About a year ago I bought the bike frame of my dreams, only to realize later that it had a small crack where the seat tube meets the bottom bracket. The guy who sold it to me covered the crack with nail polish and told me it was just a small chip in the paint that he covered to prevent rust. I was hoodwinked, and ripped off, but luckily I was able to (with the help of friends) repair the damage.
For this project you will need:
Broken Bike Frame
A Piece of Steel Tube
Work Bench with Vice
TIG Welder (and friend with welding experience)
Sand Blaster (or grinder with wire wheel)
Die Grinder and Carbide Bit
Step 1: Take the Bike Apart
Before I could do any work on the frame I had to take all the components off in the area of the damage. Actually, I had to take off almost all the components to fit it in the sand blaster. Many parts, such as the crank arms and bottom bracket require specific tools. If you don't have these most bike shops sell them. It is possible you can get someone at a bike shop to take the bike apart for you but it will pay off in the long run to buy the tools to do it yourself.
Step 2: Tape Off the Area
Before any welding takes place you will need to take the paint off. Before you do that it is a really good idea to protect the rest of the bike. I went a bit overboard with this step but, better safe than sorry.
Wrap some tape around each of the tubes of the bike about 2 or 3 inches away from the damaged area. Blue painters tape works fairly well for this but make sure you put a few layers. A powerful sand blaster may be able to get through one or two layers.
Once you have the all the tubes taped off wrap the rest of the bike with plastic and/or more tape. A plastic trash bag is a good thing to use. You don't really need to cover the entire bike like I do, but it never hurt to make sure no grit gets into any of the other moving parts.
I stuffed the bottom bracket with cardboard and taped that off too. This is necessary to protect the threads inside the bottom bracket. Damage to those threads will not bode well for the health of your bike.
For those without a sand blaster, it is possible to do this with a wire wheel on an angle grinder but it is way harder to get a nice clean edge.
Step 3: Sand Blast
Stripping the paint off of the frame before welding is crucial. Any paint will burn and create gasses that are toxic for you and damaging for the weld.
A sand blaster is the best choice for this but if you don't have access to one an angle grinder with a wire wheel will probably do the trick just fine.
Check the inside of the bottom bracket for paint too and remove that with a flap wheel or sanding attachment for your die grinder, but be careful of the threads. Paint on the inside is less likely to be a problem but in order to err on the side of caution you should remove this pain if possible.
Step 4: Making the Bandaid
Warning, this step is long, please read it all if you have never done work on a bike frame before.
In order to make a good repair you will need to have a really well shaped patch. I suggest ordering 4130 steel tube from McMaster Carr (http://www.mcmaster.com/#steel-hollow-rods/=tfxxeg) with an internal diameter equal to the outside diameter of the tube you are repairing. The best thickness of tube is probably .058 inches. The thickness is important because you want it to match the bike frame tubes as closely as possible, so that it is easier to weld.
Clamp your tube in a vice and and cut a slit down the center of it about 2 inches long with a hack saw. Now cut those pieces of tube off. You will only need one of these pieces in the end but if you are like me it won't turn out perfectly the first time.
If you have tubes that are not perfectly round use your bench vice to bend the piece of tube to shape. In my case the seat tube was slightly oversized at the bottom only so I needed to squeeze the tube one one side and not on the other to get a perfect fit.
Once you have the tube bent into a shape that will fit the frame you can start shaping it. Place the tube piece on the frame so that the cut edges are on the front and back of the frame not the right and left side. This is extremely important to the success of this repair, and I'll try to explain why.
A bike has pretty extreme forces acting on it, and most of them are generated by your legs on the pedals. This creates a force acting left and right along the bottom bracket, which translates to a force let and right along the length of the seat tube. There is much less force created in the center of the bike frame, which creates a neutral axis. You want most of your welds to be following the neural axis rather than concentrated where there is a lot of bending forces. Welding creates an extreme amount of heat, which has a tendency to weaken the steel tubes. If the welds are along the neutral axis only there will be less chance of the the frame's strength being effected. There is also a section of tube on the ends that is thicker to allow for the original welding of the bike. This makes it okay to weld where there is already a joint.
Now that we understand where it is okay to weld and where it is not, it is time to shape the patch. Start by making the bottom edge of the patch match the contours of the tubes it is meeting. This part took me a while because I was going back and fourth between grinding with die grinder and checking the fit on the frame. In order to get the best fit possible grind curves at an angle towards the inside of the tube to make room for the existing welds on the bike.
Once the bottom fits to your satisfaction create a curve on the top of the tube as well. This is not an arbitrary curve to make it look nice, it is actually critically important. A flat edge on the top of the tube will create a localized stress on the frame making it much more likely to break there. An upward facing curve will create what my bike builder friend refers to as a 'can opener'.
The final patch should be about two inches tall and look like a section from a lugged bike frame (If you are not familiar with the difference between lugged frames and welded frames look it up. This repair is only good for welded frames not lugged.)
Step 5: Welding
This is the most technical step in the process and I don't have a ton of wisdom to share here, because I am not a TIG welder. That being said, you should not be attempting this yourself unless you are an experienced welder.
Oh, and one more thing before you weld. Cracks have a tendency to propagate even after they are patched over. In order to prevent this drill a small hole at each end of the crack. This will precent the crack from growing and causing any further damage.
My strategy was to find a friend who is a welder (and specifically a bike frame welder) to do the job for me. Friends with this high level of skill are great to have but hard to find so all I can do here is wish you the best of luck.
One suggestion I do have is to buy extra tube of the same thickness and practice until you can get a good weld between two pieces.
Make sure that the welds are consistent on the bottom and sides of the patch but do NOT weld the top of the patch. Welding the top will create a heat effected zone and weaken the frame.
Step 6: Seal
Since there is not a weld on the top of the patch there may be a tiny gap. Even if you made a perfect patch there is a chance it moved slightly while welding. It is a good idea to fill this gap with a small amount of epoxy or a gap filler like bondo in order to prevent water from getting in and rusting your frame. This will not add any strength to the patch but it will help to make it last a long time.
Step 7: Paint
Painting a bike frame, or any steel object, has two purposes. First, it looks better to have painted surfaces rather than raw metal. Secondly, paint protects the steel from rust and other damage.
Before painting tape off the bike again in the same manner that you did before sand blasting. Paint doesn't stick as well to other paint so try to match the tape line pretty close. It is better to paint a small area of already painted metal than to leave anything bare so I chose to wrap the tape about a quarter inch above the previous tape line.
Once you have it taped up apply a coat of primer. Primer acts as an interface between the top coat and the raw metal. Read the label on the can in regards to when it is appropriate to add a top coat.
Apply at least two coats of you final paint color in order to create a durable coating. It is also not a bad idea to put a clear coat on top of that also to create an even stronger seal and a smoother finish.
Step 8: Put It All Back Together and Ride
Now that your frame is repaired you can put all of the components back on. I could go into detail on assembling a bike but that is a story for another day. There is a good amount of info online about how to adjust bottom brackets and derailleurs if you have never done it before. If you are not confident about this, there is no shame in taking to a bike shop to get it tuned up.
Finally your bike is ready to ride again. I was extremely relieved to have it back after a total of 4 days without it. If it is your primary means of transportation I assume you will feel the same.