Preferably the guiding axle c is loosely guided in slotted guides o for the purpose of preventing the axle c oscillating, for instance under the in-'65 uence of the bearing friction in the bearing b or under the influence of the retarding forces in the mass of the axle, when starting and when the brakes are being applied (Fig. 10). - The abutments may be made of noise-damping 70 material such as r