A security device for use on shopping carts comprises an extra wheel adapted to be installed on a cart such that it normally travels along the ground at location spaced behind the front wheels, and a locking assembly for locking the extra wheel in a turn-inducing orientation when actuated by someone pushing the cart over a raised barrier. The extra wheel is mounted on the cart by a telescopic mounting assembly comprising first and second telescoping cylinders. The first cylinder is adapted to be mounted on the cart in a vertical orientation at a position spaced behind the front wheels, and the extra wheel is mounted at the lower end of the second cylinder which is telescopically engaged in the first cylinder. Swivelling motion of the extra wheel is allowed by rotation of the second cylinder, and the locking assembly is arranged to lock the second cylinder against rotation as soon as the operator attempts to make a turn after it has been actuated by the second cylinder sliding out of the first cylinder as a result of the front end of the cart passing over the barrier.
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