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Cost less than three bucks at walmart, small masterlock combination lock. Enough to keep someone from casually picking up your helmet and walking away.
I use an inexpensive bicycle cable lock and lock it through the triple or wrapped around both sides of the risers now that I have a ride with a nacelle. Added bonus - covers the fork lock on my ride, and provides a hindrance to anyone steering should a thief come 'round. Works best with full face helmets, mind.
Cost less than three bucks at walmart, small masterlock combination lock. Enough to keep someone from casually picking up your helmet and walking away.
Way way back in high school I put a combination lock on my car's hood to prevent theft of my goodies underneath it. Then I had a engine fire and... well... it's hard to calmly open a combination lock under duress.
I don't use combination locks anymore.
On the other hand, I have been able to open many cheap combination locks with just a little pressure on the lock and a feel for when the "notches" seemed to line up. Not that anyone would go through that trouble for a helmet, but a keyed lock is just easier to use in most circumstances.