Page 114 - Moving the Earth_ The Workbook of Excavation

ROCK, SOIL, AND MUD

3.8 THE WORK

FIGURE 3.6 Tractive efficiency of surfaces.

If this information is not available, the hauler should be weighed, one axle at a time, both empty
and loaded. Different load distributions, to the front or rear of the body, can also be checked on
scales.
A highway truck may be taken to scales. Off-the-road trucks and scrapers might have to have
scales brought to them, which could prove difficult and expensive.
If there is no specific information on weight distribution and scales are not available, it may be
assumed that a rear-drive dump truck carries about 50 percent of its empty weight or 70 percent
of its loaded weight on the drive wheels. A front pull scraper, wagon, or rocker usually has 50 to
60 percent of its empty weight or 40 to 50 percent of its loaded weight on the drivers.
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On climbs, vehicle weight shifts toward rear drives and away from front drives by about 1 ⁄ 2
percent for each percent of grade. This factor increases the already considerable traction advan-
tage of the rear-drive truck in slippery climbs.
Tire Treads. Tire treads are an important variable in traction. There are a number of tread
designs, all of which eventually wear smooth. For most situations high lugs or cleats give best
traction, but smoother surfaces are better on dry sand and ice. When tire chains are used, their
effectiveness is reduced by high tread blocks.
Since a deep cleated tire gives best traction on soft loam and a smooth tire on loose sand, there
are likely to be intermediate surfaces on which they are equally effective, or perhaps ineffective.