can be seen from fig1 the chassis 10 according to the invention is constituted by two parallel lateral flanges 11 connected at their upper ends by a platform 12 intended to receive a boot and equipped at their lower ends with holes 13 for the assembly of the wheel axles respectively the outer wheels 1 and central wheels 2 in addition the chassis 10 is equipped in the area of the central portion of each lateral flange 11 with a downwardly open wide cut out or scalloping 15 extending in height from the lower edge 14 of the flange up to the level of the upper platform 12 each scalloping 15 which has a substantially rectangular shape extends substantially in the longitudinal direction along the entire length between the axles of the central wheels 2 in addition each scalloping 15 is equipped along its edge with a peripheral groove 16 having a rectangular transverse section a wear plate 20 is associated to each scalloping 15 at least partially covering a scalloping such plate being intended to be fixed on the corresponding flange 11 so as to constitute the central detachable part of the chassis as can be seen especially from fig1 and 3 each flange 11 of the chassis includes a recess 17 in its thickness around the scalloping 15 and its depth corresponds to the thickness of each wear plate 20 such that the wear.