Why do we lose balance in a bike when at a standstill?

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  • #26
rcgldr
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As far as I know the bike turns because of camber thrust.
The bike turns because the front tire points in a slightly differnt direction than the rear. There's some camber thrust effect, but not much. Motorcycle tires don't all have circular cross sections, some older ones (Dunlop K81's) had high narrow domes with nearly flat and very steep sides. Modern Dunlop tires generally have near flat centers (for longer wear), and rounded sides. Bridgestones are pretty close to a circrular arc. You feel the difference. The Dunlops flatter profile resist the initial lean on a motorcycle a bit, transition from a Dunlop to a Bridgestone and it feels like the back end is slipping out from underneath you (no lean resistance, so it takes less countersteering). Racing tires are generally like the Bridgestones, close to a circular arc.

The tires also have a bit of a overhanging lip at the edge of the tread above the sidewall, that allows the contact patch to flex a bit on the edge for better grip (only racers probably notice this).
 
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  • #27
rcgldr
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Try getting that bicycle up to about 100mph and see how the steering compares to a nice stable motorcycle designed to handle at those speeds.
This is a matter of how much trail there is. Record speeds for a bicyle following wind breakers are over 150mph. The bikes have more trail than a typical bike.

bike leans over on a contact patch that produces camber thrust. ...
inside of the tire traveling a shorter path than the middle.
At high cornering forces, tire squirm and slippage eliminate most of the camber thrust effect. Bikes turn because the front tire points in a different direction then the rear.

Now if it leans even a small degree to the left gravity pulls harder on that side and the bike falls.
Safely ignoring distances that are significatly realtive to the radius of the earth, gravity pulls equally on the bike at all times. The ground is pushing up at the points where the tires meet the ground (contact patches). If the bike leans, then the center of gravity is off to one side of the contact patches. The ground pushes up at the contact patches, gravity pulls downwards on the center of gravity, and these mis-alinged forces create a torque force, twisting the bike to make it lean and fall.

The more foward momentum the bike has the less effect gravity has at pulling the bike to the left or right.
Gravity pulls the same on the bike, regardless of the bikes speed.

This allows the rider to use less pressure on the bars to make corrections and keep himself balanced (hence less wobbling at speed).
Less movement, but more pressure is required, the gyroscopic forces in the front resist turning forces applied at the handlebars.

As I said the bike is always falling to the left or right, the rider has to make small corrections constantly to remain upright.
The steering geometry's trail effect do these small corrections for you (see my other posts). You can push a bike and let it coast by itself, and it will remain upright within a speed range (too slow and there's not enough correction force, too fast (100+mph probably) and other issues come into play, like underdamped overcorrection (speed wobble).
 
  • #28
rcgldr
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counter rotaing wheels
Huge loads on the axis of each wheel, but they counter each other, and the net result is no gyroscopic resistance to a rotation of the common axis. Counter rotating props on planes eliminate pitch - yaw coupling.
 
  • #29
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Jeff Reid said:
The bike turns because the front tire points in a slightly differnt direction than the rear. There's some camber thrust effect, but not much. Motorcycle tires don't all have circular cross sections, some older ones (Dunlop K81's) had high narrow domes with nearly flat and very steep sides. Modern Dunlop tires generally have near flat centers (for longer wear), and rounded sides. Bridgestones are pretty close to a circrular arc. You feel the difference. The Dunlops flatter profile resist the initial lean on a motorcycle a bit, transition from a Dunlop to a Bridgestone and it feels like the back end is slipping out from underneath you (no lean resistance, so it takes less countersteering). Racing tires are generally like the Bridgestones, close to a circular arc.

The tires also have a bit of a overhanging lip at the edge of the tread above the sidewall, that allows the contact patch to flex a bit on the edge for better grip (only racers probably notice this).
The bike turns because of camber thrust my friend, period. The camber thrust is a result of the the bars being turned in a direction, or as you say both wheels pointing different directions. So the result of having both weheels pointing a different direction is camber thrust which results in the bike turning.

Here is one page that describes the physics behind it http://www.tonyfoale.com/Articles/Tyres/TYRES.htm . Even a tire with a more square profile like a car will deform as the bike is leaned and produce the same effect, ie; the contact patch still has a longer distance toward the middle.

Oh and suzuki wasn't the only ones to use a smaller front wheel. A lot of the ninjas, including the very popular ninja 900 had 16in front and 18in rear wheels. Possibly some of the honda interceptors had them too. I also know for a fact that some of these bikes were used in racing.
 
