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I have a question which might be asked by honest people wanting to protect their bike from thieves, but unfortunately can also be asked by bike thieves. I hope this won't be downvoted, my intentions are entirely honest.

I have recognised a vulnerability in my bike-locking habits. I sometimes cycle to work into a commercial area with no residential buildings. This means that during the day, it's probably very hard to use an angle grinder to steal my bike (locked with a hardened steel U-lock into a solid bike stand). That would be noticed instantly because there are so many people working in these >10-story buildings in this highly dense area, and the thief wouldn't have enough time using such a tool making a huge noise.

However, I realized that using a battery powered hot glue gun, it's possible for a thief to quickly prevent me from opening my lock, with absolutely no noise, in just two or three seconds. Then the thief can steal the bike with a battery powered angle grinder, during the next night, when practically nobody is present in this commercial area (there are no residential buildings there).

Another way for the bike thief to immobilize the bike would be to add a secondary lock, that the bike thief owns, with me having no key for that lock.

Previously, I wouldn't have any defense against this kind of quite clever attack. However, recently I bought a battery powered angle grinder for sharpening lawnmower blades, and I recognised if I notice this kind of attack, I could quickly take a taxi home, get the angle grinder with cutting wheels of the proper type and count (in case I need more than one wheel) plus receipt as proof that I'm the owner of the bike (in case the police notices me using the angle grinder against my bike U-lock), and drive to work by car immediately to save my bike by cutting the lock before the thief does it. However, to be prepared against this kind of attack, I need to buy some cutting wheels, of the correct specifications, and enough many that I can be certain to cut my lock with it.

The hardware store sells these kinds of cutting wheels compatible with my battery angle grinder (I listed only wheels made for metals, there are some wheels intended for cutting rock that probably aren't applicable):

  • 1.0 mm, 125 mm, for steel or stainless steel, 1.49 EUR (so cheap I already decided I'll buy few)
  • 1.6 mm, 125 mm, for steel or stainless steel, 1.49 EUR (so cheap I already decided I'll buy few)
  • 2.5 mm, 125 mm, for metals, 1.49 EUR (so cheap I already decided I'll buy few)
  • 1.4 mm, 125 mm, diamond, for metals, 14.90 EUR

Which and how many of these would be the best choice to be certain I can cut my bike lock before the thief does that after disabling the lock with glue? I need to buy them in advance as any delay in buying them as-needed would give the bike thief more time.

Are the standard non-diamond wheels hopeless against hardened U-lock shackles? Should I have the expensive diamond wheel for cutting through an U-lock, and if so, how many diamond wheels are expected to be required?

The lock is Kryptonite Evolution Mini 5. But then again, a bike thief can disable a bike by putting an even more hefty secondary lock, so I may need to be prepared to cut through the Mini 5 plus another lock that's the most secure U-lock out there.

