5 ms·

A bike with a reverse-spinning wheel

jrockway 16y ago
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>.)
- wnoise 16y ago
  The gyroscopic effect does not _directly_ make it harder to turn, it causes a reaction force (torque, really) at right angles to the applied force. With two counter-rotating wheels, these reaction forces work against each other.
  - hristov 16y ago
    No they don't. Any spinning wheel will create a force that tries to preserve its axis of rotation. If you have two counter-spinning wheels that share the same axis and try to tilt them, they will both try to preserve the same axis of rotation and thus exert a force in the same direction.
    - wnoise 16y ago
      Have you done the experiment? "Syd12107" has claimed to in this thread: http://www.physicsforums.com/archive/index.php/t-173215.html http://www.physicsforums.com/archive/index.php/t-173215.html The result was that counter rotating gyroscopes had no net effect. The only thing that matters is total angular momentum. The resistance to being tilted is a _secondary_ effect. The push gets translated into motion at right angles to the push, and this motion gets translated into push at one more right-angle, exactly countering the original push. For two counter-rotating gyroscopes, this first-stage motion of one is exactly countered by the other, leading to no net effect. Really, truly.
      - RiderOfGiraffes 16y ago
        That exactly matches my experience and calculations.
- hristov 16y ago
  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.
  - jules 16y ago
    Which other spinning wheel effects?
    - yread 16y ago
      Precession
      - jules 16y ago
        Precession is caused by the gyroscopic effect.
        lincolnq 16y ago
        Wait, so if you have two identical wheels on one axis, spinning in opposite directions, and you rotate the axis, are you saying you'll experience precession or you won't? My intuition (and physics understanding) says you won't. But the top-level post is saying you will still experience "gyroscopic stability". Is that true?
        RiderOfGiraffes 16y ago
        You're right, your understand is right, you won't experience "gyroscopic stability." I've done it. You don't. I believe that people who believe otherwise are working on misunderstandings of perceptions. It's nototiously easy to confuse several effects in this. Careful experiments are required with tight controls on error bars, etc. It's easy to be fooled. Eric Laithwaite is a classic and high profile example: http://en.wikipedia.org/wiki/Eric_Laithwaite http://en.wikipedia.org/wiki/Eric_Laithwaite
- jules 16y ago
  The torque is tau = I*domega/dt When we have two wheels spinning in opposite directions the torque is tau = I*domega/dt + I*d(-omega)/dt = 0 The gyroscopic effect doesn't actually make it harder to turn a wheel. It's just that if you turn it in the xy-plane, it automatically turns in the direction perpendicular to the push (the yz-plane). When a human is physically turning a wheel he will try to stop that from happening, thus the feeling that it's hard to turn the wheel. Note that in particular the gyroscopic effect does not produce any force in the direction opposite to the pushing force. You have a wheel spinning one way, and say that if you try to turn it clockwise in the xy-plane then it will turn clockwise in the yz-plane because of the gyroscopic effect. If you have a wheel spinning the opposite way, and if you turn it clockwise in the xy-plane then it will turn counterclockwise in the yz-plane because of the gyroscopic effect. Adding the two effects cancels them.
- jlees 16y ago
  He's Australian, they misuse English wilfully all the time. ;)
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)
  - 16y ago
JanezStupar 16y ago
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 16y ago
  Couldn't that effect be simply because of the inertia of the wheels that they also have when they're not spinning?
  - Empedocles99 16y ago
    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 16y ago
      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.
points 16y ago
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=related http://www.youtube.com/watch?v=9gCnBBeV2ws&feature=relat... 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.
adolph 16y ago
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 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 http://www.youtube.com/watch?v=xaYDZ-RSOTA
- aplusbi 16y ago
  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).
james1071 16y ago
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.
- gokhan 16y ago
  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 :)
eru 16y ago
If you want to ride an "unrideable" bicycle, try a flevo bike (like http://home.ultramagic.net/bicycles/knakker/ http://home.ultramagic.net/bicycles/knakker/). It has a giant joint in the middle, instead of steering bars.
sh1mmer 16y ago
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/dp/0262731541 http://www.amazon.com/Bicycling-Science-David-Gordon-Wilson/...

