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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.

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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, 2022 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, 2022 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. Sep 26, 2022 at 22:33
  • While your office complex may be populated, you may be overestimating the people's amount of concern. Sep 26, 2022 at 22:34
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    Note that it is likely easier for the thieves to cut through the bike rack than through the lock. Sep 27, 2022 at 1:37

2 Answers 2

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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.

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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.

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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, 2023 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. bicycles.stackexchange.com/help/behavior Use edit to update your answers.
    – Criggie
    Jan 16, 2023 at 19:25

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