Jockey wheel

A jockey wheel is a wheel-based mechanical system used for steering or guidance, either:

• a guide wheel — usually operating as a pair — which is used to move, retain, locate and (usually) add tension to a chain, rope or cable which is then fed into the groove of another wheel, toothed wheel, cog or sprocket;
• a (frequently) retractable, smaller, height-adjustable wheel located on the front of a trailer or caravan for the low-speed manoeuvring or manual steering of said trailer-based unit when decoupled from the powered vehicle to which it was attached.