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>.)

wnoise 16y ago
The gyroscopic effect does not _directly_ make it harder to turn, it causes a reaction force (torque, really) at right angles to the applied force. With two counter-rotating wheels, these reaction forces work against each other.
- hristov 16y ago
  No they don't. Any spinning wheel will create a force that tries to preserve its axis of rotation. If you have two counter-spinning wheels that share the same axis and try to tilt them, they will both try to preserve the same axis of rotation and thus exert a force in the same direction.
  - wnoise 16y ago
    Have you done the experiment? "Syd12107" has claimed to in this thread: http://www.physicsforums.com/archive/index.php/t-173215.html http://www.physicsforums.com/archive/index.php/t-173215.html The result was that counter rotating gyroscopes had no net effect. The only thing that matters is total angular momentum. The resistance to being tilted is a _secondary_ effect. The push gets translated into motion at right angles to the push, and this motion gets translated into push at one more right-angle, exactly countering the original push. For two counter-rotating gyroscopes, this first-stage motion of one is exactly countered by the other, leading to no net effect. Really, truly.
    - RiderOfGiraffes 16y ago
      That exactly matches my experience and calculations.
hristov 16y ago
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.
- jules 16y ago
  Which other spinning wheel effects?
  - yread 16y ago
    Precession
    - jules 16y ago
      Precession is caused by the gyroscopic effect.
      - lincolnq 16y ago
        Wait, so if you have two identical wheels on one axis, spinning in opposite directions, and you rotate the axis, are you saying you'll experience precession or you won't? My intuition (and physics understanding) says you won't. But the top-level post is saying you will still experience "gyroscopic stability". Is that true?
        RiderOfGiraffes 16y ago
        You're right, your understand is right, you won't experience "gyroscopic stability." I've done it. You don't. I believe that people who believe otherwise are working on misunderstandings of perceptions. It's nototiously easy to confuse several effects in this. Careful experiments are required with tight controls on error bars, etc. It's easy to be fooled. Eric Laithwaite is a classic and high profile example: http://en.wikipedia.org/wiki/Eric_Laithwaite http://en.wikipedia.org/wiki/Eric_Laithwaite
jules 16y ago
The torque is tau = I*domega/dt When we have two wheels spinning in opposite directions the torque is tau = I*domega/dt + I*d(-omega)/dt = 0 The gyroscopic effect doesn't actually make it harder to turn a wheel. It's just that if you turn it in the xy-plane, it automatically turns in the direction perpendicular to the push (the yz-plane). When a human is physically turning a wheel he will try to stop that from happening, thus the feeling that it's hard to turn the wheel. Note that in particular the gyroscopic effect does not produce any force in the direction opposite to the pushing force. You have a wheel spinning one way, and say that if you try to turn it clockwise in the xy-plane then it will turn clockwise in the yz-plane because of the gyroscopic effect. If you have a wheel spinning the opposite way, and if you turn it clockwise in the xy-plane then it will turn counterclockwise in the yz-plane because of the gyroscopic effect. Adding the two effects cancels them.
jlees 16y ago
He's Australian, they misuse English wilfully all the time. ;)

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)
- 16y ago

JanezStupar 16y ago

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 16y ago
Couldn't that effect be simply because of the inertia of the wheels that they also have when they're not spinning?
- Empedocles99 16y ago
  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 16y ago
    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.

points 16y ago

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

adolph 16y ago

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

aplusbi 16y ago
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).

james1071 16y ago

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.

gokhan 16y ago
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 :)

eru 16y ago

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

sh1mmer 16y ago

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/dp/0262731541 http://www.amazon.com/Bicycling-Science-David-Gordon-Wilson/...