Stability Bounds for a Two-Wheel Bicycle Cargo Trailer: Hitch Load, Roll, and Cornering Speed on Dirt

Two-wheel cargo trailers are widely used behind bicycles to move goods where motor vehicles are unavailable or uneconomic, yet loading guidance is usually anecdotal. This paper derives the stability bounds from statics and rigid-body cornering mechanics for a representative trailer (tare 10 kg, hitch-to-axle distance 1.00 m, track width 0.70 m) towed by a 75 kg rider on a 15 kg bicycle. The hitch must carry 7–15% of the trailer weight for stable towing, and for this geometry that band corresponds to a load centre of mass only about 10 cm wide, centred just ahead of the trailer axle: for a 40 kg load, between 0.925 m and 1.025 m behind the hitch. Loading the same 40 kg at mid-bed (0.80 m behind the hitch) puts 25% of the weight on the hitch, and at 0.50 m it puts 49% — both far outside the stable band. In turns on dirt (friction coefficient 0.50), the trailer slides before it rolls whenever the combined centre-of-mass height is below 0.70 m; above that height roll governs, and at a combined height of 0.86 m the safe speed in a 10 m radius turn falls to 18.6 km/h. With a safety factor of 1.5 on lateral acceleration, the maximum safe cornering speed at normal stacking heights is 14.6 km/h in a 5 m turn and 20.6 km/h in a 10 m turn, independent of load between 20 kg and 80 kg. The largest load permitted by the trailer wheel ratings is 99.5 kg. The load height limit becomes the binding constraint only for tall loads at higher speeds: holding 20 km/h through a 10 m turn requires the combined centre-of-mass height to stay at or below 0.742 m, which for a 60 kg load means a stack no more than 0.93 m above the bed.