By Richard J. Cunningham
In the theoretical world of physics, if you roll an object, say a ball or a wheel, off a ramp at a given speed, it will be projected into the air at the same angle, fly
the same arc and land in exactly the same place every
time. That is the basis of the manly art of artillery. It is
so repeatable that, if you live next to Isis headquarters, it
So, why is it that when you hit the same ramp, in the
same gear, at the same rpm—lap after lap—your bike
never jumps the same way twice? Do motocross bikes live
THREE FLYING OBJECTS
When a motocrosser leaves the ground, there are
actually three objects flying through the air together: (1)
The motorcycle. ( 2) The rider. ( 3) The unsprung weight of
the motorcycle’s wheels and suspension parts. It is easy
to understand the first two, but it is the third part, the
wheels and suspension parts, that drive the equation. The
energy stored in the bike’s suspension can create havoc
if its influence is not factored into every jump. It is both
friend and foe.
Let’s take a look at the role that each of this trio plays
in the aerial ballet we refer to as motocross.
(1) THE MOTORCYCLE: If you aimed your bike at the
typical double jump (with the throttle locked in the halfway
position and the transmission in third gear) and stepped off
the bike before it reached the jump, the odds are that the
bike would backside the jump in a gentle arc (followed by
a perfect nose-first landing).
This outcome is dictated by simple physics. Racetrack
designers use this knowledge to build their jumps so that
any racer who hits the ramp at the correct speed—and
doesn’t do anything stupid—will make it to the other side
safely. A riderless bike, with the throttle held in a steady
position, will clear any jump on a motocross track. It won’t
be pretty. It may not go straight for long after landing, but
it works for the same reason that a mortar shell hits the
enemy ammo depot with amazing accuracy. Once you
THE 105MM HOWITZER
by different rules from a 105mm howitzer? Hardly. Mortal
motocrossers cannot break the rules of physics. We can,
however, creatively interpret them, and exactly how we
interpret Newton’s laws is the key to effective jumping.
A talented motocross racer, or a less talented one with
an understanding of physics, can jump flat, sky his bike or
go for distance over the same double in the same gear at
the same speed. Although the ramp angle and the speed
may be identical, a motocrosser can make his bike go long,
short or high. How so? By applying physics to the problem.
know the angle and the speed, the rest is preordained.
If you wanted your pilot-less bike to go higher, come
down at a different angle or go for distance, you would
have to alter the ramp, change the bike’s approach speed
or hop aboard and do it yourself.
( 2) THE RIDER: With a rider on the bike, the rules that
govern a 250cc artillery shell are rewritten. Remember what
we said about you and the bike being separate objects?
You may be holding onto your bike for dear life, but the
two of you are, in fact, two objects who just happen to be
flying on similar paths. Because you are not a rigid piece
of aluminum bolted to the frame, your 175 pounds of soft
tissue acts as an energy absorber. It can also be used to
store energy. Here’s how it works.