Understanding the Mechanics of Drawbar Pull

Toyota recently showcased the drawbar pull potential of its Toyota Tundra CrewMax pickup truck in a televised ad. The scene unfolds on a road adjacent to a 180-foot cliff at the Sierra Rock Quarry in Placerville, California. A steel cable connects the truck's hitch to a 6,400-pound shipping container suspended mid-air by a crane. Once the crane releases the container, the truck is tugged toward the cliff edge. Miraculously, the Tundra's tires grip the road, its rear dips slightly, and it successfully hauls the massive container up from the quarry.

While the quarry stunt is visually striking, everyday examples of drawbar pull are equally remarkable, though less dramatic. These instances, though subtle, highlight the same principles of force and traction.

One of the most common examples of drawbar pull occurs on railways, where locomotives haul lengthy freight trains. At airports, specialized tractors maneuver colossal jet aircraft, showcasing drawbar pull in action. Similarly, on farms, tractors pulling hayrides for families and friends are also utilizing drawbar pull.

These instances illustrate drawbar pull in action, but what does it truly mean? In essence, drawbar pull refers to a vehicle's capacity to generate pulling force. Specifically, it is the towing power exerted by a truck or industrial vehicle at its coupling point, measured in pounds or Newtons [source: A&G Mercury].

Now that you understand the definition of drawbar pull, you might be curious about calculating it for a specific vehicle, perhaps even your own. To learn more about drawbar pull and the formula required to determine it for your vehicle, proceed to the next section.

Calculating Drawbar Pull

Determining a vehicle's drawbar pull isn't as complex as it may seem. With the right equation and an understanding of the variables involved, the process becomes straightforward.

To calculate a vehicle's drawbar pull (DP) in pounds, multiply the motor's torque (T) in inch-pounds by the gear reduction (R), which includes the axle and transmission, and then divide by the radius of the drive tire (r) in inches. This result represents the vehicle's tractive effort, or its maximum pulling capacity.

However, this is only part of the equation. To find the net drawbar pull, you must also calculate the vehicle's rolling resistance (RR). The net drawbar pull is derived by subtracting the rolling resistance from the tractive effort, as shown in the following formula:

DP = T x R ÷ r - RR

DP = T x R ÷ r - RR

To compute rolling resistance (RR), which is the force needed to move a vehicle across a surface, multiply the gross vehicle weight (GVW) in pounds by the surface's rolling resistance (R). Rolling resistance is measured in pounds per thousand pounds of vehicle weight, divided by 1,000 (or 100 for metric measurements). You can find rolling resistance values for various surfaces in online tables. For instance, to calculate the rolling resistance of an 8,000-pound vehicle on good concrete, multiply 8,000 (GVW) by 15 (rolling resistance per thousand pounds on good concrete) and divide by 1,000. Using metric units, divide by 100 instead. This calculation yields a rolling resistance of 120 pounds, as shown in the equation below:

RR = GVW x R ÷ 1000

RR = GVW x R ÷ 1000

With the rolling resistance calculated, we can return to the drawbar pull formula. Assume our example vehicle has a motor torque of 4,800 inch-pounds (400 pound-feet), a total gear reduction of 10:1, and a tire radius of 18 inches. After performing the calculations, the tractive effort—or gross pulling force—comes out to 2,667 pounds. Subtracting the rolling resistance from this figure gives us the net drawbar pull, which for this vehicle is 2,547 pounds.