  • #30
rcgldr
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The bike turns because of camber thrust my friend, period.
Read that article again, it turns due to a combination of slip angle and camber thrust. It maybe mostly camber thrust, but not all camber thrust. There will be combinations of grip, speed, and cornering radius where it can be all camber thrust, but this not the typical case.

The article also assumes that slip angles are the result of tire slippage, which is incorrect. Tires flex when cornering, and it's this flexing that accounts for most of the slip angle, with only a bit of slippage with typical cornering forces. The slippage component comes into play with high g cornering. Both flexing and slippage reduce the amount of camber thrust, requiring more slip angle to compensate for the reduction in camber thrust.

Here's a quote from that ariticle:
When cornering at an angle of 45° and 70mph. the turn radius will be 327 ft. and at half that speed, 35mph., the radius will be a quarter of that or 82 ft. --- but as shown earlier the cone radius will be only 1.5 ft.
The camber thrust is trying to produce a radius to match the cone radius of 1.5 ft, however the actual radius is 327 ft at 70mph, 82 ft at 35mph, so camber thrust may be helping, but there's a huge difference between 1.5 ft and 327 ft, obviously camber thrust alone isn't doing much in these cases. At higher g forces corners, it's the slip angle than counts.

Oh and suzuki wasn't the only ones to use a smaller front wheel. A lot of the ninjas, including the very popular ninja 900 had 16in front and 18in rear wheels. Possibly some of the honda interceptors had them too. I also know for a fact that some of these bikes were used in racing.
Yes it was a "fad" at the time, and street bikes have been used for racing. However if the F1 (500c 2 stroke GP) racing bikes used this, it wasn't for very long.
 
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  • #31
rcgldr
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more on camber thrust
The profile of a tire reduces the camber thrust affect. Because of the circular shape, when leaned over at a certain angle, there's virtually no camber thrust, because the inside (higher up on the tire profile, smaller radius) is moving slightly slower than the outside (lower down on the tire profile, with a larger radius). For a range of specific speeds, there is an lean angle where no camber thrust effect occurs at all.

Dunlop made some high dome tires (K81, K181) with steeply sloped almost flat sides on the tire profile. The goal was to provide a larger contact patch when cornering, and a skinnier one when going straight. This is a profile that reduces camber thrust (it may be negative camber thrust at some lean angles).

Dunlop also makes tires with the opposite profile for most of it's street tires, a bit more contact patch while going straight to reduce tire wear. Bridgestones and most racing tires are near circular arcs. For racing bikes tire size is used to adjust contact patch area. The soft compounds flex enough to increase contact patch area and also reduce camber thrust, but the camber thrust reduction wasn't a goal, just a consequence of a stickier, softer tire that flexes more.
 
  • #32
rcgldr
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This thread didn't last very long.
 
  • #33
rcgldr
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Artman said:
Found a good description of the physics of bicycle riding here:Bike Physics
In this article, he atributes balance to centripital force.
The article attributes balance to steering corrections, steer inwards to reduce lean, (and implying steer outwards to increase lean).
 
  • #34
rcgldr
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Someone asked about this, so I thought I'd bring it up again with this thread.
 
  • #35
krab
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Jeff Reid said:
The article attributes balance to steering corrections, steer inwards to reduce lean, (and implying steer outwards to increase lean).
which is mostly correct when learning to ride a bicycle, since when learning, you are initially at a very slow speed. But to proclaim this and use it to sell training videos... people try to make money off anything these days. Myself and my kids learned very efficiently, with no videos (and no training wheels).
 
  • #36
rcgldr
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krab said:
which is mostly correct when learning to ride a bicycle, since when learning, you are initially at a very slow speed. But to proclaim this and use it to sell training videos... people try to make money off anything these days. Myself and my kids learned very efficiently, with no videos (and no training wheels).
Someone else posted a link to the article. I don't think anyone was suggesting buying a video. Somehow we've managed to learn how to ride bikes without any videos for many years. Part of my point is that bicycles self-correct once they're going anything faster than a slow walk.
 
  • #37
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Can someone explain what exactly the gyroscopic action is?
 
  • #38
rcgldr
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mathlete said:
Can someone explain what exactly the gyroscopic action is?
When a torque force is applied to a rotating mass, the reaction is along an axis perpendicular to the torque force.

Using a helicopter as example, there's a cyclic control that changed the pitch of the blades as they travel around in a circle. If the cyclic is tilted forwards, creating a pitch down torque force, the helicopter instead responds with a roll towards the rearward rotating blades. If the cyclic is tilted sideways towards the forward rotating blades, the helicopter pitches down.

To keep the pilot from getting confused, the cyclic control is shifted 90 degrees to compensate for this, so the pilot can just push forwards to pitch downwards.
 
