Tow tractor drawbar pull calculator
Drawbar pull required for a tow train (level and on a grade): level: DBP_N = WL × SR; grade: DBP_N = (WL × SR) + (W_grade × 98 × G) where W_grade = WT + WL (see conditions). Enter your numbers below; the formula and its source are shown under the result. Part of this calculation is derived by Warehouse Machines from sourced inputs.
Compare the rolling pull with the tractor's rated drawbar pull and the starting pull with its maximum drawbar pull. Never exceed the towing capacity on the tractor's plate.
The formulas
level: DBP_N = WL × SR; grade: DBP_N = (WL × SR) + (W_grade × 98 × G) where W_grade = WT + WL (see conditions)
Corrected by Warehouse Machines: the source prints a different form; the conditions below say what changed and why.
Supralift prints the grade term as ((WT x WL) x 98 x G). Multiplying two weights gives tonnes squared, so the 98 N per tonne per % factor cannot apply to it; Warehouse Machines uses (WT + WL), the weight of everything going up the slope. 98 = 9.81 m/s2 x 1,000 kg x 1% (our check). Supralift: for average calculations use 400 N/tonne starting and 200 N/tonne rolling; compare the result with the tractor's maximum DBP (starting from rest) and rated DBP (rolling).
Worked example: Our check: 2 t load on smooth concrete (SR 150 rolling): level DBP = 2 × 150 = 300 N. Same load + 1.5 t tractor on an 8% grade: 300 + 3.5 × 98 × 8 = 3,044 N.
[src]lookup (starting / rolling): smooth concrete 300/150; rough concrete 500/250; good macadam 400/200; smooth brick or tile 360/180; wood 400/200; hard gravel 700/350; steel rails straight 100/50
Supralift lists a starting and a rolling value per surface, in newtons per tonne of towed load.
Worked example: see drawbar-pull-required
[src]check: required rolling DBP ≤ rated (normal) DBP; required starting DBP ≤ maximum DBP; total trailer load ≤ rolling load capacity
Crown TR 4500 spec sheet: drawbar pull normal 250 lb (110 kg), maximum 700 lb (315 kg); rolling load capacity 10,000 lb (4,535 kg). The check itself is arithmetic by Warehouse Machines.
Worked example: Our arithmetic: TR 4500 pulling 10,000 lb (4,536 kg = 4.536 t) on smooth concrete, using Supralift's rolling surface resistance for smooth concrete, 150 N per tonne (second source): 4.536 x 150 = 680 N = 153 lbf, under the 250 lb normal drawbar pull.
[src]rule: braked trailers recommended if trailer load > 2.5 t or any gradient
Supralift drawbar pull diagram page.
Worked example: Supralift: 6 t nominal tractor (rated DBP 1200 N) towing 2 t on 8% slope: max 5 km/h, 1200 m permissible run per hour.
[src]1 mi/h = 0.44704 m/s; 1 km/h = 0.2777778 m/s; 1 lbf = 4.448222 N; 1 lb = 0.4535924 kg; 1 in = 0.0254 m; 1 t = 1,000 kg
NIST SP 811 Appendix B.9. Used by the calculators to show results in both unit systems.
Worked example: 28 mph x 0.44704 = 12.5 m/s.
[src]Estimate only. The capacity on the machine's data plate is the legal limit; never exceed it.
Sources
- Drawbar Pull (Towing) (Supralift materials handling glossary) (retrieved November 6, 2026)
- Average Resistances for Various Surfaces (Supralift materials handling glossary) (retrieved November 6, 2026)
- TR 4500 Series Tow Tractor Specifications (Crown Equipment) (retrieved November 6, 2026)
- Drawbar Pull (Towing) Diagram (Supralift materials handling glossary) (retrieved November 6, 2026)
- NIST Guide to the SI (SP 811), Appendix B.9: Factors for units listed by kind of quantity (NIST) (retrieved November 6, 2026)