Influence of Suspension, Tires, and Shock Absorbers on Vibration in a Two-horse Trailer

Combinations of vehicle suspensions (leaf-spring and torsion-bar), tires bias-ply (311 kPa), normal-pressure (344 kPa) and low-pressure (172 kPa) radials, and shock absorbers were evaluated for smoothness of ride and common frequencies of vibration using a two-horse, bumper-pull, tandem-axle, forward-facing trailer. Accelerations on the x-axis (dorsal-ventral, upward-downward) were measured on each wheel and each corner of the trailer while the trailer was driven over fourteen 25-mm-diameter cables spaced at 4.8-m intervals at three speeds (40, 56, and 72 km/h). Twelve combinations of suspension, tires, and shock absorbers were tested in all. Initially, five steel barrels filled with 1 000 kg of water (total mass) were positioned in the trailer to mimic the body masses of two horses. The leaf-spring suspension with low-pressure radial tires and without shock absorbers provided the smoothest ride in terms of the lowest root mean square (rms) accelerations (P < 0.05), but the leaf-spring suspension combined with bias-ply tires and without shock absorbers also had similarly low rms accelerations. The torsion-bar suspension combined with normal-pressure radial tires was the roughest (in terms of rms accelerations) combination tested. Generally, the addition of shock absorbers did not improve ride quality in terms of x-axis acceleration. Several combinations of suspension and tires (leaf-spring suspension with bias-ply, low-pressure and normal-pressure radials, and the torsion-bar suspension with normal-pressure radials) were chosen to test with four live horses riding in the trailer (horses were transported two at a time). Root mean square acceleration values (x-axis only) were measured on the left and right front trailer subframe, the left front hoof and left trunk of one horse, and the right front hoof and right trunk of another while the trailer was driven over smooth and rough roads. The leaf-spring suspension with low-pressure radial tires provided the smoothest ride. Horses traveling on the right side of the trailer experienced more vibration than horses on the left side of the trailer, perhaps because of the poor conditions of asphalt roads near the shoulders.