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TM 9-8000
factor of the 1/4-ton, 4 x 4 truck at maximum gear
29-14. General. There are many different aspects of
reduction and unloaded. Unloaded, the vehicle weighs
vehicle performance. Miles per gallon of fuel, miles per
2,625 pounds. The engine torque is 79 ft lb. The gear
hour in first, second, or other gear ratio, and the amount
reduction in the transmission in low gear is 2.798:1; in
of weight a vehicle can carry are all commonly
the transfer in low range, 2.43:1; and in the differentials,
considered factors. Other factors of equal importance
5.38:1. Total gear reduction is 36.7:1, in low range, low
include tractive factor, grade ability, and drawbar pull.
gear. Tire radius is 1.25 feet, unloaded; and slightly less,
These three factors refer to less-known performance
loaded. However, use the figure 1.25 feet for ease of
aspects that should be considered further.
figuring. Substituting all this data in the formula gives:
29-15. Performance Factors.
79 x 36.7 x 0.85
Tractive factor =
= 0.75.
a. Tractive Factor.
2.625 x 1.25
(1) Tractive factor refers to the pushing effort
(3) This figure means the tires can exert a push
the wheels can make against the ground as a result of
against the ground of 0.75 pound for every pound of
the application of torque to them through the power train
vehicle weight. If the tires do not slip, the vehicle can be
from the engine. Actually, the tractive factor is given in
pushed forward 0.75 pound for every pound of vehicle
pounds-of-push for each pound of vehicle weight.
Tractive factor can be determined by the following
b. Grade Ability.
ET x R x 0.85
(1) Grade ability refers to how steep a grade the
Tractive factor =
vehicle can climb. Grade, itself, is referred to in percent:
a 10-percent grade, or a 26-percent grade, for example.
A 1-percent grade rises 1 foot in every 100 feet. A 26-
where ET
Engine torque from dynamometer
percent grade rises 26 feet in every 100 feet.  If the
tests, with at least distributor
tractive factor is known, the grade ability can be easily
water pump operating (ft lb).
figured by use of the formula:
Gear ratio at reduction under
Grade ability = (Tractive factor - 0.015) x 100
where 0.015 equals rolling resistance per pound of
0.85 =
Efficiency of power transmission
vehicle weight (arbitrarily chosen as an average value).
(arbitrarily chosen as an average
(2) Figure the maximum grade ability of the 1/4-ton, 4 x 4
truck for which calculated the tractive factor (maximum)
was 0.75. Substituting in the formula would give:
Gross vehicle weight (lb).
Grade ability = (0.75 - 0.015)x 100 = 0.735 or
Tire loaded radius (ft).
73.5 percent.
(2) Figure the tractive factor of a 114-ton, 4 x 4
(3) The truck, unloaded and in low range, low
utility truck. This can be determined in any gear, and
gear, could go up a 73.5-percent grade; that is a grade
with the truck loaded or unloaded. The tractive factor will
that rises 73.5 feet every 100 feet. it can be seen that
increase with increased gear ratio, and will decrease as
the grade ability of a vehicle will vary with the load and
the vehicle is loaded. Generally, the tractive factor is
figured with maximum payload because this gives an
indication of the vehicle performance under the most
adverse conditions.  However, calculate the tractive


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