The lateral stability for doubles was analyzed in terms of frequency response and characteristic roots by using simple models (with the degree of freedom considered by taking into account lateral motion and yawing motion around vertical axis but excluding such factors as steering system, rolling, unsprung mass, and friction of coupling point). The value of frequency response found by actual testing was roughly in agreement with the value determined by calculation.Effects of various parameters were also analyzed. This analysis afforded a guideline toward the determination of principal particulars for basic design.The effect of the loading condition of cargo is most unstable when the first trailer is empty of cargo and the second trailer is loaded. Stability is impaired when the trailer's c.g. is excessively shifted to the rear side. Such loss of stability affects the first trailer more than the second trailer. The stability of each trailer increases in proportion as the wheel base gains in length, and with the increasing cornering power of trailer wheels.For stability, the overhang at the position of the pintle hook should be as short as permissible.