Motocross bikes of the ’70s
typically had 30-degree (60-degree)
head angles. They were called
“slack” head angles. Slack is the
converse of steep
THE MAGIC OF FRAME GEOMETRY
Frame geometry is the magic that makes it possible to
roost corners, slam berms and survive the whoops. Forget
engine technology and set aside suspension, because it
wouldn’t be possible to get past second gear without
the self-stabilizing forces that are built into a modern
motocross chassis. Without the right blend of head angle,
trail, fork offset and weight distribution, your motocross
machine would be hard-pressed to out-handle a Walmart
shopping cart. In fact, it would need a pair of Gottlieb’s
training wheels to get out of its own way.
WHY IS IT CALLED TRAIL? As will become
apparent, trail is called trail because it is derived from an
equation that measures how far behind the head angle the
tire’s contact patch is. In simple terms, how far the contact
patch “trails” the head angle.
Unlike a shopping cart, the steering axis of a motocross
chassis is angled, but the fact that its front axle and the
tire’s contact point lie well behind the centerline of the fork
creates the same trail equation.
WHAT IS HEAD ANGLE? Head angle, caster angle
and rake angle are all the same measurement. Head angle
is the difference between the front side of the steering axis
and a theoretical line drawn perpendicular to level ground
Motorcycles need to be stable at high speed, so it is
necessary to slow the steering way down by raking the
head angle forward. The degree that the steering axis is
raked forward is called the “head angle.” A chassis with a
slacker head angle steers less when you turn the handlebar and wants to remain in a straight line (think chopper).
A steeper head angle turns quicker, feels lighter at the
handlebar and is less stable at speed (think trials bike).
Head angle works in conjunction with “fork offset” to
cause the front wheel to steer into a turn when it is leaned
and straightens out as you exit.
HOW DO YOU MEASURE HEAD ANGLE?
You measure the head angle from an imaginary line
perpendicular to the ground to where it intersects the
angle of the steering axis. The numbers are based on a
90-degree right angle. There is some confusion among
designers as to which side of the fork the head angle
To understand what motocross bikes are all about, you
need a rudimentary understanding of the ins and outs of
frame geometry. Read on and discover what handling is
WHAT IS CASTER AND WHY IS IT SO
IMPORTANT? “Caster” is the effect that causes the
swiveling wheels of a shopping cart to steer in
the direction you deflect the cart. Motocross bikes use
the same physics of caster as shopping carts.
WHAT IS TRAIL AND WHY IS EVERYONE
TALKING ABOUT IT? “Trail” is what causes the front
wheel of a motorcycle (or shopping cart) to align itself with
the direction that the vehicle is traveling. Trail exists when
the contact patch of the tire is well behind the steering
In the case of a shopping cart, the steering axis is
vertical and the axle is offset about an inch behind the
axis. When the cart is pushed, the wheels instantly swing
into alignment. The same holds true for a motocross bike.
Take your hands off the bars and your motorcycle’s front
end will snap into a straight line for the same reason.
“Caster” is the effect; “trail” is what makes it happen.