A vehicle having a wheel which is movable between a lower position and a raised position by a wheel retraction and lowering mechanism is described, the mechanism including a guide to determine the path of the wheel during retraction, said guide being in the form of a pillar, and characterised in that a lower portion (c;16) of which is slidable relative to an upper portion (B;14) whereby the length of the pillar is less when the wheel is in its retracted position than when it is in its lowered position.