straightening out, and the bike goes into
a very sharp turn and falls.

As an addendum, that's why recumbent bikes stick with a rear drive and front
steer, which requires very long chains, rather than driving the front and
steering with the rear (which at first blush seems like a much simpler
configuration - the pedals are up near the front wheel, and the hands are near
the rear).

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barkingcat
this leads me to wonder - has there been a bike design where the rear wheel is
allowed to freely "steer" as well as the front wheel? sorta like a crabwalking
bike able to side pedal when needed.

Modern automobiles have incorporated rear wheels that turn together with the
front wheel (I believe in opposite directions during a turn) in order to
decrease the turning circle and improve maneuverability - I'd kill for a bike
that has an electro-servo controlled rear wheel that turns to help the biker
corner, etc.

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bch
See "swingbike" -- they're fun:
[http://blog.makezine.com/archive/2008/01/sweet_swing_bike.ht...](http://blog.makezine.com/archive/2008/01/sweet_swing_bike.html)

In this next picture, note the seat post (not the sissy bar) is actually
inserted into something like a typical headset (where a fork and stem would go
on the front of a bicycle) that pivots when the pin on the underside of the
frame is removed. <http://www.swingbikerider.com/4-24-05%20003.jpg>

~~~
barkingcat
That looks pretty cool! and insane too!

~~~
bch
Yup -- they're pretty fun... The bicycle has had nearly 200 years of
evolution, though, and the reason most bicycles aren't built like this is it's
simply impractical. If the fun/coolness is worth it to you though, it's
worthwhile :)

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adolph
The article seems to be something of a joke. Yes, caster angle is important
for stability, but the Gyrobike training wheel seems to illustrate how
gyroscopic forces can also be helpful.

See: <http://www.youtube.com/watch?v=Cbfe2_2DDc0>

Additionally, stationary trainers like rollers that don't allow for the
steerage correction described by the author illustrate how at speed something
other than castor angle is assisting in stabilizing the bike.

See: <http://www.youtube.com/watch?v=xaYDZ-RSOTA>

~~~
aplusbi
I've been meaning to set up some gyroscopically neutral rollers and see if
it's possible to ride them...but I've been lazy (and my neighbors hate my
rollers).

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points
A related note is the reverse steering bike which is impossible to stay up on
unless you 'relearn' how to ride a bike.

[http://www.youtube.com/watch?v=9gCnBBeV2ws&feature=relat...](http://www.youtube.com/watch?v=9gCnBBeV2ws&feature=related)

I think that proves things a bit better - we stay up on a bike because we
learn to notice any slight chance we're falling, and counteract that with
steering to bring us back to upright.

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JanezStupar
I've spent quite some time racing Downhill Mountainbikes - and believe me -
the gyroscopic effect IS there - and it is very noticeable (DH bikes tend to
have MUCH heavier wheels).

Riding lighter wheels has a lot of effect on acceleration and braking.
Handling also improves ALOT with lighter wheels.

It also contributes to stability. But you do need to ride faster than this
"wuss" (pun intended) moving at snail speed.

~~~
jules
Couldn't that effect be simply because of the inertia of the wheels that they
also have when they're not spinning?

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Empedocles99
That might be the case... I mounted a heavy U-lock on the front forks of a
mountain bike, made steering very heavy. Felt pretty similar to a heavy wheel
to me.

Of course this is all subjective (my observation, and the parent's downhilling
observation).

Definitely easier to steer lighter wheels regardless of speed.

~~~
mikeklaas
Although, when the total weight of the bike is high, it is better to have
weight up front too (front+rear panniers). Otherwise, steering feels flimsy.

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sh1mmer
If this kind of stuff interests you there is an excellent book on the physics
of bicycles [http://www.amazon.com/Bicycling-Science-David-Gordon-
Wilson/...](http://www.amazon.com/Bicycling-Science-David-Gordon-
Wilson/dp/0262731541)

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eru
If you want to ride an "unrideable" bicycle, try a flevo bike (like
<http://home.ultramagic.net/bicycles/knakker/>). It has a giant joint in the
middle, instead of steering bars.

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james1071
The castor effect is what makes a bicycle self-steer. Bicycles with small fork
rake are very twitchy-bikes with big fork rake are unresponsive.

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gokhan
Yep. Sports motorcycles have small rake (should be responsive on the race
track) while choppers have big rake (long, straight journeys on Route 66). I
prefer a KTM 990 Adventure, just in the middle, though :)

