A Sustained Draft Bound for Donkey and Ox Carts: Working-Day Budgets, Grade Limits, and the Overload Line

Animal carts are commonly loaded by habit, and habit regularly overloads the animal. This paper computes a working-day draft budget for a donkey and a single ox from stated assumptions: sustained draft equal to 10% of body weight (176.6 N for a 180 kg donkey, 490.5 N for a 500 kg ox), reduced by a factor of 0.70 in tropical midday heat (123.6 N and 343.3 N), against a dirt-track rolling coefficient of 0.06 and a cart tare of 100 kg. On level dirt the gross mass limits are 300.0 kg for the donkey and 833.3 kg for the ox; at a grade of 8% they fall to 128.6 kg and 357.1 kg — for the donkey, a payload of only 28.6 kg. Daily output on a five-hour cool-hours day is 1.50 tonne-kilometres for the donkey on the level, falling to 0.21 tonne-kilometres at 8%, and 5.87 tonne-kilometres for the ox on the level, falling to 2.06 at 8%. A midday-only day cuts the donkey's level output to 0.49 tonne-kilometres and leaves it unable to move even the empty cart at an 8% grade. The overload result is plain: a commonly loaded cart of 300 kg gross (100 kg cart, 200 kg payload) requires 176.6 N on level dirt — exactly the donkey's entire sustained draft — so it exceeds the donkey's sustained draft on any grade above 0.0%, needs 294.3 N at 4% and 412.0 N at 8%, and in midday heat is over the limit even on the level. On level dirt, a full 200 kg payload is a zero-grade load for a donkey.