The invention relates to a castor which can rotate about a vertical axis. Said castor is mounted on a lever arm and can be locked by means of brake blocks which are located on the castor housing or support, the locking of the castor wheel or the frictional engagement of the brake blocks taking place by the action of a spring which acts on the lever arm. Castors of this kind are used in particular on mobile chairs. They lock the chair when relieved of load and unlock it automatically when loaded. Hitherto, there was the problem of matching on the one hand the opposing forces of the weights of the chair and the chair user and on the other hand the rolling away resistance to be be overcome by this loading which varies considerably in each case by means of the spring force despite the contrary effects in such a way that the travel wheel remains nevertheless functionally reliable and uncomplicated. This object is achieved according to the invention in that the rolling away resistance which originates from the spring is variable in such a way that the spring is inserted in the roller housing or support in an interchangeable and/or readjustable plug-in arrangement. <IMAGE>
