TRACTIVE EFFORT
Torque developed by an engine is multiplied by the
transmission and rear axle gears.
These gears act like levers, i.e. they increase
torque in the same proportion as do the gears of the
transmission or rear axle.
Trucks have different demands for the pulling power
depending on the conditions. It takes more effort to
start a truck than to keep it moving - more effort to
climb a hill than to travel on a plane - more effort to
pull through mud and sand than to travel on the a paved
road. The difference in gear rations gives the truck the
extra power that is necessary to overcome obstacles like
these. For example, if a five speed transmission has a
6.40 to 1 ratio in low gear this means that in
transmitting the torque through the transmission gears
the torque is multiplied by 6.40. Likewise, in any other
gear the torque is multiplied by the corresponding
ration.
As is the case with the transmission, the rear axle
gears can be compared to a simple lever. They increase
engine torque in the same proportion as the gear ratio.
The rear axle ratio is the number of engine
revolutions for each rear wheel revolution when the
transmission is operating in direct drive. If the
truck has a rear axle ratio of 6.166 times to one
revolution of the rear wheels and increase the torque in
the same ratio. However, more engine revolutions per
rear wheel revolution means reduced speed. A high gear
ratio is good for speed and economy.
Tire rolling radius is the distance from the center
line of the rear axle to the point where the tire
rests on the road. Its effect is to decrease pulling
power. The larger the tire and the greater the rolling
radius, the greater is the decrease in pulling power and
truck ability. It is an advantage to have smallest
possible diameter tires to complete the job, keeping in
mind that they should have sufficient capacity to carry
the load when maximum pulling power is desired. On the
other hand, larger tires will have the effect of
decreasing the power but will increase road speed if the
engine is operating at the same RPM.

Tractive Effort (RimPull)
The force exerted by the tires on the road is engine
torque transmitted through the clutch, transmission,
propeller shaft, rear axle, wheels and tires. This force
propels the vehicle and is known as Tractive
Effort or Rim Pull. The friction generated in the gears
and bearings in the transmission, propeller shaft and
rear axle causes a loss of torque. This loss is
compensated for by introducing an efficiency factor in
calculation. The efficiency factor is 0.92 of 0.90,
depending on whether the vehicle is operating in direct
drive of the transmission or in one of the other speeds.
The following formula is used for calculating the
tractive effort:
Engine torque x
Efficiency Factor x Transmission ratio x Rear axle
ration
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Tire rolling radius = Tractive Effort
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