Calculating Drawbar Pull
Calculating the drawbar pull of a vehicle doesn't have to be complicated. In fact, once you know the equation and how to find each of the factors involved, it's rather simple.
To find the drawbar pull (DP) of a vehicle in pounds, you'll need to multiply the torque of the vehicle's motor (T) in inch pounds by the gear reduction (R) (including the axle and transmission) and divide that number by the radius of the drive tire (r) in inches. The resulting number represents the tractive effort of the vehicle. In other words, it's the gross pulling force the vehicle is capable of.
But that's not all of the information you'll need. To determine the actual (or net) drawbar pull, you'll need to know how to figure out the rolling resistance (RR) of the vehicle. Drawbar pull is the tractive effort minus the rolling resistance. The equation looks like this:
DP = T x R ÷ r - RR
DP = T x R ÷ r - RR
To determine the rolling resistance (RR) -- the force necessary to move a vehicle over a surface -- multiply the gross vehicle weight (GVW) in pounds by the rolling resistance of the surface (R). The rolling resistance of the surface is measured in pounds per thousand pounds of vehicle weight, divided by 1,000 (100 if you're using metric measurements). The rolling resistance of several different surfaces can be found in tables like this one that you can easily locate online. So, for example, to determine the rolling resistance of an 8,000-pound vehicle on good concrete, you would multiply 8,000 (GVW) times 15 (pounds per thousand pounds of rolling resistance on good concrete) and divide that number by 1,000 (a constant). If you're using metric figures, you'll use 100 as your constant. After you do the calculations, you'll find that the rolling resistance of our example vehicle is 120 pounds. The equation looks like this:
RR = GVW x R ÷ 1000
RR = GVW x R ÷ 1000
Now that we know the rolling resistance, we can get back to the drawbar pull equation. Let's say that the torque of our example vehicle's motor is 4,800 inch-pounds (400 pound-feet) and it has an overall gear reduction of 10:1 along with a tire radius of 18 inches. Again, complete the calculations, and you'll discover that the tractive effort -- or gross pulling force -- of the vehicle is 2,667 pounds. To find the net drawbar pull, subtract the rolling resistance from the tractive effort. In this case, the drawbar pull of our example vehicle is 2,547 pounds.
Now that you know the equation and how to find all of the factors involved, it shouldn't be difficult to calculate the drawbar pull of your own vehicle. If you'd like to read more about towing, weight and other related topics, follow the links below.
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More Great Links
- A&G Mercury. "A&G Mercury Tow Tractors: Information Guide." April 20, 2005. (Oct. 9, 2008) http://www.agmercury.com/Tow%20Tractor%20PDFs/ Tow%20Tractor%20Sales%20Information.pdf
- Mraz, Stephen J. "Engineering an ad." Machine Design. Jan. 10, 2008. (Oct. 9, 2008) http://machinedesign.com/ContentItem/71927/Engineeringanad.aspx
- The Raymond Corporation. "Glossary." (Oct. 9, 2008) http://www.raymondcorp.com/solutions/glossary.cfm?selector=D
- Webtec Products Ltd. "Draw Bar Pull." (Oct. 9, 2008) http://www.webtec.co.uk/techinfo/equation/eqnfr/eqn03.htm
- Webtec Products Ltd. "Road Rolling resistance." (Oct. 9, 2008) http://www.webtec.co.uk/techinfo/equation/eqnfr/eqn14.htm