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 The robot head
A. Single-wheel vs dual-wheel heads.
The robot head refers to that part that throws out that
ball. It generally consist of a tube through which the balls are fed, with
one or more motorised rubber wheels (commonly referred to as 'throwing
wheels') at the exit that 'throws' out the ball with the required spin and
speed.
 The
single wheel heads have a single rotating wheel that ‘throws’ the ball out,
making contact on only one side of the ball. Dual-wheel heads (as pictured
on right) have 2 wheels
on either side of the ball, which can spin at different speeds, and
therefore can generate a greater variety of spin and speeds of the ball.
Multi-wheel robots have 2 main advantages:
1. Spin and speed can be set independently.
Although this may seem like an absolutely essential feature, in reality it’s
not quite as straight-forwards. The extent to which to which a robot have
truly independent speed from spin varies from one model to another, and is
some setting may produce balls which are not really realistic in real play.
Still they no-doubt offer more flexibility, so it’s worth weighing up the
pros and cons, to see if it’s worthwhile.
Pros:
You should be able to generate any heavy or light spin for both slow and
fast balls. For example you can generate a short very heavy backspin,
despite the ball being very slow, something that’s hard to do with a
single-wheel robot. Similarly you can generate fast balls with only a
moderate amount of spin, which may be common against certain opponents.
Cons:
Although with a single wheel robot you don’t have the same flexibily, in
reality you can probably generate ‘most’ balls quite closely, and probably
close enough for most players. In real play the spin is generally heavier
for fast balls compared to slow balls anyway, so the balls from the single
wheel robots are not entirely unrealistic.
In addition to this the speed and spin of the 2-wheel robot are not entirely
independent anyway, as there is still some interaction between the 2, which
is better for some robot than others.
You should also consider the extra complexity of the 2-wheel robot, making
it harder to setup and a little more likely to breakdown (the more features
the more that can go wrong).
2. They can produce no-spin balls
Pros:
For many people this may not be important, as it’s more common that people
want to practice handling spin, not no-spin. For some people this may be
important, for example long pimple player, who generally have more trouble
handling no-spin than spinny balls. For these people this may be a ‘must’
Cons:
In real life it’s rare to get a no-spin ball… virtually all balls do have
some spin however slow. Spin from a single-wheel robot can be reduced by a
significant amount by bouncing the ball off the table or projecting it from
a longer distance, which may already produce balls close enough to make them
suitable for the exercise.
Are they worth it?
So does this make 2-wheeled robots a waste of money?
Not at all. In general the 2-wheel robots definitely have more flexibility than single
wheel ones and can generate some balls that single-wheel robot cannot or
struggle to do. They
do tend to be more expensive and more complex though, so you should weigh up
exactly how important these extra features are, and make a decisions on
whether it’s worth the extra money.
Continue on to
Robots - Programmable
versus non-programmable | 
