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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 ste

by pge 16y ago

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).

barkingcat 16y ago
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.
- khafra 16y ago
  I had a bicycle like this[1] as a kid--with the rear wheel and pedals on a hinge. It wasn't significantly better at cornering--a good understanding of countersteering[2] should explain why. [1] (sorry, can't find a better reference/illustration with a cursory googleing) http://www.faqs.org/patents/app/20090014981 http://www.faqs.org/patents/app/20090014981 [2] http://en.wikipedia.org/wiki/Countersteering http://en.wikipedia.org/wiki/Countersteering (this article may not be sufficient for a thorough understanding of countersteering, but it's a good start)
- bch 16y ago
  See "swingbike" -- they're fun: http://blog.makezine.com/archive/2008/01/sweet_swing_bike.html http://blog.makezine.com/archive/2008/01/sweet_swing_bike.ht... 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 http://www.swingbikerider.com/4-24-05%20003.jpg
  - barkingcat 16y ago
    That looks pretty cool! and insane too!
    - bch 16y ago
      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 :)
- RiderOfGiraffes 16y ago
  The reason some vehicles have steerable rear wheels is because there are two rotational effects required when turning. One is to make the vehicle rotate, the other is to make the vehicle move in a circle. If you only steer the front wheels then when the centripetal force is correct for turning the corner, the rotational force is much larger than necessary. This leads to excessive scrubbing of the tires. Making the rear wheels steer slightly in the direction of turn the rotational force is reduced, similarly reducing the scrubbing. As a side benefit the wheels can be turned in the opposite direction at low speed, increasing the manoeuvrability.
adolph 16y ago
The one-directional stability in steerage of wheeled vehicles (bikes, shopping carts, cars and non wheeled vehicles like boat rudders) is affected by more or less asymmetry between the center of pivot and the center of force. See: http://en.wikipedia.org/wiki/Caster_angle http://en.wikipedia.org/wiki/Caster_angle
herdrick 16y ago
Actually it shouldn't matter to stability whether a recument (or other) bike is front or rear drive. In fact it appears some recuments are front drive. I would think that rear drive is prevalent because that's the wheel carrying most of the weight. Also I think the larger effect on your experiemt is the "trail" thing the author mentioned. If you spin the front wheel 180 degrees (possible for some bikes) and try again I think you'll mostly see the reverse result: pushing the bike backwards will let it roll a while, but pushing it forwards will result in immediate crash. But the leaning effect you mention could be important too, I don't know.
- pge 16y ago
  there are two effects, the lean and the trail. A front-steering bike with trail is self-correcting through both effects. A rear-steering bike with trail will correct itself to a straight line as long is it is perfectly vertical, but will be unstable as soon as it leans. When it leans, it will go into a hard turn and collapse. There are front-drive recumbents, but there are no rear-steering two-wheeled recumbents (it is possible to address the lack of lean stability on a tricycle configuration).

by pge 16y ago

barkingcat 16y ago

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.

khafra 16y ago
I had a bicycle like this[1] as a kid--with the rear wheel and pedals on a hinge. It wasn't significantly better at cornering--a good understanding of countersteering[2] should explain why. [1] (sorry, can't find a better reference/illustration with a cursory googleing) http://www.faqs.org/patents/app/20090014981 http://www.faqs.org/patents/app/20090014981 [2] http://en.wikipedia.org/wiki/Countersteering http://en.wikipedia.org/wiki/Countersteering (this article may not be sufficient for a thorough understanding of countersteering, but it's a good start)
bch 16y ago
See "swingbike" -- they're fun: http://blog.makezine.com/archive/2008/01/sweet_swing_bike.html http://blog.makezine.com/archive/2008/01/sweet_swing_bike.ht... 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 http://www.swingbikerider.com/4-24-05%20003.jpg
- barkingcat 16y ago
  That looks pretty cool! and insane too!
  - bch 16y ago
    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 :)
RiderOfGiraffes 16y ago
The reason some vehicles have steerable rear wheels is because there are two rotational effects required when turning. One is to make the vehicle rotate, the other is to make the vehicle move in a circle. If you only steer the front wheels then when the centripetal force is correct for turning the corner, the rotational force is much larger than necessary. This leads to excessive scrubbing of the tires. Making the rear wheels steer slightly in the direction of turn the rotational force is reduced, similarly reducing the scrubbing. As a side benefit the wheels can be turned in the opposite direction at low speed, increasing the manoeuvrability.

adolph 16y ago

The one-directional stability in steerage of wheeled vehicles (bikes, shopping carts, cars and non wheeled vehicles like boat rudders) is affected by more or less asymmetry between the center of pivot and the center of force. See: http://en.wikipedia.org/wiki/Caster_angle http://en.wikipedia.org/wiki/Caster_angle

herdrick 16y ago

Actually it shouldn't matter to stability whether a recument (or other) bike is front or rear drive. In fact it appears some recuments are front drive. I would think that rear drive is prevalent because that's the wheel carrying most of the weight. Also I think the larger effect on your experiemt is the "trail" thing the author mentioned. If you spin the front wheel 180 degrees (possible for some bikes) and try again I think you'll mostly see the reverse result: pushing the bike backwards will let it roll a while, but pushing it forwards will result in immediate crash. But the leaning effect you mention could be important too, I don't know.

pge 16y ago
there are two effects, the lean and the trail. A front-steering bike with trail is self-correcting through both effects. A rear-steering bike with trail will correct itself to a straight line as long is it is perfectly vertical, but will be unstable as soon as it leans. When it leans, it will go into a hard turn and collapse. There are front-drive recumbents, but there are no rear-steering two-wheeled recumbents (it is possible to address the lack of lean stability on a tricycle configuration).