balance

david said:

Can somebody please explain why a moving bicycle is easier to balance
than a stationary one.

Because when it's moving you don't have time to think about keeping it
upright.

HTH!

david said:

Can somebody please explain why a moving bicycle is easier to balance
than a stationary one.

Maybe you can work this out for yourself!!

Think about - what is the difference between a moving bike and a
stationary one (ie. what two big round things are doing something
different in each case??). That might start you on the right track,
yes?? 😉

Cheers,
Abby

david said:

Can somebody please explain why a moving bicycle is easier to balance
than a stationary one.

Take off the front wheel and hold the wheel by the axle. Then start it
spinning. Now try to turn the axle sideways 90 degrees , as it would do
when falling over when fixed to the bike.

See how hard it is.

Also when riding you have the opportunity to keep shifting the point of
contact on the road to keep it under the center of gravity. ie, you can
weave around slightly, or a lot.

david said:

Quoted message said:

Can somebody please explain why a moving bicycle is easier to balance
than a stationary one.

Momentum is an additional force to keep you balanced.
Momentum forces that hold you up in corners are in order of magnitude
are acceleration, braking, and coasting (from pure velocity and
momentum alone).
Too bad you can't spin in hard corners for acceleration, or you will
hit pedals on pavement.

So without accleration or braking forces, you have to use body lean
with bike more erect, versus inline bike (/and body) lean which is most
efficient if going fast enough. Only good reason for compact cranks
other than phsical size constraints, but larger cranks are better
leverage and leg muscle extentions most of the time spinning.

david said:

Can somebody please explain why a moving bicycle is easier to balance
than a stationary one.

Actually, it is impossible to balance a stationary bicycle.

david said:

Can somebody please explain why a moving bicycle is easier to balance
than a stationary one.

See http://sheldonbrown.com/brandt/gyro.html, but that mainly refutes
the gyroscopic theory rather than describing what the real mechanism is.

You can also search the archives of r.b.t. Below is an explanation I
gave to someone before, but I'm going to repeat it since I'm no longer
so sure how correct it is.

Quoted message said:

I think it's because the bike has negative caster, which gives it
directional instability, but keeps it upright.

Quoted message said:

If you lean a bike to the left while wheeling it slowly along holding
onto the saddle, you will see that the front wheel steers to the left.

Quoted message said:

This is because the contact patch is in front of the steering pivot
(negative caster).

This I think is false, the contact patch I have since learned is behind
the the projection of the steering pivot onto the ground. So is it true
to say that a bike has "negative caster"? Usually people talk about
"trail" in the context of bikes, not caster. I'm a bit unclear about all
this.

I think the key thing is the way the fork "flops" as the bike is leaned
to the side, something that's not applicable to a four-wheeled vehicle.
If you built a car with two bicycle head tube/fork assemblies at the
front joined by a steering rack, I suspect it would have self-centering
positive-caster steering since the projection of the steering axis onto
the ground is behind the contact patch.

Quoted message said:

Uncorrected, the bike goes further and further off course. This is
what I mean by "directional instability". Cars have postive caster
(contact patch behind steering pivot) which is why their steering
self-corrects-- you can turn the car, and let go of the steering
wheel, and it winds back to the centre position by itself.

Quoted message said:

Back to bikes. If a bike is going round a left-hand bend, its centre
of mass is naturally thrown out to the right. This effect is commonly
called "centrifugal force".

Quoted message said:

So, bike leans to the left, that makes it steer left, which makes it
lean right again, correcting the original left-lean. That's the basis
of the stability.

Quoted message said:

If you ride a bike through a puddle, and then continue slowly, and
then look at the wet trail made by the tyres, you see a straight
footprint made by the rear tyre, and usually a sort of sinusoidal one
crossing backwards and forwards over it made by the front tyre. These
are the small left-to-right corrections of the steering that are
keeping the bike upright.

Quoted message said:

I have also heard that bikes have been built with counter-rotating
flywheels attached to the wheels. The flywheels are smaller in
diameter but have the same moment of inertia as the wheels. They
should counteract any gyroscopic effects. The conclusion of the
experiment was that the bikes were no harder to ride than normal.

Quoted message said:

I do not have a link to this experiment though.

david said:

Can somebody please explain why a moving bicycle is easier to balance
than a stationary one.

See:
http://sheldonbrown.com/brandt/gyro.html

Art Harris

ddog said:

david said:

Quoted message said:

Can somebody please explain why a moving bicycle is easier to balance
than a stationary one.

Momentum is an additional force to keep you balanced.

Momentum isn't a force.

I will attempt to explain.

Momentum is the product of force and velocity. A speeding bullet has a
certain momentum. The same speed and a heavier bullet has more momentum;
so does the same bullet at a higher speed.

Impulse is the product (integral strictly) of force and time. If you
give something a knock with a hammer, or with a puff of wind, you apply
a bit of force for a bit of time, which adds up to an "impulse".

Newton's second law of physics tells us that a given impulse will
produce a given change of momentum.

