Sentence:  He slapped the reins on the back of the powerful gray horse and held on as the sulky 's <WSD>wheels</WSD> hit a pothole and came out with a jolt and went on . 
Meaning of the word marked with <WSD> tags:  a simple machine consisting of a circular frame with spokes (or a solid disc) that can rotate on a shaft or axle (as in vehicles or other machines)