Before revision: As an approximation, the following formula can be used (for a two-cylinder locomotive):As with any physical formula, units of measurement must be consistent: pressure in psi and lengths in inches give tractive effort in lbf, while pressure in Pa and lengths in metres give tractive effort in N. where t is tractive effort d is the piston diameter in inches (bore) s is the piston stroke in inches p is the working pressure in pounds per square inch w is the diameter of the driving wheels in inches The constant 0.85 was the Association of American Railroads (AAR) standard for such calculations, and overestimated the efficiency of some locomotives and underestimated that of others.
After revision: in pounds-forceAs an approximation, the following formula can be used (for a two-cylinder locomotive):As with any physical formula, units of measurement must be consistent: pressure in psi and lengths in inches give tractive effort in lbf, while pressure in Pa and lengths in metres give tractive effort in N. where t is tractive effort d is the piston diameter in inches (bore) s is the piston stroke in inches p is the working pressure in pounds per square inch w is the diameter of the driving wheels in inches The constant 0.85 was the Association of American Railroads (AAR) standard for such calculations, and overestimated the efficiency of some locomotives and underestimated that of others.