Damage without stable crane outrigger pad.

Whenever a mobile crane or a hook and ladder fire truck needs to extend its boom or ladder out a long distance or with a heavy weight, it requires additional support to prevent it from toppling over. This support is provided by retractable outriggers.

Outriggers are sturdy metal legs that extend the footprint of the mobile equipment out to cover a larger area. They can either extend directly from the truck to the ground at an angle, or extend horizontally outward from the truck with a vertical leg that makes contact with the ground. In either case, the outriggers must lift all the weight of the truck up off the tires to provide maximum stability. Without the outriggers, the equipment is considered to be “on rubber” and with them it is “on outriggers.” The operator must obey different loading restrictions in each case.

Stability is dependent on the equipment’s footprint and center of mass. The footprint is the total area enclosed by the support structures of the equipment. The center of mass is the point at which the equipment would balance if it were set on top of a single point of support. If the center of mass is inside the footprint, the equipment is stable. If the center of mass is off the edge of the footprint, the equipment will topple.

So it’s not hard to imagine how a crane could become unstable when extending its boom with a heavy weight hanging from it. Since not extending the boom isn’t really an option, there are only two ways to provide extra stability:

Use a counterweight to help shift the center of mass back over the footprint

Make the footprint bigger.

All cranes have counterweights, but there is a limit to how heavy they can get. Too heavy and the crane wouldn’t be able to move, and the ground would have trouble supporting it. Simply building the crane with a wider footprint would prevent it from traveling on public roads. Using retractable outriggers to extend the footprint therefore is essential to the crane’s stability.

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