In general two plans are followed in building a motor truck. The first
is to build a rigid truck so that it will resist all shocks and
distortions that come from rough and uneven roads. The second plan is to
build a flexible body so that the chassis will "give" rather than resist
when subjected to hard strains. Although the rigidly-built truck may be
entirely satisfactory for most forms of trucking, it is practically
impossible to build one on the rigid principle that will stand up under
the heavy strains to which a logging truck is subjected unless it is to
be operated over good paved roads. When only ordinary unpaved public
roads are available, flexibility is one of the most important
characteristics to look for when selecting a truck. Where the operator
is hauling over his own pole or plank road this consideration does not
play so important a part, as the road bed then is more likely to be free
from holes and irregularities.
All makes of trucks are more or less alike in general construction,
differing only in minor details, so that the personal whims of the buyer
will largely determine the kind he will select. It is advantageous to
have as long a distance as possible between the driver's seat and the
bunk over the rear axle, in order to allow more of the load to be
carried by the truck, and less by the trailer, giving better traction to
the drive wheels, but necessitating extra strong rear springs and axles.
The type of power transmission best suited to the use of the logging
truck is a question that has received a great deal of attention. There
are three general methods of transmitting the power: (1) by chain; (2)
by worm drive, and (3) by internal gear drive. Each has its advantages.
It is claimed by many that the chain drive saves many hours of
"shut-down time" due to the fact that if anything breaks in the
transmission, it will be a link in the chain as this is the weakest
point. It is then only a matter of a few minutes to insert another link.
With the worm driven vehicle, a break in the transmission requires an
expensive shut-down before the matter can be repaired. The worm drive,
on the other hand, very seldom breaks if proper care is used.
The chain drive also allows the replacement of the sprocket with one of
a larger or smaller diameter thereby giving a higher or lower gear
ratio, which cannot be done with the worm gear. This seems to be of some
advantage to an operator when changing his setting from one with a short
haul and steep grades where a low gear ratio is required, to one where
the haul is long and fairly level, and where speed in transit is an
advantage.
On the other hand, in starting on slippery grades or wherever the
traction is poor, the worm drive will give better traction than a chain
drive because there is difficulty in taking up the slack that is always
present in the chain before letting in the clutch fully. The slightest
jerk given to the wheels when the slack is taken up is likely to cause
them to spin, thereby losing all the tractive power of the drive wheels.
In the worm gear there is no slack to take up and the power can be
applied more gradually, thus reducing the chances of spinning the wheels
and losing the traction.
The question of the weight of the truck used for logging purposes is not
as important now as it will be in the future. Laws are being passed in
nearly every state limiting the maximum weight to be carried on each
wheel by trucks using state or county roads so that the total weight of
the truck without load will be important. When operating over state or
county roads the load is limited to from 2400 to 3000 feet, B. M., of
Douglas fir, depending upon the locality. In such cases, it is an
advantage to have a lighter truck, say one of 31/2 tons capacity. By
adding additional leaves to the rear springs of a truck of this capacity
it may be made to carry a larger load than it would be possible to put
on a 5-ton truck and still comply with the law. The pulling power of the
31/2-ton truck and the 5-ton truck is practically the same so that the
difference in dead weight between the two may be carried in a profitable
manner by adding four or five hundred feet B. M. of logs. Another
advantage of the lighter weight truck is _speed_. The 31/2-ton truck is
geared to make from 14 to 16 miles an hour, while the 5-ton truck is
usually limited to from 10 to 12 miles an hour.
Whenever the legal weight limit does not enter into the problem, as in
operating over a pole or plank road for the entire distance, it is, of
course, advantageous to carry the largest loads possible. In such cases
a 5-ton truck with an 81/2-ton trailer is the most profitable investment.
This allows a much larger load to be carried in proportion to the
overhead charges. The disadvantage of the 5-ton truck is that it is very
heavy and unless the roads are good, it will easily sink into the ground
and cause trouble. A common fault of the 5-ton truck today is the
overweight of the front end, which is too heavy for the width of tire on
the front wheels. This can be very easily overcome by the use of wider
tires.