Derogatory term for diminutive small or tatty spare motorcycle used to stand in for a proper machine.
Common to owners of vintage British motorcycles. Assumed 'Crunch' refers to accident or broken down status.
'Brit' biker would be embarrassed seen riding small Japanese bike or 'Jap Crap'. Hence importance of term 'Crunch' to offer explanation for such odd behaviour as only due to being hard up whilst simultaneously affirming existence of other superior machine.
Honda stepthru C50/C90 or 'granny basher' is a typical. 'Crunch.'
'Hey, what's with the plastic crap?'
'Yeah, its my crunch, the old nail's in bits'