TRACTORS 207 
type increases as the road surface becomes worse. The some- 
what comphcated construction of the crawler traction device 
is its only disadvantage as compared to wheeled tractors. 
Modern crawler-type tractors are driven by internal combustion 
motors, and those used in logging may be divided into two groups, 
namely, the "full crawler tractors" or those mounted only on 
two traction members, and those having the front part of the 
machine supported on a pair of sleds for snow and iced roads or on 
steering wheels.^ 
The tractors of the last group require fairly good logging roads; 
hence they are not practical for skidding purposes. They often 
are used by loggers in the Northeast, both for hauling timber 
and for carrying supplies to the camps. Crawler tractors with 
steering wheels or sleds, have approximately 40 H. P. on the 
draw-bar, a speed ranging from 2 to 6 miles per hour, and weigh 
about 10 tons. Although they are used chiefly for draw-bar 
work they also have a platform about 5^ by 9 feet in size 
on which a portion of the load may be carried. In New Eng- 
land and eastern Canada these machines have largely superseded 
steam log haulers which had about 100 draw-bar H. P., a working 
speed of about 4^ miles per hour, and a weight of approximately 
18 tons. 
"Full crawler tractors" are the type best adapted to logging 
because they are not handicapped by front wheels or sleds, and 
they are suitable for work both on very poor roads and on the 
forest floor. The great flexibility of these machines, which is 
important for work on uneven surfaces, is provided by an 
independent, though limited, vertical oscillation of the traction 
members, and also by the "three point" suspension of the body, 
the front part of which is pivoted to the middle of the cross 
equalizing bar, the ends of which rest on the frames of the 
crawler members. Spiral springs are commonly used for better 
cushion. 
Obstacles, such as stumps and stones, from 12 to 18 inches 
high, may be readily overcome by crawler tractors. 
Full crawler tractors may be divided into three classes, accord- 
ing to size, typical machines for each group having the following 
power : 
^ The crawler tractor with a single steering wheel is not adapted for log- 
ging though formerly it was occasionally used. 
