Robot Mechanisms and Mechanical Devices Illustrated - Profe Saul

134 Chapter 4 Wheeled Vehicle Suspensions and Drivetrains
Figure 4-3
wheel hub
Geared offset
For the raised layouts, the drive axle is coupled to the wheel through a
chain, belt drive, or gearbox. The US Army’s High Mobility
Multipurpose Wheeled Vehicle (HMMWV, HumVee, or Hummer), uses
geared offset hubs (Figure 4-3) resulting in a ground clearance of 16"
with tires that are 37" in diameter. This shows how effective the raised
chassis layout can be.
WHEEL SIZE
In general, the larger the wheel, the larger the obstacle a given vehicle
can get over. In most simple suspension and drivetrain systems, a wheel
will be able to roll itself over a step-like bump that is about one-third the
diameter of the wheel. In a well-designed four-wheel drive off-road
truck, this can be increased a little, but the limit in most suspensions is
something less than half the diameter of the wheel. There are ways
around this though. If a driven wheel is pushed against a wall that is
taller than the wheel diameter with sufficient forward force relative to the
vertical load on it, it will roll up the wall. This is the basis for the design
of rocker bogie systems.