Motorcycle and Bicycle Crash Analysis- Wobble, Weave and Capsize
28th August 2011 by John Sedgewick
Motorcycle and Bicycle Crash Analysis – Wobble, Weave and
Capsize
Motorcycles and bicycles stay upright for the same reasons,
so they also crash for the same reasons. This is true despite their difference
in weight and the difference in the speed at which they commonly travel. Although
the rule holds true for crashes which involve two wheeled cycles and other participants such as pedestrians
and other drivers, in this discussion we are talking only about crashes that
involve a bike, unaffected by the actions of people other than the rider and
any passenger who may be aboard.
The most simple form of bike crash is called a capsize. It
happens when the center of gravity of the bike and rider get so far outside of
the contact points between the tires and the ground that the operator cannot
overcome the force of gravity. A capsize is really just a form of falling over
sideways. As simple as it is, understanding the basics of the capsize is
necessary to understanding the more complex forms of crash.
The three critical components of the capsize are the two
places where the tires meet the road and the one single point where the center
of gravity of the rider/bike combination is located. Starting with a non-moving
two-wheeled bike standing straight up with a single rider, it is easy to
visualize these three components. The places where the tires meet the road will
be patches that are roughly square. They are where tires are slightly squashed
against the pavement, and are roughly square because gravity pulls the bike and
rider down and the compressed air pumped into the tires is, by design, not
powerful enough to maintain perfect roundness of the tires. The force of the air
in the tires makes them hard enough to avoid going entirely flat, but soft
enough to provide a cushioned ride, so the tires go slightly flat and are
slightly spread out on the road. Theoretically, the two contact patches are of
equal size, as both tires share the burden of holding everything up. However,
assuming that the tire pressures are equal but the rear of the bike is loaded
more heavily than the front, the rear contact patch is probably a little bigger
than the front one. Also, assuming that the combined weight of the bike and
rider is evenly distributed to the left and right, the tire/road contact patches
will be centered on the front-to-back centerline of the bike.
Having visualized the two tire/road contact patches, the
center of gravity is remains to be considered. The center of gravity is the point around
which the combined bike/rider would balance in every direction if the
combination could be supported there (which can only occur in theory). To visualize
it, consider any two-wheeled bike sitting on a flat roadway and viewed from the
side. The left/right position of the center of gravity will be close enough to
the middle of the space between the two wheels (or road contact patches) that
there is no chance of the bike tipping over frontwards or backwards (not so
with a unicycle!). Considered from the front or rear, the center of gravity of
this bike will be directly above the bike’s front-to-back centerline (if you
suspended a lead weight from the center of gravity, it would hang straight down
to the left/right centerline of the bike). As long as gravity is pulling the
rider/bike straight down on the front-to-back centerline of, the gravitational
pull will be aligned with and resisted by the two road/tire contact patches and
the bike will stay upright. If the rider leans left, he or she moves the center
of gravity to the left of the front-to-back centerline of the road/tire contact
patches, and the contact patches can no longer overcome the pull of gravity.
The rider will have to put his left foot down, or the bike will fall over in a
capsize.
Beyond a bike capsize from a stationary position, the
mechanism of single bicycle and motorcycle crashes get more complicated fast. The
same three components, two road/tire contact patches and the center of gravity
of the bike/rider, are the critical factors, but there is a lot of changing
going on with a moving bike.
For example, when the bike is still, the shape and size of
the road/tire contact patches does not change. When the bike is in motion, it
changes constantly. When the bike accelerates, the load on the front wheel
drops (which is what makes the “wheelie” possible). As the load on the front
wheel drops, the rear contact patch gets bigger and the front contact point gets
smaller (until, in a wheelie, there is no front contact patch at all). When the
bike leans to the right, the contact patches move to the right (part way up the
sidewall, similar to what happens when a sailboat heels- that part of the hull
which is supports a heeling boat is different than that which supports a boat at
rest). The angle of lean (or heel) and the front/back weight distribution are
just two things that affect the size and shape of the contact patch. Wind and the
hardness, flatness, roughness, and stickiness of the road surface, which also
vary constantly, all change the shape of the road/tire contact patches and
affect the stability of the bike.
Wobble and weave are two specific forms of instability which
are characteristic and virtually unavoidable in two wheel bikes and
motorcycles. They will be explained in more detail later.
Few motorcyclists or bicyclists think about or understand
the dynamics of capsize, wobble or weave, but they all have an instinct as to
how to manage them to avoid crashing. When a crash does occur, an engineer or
other expert may be called upon to re-create or reconstruct the events leading
to the crash. Did the bike go down strictly due to operator error, or was a
defect in a tire, or the design of the bike
play a significant role? If wind and road conditions play a part in a bike
tipping over, it is not hard to see that fork construction, tire performance
and bike suspension also play a big role. A crash reconstructionist must be
prepared to analyze all of these factors, as well as issues of maintenance and
repair, to be able to explain the cause of a bike or motorcycle crash.
If you or a close friend or loved one have been hurt in a bike
crash, be sure that your case is evaluated by an engineer and an attorney who can
identify the factors that lead to crashes and properly analyze them. At Berman
& Simmons, we have the experience, resources and will to understand your
case and to pursue all of the remedies that are available under the law. We are
happy to answer your questions about your potential case and our expertise. All
of our work is done on a contingent fee basis, which means that there is no
cost to you unless and until we win.


