Tractor Mechanics, Stability & Weight Transfer Notes

Last reviewed 1 Oct 2026 · 8 min read

Tractor performance

A tractor converts engine (brake) power into drawbar power to pull implements. Not all the engine power reaches the drawbar.

Power at different points

Power	Definition
Brake power (engine power)	Power at the engine flywheel (dynamometer)
PTO power	Power available at the PTO shaft (about 85–90 % of the engine power)
Axle power	Power at the driving axle after the transmission losses
Drawbar power	$P_{d} = F_{d} v$ — the useful pull × travel speed

P_{d} = F_{d} \times v (F_{d} in kN, v in m/s, P_{d} in kW)

Transmission efficiency about 90–92 % (gear losses, clutch, differential, final drive).
Tractive efficiency $η_{t} = \frac{P _{d}}{P _{a x l e}}$ — depends on rolling resistance and wheel slip: about 75–90 % on concrete, 70–80 % on firm soil, 55–65 % on tilled soil, 40–60 % on soft or sandy soil.
Overall drawbar efficiency $\approx$ transmission efficiency × tractive efficiency = about 55–75 % on firm soil, and 40–55 % on tilled soil; the drawbar power is thus roughly 45–65 % of the PTO power in field work.

Worked ExampleExample — drawbar power

A tractor of 45 kW PTO power with a transmission efficiency of 92 % and a tractive efficiency of 70 % on firm soil has a drawbar power of $45 \times 0.92 \times 0.70 = 29 kW$ . At a speed of 6 km/h (1.67 m/s) the drawbar pull is $29/1.67 = 17.4 kN$ (about 1770 kgf).

Rolling resistance

Rolling resistance is the force needed to move the tractor along the ground, caused by tyre deformation and soil compaction.

R_{r} = C_{r r} W (or μ_{r} W)

where $W$ = weight on the wheel and $C_{r r}$ = coefficient of rolling resistance: concrete 0.01–0.02, firm dry soil 0.04–0.08, tilled soil 0.1–0.15, soft or sandy soil 0.15–0.3. Larger-diameter, wider tyres and lower inflation (on soft ground) reduce it.

Traction and wheel slip

Traction (tractive force, gross traction) $F_{T}$ is the horizontal force developed by the driving wheels against the soil by shear of the soil. The net traction (drawbar pull) is:

F_{d} = F_{T} - R_{r}

The traction depends on the weight on the driving wheels, the soil strength, the tyre tread, and the slip:

F_{T} = μ_{t} W_{d}

where $μ_{t}$ = coefficient of traction (the tractive force coefficient), which rises with slip up to a peak and then flattens: about 0.3–0.5 on tilled soil, 0.5–0.7 on firm soil (up to 0.8–0.9 on concrete at lower slip), 0.2–0.3 on soft soils; $W_{d}$ = dynamic weight on the driving wheels.

Wheel slip

Slip (%) = \frac{v _{t} - v _{a}}{v _{t}} \times 100

(where $v_{t}$ is the theoretical speed without slip and $v_{a}$ the actual speed under load).

Too little slip (< 5 %) means a lightly loaded or over-ballasted tractor, wasting power in rolling resistance.
Too much slip (> 15–20 %) wastes energy and fuel, damages tyres and soil.
Best operating slip: about 8–12 % on firm soil (and about 10–15 % on tilled ground), at which the tractive efficiency is highest; for 4WD on soft ground 12–15 %.
Remedies for excessive slip: add ballast, lower tyre pressure, reduce the load (smaller implement or shallower depth), use 4WD, fit duals or tracks.

Tractive efficiency

η_{t} = \frac{F _{d} v _{a}}{F _{T} v _{t}} = \frac{F _{d}}{F _{T}} \times (1 - s)

with $s$ = slip fraction.

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