don't its wheel provide the same amount of friction?
To make it harder for them to tip over.
the capacity of a travelling crane relates to its resistance to tipping, and is dependent upon wheelbase, which can be extended by use of the outriggers, and counterweight (the big boxy thing at the back of the crane). Every lift requires a firm and stable platform which is what the outriggers provide. For every lift (weight) and reach (how far from the slew point), there is a limiting capacity factor. This is required to be posted within the operator's cab. The actual load lifted may be monitored by a load cell on a running rope (or other location), and through a computer control, limit the actual capacity of the crane.
