Instructables
Picture of Unlock 4 digit cable lock
My brother's friend found my 6ft On Guard cable lock and promptly reset the combination to something he was too lazy to remember. so I cam up with this method to find the lost combination and save the world (my world)
This is the semi-hard way. it's not the exhaustion method (trying every possibility) but it takes a little brain power and reasoning.
I know there's a lot of stuff on line about how to unlock a cable lock but these methods only work for the drug-store variety of locks that have low tolerances and a lot of wiggle room. (i'll explain why later) This one works on the nicer kind. the ones that have half inch braided cables and much tighter tolerances.

Footnotes:
Don't try to use this to steal a bike. It won't work. you can't put enough tension in the system to feel the shape of the disc using your muscles while rotating the tumblers. If that's your reason for being here, kindly piss off.

Edit (2014): I have since retired this lock. After moving to the Roger's park 'hood of Chicago, it has become rapidly apparent that cable locks are not only easy to crack but easy to cut. Kryptonite's higher-end locks still prove to be beyond my ability to crack. their tolerances are tight making the lock seize under tension. the only way to open one seems to be through exhaustion. anyone good with arduino want to make a robot?

I now use a keyed on-guard U-lock.
 
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Step 1: Understand what's inside

Picture of Understand what's inside
inside there are four rotating plates with a notch the shackle to slide through. When you scramble the lock, the plates slide to block the shackle from coming out.

Step 2: Tension the lock

Picture of tension the lock
I wrapped the lock around a chair (pictured) sliding the cables apart increases tension in the system. you need tension to press the teeth of the shackle against the lock discs. this lets you feel the shape of the discs better and decide when one of the teeth isn't pressed against steel disc and is just hanging out in open air.
chuckdav1 month ago

it didnt work for me. i have a kryptonite 1018 cable lock.

ltnemo2000 (author)  chuckdav12 days ago

Some locks can be tricky, especially higher end ones. 2 suggestions:
1. Dump lube into it. You'll be able to feel fine detail in the tumblers better if they slide more easily. You may even want to soak it in mineral oil over night.
2. get a pen and pad and write out everywhere each tumbler catches. this way rather than having 10^4 possible combinations, you'll be left with 4^4 which is actually feasible for exhaustion method.

Hope this helps. If all else fails, smash it with a hammer and send me pics of the internals!

TinaC11 month ago

Thank you! You just made me a hero to my 13-year-old. :-)

blows.blows2 months ago

I bought a 2nd hand bike that had a large 4 digit cable lock - Also onguard, just like the one in the article. I pulled on the cable and felt for the notches. The notches were either there on not so very easy to tell.

I had the lock open in about 15 seconds.. and that's as a first timer!

I am getting something with a key straight away!

CatsConcert2 months ago

Perfect! Worked like a champ.

I forgot my code to my onguard how do I reset it

mpeddicord7 months ago
Thank You for this! I fix up bikes people don't want anymore. This had a BELL Kevlar cable, cracked the combination in about 10 mins. Now instead of using my cable cutters someone will have a really nice lock for this bike I'm rejuvenating! COOL!
azharbass made it!8 months ago

WOW thanks a million! We have a lock that we forgot the combo to for almost 10 years now, and with this 'ible I was able to open it just now! It just took me 2 chairs and a vise-grip (the tumblers were hard to turn) and not even 2 min later it popped open. When you said that it's going to fly apart, I half believed you. I almost got smacked in the face when it sprung apart! It's kind of creepy to think that just anyone with determination can open these things so quickly and easily...

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yiucycle1 year ago
work like charm. found a combination lock and took 5 mins. i just pull the cable apart with my hand and work from right to left. probably wont lock a expensive bike or locks stuff overnight knowing it only take couple mins to unlock.
strain11 year ago
This worked for me!
tbreslow1 year ago
THANK YOU!!! It's kind of scary how easy that was.
blodefood1 year ago
Great 'ible! Sounds like it would take too long for a bike thief anyway. But, to someone who needs the lock, oh, so useful!
jcdiddi2 years ago
THANK YOU THANK YOU THANK YOU
your right. it did startle the crap out of me.
alumofcu2 years ago
I just unlocked mine after 4 years or so. As it was getting closer, I realized....last 4 of ssn...not my birthday month and day....so funny but this method got me to that point and voila! thank you!
dodo1012 years ago
Just found one of those lock, used your method, it worked! Thanks, now I have an awesome free bike lock :)
amitchell133 years ago
YES! Thank you!! I had one of these locks that I put on a year ago and had since lost the combination! Thanks to YOUR tips, I just got it back open! :D
Wow! Thank you so much :-)
I just bought a lock from walmart which wouldn't open with its factory default passcode, but now unlocked it with this instructions.
shadnlyd3 years ago
ding! ding! ding! +1 for use of the word "onomotopeia";