Stinky-Dye Bike Lock

The standard bike lock is a simple U-shaped piece of hardened steel with a locking cross piece. Hardened steel is tough to cut, but modern tools make this much less of a chore than it used to be. For example, a Chinesium angle grinder complete with cut off disk will get you through a lock in less than 5 minutes for <$40. If you want to avoid sparks and the need for mains electricity, then an electrohydraulic rebar cutter will chop through in 1 minute with nothing more than a "thunk". Alternatively, a good cordless drill can make short work of the lock face-plate with judicious use of a tungsten carbide bit... the only downside there is the ferocious self-feed once you break through to the brass. There's other methods, the hydraulic car jack etc. but all the quick methods require destruction of parts of the lock.

To discourage this, let's make the lock double-walled. The shackle is now steel-void-steel with the walls still being relatively robust >1mm thick to avoid accidents. Between the walls, we fill the void with a liquid. The liquid should be under significant pressure such that cutting through the outer layer causes a spray through the newly-made hole. The liquid should get all over the place, especially on the perp.

The choice of liquid is the fun part. Shops have been fitting clothes with dye-labeled tags for decades, but this isn't a huge deterrent to the average late-night scumbag so I propose the addition of a really nasty smelling substance, putrescine or thioacetone for example. Efforts should be made to make the liquid sticky and persistent so it hangs around on any tools/clothes for as long as possible.

If this proves effective then the hardened steel can be replaced by a tough aluminum since the cut resistance of steel is no longer required.