juhist
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    It seems a bit strange to ask if we know if inexpensive products you already purchased will work for your use case. Is this a setup for a self-answer? It's OK if it is, but this seems odd. – Paul H Sep 26 '22 at 18:22
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    Most bike thefts are opportunity attacks. The amount of time and effort using the method you describe is incredibly rare and unless you're locking a 5-figure bike outside in the same spot on a daily basis, I have no idea why one would go through this much speculation on the edge case vulnerabilities of a lock. – Gabriel Sep 26 '22 at 19:13
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    @Gabriel: bike thieves do that on the regular around here. Putting glue in a lock is super cheap and easy, and if they don't even bother coming back it's no sweat off their back. – whatsisname Sep 26 '22 at 22:33
  • While your office complex may be populated, you may be overestimating the people's amount of concern. – whatsisname Sep 26 '22 at 22:34
  • How would you prove the bike is your bike when you return to cut the lock? – Andrew Henle Sep 26 '22 at 23:19
  • @whatsisname yeah, but I daresay an office complex is relatively good in that regard. Sure, many of the people working there will be to deep in their own business to care, but there will likely also be clerks doing boring work, easily distracted by the screeching of an angle grinder, and eager for an excuse to focus on something more interesting like ringing up the police. And the thief will have it more difficult to abort-and-blend-into-the-crowd there, compared to a residential- or shopping area. It could still be done, but it definitely requires some extra audacity. – leftaroundabout Sep 26 '22 at 23:34
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    Note that it is likely easier for the thieves to cut through the bike rack than through the lock. – Ray Butterworth Sep 27 '22 at 01:37
  • I would expect thinner to be quicker, as you have to grind away less material. The normal abrasive grit of cutting wheels (aluminium oxide or silicon carbide) is harder than hardened steel anyway. BTW the easiest, most compact way to immobilise someone else's lock is superglue - not heat source required. The fix for that is a soak in acetone (e.g. extra strength nail polish remover); you might be able to fill the lock with it repeatedly and shake it out – Chris H Sep 27 '22 at 14:44
  • @PaulH - "will my cheap tools do in an emergency?" seems reasonable, especially as testing would destroy an expensive lock. I briefly had a failed D-lock and would have tried a few things (first, liquid nitrogen to make it brittle) but had to give it back to get my refund. – Chris H Sep 27 '22 at 14:47
  • @RayButterworth a devious 2-visit approach used by bike thieves round here: Cut the rack while empty. Cover the cut with a sticker (perhaps one promoting cycling - there were some available free that happened to be the right sort of size). When the thief comes back and a nice bike is locked up to the rack, remove or slit the sticker, and bend the rack enough to get the bike off. That works for surprisingly big racks. Locking through the chain- or seat-stays to secure the back wheel also means the bike can't be ridden so is some mitigation - but if they turn up in a van that's no help. – Chris H Sep 27 '22 at 14:54
  • @AndrewHenle I'm sure the OP isn't in the UK, but similar schemes to our Bike Register database exist in other places - record your serial number there when you buy the bike (selling 2nd hand is supported, and indelible security marking kits are available) – Chris H Sep 27 '22 at 14:56
  • In my major city (Seattle), bike thieves cut locks with crowds around, watching and recording, and no one lifts a finger to stop them. Calling police would be useless, because they won't show up, thief will not be prosecuted, etc. etc. – kreemoweet Jan 16 '23 at 20:07

2 Answers2

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Steel is a bad match for diamond power grinders, the diamond chemically reacts with the iron, wasting the diamonds. Otherwise it doesn't matter much which wheel you use as long as it is the thinner cutting type and not the thicker grinding type. Extra thin takes less power and cuts faster but is more fragile. Modern abrasives on a power tool don't care about the heat treatment of the steel. The alloy of steel has more to do with abrasive wear resistance, but that applies to most abrasives equally.

I don't know of any companies that are still using old fashioned natural abrasive in their wheels, all you find now are aluminum oxide(general purpose) and silicon carbide(for non-ferrous; brass aluminum and such). There are some slight formula differences that high-production manufacturers would care about, but totally insignificant for cutting a lock.

As for super abrasives, they are favored for high production because they last longer and companies pay someone to change out worn wheels. Diamond is for grinding of non-ferrous, ceramic, rock, carbide, and non-power(cold and slow) grinding of ferrous. cBN, cubic boron nitride, is used for power grinding ferrous metals because it doesn't react at the high contact temperatures.

Max Power
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The specific type of cutting wheel is not as important as the material being cut. That being said, as already mentioned, a diamond wheel is best for other things not steel and alloys of steel.

Have you considered another u lock? While it may not stop the thief from being able to put a secondary lock on your bike there are now bike locks that cannot be cut with a grinder in a reasonable amount of time with a portable grinder. A lock like this may be enough to keep your bike safe even if the thief has plenty of time.

Ted Hohl
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Oliver Wright
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    While your suggestion is an interesting one, you're not actually answering the question except as an aside; this would probably be better posted as a comment. – DavidW Jan 16 '23 at 16:36
  • Welcome to the site - please take a moment to read the [tour] and also, you appear affiliated with the site linked. If so, you must disclose that affiliation within your answer. http://bicycles.stackexchange.com/help/behavior Use [edit] to update your answers. – Criggie Jan 16 '23 at 19:25