[Note: I no longer believe that one of the bike locks had an aluminum cross-piece; I think it was just non-hardened steel.]

With regard to the recent thread on breaking bike locks with extreme cold:

Yesterday, I and an intrepid band of fellow adventurers tried a few experiments with bike locks and liquid nitrogen (LN).

We had two locks, both of "U"-type: a 14-year-old Citadel lock (top of the line in its day) and a more recent Master U-lock. Before the LN experiment, the Master lock had been forced open with a hydraulic jack.

Both locks have (apparently) hardened steel U's. The Citadel lock also had a steel cross-bar; the Master's cross-bar was (amazingly) aluminum. A few minutes with a hacksaw would have been sufficient to cut the Master's cross-bar---fellow experimenter Joe Chew cut halfway through with a power saw in less than 30 seconds.

We did not try to duplicate an actual bike theft situation---when we wished to hammer on a lock, we simply placed it on a metal plate on the ground and banged away. (More on this later).

First, we tried breaking the metal "U" from the Master lock. I banged on it quite hard with a light (2-lb or so) chisel-headed steel hammer, with no effect. Then, we immersed one end of the U in a cup of LN for about 1.5 minutes, and I hammered away again. Again, there was no apparent effect---certainly no shattering or cracking.

We then assaulted the Citadel U-lock. With the cross-bar in place and the lock engaged, we immersed one end (with the locking mechanism) in the cup of LN for about 2 minutes. I then hammered on the cross-bar for about 20 seconds. At about the 5th blow, a longitudinal crack appeared on one side of the lock, directly below the point being hit. A similar crack appeared on the opposite side, where the lock rested against the metal plate. Subsequent blows enlarged the crack somewhat but did not completely break the lock. At this point the lock was still engaged. We then immersed the lock for another minute, and I hammered at it again. About 8 blows sufficed to break the end of the lock completely apart---the lock "shattered" in the sense that the end of the lock broke into small shards, but the blows were much harder than would have been required to break a column of glass. As before, once the bar had warmed up a bit, continued hammering had no effect---even though fractures were clearly visible in the metal. I, and the observers, were certainly convinced that the LN treatment was necessary to allow the lock to be broken this way.

Finally, we moved to the cross-bar of the Master lock. The end was immersed for about 1.5 minutes, and then Joe Chew broke it with a single moderate blow of the same hammer. As I mentioned, this bar was aluminum rather than steel. A couple of blows utterly destroyed the end of the bar. Again, once the bar warmed up, further blows had little effect: visible nicks and dents appeared under the hammer blows, but there was none of the cracking and breaking into shards that was apparent when the bar was near LN temperature.

What did we learn?

1. Different locks are different. The Master U-lock, with an aluminum cross-bar rather than the Citadel's steel, was much easier to break (and in fact could have been sawed through, if we'd wanted to).

2. Liquid Nitrogen temperatures do serve to embrittle at least some steel and aluminum enough that these locks can be broken with a few hard hammer blows.

3. The most vulnerable part of these locks (to this kind of assault) is the end of the cross-bar, where the locking mechanism is. Here the metal casing is at its thinnest.

4. The "U" portion of the locks is, as far as we can tell, invulnerable to this sort of attack. (In both cases, the U portion appeared to be hardened steel). Presumably, the steel of the cross-bar was vulnerable only because around the lock it is much thinner than the steel in the U portion. But perhaps more importantly,

5. Although LN enabled us to break both locks, I doubt bike thieves actually use this method, unless they do it just for kicks. A hydraulic jack made short work of the Master and would probably have worked on the Citadel as well. A hacksaw would have worked on the Master. Given the speed and surety (and relative inconspicuousness) of the hydraulic jack method, it seems unlikely that a bike thief would prefer the LN method, requiring a source of liquid nitrogen, several minutes of waiting, and wailing away with a hammer for 30 seconds or so.

On the other hand, if I actually need to break my U-lock sometime (if I lose the keys, for example), I'll consider the LN method, simply because I have easy access to LN and I don't have one of the tiny hydraulic jacks the thieves use.

Note: As I mentioned above, we did not try to duplicate the physical setup of a locked bike. Actually "liberating" a locked bike would require either a great deal of LN (to be poured on the mechanism) or a container shaped in such a way as to allow a portion of the lock to be immersed. This would require a certain amount of ingenuity, but does not pose any great difficulties: I could certainly find or make such a container in half an hour or so. More difficult would be finding or making a suitable surface to hammer against. It's possible that the head of a sledgehammer, or some similar weighty and hard metal object, would suffice as an anvil, but some ingenuity would be required to find a way to hold it in place. Again, though, if pressed I'm sure I could work this out, so I'm sure a thief could too. But I doubt a thief would go to the trouble, since other methods are available for breaking these locks.

Additional note: my officemate just told me that he once broke a Kryptonite lock with LN, pouring the LN on the lock rather than immersing the lock. His experiences are in line with my observations on the "Citadel" lock--- it required several hard blows, but did eventually fracture. He said the lock broke at the hole where the U inserts, rather than at the very end where the key inserts. I took a few photos to document the results of our little experiment, but I don't have plans to distribute them or anything. If you have a burning desire to have one, though, please get in touch.

--Phil Price

Oh, one final note: We used an un-insulated 10-oz wax paper cup to hold the LN during lock immersion. Over the entire course of the "experiments", we used perhaps 4 cupfuls of liquid nitrogen.