My Testing Methodology for Angle Grinder Resistant Locks

I thought I would create a new post on my testing methodology for angle grinder-resistant bike locks.

This will keep a lot of detail out of the main roundup page, so I can just concentrate on how to choose the right lock for your needs over there.

It also means I can go into a lot more detail here!

So how do I test the locks?

Each lock is now attacked (to destruction) in multiple different ways:

1. Halfway up the shackle (where most thieves are likely to attack)
2. Where the shackle meets the crossbar (a weak point on most locks)
3. At the crossbar
4. With a selection of different diamond discs
5. One-handed with a battery-powered grinder minus guard (to replicate real-life conditions)

Moreover, I don’t just test one version of each lock. I test multiple versions of each lock over extended periods. In fact, I never really stop testing them!

This is for 2 reasons:

Firstly, there seems to be a lot of inconsistencies (in protection) across different copies of the same locks.

And secondly, I have found serious faults in some of the locks (such as the Kryptonite New York Diamond and Abus Granit Super Extreme 2500), and I need to test multiple versions of these locks before I can confirm that the faults have been fixed (or not).

Plus, of course: building up multiple sets of data over multiple versions of the different locks helps give a much more accurate picture of how secure they really are.

The Overall Aim of My Tests

The overall aim of my destruction tests is to be able to accurately determine which bike locks are the most angle grinder-resistant.

So that means I am most concerned with eliminating any variables that are not related to the fundamental angle grinder resistance of the locks from the tests. Such as variations in the power of the angle grinder or movements to the locks as I cut them.

And that’s why I tend to tape the locks down and use a mains-powered grinder…

Battery vs Mains-Powered Angle Grinders

Most of my tests are conducted with a mains-powered angle grinder. Some people complain that this makes the tests unrealistic, because battery-powered grinders (which thieves would use in the street) are inherently less powerful.

This is not true.

There are plenty of battery-powered angle grinders that are just as powerful as my Makita GA5030R.

Of course a high-powered angle grinder is going to drain the battery pretty quickly (more on this below).

But have no doubt: if my mains-powered Makita GA5030R can cut one of these locks in a couple of minutes or less, a similarly powerful battery-driven angle grinder will have no problems either.

It’s also worth restating here: the tests with a mains-powered angle grinder are not actually meant to replicate real-life street conditions. They are meant to make it easier to compare different locks by eliminating variations in power that might affect the cutting times.

It’s very easy to not notice that your battery is running low and the power is reduced until it’s too late and then the test is invalid.

However, for those that are specifically interested in how these locks fare individually in real-world conditions, I have also started to test them against a battery-powered grinder with the guard off and one hand holding the lock (which is how a thief typically does it).

Immobilized vs. Hand held Locks

In most of my tests the locks are immobilized with tape on a chair in order to reduce any unusual movement that might affect the cut time and make comparisons between different locks more difficult.

You might think when the locks are immobilized like this, they are easier to cut. But that’s not necessarily the case, as two-handed operation of the angle grinder allows you to exert extra force, which can actually wear out the discs quicker and extend cutting time.

Unless you’re using diamond discs…

Diamond Discs vs Abrasive Discs

Most of my tests are carried out with regular abrasive discs. This is because these are the discs that are used by bike thieves.

And the reason that thieves use these discs? They are incredibly effective against regular bike locks.

For example, the Kryptonite Fahgettaboudit Mini u-lock is the most secure regular bike lock available, with an unrivaled 22 mm thick shackle. But that shackle can be cut in just 13 seconds with an abrasive disc.

Conversely, thieves don’t use diamond discs against regular bike locks because they are not very effective.

It could take me over 5 minutes to cut the Kryptonite Fahgettaboudit with a diamond disc!

But with these anti-angle grinder bike locks performing so well against regular abrasive discs, many people think that diamond discs are the way to defeat them.

So I test each lock against a variety of diamond discs: those designed for ceramics, those designed for metals, and multipurpose discs that can take care of both concrete and steel rebar.

