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Technical Article
Kart Steering,
Physical Forces and Setup - Theory and Practice
by James Hughes
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2
Karting 'Forces'
During
Acceleration/Deceleration
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obvious forces, and are a caused by the tyres exerting a force on the
track, either forwards or backwards, with the result being to brake or
accelerate.
Its is important to
remember that this force is in the same plane as the track, that is, it
is below the karts centre of inertia. For this reason, the force exerts
a turning moment (or torque) on the entire kart. During acceleration
this torque causes a weight transfer to the rear of the kart, and
during braking it causes a weight transfer to the front of the kart.
There is no actual movement of any mass, but the torque effectively
forces the appropriate part of the kart 'harder' down on the track. It
is possible to calculate the amount of weight transfer if we know the
acceleration and the distance from the centre of inertia to the rear
wheels, but that is beyond the scope of this article. Figure 4 shows
the rotational torque while accelerating.
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Figure 4
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While Cornering
During cornering the driver
feels like he is being pushed outwards from the kart. This is actually wrong, he
is not being throw out but is simply trying to move in a straight line. The
tyres of the kart are producing a grip which imparts an angular acceleration on
the kart (and driver), forcing the kart to corner. It is this angular
acceleration that the driver feels. The force which the ground imparts on the
kart to make it corner is known as the Centripetal force, and it always acts at
towards the centre of the imaginary circle we are cornering round. It important
to remember that there is NO SUCH THING as centrifugal force.
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In figure 5, we see
that the centripetal force is split into two components, a vertical and
a horizontal. The horizontal force we have just described, but the
vertical can be regarded as the cornering equivalent of the forward
acceleration case. Because the centripetal force is acting on the kart,
it imparts acceleration to it, and again, this acceleration is acting
at ground level. Therefor a torque effect is again produced, but this
time it is acting across the kart, and we get a weight shift to the
outside of the kart (vertical component Y in our diagram). This weight
shift also helps the inside wheel lift, as the weight shift reduces the
weight on the inside wheel by an equivalent amount. This force has not
been show on the diagram to aid clarity, but is simply in the opposite
direction over the inside rear. In fact once cornering is initiated,
this weight shift is more important to raising the inside wheel than
the steering geometry.
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Figure 5
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The distance between the rear
wheels affects how the centripetal force is distributed over the horizontal and
vertical components. In the diagram, F1 is the centripetal force spread over a
wide track, F2 over a narrow track. In proportion to the Y components, X1 is
higher than X2, meaning that as track is increased, more centripetal force is
distributed as a sideways force in relation to the weight shift. This means a
wider track produces less weight shift to the outside rear, and more sideways
force. A narrow track increases the weight shift and decreases the sideways
force. Therefor a narrow track is less likely to exceed the grip of the tyres
when cornering than a wide track. Consequently, the grippier the tyres used, the
wider the stance can be before the grip is exceeded.
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On final force to
consider is a torque around the vertical axis experienced when
accelerating during a corner. It is common knowledge that braking while
cornering on a kart causes massive understeer (the kart attempts to
continue in a straight line) while accelerating can improve cornering.
This at first seems counterintuitive, since normally when accelerating
there is a weight transfer to the rear, which you would expect to try
to push the inside rear back onto the track. However, this weight
transfer is dwarfed by the torque around this vertical axis caused by
the fact that only one wheel rear wheel in on the track, and this wheel
is offset from the centre of inertia.
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Figure 6
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The further this wheel is
from the centreline of the kart (and therefor the centre of inertia), the
greater the turning moment, and the more likely the kart is the overcome the
grip of the tyre on the track . This causes the back to break away - oversteer
when accelerating and understeer when braking.
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