Robot Mechanisms and Mechanical Devices Illustrated - Profe Saul

The Romans extensively used two wheeled carts, pulled by horses.
Pull on the right rein and the horse pulls the cart to the right, and vise
versa. The two wheels on the cart were mounted on the same axle, but
were attached in a way that each wheel could rotate at whatever speed
was needed depending on whether the cart was going straight or around
a corner. Carts got bigger and eventually had four wheels, two in front
and two in back. It became apparent (though it is unclear if it was the
Romans who figured this out) that this caused problems when trying to
turn. One or the other set of wheels would skid. The simplest method for
fixing this problem was to mount the front set of wheels on each end of
an axle that could swivel in the middle (Figure 6-1). A tongue was
attached to the axle and stuck out from the front of the vehicle, which in
turn was attached to a horse. Pulling on the tongue aligned the front
wheels with the turn. The back wheels followed. This method worked
well and, indeed, still does for four wheeled horse drawn buggies and
carriages.
Figure 6-1
wheels
Pivot mounted front
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