Note that momentum has direction as well as magnitude (which is why it
is often represented with a vector). In other words, if the impulse hits
the bullet exactly head on, it will change the magnitude of its velocity
but not its direction. If it hits the bullet exactly from the side, the
bullet's velocity will change direction only and not magnitude. From any
other angle, you will get a bit of both.

You can think of sources of instability as random impulses-- puffs of
wind, perhaps collisions with small particles in the air.

If the instability impulses are the same, then a speeding bullet will
effectively be perturbed less the higher its momentum. This is just
because the change of momentum due to each impulse will be small
compared to the momentum the bullet has to start with. Each impulse
still results in the same change of momentum.

This kind of effect will happen with a bicycle as well-- the faster or
heavier you are the less you will be affected by puffs of wind, road
imperfections, etc. But they aren't enough on their own. Bicycles are
not ballistic-- you maintain constant control of them at all times (well
so you hope), and you can ride a bike for hours and hours and arrive
back at your front door, exactly on target. More like a guided missile
than a bullet.

As a bike leans to the left, the arrangement of the steering pivot makes
it steer to the left. Steering to the left makes it lean back up to the
right (going round a left-hand curve always throws you to the right--
centrifugal force), correcting the initial error to the left. This gives
us a certain amount of stability, and with the addition of a rider, the
bike can be controlled. I believe this is the basic mechanism, but keep
an eye on this thread because I could be wrong.

Dan said:

david said:

Can somebody please explain why a moving bicycle is easier to balance
than a stationary one.

Actually, it is impossible to balance a stationary bicycle.

Oops. I guess this guy didn't get the memo. While he does bounce a lot
of the time, there are instances where he is truly stationary.

<http://video.google.com/videoplay?docid=-8727079186070973755&q=bike+trials>

not the best example...I'm still looking the video of a French trials
rider standing still on a boulder for ~30 sec as he planned his line.

then if you want to count track stands on fixed gear bikes, then a lot
people (even yours truly) can remain standing while only moving ~5 cm
back and forth for pretty much as long as we need to. I should also
note that riders more skilled than myself can do this on freewheeled
bikes (two buddies can do it on flat terrain!)

\\paul

Paul Hobson said:

Dan said:

david said:

Can somebody please explain why a moving bicycle is easier to balance
than a stationary one.

Actually, it is impossible to balance a stationary bicycle.

Oops. I guess this guy didn't get the memo. While he does bounce a lot
of the time, there are instances where he is truly stationary.

<http://video.google.com/videoplay?docid=-8727079186070973755&q=bike+trials>

<http://video.google.com/videoplay?docid=-8551222727971975773&q=bike+trials>

maybe that one's better?

Paul Hobson said:

Dan said:

david said:

Can somebody please explain why a moving bicycle is easier to balance
than a stationary one.

Actually, it is impossible to balance a stationary bicycle.

Oops. I guess this guy didn't get the memo. While he does bounce a lot
of the time, there are instances where he is truly stationary.

<http://video.google.com/videoplay?docid=-8727079186070973755&q=bike+trials>

<http://video.google.com/videoplay?docid=-8551222727971975773&q=bike+trials>

maybe that one's better? (skip ahead to 2 min into the video)

david said:

Can somebody please explain why a moving bicycle is easier to balance
than a stationary one.

Dear David,

The common but mistaken theory is that the two wheels act as
stabilizing gyroscopes. Tipping the axles in any direction to fall
over requires overcoming their angular momentum.

You can feel the angular momentum by taking a front wheel out of the
fork, holding it by the axle, and spinning it--the axle will resist
your efforts to tip it. It's acting as a large, slow gyroscope.

The angular momentum helps, but it hardly explains why any clumsy oaf
(like me) can easily balance a bicycle at absurdly low speeds, with
the wheels barely turning and their angular momentum obviously next to
nothing. As soon as you push off from a standing start, you're able to
balance a bicycle.

The trick is that you keep moving the front tire's contact patch back
and forth to keep it under your center of gravity.

If the tire is moving, it's easy to move it under your center of
gravity.

If the tire isn't moving--well, you can't move a tire if it isn't
moving, can you?

This is why snow bikes with skis instead of wheels work. Obviously,
the skis have no angular momentum. The rider just keeps steering the
large front contact patch back and forth under his center of gravity.

A unicycle's tire can move, but two problems make it much harder to
ride. First, there are no handlebars to give you control. Second, the
tire moves so easily that you have to learn how to stop it from
squirting out from underneath you.

To balance a motionless bicycle, the first trick is to [censored] the front
wheel to one side. If you watch riders doing trackstands on fixed gear
bikes, most of them are cheating by rocking back and forth very
slightly. Even the tiny movement makes it much easier to move the
cocked front wheel from side to side.

(If you have a normal freewheeling bike, practice on a faint uphill,
which will let you rock back and forth against the pedals. Lower tire
pressure also helps by giving a slightly broader base.)

Really good riders can balance on two motionless wheels--they shift
their weight so delicately that they never go out of balance enough to
need to move the contact patch.

Fantastic riders can balance sitting down, no hands, on two motionless
wheels.

