Drawbar Power

Power is force times velocity, and at the drawbar that is pull times travel speed. The equation is elementary; the reason it matters is that the drawbar is the last link in a chain that has already lost a third of the engine's output before the implement feels anything.

A tractor is rated three ways and the numbers are not interchangeable. Engine power is what the flywheel makes. PTO power is what survives the transmission to the shaft, typically around 85–90% of engine power. Drawbar power is what survives the transmission AND the tyres, and it is the smallest of the three: roughly 75–85% of engine power on concrete, 65–75% on firm soil, and as little as 50% on soft or loose ground. The losses are rolling resistance and wheel slip, both of which are functions of the surface rather than the machine. Sizing a tractor by comparing an implement's drawbar requirement to an engine rating is the classic error, and it produces a tractor that is comfortably a size too small.

The trade shortcuts are worth deriving once so they stop being magic. In imperial, drawbar hp = pull in lbf × speed in mph ÷ 375, because one horsepower is 33,000 ft·lbf per minute and a mile per hour is 88 feet per minute, so $88/33{,}000 = 1/375$. In metric, kW = kN × km/h ÷ 3.6, which is just the km/h to m/s conversion wearing a different hat. Both are the same product this page computes, expressed in the units the specification sheet happens to use.

The design consequence is that draft and speed trade against each other at constant power. A tractor that cannot pull a given implement at 8 km/h can usually pull it at 6, and the operator's instinct — drop a gear — is the correct response to a power limit. What it cannot fix is a TRACTION limit, which looks similar from the seat and is entirely different underneath: when the tyres cannot transmit the pull, more throttle produces more slip and no more work. Distinguishing the two is what the wheel slip measurement is for, and getting it wrong means adding power to a machine that needed ballast.