Working out the Left-right-left combination to a lock

Conventionally, a combination lock is opened by turning the dial two turns to the right, then to the first number, one turn left, and left to the second number, and then right again to the last number. This series of turns causes the rotors of the locks to have their notches aligned with each other (see wooden combination lock model)

However, there's nothing that prevents you from starting the combination counterclockwise instead of clockwise, and conventional combination locks can indeed be opened this way.
The left-right-left combination lock trick

Because the tabs on the rotors have non-zero thickness, slightly less than a full turn is needed to catch subsequent rotors. On account of this, to open a combination lock by turning left right left instead of right left right, the first number needs to be several numbers further to the right, and the second number needs to be several to the left.

The table below shows several locks that I worked out the left-right-left combination for. Note that for the Dudley lock, the left-right-left has the first number increase and the second number decreased -- exactly opposite of the other locks. This is because turning clockwise on a dudley increases numbers, but decreases them on the other locks.

The last number of the combinations doesn't change because the front most rotor is directly coupled to the dial, so the direction of approach doesn't change its position with respect to the dial.

Lock type
Combinations
Difference RLR to LRL
Dudley  Right-left-right: 30 - 14 - 47 First number increases by 14;
second number decreased by 10
Left-right-left:44 - 10 - 47

Master  Right-left-right:15 - 17 - 31 First number decreased by 7;
second number increased by 1
Left-right-left: 8 - 18 - 31

Dollar  store  
lock 1
Right-left-right:  5 - 26 - 7 First number decreased by 7;
second number increased by 2
Left-Right-left: 38 - 28 - 7

Dollar  store
lock 2
Right-left-right:27 -  3 - 24 First number decreased by 8;
second number increased by 4
Left-Right-left:19 -  7 - 24

I was hoping that the relationship for different locks of the same make would be the same, but it appears that at least for the two dollar store locks that I tried, the difference for the left-right-left combination is not quite identical.

The differences between left-right-left and right-left-right combinations are similar for the master lock and its knockoffs. So a best first try would be to apply the differences from the table above to your combination and try each one to see if any one of them opens the lock going left-right-left. Don't forget to turn the lock a full turn clockwise after the first number - it sounds silly, but I forgot to to do this often.

Determining the middle number

Start by dialing the first two numbers of the combination into your lock as you would normally. After the second number, turn left dial right by a whole turn. At some point past the last number of the combination, there will be a slight tick and increase in resistance on the dial. The exact position where this happens is the value of the second number in the left-right-left combination. Unfortunately, the slower you turn, the harder it is to notice this tick. It's easier to feel and hear this tick if you turn the dial faster, but that makes it hard to spot exactly where the dial was when you do that. It will take you several tries to work this out regardless, so might as well go fast for some of the tries.

You can verify that the second number for left-right-left is correct doing the following set of turns on the lock: Two turns right, to the first number. Then turn one and three quarter turns to the left, and then right (by three quarters of a turn) to what you believe is the middle number for left-right-left, and then reverse direction to the left to the final number. If this opens the lock, you have found the middle number for the left-right-left combination.

Determining the first number

To work out the first number, one could in theory try to listen and feel for the tick where the third rotor is engaged by the second rotor. I was unsuccessful with this method so I can't really recommend it But if you know the middle and last numbers, it doesn't take that many tries to work out the first number for the left-right-left combination by trial and error. Just try the first number from the original combination, followed by your just derived middle number followed by the last number. This probably won't work, but just keep decreasing the first number by 1 (or increase in case of a Dudley), and try the combination again. Keep iterating on the first number until you find a combination that opens the lock.

Optimizing the combination

Once you have a combination that opens the lock, try varying each number up and down a few and try these. Most locks will open with numbers off by one from the combination as well. For example, suppose you found the left-right-left combination that works to be: 10-31-24, Try opening it with 9-31-24, and if that works, try 8-31-24. Similarly, try it in the other direction, trying 11-31-24, and if that works, try 12-31-24. Now, you may find that the lock opened for 10-31-24, 11-31-24, and 12-31-24. In that case, the best combination to use would be 11-31-24 because that leaves the greatest margin of error for dialing in the number. Apply the same method to get the best value for the middle number.

What's the use?

And what is the use of working out the left-right-left combination of a lock? Well, mostly it's just something cool to do. But it could be useful in that you could write the left-right-left combination down near the lock, and other people, after trying it, would assume it's the wrong combination. But you, knowing that it's a left-right-left combination, will know how to open the lock without having to remember the numbers.


For more on the mechanics of a combination lock,
check out my Wooden combination lock model


See also:


Wooden air engine

Pen shaking centrifuge

Wooden burr puzzles

Wooden tank thread
vehicle

modular marble machine

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