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Moving bike stabilizes but standing bike falls!by hellboy4444
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#1
Nov811, 12:09 PM

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A bike "B1" is on the go. Another one, "B2" is standing upright and still.
Which one is more stable, in the sense that, if I push one, which one shall fall down more easily? 


#2
Nov811, 03:58 PM

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The moving bike has rotating wheels  so it is gyrostabilized.
I remember letting a 2wheeler roll down a hill as a kid, I expected it to fall over almost right away but it went all the way down the hill first (about 20m). 


#3
Nov811, 07:15 PM

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The moving bike is stable because of steering geometry. You could replace the wheels and tires with circular skate blades that don't rotate, give it a shove on an ice rink and it would still be stable. Most of the stability is due to trail (weight distribution on the front tire also has an effect). Trail is the distance from the extention of the steering axis to the ground, back to the contact point between the tire (or skate blade) and ground. Generally, the longer the trail, the slower the bike can move and still be stable. What the trail does is cause the front wheel to steer inwards in response to a lean of the bicycle. This results in selfcorrection that returns the bike to an upright position (although it's velocity direction will have changed somewhat).



#4
Nov811, 08:38 PM

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Moving bike stabilizes but standing bike falls!
Lowel and McKell 1982
(which actually puts it after I last looked  so I'm out of date!) http://fisica.ciencias.uchile.cl/~go...ability_82.pdf ...conclude that the "castor" (trail) of the front wheel is the most significant contribution to overall selfstability. If gyroscopic effects are ignored, the bicycle is almost selfstable  a small perturbation that may otherwise push it over just leased to a circular path  except that oscillations grow in magnitude (it gets more and more wobbly) till it does fall over. The gyroscopic effects improve stability but increase the tendency to wobble. Though the graphs show a smaller amplitude of wobble at a higher frequency. The biggest contribution to a ridden bike's stability is the rider reacting to smooth out wobbles and correct lean. The rider is also the biggest contribution to the bike instability by raising the center of mass. Motorcycles have a significant gyroscopic effect, being heavier and faster,  the trail contributes strongly to stability while maneuvering. You'll feel it through the handlebars. The lessstable configurations seem to be more "cool" though(?) The practical research has mostly been done with a rider though. This has turned out to be more interesting  and a good example why answering these questions is a good idea. 


#5
Nov811, 11:11 PM

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Nonstable and stable modes demonstrated in videos of a riderless bicycle at tudelft. The bike is instrumented with some sensors and a lap top attached behind the seat to record the readings.
treadmill measurments .htm The test results at 30 kph (8.33 m/s) conflicted with the mathematical model they made for that same bike that calculated the bike would be in capsize mode (slowly fall inwards) at 8 m/s and faster speeds, but the video showed the bike to be very stable at 8.33 m/s. http://home.tudelft.nl/index.php?id=13322&L=1 


#6
Nov911, 12:59 AM

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Bikes are not mainly stable due to gyroscopic stabilization.
When the bike tilts, the front wheel tends to precesses, turning the steering column and front wheel into the turn, tending to right the bike. That all there is to the basics of stability of a standard bicyle. 


#7
Nov911, 03:47 AM

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The capsize mode predicted by theory above certain speed is not very prominent and remains around zero. It basically means that the bike fails to straighten up and recover from the turn. You cannot show this on a treadmill, because the bike cannot do a long continuous turn there. My own idea: The perturbation they introduce on the treadmill above capsize speed is more like the weave mode for low speeds, which might explain why it doesn't grow at high speeds, and the bike appears stable. Others did experiments with releasing a bike above capsize speed bike in free terrain, and the bike remained in a circular turn until it slowed down to stable speeds. Then it straightened up again. I suggested to use electric powered bikes to see what happens if the bike doesn't slow down. 


#8
Nov911, 04:51 AM

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#9
Nov911, 06:58 AM

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I'm thinking the reason for the transition into "lean" stablity (tendency to hold a lean angle) at high speed is due to the gyroscopic forces (at front and rear tire) resisting any change in lean angle, and dominating the selfcorrection tendencies. Also at high speed, the rate of precession and reaction to leaning (roll axis to pivot axis coupling) of the front wheel will be reduced, because of the relatively large amount of angular velocity in the tires forward rotation compared to the angular velocity along the roll (lean) axis. The bike may actually be falling inwards or outwards, but the rate of change in lean angle is so small that it's imperceptible by the rider. 


#10
Nov911, 07:23 AM

P: 23

Are you sure? I don't think so. What do you think? 2. "....This results in self correction that returns the bike to an upright position" Do you mean that the "selfcorrection returns the bike to an upright position"? Really? I don't get it. What exactly returns the bike to an upright position? Honestly, I don't believe the bike shall return to an upright position ever! If at all, it may tilt more instead. What do you think? 


#11
Nov911, 08:10 AM

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http://en.wikipedia.org/wiki/Twomassskate_bicycle There are kits to replace the wheels on a bicycle with skis (downhill usage), and while these are ridable, they aren't self stable (the skis will often slide instead of turning). The reason for selfstability is that the front tires turns into the direction of lean, and this results in literally steering the tires back under the bicycle. The tires apply an outwards force to the ground, with the ground applying an equal and opposing inwards force that steers the wheels back under the bike (since the inwards force from the ground is applied below the center of mass of the bike) and straightens the bike back upwards (although the direction will have changed, depending on the amount of disturbance that caused the lean). 


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