Date: Around 1993
If you are on a motorcycle going down the highway, and you press
on the inside of the left handle grip, why do the wheels turn to the right,
but you turn to the left?
1991: This only happens, by the way, above a certain speed. If you are
going slow enough, the bike will turn in the direction you would expect. The
reason the motorcycle behaves as you describe at speed is due to a phenomena
called precession. For a thorough explanation of precession, I recommend you
look it up - it is a fairly complex subject. You can observe precession with
a gyroscope. I will give you a quick explanation, though. When you try to
turn a rotating object, you produce a force at 90 degrees to the direction of
rotation. When you push on the left handlebar and turn the front wheel of the
bike to the right, you produce a force which makes the whole motorcycle lean
to the left. It is the leaning to the left that turns the bike.
Gyroscopic precession is one of two factors that cause the cycle to turn opposite the
direction the handlebars are moved. A great explanation of precession is given in a
book called "Thinking Physics" by Lewis Epstein. However, the precessional force
acting to tilt the wheel left when you move the handlebars right, is diminished
by the fact that the front forks are not vertical, but angled forward. To exaggerate
this, imagine that the front forks were completely horizontal. Turning the handlebars
right would simply cause the wheel to tilt right, but precession would cause the
wheel to try to angle right as viewed from above. Here now, would be a tendency for
the front wheel to track to the right, as wanted. This fact is apparent in choppers
which have long front forks. A second major force occurs when the wheel is deflected. When a
cycle leans left or right, it tilts about a pivot point where the tires
contact the road. If the wheel is forcefully deflected to the right, the wheel will
immediately begin tracking to the right (slightly) even though precession
makes the wheel lean left. As the cycle begins this slight change in direction (to the right),
centrifugal force immediately throws the bike and rider to the outside of the turn,
making it tilt about its pivot point, and lean to the left. Once you lean left,
you're going left.
Winter 2009-2010 Up-date
The respondent does not answer the question correctly. The question asks why the
motorcycle turns to the left when the front wheel is turned to the right.
A little background: first, motorcycles travel at speeds from low (gyroscopic
forces are small) to high (gyroscopic forces are large). Most riding is conducted
somewhere in the middle where gyroscopic forces influence steering, but not hugely
At low speed, roughly speaking, there are three motorcycle motions that can be
understood easily: a motorcycle can move along upright and straight, it can turn
in a smooth arc or circle while maintaining a constant lean angle, or it can fall
1) If a motorcycle is to continue in a straight line upright, it is necessary for
the center of mass to remain directly above where the tires meet the road.
2) If a motorcycle is leaning and moving along a smooth circular path, it is
necessary for the "sideways" forces to balance: the tilt of the motorcycle creates
a centripetal force, which provides the inward (toward the center of the circle)
acceleration to keep the motorcycle moving in a smooth circle. Bicycles must do
this too, as must people sprinting around a circular running track, they must lean
into the turn. For a constant curve to the left, the motorcycle must lean to the l
eft so that gravity acting through the center of mass exerts a force toward the center
of the curve.
3) If forces are not balanced then the tilt or lean of the motorcycle is unstable
and the motorcycle will fall over. If a tire loses traction, a motorcycle or bicycle
can fall over in a fraction of a second.
At low speeds, if the left handlebar is pushed, the front wheel does indeed turn to
the right a small amount, and the contact where the tires meet the road move to the
right. But the center of mass of the motorcycle continues along pretty much the same
line and thus the motorcycle starts to lean to the left. This is good: a left lean
allows it to travel in a curve. If the handlebar is continued to be pushed hard, the
lean of the motorcycle will get larger and larger and the motorcycle will slam to the
ground on the left side. Otherwise the motorcycle will be leaning to the left and
continue a smooth turn to the left.
(At this point, a push of the right handlebar turns the front wheel to the left, the
tire contact patch moves from the right toward the center of mass of the motorcycle,
and the motorcycle "straightens up.")
In addition to these simple balancing forces, there are gyroscopic forces present due
to the rotation of the wheels and engine parts. These gyroscopic forces influence the
effort required to turn the handlebars, and also influence how the motorcycle tilts or
leans or falls over when it is unbalanced. The gyroscopic forces become large at high
highway speeds, where it is necessary to push the handlebars very hard to execute fast
left and right turns by leaning the motorcycle fast left an fast right.
The principle is the same though, but exaggerated: at high speeds, due to gyroscopic
forces and the geometry of the fork suspension, a motorcycle will tend to be stable on
two wheels. The motorcycle must still lean in the direction that it is curving.
Considerable effort on the handlebars is required to lean (or un-lean) the motorcycle
moving at high speed to make it go in the desired direction.
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Update: June 2012