Re: Gyroscopic forces of motorcycles and other esoteric stuff?


cavelamb wrote:
Can anyone explain the physics of this a bit mor formally.
In relatively small words?

Steering right to go right is *direct steering*. Steering left to go
right called "countersteering".

To start with, a motorcycle is unstable in the roll axis. If you get
off, let go of the handlebar, and walk away, the motorcycle will fall
over. So, hold onto those bars or put a foot down if you're not moving,
unless you get good at using the handlebars to keep pushing the machine
upright while you have both feet on the pegs.

This drill tends to explode the theory that gyroscopic effect has
*anything* whatever to do with motorcycle stability. There's NO
gyroscopic effect if the wheels aren't turning!

The front tire contact patch is what's keeping the motorcycle vertical,
as you apply pressure to the handlebars to push it against the
pavement.

The initial heavy feel in the steering as you start rolling is due to
the *rake* of the front forks. Rake is only one of the three factors in
the steering geometry of a motorcycle. Rake is the same as *castor*, or
the angle of tilt of a car's king pin from the vertical.

A car that has 5 degrees of castor in the kingpins is really hard to
steer at low speeds. Ask anybody who ever tried to parallel park a
Triumph TR-4A sportscar.

But, to steer a car right, you turn the wheel to the right, just as you
turn the motorcycle's handlebars to the right when you're riding at low
speed and want to go to the right.

While you are riding a motorbike down the road at a speed of more than,
say 5 tp 7 mph, the front tire contact patch is normally trying to keep
the motorcycle chassis, from the steering head back, from flopping over
to the right or the left.

The force generated by the front tire contact patch is multiplied over
a short distance of about 3 to 5 inches. This distance is called
"trail".

Trail is the second factor of a motorcycle's steering geometry. The
third factor is steering stem offset from the raked axis of the fork
tubes. If the fork tubes are placed
an inch or so forward of the steering stem axis, this reduces trail by
an inch or so and makes the machine steer easier.

Let's take two hypothetical motorcycle steering geometries and relate
them to wheelbase length.

A. This motorcycle that has 3 inches of trail will generally have the
forks raked at about 24 or 25 degrees from the vertical. It will
generally have a short wheelbase (the distance between front and rear
axles) of about 52 to 54 inches.

B. This motorcycle that uses 4.5 to 5.0 inches of trail will often have
the forks raked to 27 to 29 degrees from the vertical. The wheelbase
will be longer, from 56 to 60 inches.

Motorcycle A will go around a tight corner without leaning as far over
as motorcycle B.

Motorcycle A feels light and nimble at all speeds, motorcycle B feels
heavier steering and has to be leaned over further to go around the
same curve. Motorcycle A usually runs away from motorcycle B in tight
twisties.

But, let's get back to the mystical gyroscopic effect. When *does*
gyroscopic effect come into play, if it isn't doing anything at all
when the wheels aren't turning?

There are two effects we want to look at. The first is gyroscopic
precession, and the other is is angular momentum.

Take your right hand and hold your thumb up and back at an angle of
about 25 degrees. Point your index finger staight forward, and point
your middle finger to the left.

Your thumb is the steering axis. Your index finger is the direction of
travel of the motorcycle, but it's also the yaw and roll axis of the
machine. Your middle finger is the front axle, which is also parallel
to the handlebars.

So, let's countersteer this three fingered model into a bank to the
right. If your middle finger is the same as the handlebars, pulling it
back towards you is the same as pushing the right hand side of the
handlebars forward.

Notice what happens to your index finger. It *yaws* a little off to the
left, but your index finger and thumb *roll* toward the right,
countersteering your hand into the right turn!

The yawing of the motorcycle actually happens when you countersteer.
The front wheel tries to *out track*, heading the machine off in the
*wrong* direction as you countersteer to the desired bank angle.

When you remove the countersteering force, the front wheel
automatically realigns itself with the rear wheel and tries to
stabilize the chassis into the desired bank angle.

Now, what does that have to do with gyroscopic precession? If your
front wheel is a gyroscope because it's rotating, whenever the front
tire is not in contact with the pavement, the wheel will *precess* in a
manner *exactly* like countersteering the motorbike into a turn.

So, gyroscopic precession is working in the same three dimensional axes
as countersteering. It's working with you. No problem, if you're not in
a speed wobble or speed weave at that moment. I went into speed wobbles
and speed weaves in other posts.

The last issue is angular momentum, or *flywheel effect*. Flywheels
that store energy between the pistons firing on your car are large
diameter disks. So are the wheels of your motorbike, effectively. The
larger diameter the wheel, the more mass is distributed at a greater
distance from the axis of rotation of the wheel.

And, besides mass at a distance from the center of rotation, the
*other* factor in angular momentum is the speed of rotation. Engineers
don't rate this rotation in revolutions per minute, they do it in
*radians per second*.

If you rotate a wheel at *twice* as many radians per second, you'll get
*four* times as much angular momentum from the wheel.

So, if you're going 60 mph and you feel a certain resistance to
steering the motorcycle, you'll feel four times as much resistance to
steering when you're going 120 mph.

If you want to see the formula for angular momentum, google up
"wikipedia" +"flywheel". This NG doesn't do Greek letters, so I won't
try to cut and paste them.

So those are the two times when the gyroscopic effect comes into play.
When the tires are off the ground and when the motorcycle is going very
fast.

But there's NO gyroscopic effect coming from the chassis while the
motorcycle usn't moving.

If you wanted a build a motorcycle that would just stand there
motionless without the side of centerstand down, you could install a
gyroscope in place of the engine and spin it up around 250,000 RPM and
it would just stand upright, magically.

Until the Earth rotated underneath it...

.

Relevant Pages

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