>Actually I don’t think that I “hit” my arm. I think the bruise was caused
>by the force of the impact on my wrists/hands. I really don’t think that
>there is any protective gear that would have helped “protect” my arm from
>this (relatively minor) injury.
First off, we assume that all unicyclists are smart enough [there are
plenty of counter examples, but no one will convince them until they or a
good rider they know injure their head while riding] to wear a helmet
whenever riding.
In a fall there are two main components of force on the human body,
straight down due to gravity, and parallel to the riding surface (usually
close to horizontal).
The gravity component:
It is true that there isn’t much that can be done about gravity, unless
one wears a balloon suit which simulates the protection of an auto air
bag. Let’s say the balloons offer 15 cm of cushion (before popping)
vs. less than a mm of soft flesh over the hands, knees, etc. In both
cases, the vertical velocity component is the same. The
de-acceleration while wearing the balloons is much smaller since the
balloons can be compressed 15cm before they pop. The de-acceleration
without balloons is very large since the rider’s flesh compresses less
than a mm over bony areas without doing significant damage. The
magnitude of the de-acceleration into the ground is what causes
damage. If the acceleration is large enough, the death of the rider
may result, as in a case without protection. If the acceleration is
small enough, no significant damage occurs, as may occur with the
balloon protected body, assuming the balloons reduce the velocity to
zero with or without popping.
A more practical protective gear might be 1 cm of dense foam sewed into
clothing for complete body protection (used in addition to helmets, wrist
protectors, knee pads, elbow pads, etc.). The 1cm of dense foam should
compress at least 5 mm on impact, probably at least 10 times as much as
unprotected flesh near a joint without significant damage.
The danger of a vertical fall increases rapidly as the height of the
unicycle seat increases.
The (linear) momentum component:
Rolling into a ball just prior to impact will convert the linear momentum
into rolling momentum until the rider stops moving due to air friction and
rolling friction with the riding surface. Some damage may occur because no
rider can become a perfectly round ball or wheel and simply roll to a
stop. However, proper protective gear could minimize any such “bumpy
rolling” damage.
Sometimes a fall occurs so suddenly that the most the rider may be able to
do is place his arms between himself and the impact point. Remember that
we are only considering the impact component parallel to the riding
surface in this section. In this type of fall, no rolling occurs and the
much stronger sliding friction applies. The friction of bare hands and
knees against concrete or tar is so high that sections of skin tear off
because skin doesn’t really slide on these surfaces without the
lubrication of torn skin fragments (and probably not even then). Since the
rider doesn’t really slide agianst the rider surface, his body
de-accelerates (parallel to the rider surface) very quickly and the high
de-acceleration can cause a great deal of damge, and even death, depending
on how fast the rider was moving parallel to the ground at the time of
impact. (No obstacle is needed to cause horrific damage.)
On the other hand, hands protected by wrist protectors and knees protected
by hard covered knee pads have a very small sliding friction against
concrete and tar. A small amount of the outer surface of the wrist
protector and knee pads is ground off and provides lubrication between the
protectors and the riding surface. All the potential energy (parallel to
the riding surface) of the rider wearing hard protection is converted into
grinding a small amount of the protector off and into sliding friction
between the rider’s protectors and the riding surface. The de-acceleration
parallel to the riding surface is very small since the rider simply slides
over it with very little friction. No damage due to linear momentum
parallel to the riding surface occurs (unless an obstacle is hit).
Note: Sliding friction is usually lower than “gripping” friction due to
the lubricating effect of small particles torn from either or
both surfaces.
Sincerely,
Ken Fuchs <kfuchs@winternet.com
