2. A vehicle including said wheel set forth in claim 1 and including an axle at which said wheel is mounted for rotation about its axis and for supporting a load on said axle.

3. A vehicle as claimed in claim 2 wherein said axis is inclined relative to the horizontal when said vehicle is disposed horizontally, the concave surface of said outer member facing downwards.

4. A vehicle as claimed in claim 2 and including means for adjusting the inclination of said axis relative to said vehicle.

5. A vehicle as claimed in claim 2 and including a second wheel of conventional construction, said second wheel being mounted at said axle upon which said wheel of the invention is mounted, said second wheel being arranged to be movable relative to said wheel of the invention to a ground engaging position in which it supports or substantially supports the load on said axle but is movable to an out-of-use position whereby said wheel of the invention supports the load on said axle.

6. A vehicle as claimed in claim 5 and including means for adjusting the inclination of said axis relative to said vehicle, said second wheel being mounted co-axially with said wheel of the invention whereby the selection of which one of said two wheels at said axle is to be arranged to support the load is effected by adjustment of the angle of inclination of said axis.

7. A vehicle as claimed in claim 2 and including means capable of restraining deflection of said outer member whereby under an increased load said outer member is restrained from deflecting to increase the area of surface contact between said outer member and the ground.

8. A vehicle as claimed in claim 7 wherein said means comprises an arm mounted for pivoting about said axis of said wheel, a roller rotatably mounted at an end of said arm, and means operable to secure said arm in a position in which the axis of said roller is vertically below said axis of said wheel and said roller is in contact with the inner surface of that portion of said outer member which contacts the ground and restrains that portion from movement from its undeflected position.

9. A vehicle as claimed in claim 7 wherein said means comprises a disc member securable in the concave part of said outer member of said wheel, the periphery of said disc memBer engaging the inner surface of said outer member to restrain said outer member from deflection.

10. A vehicle as claimed in claim 9 wherein said disc member comprises a plurality of parts connected together in a manner enabling the configuration of said disc member to be changed from that of a disc towards that of a cylinder, means being provided to effecte the change in configuration of said disc member to its disc shaped configuration and for retaining said disc member in that configuration.