A bob and wheel is a stanza ending used in some Middle English alliterative poetry. The bob is a very short line, often with only one strong stress, and the wheel is a group of four short rhyming lines that follows it. This ending adds a quick shift in rhythm and often marks a turn in tone or thought.
Device used at the end of the main stanzas in alliterative verse such as Sir Gawain and the Green Knight. The 'bob' is a short, one-stress line followed by the 'wheel' - which is a quatrain rhyming a-b-a-b e.g.
Example
The hall was dark; the winter wind blew cold
The embers dimmed and turned to ash and coal
Then came a spark
That warmed the listening hearts of all the old
And set the silence spinning into gold