PURPOSE: To select the weight ratio of two axles in a suspension device such as a truck crane whose one axle is a single wheel type steering axle by inserting a fluid pressure cylinder between a beam for supporting the steering axle and a car body and switching the actuation and non-actuation of a cylinder.

CONSTITUTION: In a large vehicle with adjacent four axles 7, 10, 8, and 9 whose forefront and last axles are single type steering axles 7 and 10 and double wheel type non-steering axles 8 and 9, the weight ratio of the steering axles 7 and 10 is freely changed in two ways by supporting the respective axles 7, 10, 8 and 9 using an equilibrium beam consisting of five pairs of beams 15, 17, and 19, inserting fluid pressure cylinders 53 and 66 between the beams 15 and 19 that support the steering axles 7 and 10 and a car body 1, and switching these cylinders 53 and 66 into the actuation and non-actuation status. As a result, for example, since the axle is weight ratio of a construction vehicle such as a truck crane can be changed in each of advantage values depending upon the differences in the running conditions at field movement and general road running, facility on practical use is exceedingly high.