Bicycle O-lock From Disc Brake Rotor

This O-lock or caliper style lock is made from an old worn out disc brake rotor, 160mm diameter.

With an angle grinder or dremel with a cutting disc, cut through the supports so that you're left with just the outer circle. Resist the temptation to throw the cut-out centre like a ninja star - it is jagged and sharp and, if not careful, you may well injure yourself just picking it up to place it directly in your scrap metal bin.

Cut through the centre to create two semi circles.

Switch the cutting disc on your grinder to a grinding disc to smooth out the cuts on the circle so that no sharp jagged edges remain. A bench grinder or a file could also be used.

This particular rotor from Tektro features 6mm holes all around the braking surface, so I didn't need to drill any holes. Put a 6mm diameter bolt through a hole at the end of each semi circle. Fix a nylock nut on it and tighten until the arms are tight against each other but still able to pivot, so that they can be opened and closed.

Cut off the end of the bolt leaving about 3mm excess. With the head of the bolt against something hard (like an anvil) pound the 3mm excess until it flattens over, making it impossible to turn the nut off the end.

Drill out the head of the bolt so that a hex/Allen key cannot grab it.

Fit a length of inner tube around the lock to prevent it from scratching your bike frame.

The lock is now ready to use with a padlock.

For a more robust lock, a thicker motorcycle brake rotor could be used. A shroud around the padlock would also enhance security.