  • #39
Chi Meson
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If you have ever ridden a bike on rollers, you will find out how very little help "conservation of angular momentum" will be for balance.

As opposed to stationary trainers, rollers are thin barrels that the wheels sit on. The rear wheel spins normally, which turns the rear roller, which is attached to a long band which turns the front roller which turns the front wheel. There is just as much angular momentum as when on the road. The wheels are not kept in place "side-to side" and consequently, a novice bicyclist will fall repeatedly, no matter how fast the wheels are spinning.

The reason why rollers are more difficult than actually riding, is you just can't lean! Leaning will turn the front wheel which will send you 10 inches to the side and off you go!

So angular momentum does connect the lean with the turn of the front wheel, but it does nearly nothing to keep us "gyroscopically" upright.
(Basically, everything Krab has said is Bang on.)
 
  • #40
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krab said:
The gyroscopic effect is important; it allows one to affect a bike's lean by applying steering force. But it is not the effect that explains why it is possible to balance while moving and practically impossible when stationary. There are in fact 3 phases: 1. When stationary, it is hard to balance; 2. When moving too slowly for gyro effects to matter (below a fast walking pace), you tend to meander around while balancing; 3. When moving above a walking pace, it's easy to balance with hardly any meander, one can also ride with no hands.

Experiments have been done where the gyro effect is cancelled by a counter-rotating wheel. It is still possible to ride such a bike.
Another question: let's say we only have a bicycle wheel, a dinner plate or another circular object. If we roll this object pretty slow down the floor, it will very well hold balance. What is the main reason for this? The gyroscopic effect?
 
  • #41
rcgldr
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Tullebukk said:
Another question: let's say we only have a bicycle wheel, a dinner plate or another circular object. If we roll this object pretty slow down the floor, it will very well hold balance. What is the main reason for this? The gyroscopic effect?
It doesn't hold balance for very long. Gyroscopic forces will reduce the rate of lean, but a wheel will lean over until camber thrust counteracts the leaning torque due to gravity pulling downwards at the center of mass, and the upwards force at the contact patch, or until the wheel slips and falls.

I don't know if I mentioned this before, but I can balance a 10 speed for about 10 to 30 seconds without moving. This is because steering the front tire translates the contact patch sideways. Steer left, and the contact patch move right if not resisted. With the front tire on the ground, the contact patch doesn't slide much, so steering left moves the front end left. It's enough movement to balance a bike, but it's difficult. Velodrome racers can remain still for very long times, as this is part of the tactic used to try and get the other racer(s) ahead for those racers that want to start from behind (draft).
 
  • #42
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Let me throw in a controversial twist to this discussion. Someone pointed out that gyroscopic effects plays no part in bicycle's balancing and threw in the counter rotating added wheels argument to back it up. Someone disagreed with him and said if anything the gyroscopic stability will increase. The first guy insisted that angular momentum is a vector hence this cancels out blah blah blah (they lost me somewhere. I agree with whoever say gyroscopic stability will improve, intuitively though cause my phyisic background is wanting. Now to the twist and this is helped by a chap who said motion in itself yield stability and called to meind ice skaters among other wheel-less locomotion.
Mine is simple. Riding a bike a structured perpetual falling to a particular direction much like a satellite in orbit. Take a flag pole and stand it on a flat surface. It is highly unstable (an inveted pendulam-like) Now tip it to fall to one direction. It is extremly had now to make it fall to any other direction. Left to its own divices it will hit at an exact spot. Slowing a bicycle to a stop is analogous to standing the flag pole on its foot again. how about that for food for thought?
 
  • #43
sophiecentaur
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I can't find any comments in this thread, about the effect of castor action and the fact that a line through the top bracket (the steering axis) on all stable bikes, when produced, always meets the ground ahead of the tyre footprint. This will always cause the wheel to steer into a lean. The result, when traveling forward, will be to produce a force on the ground, 'inwards' and a corresponding moment to turn the bike upright. The faster the bike is travelling, the more this effect will be.

Whilst the gyroscopic effect may be significant on morotbikes, it will be very small on light wheeled bikes - particularly with small diameter wheels, whereas the castor effect only depends on the distance between footprint and the forward produced line of the steering axis on the ground. Folding and 'delivery' bikes have very small front wheels but are still stable. It even works on kids' scooters with plastic wheels of less than 10cm diameter where angular momentum is extremely small.
There is more to this than just one mechanism at work, I'm sure.
 
  • #44
rcgldr
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I can't find any comments in this thread, about the effect of castor action.
Both Krab and I mention this effect (trail), starting with post 21 in this thread.
 
