DIY Bicycle U-Lock

Having packed up all of my earthly belongings into the back of an ancient 1992 Ford Ranger, I clunked and bounced along the 689 miles between San Diego, CA, and Provo, UT. After almost 10 hours of driving, I pulled off the freeway and onto a city street, when my truck started making the most ungodly clanging noise I have ever heard. Then, one block from my destination, the engine seized and belched a thick black smoke, tainting the crisp Autumn air. For my first month in college, I was forced to travel solely by bike.

I soon realized that I had left my only bike lock back in San Diego, and would now have to go buy one. At the local bike shop, I was blown away to see the price tags: $30 for a decent cable lock, $20 for the cheap stringy kind that wouldn't hold up to a pair of safety scissors, or a whopping $45 for the strongest option--a hardened steel U-lock.

I knew that I wanted a U-lock, but I definitely didn't have the money for one, especially not with a hefty car repair bill to pay, so I looked for other options. As a student at Brigham Young University, I knew I would have access to the student machine shop--so I decided to make my own!

For under $20, anyone, even without access to a fancy machine shop, can make their own U-lock.

Materials to buy:

32" of 1/2" diameter mild steel rod -- $8

7" of 3/16" x 1" mild steel bar -- about $3 (you might have to buy a full 3' piece)

A hardened steel lock of your choice (I show a simple combination lock, $2.50, but not very strong at all)

Tools:

I had access to a full machine shop, so I ended up using:

-A milling machine with a 9/16" 4-fluted endmill (If you have a band saw, angle grinder, or even a hacksaw with a metal cutting blade, you could use that instead)

-A drill press with a 5/16" bit

-An acetylene torch (anything that could be used to heat metal until pliable, a propane torch would probably be fine)

-A vise/clamps and pliers, to bend the heated steel rod

With my truck out of commission, I had to strap it all to my bike and bike to the shop.

Make the locking end:

First, you'll want to machine one of the ends to be the receptacle for the lock.

-Flatten out the last inch

-Cut it down until it is 1/4" thick

-Depending on the lock you use, you'll want to adjust the hole size. My lock has a 1/4" shackle, so I drilled a 17/32" hole

Now for the fun part: heating and bending the steel!

To set up your jig:

-Find something circular that you can bend the pipe around. I used a 5 1/2" steel pipe, this resulted in a 6" wide U-lock.

-Attach your "form" (the pipe) to your table using heat-resistant clamps.

-I put another clamp on top of the pipe to act as a guide.

-Mark the halfway point of your rod. I scored it with an awl. This can be lined up with your "guide clamp"

Heat it and bend it:

Now you're ready to heat the rod and bend it. Aim to heat a 10" section centered on your halfway mark--this will give you enough to bend it comfortably.

CAUTION! Make sure to bend the rod so that the bend is parallel to the machined end (see pictures).

Once you have a good U-shape, it's time to bend the angled portion on the other end.

-Clamp your U to the table (let it cool first!)

-Heat the last 2 inches of the un-machined end

-Bend the last 1" to a 45 degree angle.

CAUTION! Make sure to leave space so that your cross-bar (step 4) can sit flat. Start your 1" bend above where the cross-bar will want to rest.

Don't worry about having it in a perfect U shape, because that can still be adjusted after it cools.

While letting the U shape cool, you can machine the cross-bar, the piece that holds it all together.

Cut the length:

You want the cross bar to be about 1-1.5" longer than your U is wide. I made a 5" wide U, so my cross bar is about 6" long.

Drill the round hole:

You'll want to make the round hole 17/32" or larger, just barely bigger than your 1/2" diameter rod. If you don't have bits that go by 1/32 of an inch, a 9/16" bit should work just fine.

Center the hole with respect to the width of your bar, and position it about 3/8" away from the end.

Machine the channel (elongated hole):

The trickier part is the elongated hole. It's not essential that the hole is elongated, it could be just another matching hole, but then the crossbar will have free movement up and down your U-Piece. An elongated hole allows the cross bar to sit comfortably on the end of the U-piece, without much jiggling around.

I used a 3/8" endmill to bore out the elongated hole, but if you don't have access to a milling machine, here's what you might try:

-Drill two 3/8" holes with their centers about 7/16" apart

-Use a scroll saw with a metal-cutting blade in it to knock out the center piece

Adjust the U piece to match the crossbar:

Once you have the crossbar made, you can manually bend the "forks" of the U-piece so that they go nicely into the holes of the cross bar. It's a little tough to bend, but can be done by hand.

Rough the buff:

First, buff out all the deeper scratches with high-grit sandpaper or emery cloth.

Polish:

Then, use a buffing wheel and compound to make it look fancy.

With the awkward shape of the U piece, it's sometimes hard to polish the inner surfaces. One approach is to feed the U into the wheel, which allows you to reach almost every point on the inner surface.

Travel the world:

With your lock custom made to the right size, you can lock up anywhere!

If you do have concerns about thieves, a simple solution that makes it look just like your standard U-lock is to place your helmet over the locking mechanism. Thieves know to not even try to bust a U-lock, so your bike will still be the last on their list.

As for me, my truck is back up and running, but now I have no reservations leaving my bike out in the open. With 4 months of usage now, my U-lock is still going strong!

Improvements:

The main weakness of this lock is it's lack of weather resistance--it rusts fairly quickly when exposed to water. I don't have much experience with weatherproofing metal, so please comment and give suggestions on weatherproofing!