I have a smal bicycle lock. I put the lid over the mirror (so the glass part 'pokes' through the visor hole) and lock the little cable through the front of the lid, and around the grip - I've got wrap around handguards so it's all secure. I would imagine, here in blighty some twxt would just cut the chinstrap, but it's just supposition.

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Never underestimate the power of stupid people in large groups.