  • #45
A.T.
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gyroscopic effect ... castor effect ...

There is more to this than just one mechanism at work, I'm sure.
There is more indeed. Guys from the TU Delft build a "bike" that eliminates the gyroscopic effect and the castor effect.... and it still balances itself:

 
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  • #46
sophiecentaur
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That link is good and very inventive in its thesis. (I wonder how many takes they did to get that bike to stay up so well) At least it puts to bed the gyro theory, which applies to very few cases and which, I think only applies a damping / reactive force rather than a restoring force. If the gyro action were actually to bring the bike upright, would not the force be downwards again - by the same precession argument- as the rotation would then be the other way? Once a bike has leant into the curve, after the (truly) restoring couple is there until it is actually upright again.
"Turning into a fall" is a very good way of putting things; both "trail" and castor action, will achieve this. Their model has this forward pointing rod, which achieves the same thing. BUT how many of the bicycles we see on the road are loaded that way?

They do not dismiss trail as a mechanism so they are not disagreeing with my contention that it is due to trail on 'real bikes'. They just achieve the 'leaning in' by a different mechanism.
Funny thing is that I tried a 'butcher's delivery' bike, once. That had a large load out over the small front wheel. It was a real mother to ride.
 
  • #47
rcgldr
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Guys from the TU Delft build a "bike" that eliminates the gyroscopic effect and the castor effect.... and it still balances itself.
As explained in that video, the front wheel was weighted to produce the equivalent of trail effect, using weight distribution to cause the front end to "fall inwards" more than the bike. I would have liked to see a true "2 skate bicycle" being tested on a ice rink, to show gyroscopic forces are not required.

TU Delft also ran into a conflict between their math and their testing of an actual bicycle regarding capsize speed, I don't know if they've since resolved the issue. Link to link to article, showing image of bicycle:

http://www.tudelft.nl/live/pagina.jsp?id=95c52a8b-37c2-4136-ad98-97aea768d9b7&lang=en&binary=/doc/Koo06.pdf [Broken]

page 4 of this article includes a graph where the upper limit of the "stable" range is just below 8 m/s = 28.8 kph:

http://home.tudelft.nl/fileadmin/UD/MenC/Support/Internet/TU_Website/TU_Delft_portal/Onderzoek/Wetenschapsprojecten/Bicycle_Research/Dynamics_and_Stability/doc/Koo06.pdf [Broken]

link to treadmill video where 30 kph is described as "very stable", even though it's greater than the 28.8 kph end of the "stable" speed range from the graph in the artitcle linked to above. My guess is this is due to the fact that the tires are not infinitely thin disk, and when leaned, the fact that the contact patch is on the side of the tire results in a outwards torque that keeps the bike from falling inwards as predicted by the capsize speed shown in the graph.

http://www.tudelft.nl/live/pagina.jsp?id=0cc5c910-a1ee-40a8-92cb-bf4a2ac54bd0&lang=en [Broken]
 
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  • #48
sophiecentaur
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Is there any more need to 'prove' that gyro forces are irrelevant?
 
  • #49
A.T.
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At least it puts to bed the gyro theory,
It doesn't. It just says that there is more to it, and that there are others ways to achieve self stability.

If the gyro action were actually to bring the bike upright, would not the force be downwards again - by the same precession argument-
Sounds like you simply don't understand the "gyro theory" here. The precession is not supposed to bring the bike upright directly, it merely turns the front wheel into the direction in which the wheel falls over.

200px-Gyroscope_wheel-text.png


Here a more complete explanation of the different theories:


"Turning into a fall" is a very good way of putting things; both "trail" and castor action, will achieve this.
So will the gyro effect. But what is the difference between "trail" and "castor action" again?

They do not dismiss trail as a mechanism so they are not disagreeing with my contention that it is due to trail on 'real bikes'.
On real bikes the gyro effect also plays a role.
 
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  • #50
rcgldr
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"gyro theory" here. The precession is not supposed to bring the bike upright directly, it merely turns the front wheel into the direction in which the wheel falls over.
From what I recall, although precession turns the front tire in the direction of lean, at some speeds, the precession effect produces insufficient turn in to correct the lean, so trail is needed as well. Trail alone without gyroscopic effect can be enough to correct a lean angle.

What is the difference between "trail" and "castor action" again?
Trail is the distance from where the pivot axis intercepts the ground back to the contact point between wheel and ground. Trail is normally used to refer to what happens when you lean a castored wheel (the wheel turns in the direction of the lean). Normally castor effect refers to the tendency of a vertical castored wheel to pivot away from the direction of motion so it lines up the wheel with the direction of travel.
 
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