Really annoying acrobats can balance standing on the seat, or doing a
single-arm handstand on the seat.

Cheers,

Carl Fogel

Quoted message said:

david said:

Can somebody please explain why a moving bicycle is easier to balance
than a stationary one.

Dear David,

The common but mistaken theory is that the two wheels act as
stabilizing gyroscopes. Tipping the axles in any direction to fall
over requires overcoming their angular momentum.

You can feel the angular momentum by taking a front wheel out of the
fork, holding it by the axle, and spinning it--the axle will resist
your efforts to tip it. It's acting as a large, slow gyroscope.

The angular momentum helps, but it hardly explains why any clumsy oaf
(like me) can easily balance a bicycle at absurdly low speeds, with
the wheels barely turning and their angular momentum obviously next to
nothing. As soon as you push off from a standing start, you're able to
balance a bicycle.

The trick is that you keep moving the front tire's contact patch back
and forth to keep it under your center of gravity.

If the tire is moving, it's easy to move it under your center of
gravity.

If the tire isn't moving--well, you can't move a tire if it isn't
moving, can you?

This is why snow bikes with skis instead of wheels work. Obviously,
the skis have no angular momentum. The rider just keeps steering the
large front contact patch back and forth under his center of gravity.

A unicycle's tire can move, but two problems make it much harder to
ride. First, there are no handlebars to give you control. Second, the
tire moves so easily that you have to learn how to stop it from
squirting out from underneath you.

To balance a motionless bicycle, the first trick is to [censored] the front
wheel to one side. If you watch riders doing trackstands on fixed gear
bikes, most of them are cheating by rocking back and forth very
slightly. Even the tiny movement makes it much easier to move the
cocked front wheel from side to side.

And with practice you can reduce this angle to almost nothing, so it
appears that you are just standing there.

Quoted message said:

(If you have a normal freewheeling bike, practice on a faint uphill,
which will let you rock back and forth against the pedals. Lower tire
pressure also helps by giving a slightly broader base.)

After a while the incline is no longer needed and a small pebble or
crack in the road will be enough to provide the needed resistance. If
you find yourself pointed down a slope, all this can be accomplished by
careful control of the front brake such that you sort of bounce against
it. IMO mountain bikes, soft tires, and suspension forks make it
harder.

Quoted message said:

Really good riders can balance on two motionless wheels--they shift
their weight so delicately that they never go out of balance enough to
need to move the contact patch.

And if a gust of wind comes along, they can always resort to the above
techniques.

Quoted message said:

Fantastic riders can balance sitting down, no hands, on two motionless
wheels.

With the steering all the way to the side I presume. This is a trick I
use to wow the kids down at the LBS so I can get free access to their
truing stand!

Quoted message said:

Really annoying acrobats can balance standing on the seat, or doing a
single-arm handstand on the seat.

No motionless balancing here, but it is safe to say he would have no
problems:

Joseph

In article <[email hidden]>,

Wilfred said:

david said:

Can somebody please explain why a moving bicycle is easier to
balance than a stationary one.

Take off the front wheel and hold the wheel by the axle. Then start
it spinning. Now try to turn the axle sideways 90 degrees , as it
would do when falling over when fixed to the bike.

See how hard it is.

Gyroscopic forces are not what keeps a bike upright.

In article <[email hidden]>,

Absent Husband said:

david said:

Can somebody please explain why a moving bicycle is easier to
balance than a stationary one.

Maybe you can work this out for yourself!!

Think about - what is the difference between a moving bike and a
stationary one (ie. what two big round things are doing something
different in each case??). That might start you on the right track,
yes?? 😉

You are on the wrong track.

In article <[email hidden]>,

Ben C said:

david said:

Can somebody please explain why a moving bicycle is easier to
balance than a stationary one.

See http://sheldonbrown.com/brandt/gyro.html, but that mainly refutes
the gyroscopic theory rather than describing what the real mechanism
is.

Jobst really should rewrite this FAQ entry as it does not actually
answer the question of "what keeps the bike upright?" It is at best a
partial answer (refuting the common belief that the bike is kept upright
by gyroscopic forces) and contains at least one major inaccuracy
(regarding the relative location of the contact patch to the steering
axis).

In article <[email hidden]>,

Dan said:

david said:

Can somebody please explain why a moving bicycle is easier to
balance than a stationary one.

Actually, it is impossible to balance a stationary bicycle.

It's not impossible. It's possible to balance a motionless bicycle for
a few seconds. People think of track standing as "stationary" but of
course it is not.

"david" <[email hidden]> wrote in message
news:[email hidden]...

Quoted message said:

Can somebody please explain why a moving bicycle is easier to balance
than a stationary one.

There's a Brit named Newton you should ask. Wait, he's been dead for
several hundred years.

Quoted message said:

david said:

Can somebody please explain why a moving bicycle is easier to balance
than a stationary one.

Dan said:

Actually, it is impossible to balance a stationary bicycle.

Tell that to the messengers poised at the stoplight. I can see them now
out my window.

--
Andrew Muzi
www.yellowjersey.org
Open every day since 1 April, 1971