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: