Mike and Joe with Ramming Speed at BattleBots in May '01
Traction. What is it? A dictionary would tell you
it is the adhesive friction of a body on a surface on which it moves. Adhesive
friction? Their are actually many types of friction involved, each of them
suggesting certain design parameters for your wheels.
Friction
Friction is defined as the resistance to
motion between two surfaces. There are four basic types of friction.
Rolling Rolling Friction is the friction
you have to overcome just to keep round objects moving. It is due to
dissipation of energy from making and breaking contact between object and
surface. Balls and Wheels normally offer less friction than sliding (ball
bearings, tires). A
rolling wheel experiences only rolling friction.
IF the tire did not deform, it contacts the surface at a line. Wheels rolling
without slipping implies that the tire surface in contact with the road is at
rest relative to the road. Keep in mind
this is not the friction that pushes the object the wheel is attached to, that
would be static friction.
Static If
you apply torque ("Rotational Force") to speed up or slow down, the tire
interacts with the floor through the Static Frictional Force. So
when you accelerate, the motor transmits forces to the axle, turning the tire,
which pushes against the floor through static friction.
If
too much torque is applied, the wheels spin relative to the floor, creating
kinetic friction.
Kinetic
(Sliding)
If you lock or spin the wheels, instead of static friction, you have Kinetic Friction,
the resistance to motion between two surfaces which are moving across each
other,
which is smaller than static friction (melted rubber lubricates, debris, etc).
Adhesive
Grip is a result of the deformation of the wheel's rubber that allows it to
'surround' irregularities of the floor surface. This surrounding effect
provides more surface contact between the rubber and the floor, and it adds
the ability to 'push against' those irregularities rather than simply rely on
the coefficient of friction between the surfaces. This is often referred to as
molecular adhesion.
Internal
The resistance to motion within elastic objects (tires get warm from
internal friction as they flex).
The amount of friction between two surfaces
is called coefficient of friction, which depends upon:
substance of
material
roughness of
the surfaces
amount of
force pushing the surfaces together
presence of
lubricants
Coefficient of Friction
The term coefficient of friction is
defined as the maximum force that can be created by a tire on a given surface
condition, divided by the weight on the tire.
Thus, the
maneuvering ability of a bot depends primarily upon floor
surface and bot weight. As the bot accelerates or slows down more rapidly, it demands
greater traction forces from the tire-floor combination. The tire and
floor
combination will produce these forces up to the friction limit.
Changing Traction
So, what are the variables involved with traction as it
applies to combat robots?
The driving surface.
We cannot change the surface itself, it can be considered a constant.
Presence of lubricants
It would be possible to include mechanisms to 'clear' the floor, removing
debris that could function as a lubricant (bits of rubber, metal, etc).
Weight
The heavier your robot, the more weight being placed on each wheel, more force
applied to the floor, more friction. Distribution of weight can also play a
part in determining which wheels have more traction.
Wheel Material
We have a lot of control in this. Choosing a softer compound will increase the
adhesive friction, helping to prevent
sliding friction. Especially when considering the debris that can accumulate,
a soft compound will be more likely to deform around the debris, and allow the
rest of the surface of the wheel to contact the floor, where a harder compound
will tend to ride up onto the debris, reducing surface contact, and therefore
traction.
Wheel Size
It is often argued that wider tires do not increase traction, and it is true
that static and kinetic friction are independent of surface area. Where
surface area does play a part, however, is in determining when static friction
changes to kinetic friction. This is related to adhesive friction, which would
be area dependent (more surface area=more irregularities to push against).
More width also increases the probability or likelihood
of making contact with the floor, especially when used with a soft compound
that rides over debris.
