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Technical
terms explained.
Aeration:
condition where air bubbles are
present in a liquid.
Air-gap:
distance between top of oil in a
fork leg, and top of the fork leg.
Air-spring:
suspension using progressive compressibility
of air as a springing medium.
Anti-dive:
system used to resist nose-down
pitch under braking.
Anti-squat:
extent to which a motorcycle resists
tail-down attitude under acceleration.
Bottoming-out:
condition when all suspension travel
has been used up.
Castor effect:
self-aligning effect of steering
system with positive trail.
Coil bound:
condition where spring coils bear
against each other.
Compression (or bump) damping
effect resisting compression of
suspension.
Damping:
(usually hydraulic) system for resisting
vertical suspension movement.
Droop:
extent to which suspension compresses
under weight of stationary vehicle
alone.
Fade (go off)
condition where damping effect is
reduced by heating of the damping
fluid.
Free length:
the 'natural' length of an unloaded
spring.
Geometry:
the angles and lengths used in chassis
design.
Inertia:
tendency of a moving object to continue
moving at the same speed in the
same direction.
Momentum:
mass times velocity.
Offset:
in telescopic forks, the distance
between the steering head and the
fork legs.
Orifice:
passage of precise size through
which oil passes as a shock absorber
moves.
Oversteer:
condition in which opposite lock
has to be applied to keep a vehicle
on course through a turn.
Patter/judder:
low frequency vibration emanating
from a wheel or suspension.
Pre-load - mechanical:
amount a spring compresses when
fitted to a shock absorber.
Pre-load - static:
mechanical pre-load, plus additional
compression due to weight of vehicle.
Progressive rate:
one-piece spring which compresses
less readily as more loading is
applied.
Pro-squat
extent to which a motorcycle resists
tail-up attitude under acceleration.
Pumping down:
condition where the ride height
of a vehicle lowers progressively
over a series of bumps.
Pumping up:
where ride height rises over a series
of bumps.
Rake (head angle):
angle between the steering head
and the vertical.
Rebound damping:
damping effect resisting extension
of suspension.
Rising-rate:
suspension in which the wheel rate
rises as the suspension compresses.
Sag:
extent to which suspension compresses
under weight of stationary vehicle
and rider.
Self-aligning torque:
self-centering steering force caused
by positive trail.
Spring rate:
amount of force needed to compress
a spring a given distance.
Stiction:
'static friction': the reluctance
of mechanically loaded parts to
move against each other, esp. in
forks.
Tank-slapper:
violent lock-to-lock oscillation
of the handlebars.
Topping-out:
condition where suspension is fully
extended.
Trail:
distance between the front contact
patch & the imaginary point
where the steering head centerline
would intersect the ground.
Understeer:
condition in which extra steering
effort is required to keep a vehicle
on course through a turn.
Unsprung mass:
weight of wheels, tyres, brakes
(proportion of) suspension, etc.
Wallow:
oscillating, yawing movement of
a motorcycle, esp. through a turn
- like 'rowing' the bike.
Weave:
(often rhythmic) straight-line instability.
Weight transfer:
amount by which a wheel is loaded
& unloaded under braking or
acceleration.
Wheelbase:
distance between front and rear
axles.
Wheel rate:
force needed to deflect a wheel
vertically by a given distance.
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