Different discs perform differently against different bike locks. But (thankfully) none of them are a magic bullet for defeating angle grinder-resistant bike locks. Because while one disc will cut through each lock eventually:

1. They usually take quite a lot of time
2. They’re extremely noisy and unpleasant to use
3. They’re often blunted by cutting these locks

And don’t forget: compared to abrasive discs, they are rubbish against regular hardened steel bike locks. And a thief is unlikely to be carrying a selection of different discs for different locks. They are far more likely to be carrying whichever discs will easily cut the vast majority of locks.

And those are abrasive discs.

So I don’t think diamond discs are something we need to particularly worry about. But I test them anyway. And there is a vast difference in the way the different locks perform against different diamond discs.

Note: with diamond discs, taping them down (so you can use two hands on the angle grinder) makes a big difference in how long it takes to cut the locks.

For example, it took me over 20 minutes to cut the Kryptonite New York Diamond when I was holding the lock in one hand and using the angle grinder in the other hand. When the lock was taped down and I could use two hands on the angle grinder, I was able to cut it in 3 minutes!

Attacking the Middle of the Shackle

Every lock is cut multiple times at the middle or the top of the shackle. This is where a thief is most likely to attack, so it’s important that we establish how well protected each lock is at this point.

Once cut the first time, I measure the gap that can be made by prying the two parts of the shackle apart. If it’s big enough to slip over the bike and whatever the bike is locked to, this is a big problem, as the lock only needs to be cut once to steal your bike!

Attacking the Base of the Shackle

Most of the locks are slightly weaker at the base of the shackle (where it meets the crossbar). This is usually because there is significantly less anti-angle grinder material at the base of the shackle.

However, in my testing, I’ve also found some locks that were completely unprotected at the base of the shackle (the Kryptonite New York Diamond and the Abus Granit Super Extreme 2500).

Both brands put the issue down to faulty batches and have since claimed to have fixed the problem. However, they haven’t publicly given any information about how many locks were faulty or how to identify them.

As a consequence, a lot of my testing concentrates on testing the locks at this point (especially the offending brands) to determine whether any weaknesses persist or new ones arise. I have tested some of the brands 5 or 6 times here.

Attacking the Crossbar

This is not a point on the lock that thieves will generally target. However, since many people have asked whether it might be a weak point, I have introduced it to my testing procedure, using one hand and no guard on the grinder for better access.

Every lock is cut once here. Then, like the shackle, I measure any gap to establish whether the lock could be slipped off the bike without a second cut.

Attacking “Street Style”

I have also started attacking every lock as a thief would do in a real-life street theft situation. That means:

1. Battery-powered angle grinder
2. No guard on the angle grinder
3. Hold the lock in one hand and the angle grinder in the other hand.

While I don’t think that such tests make it easy to compare the different locks (there are too many instances where the power might fade or a movement of the lock might undermine the results), they do provide some red lines.

How I Determine the Final Security Rating

There is no complicated mathematical formula for determining the security rating each bike lock receives.

And while I do provide all the raw data for you to pore over, I deliberately put the locks in quite broad security tiers to discourage people from obsessing over which is definitively the most angle grinder-resistant lock and basing their purchase solely on that criterion.

This is because:

1. Even though I try to make the tests as scientifically definitive as possible, there will always be variables that are out of my control, so the numbers are not infallible.
2. Once a lock can stand up to 3 or 4 discs, does it matter whether the final number is 5 or 10? No thief is going to bother at that point.

Beyond the security level of the lock, it’s important to also think about the price and how practical the lock is to use on a day-to-day basis, and I want to encourage those considerations before any purchase.

And of course price and practicality go into my overall ratings.

Having said all that, there are certain things worth mentioning about the security ratings:

If an angle grinder can be defeated with one disc, then it is obviously not fit for purpose. It doesn’t matter if it’s only at a very specific point and the main part of the shackle requires 10+ discs.

That's why I couldn’t recommend the Kryptonite New York Diamond or the Abus Granit Super Extreme 2500 while they were defeated so easily at the base of the shackle. And to be honest, I still reduce their score slightly, while I’m not 100% sure the issues are fixed.

Most rating weight is given to the security at the middle and top of the shackle, as that is where the vast majority of thieves will attack. However, the performance at the base, the crossbar, and against diamond discs can move that rating upward or downward slightly.

Questions or suggestions about how to improve my testing process are welcome below…