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TOE:
Toe-in or toe-out is a measurement made when the kart is
stationary and the steering is straight ahead. It means the
wheels are pointed in towards each other at the front (toe-in)
or at the rear (toe-out). Toe is expressed in mm, the figure
being the difference between the centre line of the tyres at the
front and rear. When the front measurement is less than the rear
measurement, the result is 'toe-in'. There are many out there
who will argue that the wheels should be set parallel (zero toe)
in order to avoid 'snow ploughing' and get the best speed on the
straight. This is not necessarily so, as you will read in a
little while.
Toe adjustments
must be made equally on both tie rods with the pitman arm locked
in a central position, otherwise the one sided toe will 'split'
when the kart is driven. This will result in the steering wheel
being crooked at best, and the camber angles and cornering
weights being out of whack. Toe and camber adjustments are best
made using special equipment such as alignment plates. These
plates are available from any kart shop and are fitted in place
of the wheels. They allow ease of measurement with a tape
measure to the degree of accuracy which is needed for wheel
alignment. All steering adjustments should be made while keeping
'tracking' in mind. This means that all the wheel angles,
including those of the rear wheels, should correspond with
whatever is needed to keep the kart running in a straight line.
Always use the rear axle as a reference for all adjustments to
the front end, but before this can be done, the leveling and
centering of the rear axle in the chassis must be checked.
CAMBER:
Camber is a very important angle, being the angle at which
the tyre surface is presented to the racing surface. Wheels that
are inclined together at the top are said to have negative
camber and vice versa. The usual setting for racing tyres is to
have a little negative camber in order to generate the most
grip. Camber angles impart a sideways force called camber
thrust, and it is very important to understand this.
KPI:
KPI (King Pin Inclination) is the angle of the king pin when
viewed from the front. In karting, the kingpin should be
inclined towards the centre of the the kart at the top (positive
KPI) and is usually a similar angle to the caster angle,
although an adjustment of one usually affects the other. Caster
and KPI work together to apply a jacking effect when the kart is
steered. KPI is sometimes called SAI (Steering Axis
Inclination).
CAMBER
THRUST:
Camber thrust is a force generated in a tyre when it is run
at an angle to the racing surface. This is best understood when
you consider that the cornering forces in a motorcycle are
generated by leaning the bike over, thereby increasing the
camber angle. The wheel effectively assumes the form of being
part of a cone, and tries to roll in a circle like a cone would.
Wheels with negative camber will try to roll towards each other,
but the chassis resists this tendency. The result is that the
tyres tend to scrub on the road, thereby slowing the kart
perceptibly in a straight line. This scrub can be cancelled by
toeing the wheels out slightly. So, those who argue the wheels
should be parallel for the best straight line speed are only
correct when the camber angle is zero. This is very important in
low powered classes where every little bit of power conservation
helps reduce lap times.
CASTER:
Caster (or castor, although that refers more to the type of oil
you should be using in the engine) is the angle of the kingpin
when viewed from the side. In karting, the kingpin should always
slope back at the top, and this is called positive caster.
Caster is an angle which gives directional stability to the kart
in the straight ahead position, and which contributes to weight
jacking when the steering is turned.
WEIGHT
JACKING:
The effect of Caster and KPI is to make the inside wheel 'heavy'
when the steering is turned. By this we mean that the inside
wheel is pushed down by the geometry and the outside wheel
raised. This jacks the weight diagonally across the kart, with
the inside front and outside wheels taking most of the weight.
Centrifugal force will then transfer the weight to the outside
front wheel, causing the inside rear wheel to momentarily lift
off the racing surface. This allows the kart to turn into the
corner, rather than trying to push straight ahead if both back
tyres retained equal grip. This effect is easily seen when
driving in the wet and it is difficult to get enough cornering
force to make this jacking effect work well, and gross
understeer or push is the result.
Ackerman
geometry causes an increase in toe-out as the steering is turned
and this is more of a consideration in karting than having the
wheels follow a nice 'correct' line around a corner. More
Ackerman effect is needed on sticky tyres and with more powerful
engines as it allows a more aggressive attack on corners. This
same aggressive Ackerman will result in extra drag in corners on
a low powered kart, particularly on slow corners where the
excessive toe out may cause the inside tyre to scrub exiting the
corner affecting speed down the following straight. If you
suspect this may be happening, very carefully inspect the inside
front tyre for evidence of 'reverse' scrub marks, that is
evidence of the tyre being worn from the inside out, rather than
vice versa.
The weight
jacking effects of caster can be increased or decreased by
changing the front track. Widening the front will increase the
jacking effect at the expense of heavy steering and a lessening
of steering feel, whilst narrowing it will have the opposite
effect. Widening the rear track will allow the jacking effect to
'float' the inside rear wheel more easily, thereby
assisting turn-in.
As a general
rule, the front track should be run as narrow as will allow
adequate weight jacking, whilst the rear should be run as wide
as grip will allow. This will result in a flat cornering,
predictable chassis, which hopefully will not bog the engine
with grip in mid corner and slow the kart on the following
straight.
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