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I forgot to bring my padlock for this trip so my hostel in Taipei lent one to me until I buy one, which I keep forgetting to do.

The one they lent me is a generic "NO. 20" 4-barrel combination lock. There's no brand name in the instruction sheet. It has a feature where you can set or program the lock's combination.

my borrowed padlock

I used the padlock fine about three times. But now I cannot get it to open just a couple of hours after last using it.

I did not forget the code. I set it to a year in the 20th century. I'm not sure the order of the last two digits but I've tried both many times, and I've even gone systematically through all 100 possibilities that begin with 1-9-?-?!

Is it a known problem with this kind of cheap padlock to "forget" or spontaneously change their combination?

(Yes I know I can just cut it off. But I'd feel like a bit of an idiot especially since the lock is not mine and is on loan. In fact the hostel must not own a pair of bolt cutters like I thought most do. They've asked a locksmith to come! I fear this may prove expensive, which is worse than just feeling like an idiot...)

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  • 4
    If the "locksmith" just so happens to be the hostel owner's cousin/friend/business partner etc, this could be an ingenious scam I've not heard of before... Surely buying boltcutters would be cheaper than bringing round a locksmith? In those few hours since last using it, is it possible someone could have tampered with it?
    – user56reinstatemonica8
    Commented Aug 5, 2016 at 8:42
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    One other thing I'd try if it's not too late - sometimes the pins (or whatever is on the inside) don't quite slip in right, so it's worth trying resetting each number from both directions, e.g. if your year was 1972, try 0972 then 1972 again, then 2972 then 1972 again, then 1872 then 1972 again, then 1072 then 1972 again, and so on, and feel or listen for any kind of click
    – user56reinstatemonica8
    Commented Aug 5, 2016 at 8:47
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    ...oh and obviously don't forget to try pushing the bolt in and waggling a little before trying to pull it out to open it each time, in case it's got caught somehow
    – user56reinstatemonica8
    Commented Aug 5, 2016 at 8:55
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    I had this happen with a lock like that. The inside wheels got out of whack and so the combination did not get triggered. I had to gently play with the wheels until the clickys worked; the numbers were not lined up, some of them were in half-position.
    – Gayot Fow
    Commented Aug 5, 2016 at 9:05
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    Cheap combination looks are often easy to open. Pull on the shackle and fiddle with the wheels. You can often feel a resistance when moving out of the correct position. I managed to do that with all the locks I own with no prior lock picking experience (though most of them only had three wheels).
    – CodesInChaos
    Commented Aug 5, 2016 at 14:51

2 Answers 2

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Yes, I'm afraid it does happen, I have a bike lock that has done this three times and counting now! Most typically a digit at the beginning or the end of the combination increments or decrements by one, so if your combination was 5555, try 6555, 4555, 5556 and 5554.

Also, if this didn't work, I'm fairly sure buying a bolt cutter and a replacement lock would be cheaper than calling in a locksmith.

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  • I tried your trick to no avail. I've emailed the hostel owner to see if he has some bolt cutters that his young staff might not know about. I stayed at the same place 2.5 years ago and had his contact though he hasn't been around yet this time.
    – hippietrail
    Commented Aug 5, 2016 at 9:58
  • Getting a locksmith is ridiculous and even bolt cutters are overkill for this kind of cheap lock. A couple of minutes with a hacksaw will suffice (make sure the hostel staff know what you're doing in advance, since you will be trying to break in for a little while). Or just use a couple of spanners.
    – David Richerby
    Commented Aug 5, 2016 at 10:25
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    @DavidRicherby: That's probably true. At the hostel I work for in Sydney we always use the bolt cutters since we have them there for this job. Even those are not enough for toughened locks with larger diameter shackles. But this lock is tiny and weak looking.
    – hippietrail
    Commented Aug 5, 2016 at 11:41
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    @pnuts: I broke it with two screwdrivers without seeing the video. Still feels like cheating (-;
    – hippietrail
    Commented Aug 5, 2016 at 12:43
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    I'll diagnose it at my leisure after my cold is gone. I just hope nobody steals my stuff out of my locker now (-;
    – hippietrail
    Commented Aug 5, 2016 at 13:06
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Here's a trick I learned many years ago when I used to work in a stationary/office supply shop, it always helped me unlock ANY padlock with a combination. You'd be surprised of the number of people who come back the next day to return the padlock/briefcase because they forgot the combination. This should be useful to any traveler since padlocks are always there and the chances of forgetting the combination is high. Even worse, they do change their own combination at times as you faced.

First, Here's a diagram so we know what are we talking about:

enter image description here

Step 1:

Push the rotating dial to one side, making the space between the wheel and the lock larger so you can see through. You will find that the wheel is inserted in a bar/rod. The bar should be visible at this point.

enter image description here

Step 2:

Move the rotating dial one number at a time, until you find a groove in the bar. It should be a small groove/dent, there might be more than on groove, any one will do. Do the same on the rest of the wheels, making sure that all wheels now are on the digits where the same groove was visible. You might need a flash light if the space is too small.

enter image description here

Step 3:

Now that all wheels are aligned as in step 2, move all the digits one number up. So, if the combination from the previous step was "111", then move it to "222". Or if it was "157", the move it to "268". Try to unlock, if not unlocking, then move them 1 step up, and so on. One of these 10 combinations will unlock it for sure.

The above is only to help you in case you forgot your own combination, not to steal bikes or so :)

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    This would make a good lifehacker answer.
    – simbabque
    Commented Aug 5, 2016 at 14:09
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    It also often works to apply constant pressure to the lock, slowly turn the wheels and feel and/or listen for the different click as the groove aligns the pin on each disk. High-end locks would be made so they don't turn with pressure applied, but the simpler ones will turn and the pins will slide against the rings, snapping into the grooves.
    – Jan Hudec
    Commented Aug 5, 2016 at 18:49
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    @hippietrail we changed our minds :D
    – Nean Der Thal
    Commented Aug 6, 2016 at 12:28
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    pretty sure bike thieves already know this one.. and in case it leads to an insurgence of new bike thieves, perhaps the old ones will be sufficiently discouraged and the whole bike thieving industry will collapse upon itself.
    – Eaten by a Grue
    Commented Aug 6, 2016 at 16:07
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    If you lock your bike with a cheap lock like this, you shouldn't be surprised if it is stolen.
    – Mark Stewart
    Commented Aug 6, 2016 at 19:17

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