Introduction: Bandage a Broken Bike Frame

Picture of Bandage a Broken Bike Frame

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

Hack Saw

Work Bench with Vice

TIG Welder (and friend with welding experience)

Sand Blaster (or grinder with wire wheel)

Die Grinder and Carbide Bit

Spray Paint

Epoxy

Step 1: Take the Bike Apart

Picture of 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

Picture of 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

Picture of 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

Picture of 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

Picture of 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

Picture of 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

Picture of 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.

Comments

태영최 (author)2014-11-25

Why don't you use some woven carbon fiber with epoxy? This way seems more easy and lightweight.

CobyUnger (author)태영최2014-11-25

I have done that in the past with carbon frames but didn't want to mix materials. That is a good idea though.

throbscottle (author)2014-09-04

I assume there is a good reason, but I don't understand why you made a patch instead of welding along the crack????

zack_man888 (author)2014-09-03

My area of expertise is not specifically in bike-frame welding, so let's get that out of the way lol. But something for your consideration if you have it available is to possibly use a MIG (wire) welder to finish off the top of the patch. The wire welder will be able to fill the hole with a lot less heat than the TIG welder and will give you a true seal. The only reason I am saying this is that epoxy/steel transition points tend to crack and allow moisture in over time because of the difference in the two materials in expansion/contraction due to heating and cooling. If the heating of the metal still concerns you, you may want to look into annealing the frame after welding to regain some strength, but I don't see it being an issue.

zack_man888 (author)2014-09-03

I've never used a sand blaster to perform this kind of work. I don't even own one. I have always been happy with hand-paper (sanding sheets) and a various assortment of wire wheels and sanding flap disks for the grinder and drill. But if you have a sand blaster you might as well use it. Great job on the repair by the way.

Tangski (author)2014-09-03

Good work buddy! This stuff is good to know for everyone. One day, gas prices will put a ton more bikes on our roads, and knowing how to fix them will be important information to have~! I just learned a couple of tips here, and that is information not lost for other applications. Thanks and enjoy your 'new' bike. Looks like you love to ride:) IN Lethbridge today, we are having the world bikers event for the Alberta stages. Very cool and I can't wait to see them race~! Maybe you can catch it on sportsnet too. It does get a lot of viewers.

Cheers!

Vicki

anihver (author)2014-09-02

Good job though I would have put a sleeve inside, drilled some holes in the frame and plug welded it. Also with the cranks out you could have welded the bottom of the sleeve giving it much more strength..
Running the sleeve as high as you could go still giving enough room for the seat.
Big thumbs up for not dismissing the bike and having the initiative to fix it!

CobyUnger (author)anihver2014-09-03

We didn't want to cut the bike apart more than necessary. An internal sleeve woudl have been great but it also would have taken much longer.

supersabba (author)2014-09-03

great tutorial. and great bike. congrats. btw: I hope you got back to the guy who sold it to you and reclaimed part of your money back.

CobyUnger (author)supersabba2014-09-03

If only, if only. I bought it from a guy at a bike swap. This was the only frame he had. There is no way to find him again.

dan.blake.7583 (author)2014-09-03

Why in the world would you epoxy half of the 'patch', instead of weld it !?!? There is a difference between knowing how to weld, and being a welder. A 'welder' (yes me) would not have let the 'patch' slip out of position, he would know how to properly clamp it, an the progression of welding spots to maintain position, and control heat build up.

CobyUnger (author)dan.blake.75832014-09-03

No matter how tight the weld is, there will be a gap small enough for water to leak in and rust the frame. Welding the top of the gusset would create a heat effected zone on thin tube and weaken the frame. The epoxy is there to prevent any possible rust.

moosejaw23 (author)2014-09-02

Man, I don't blame you for being blinded by Bianchi love… That's a gorgeous frame. Nice job!

FriendOfHumanity (author)2014-09-02

Beautiful job and a very nice Instructable; happy riding.

rimar2000 (author)2014-09-01

Very good work!

skloon (author)2014-09-01

just needs some Celeste 227

seamster (author)2014-09-01

Nice work! Impressive work to make this old beauty road-worthy.

I love old ten-speeds like this. Great read, thanks!

bensmiller02 (author)2014-09-01

nice job I was looking at a very similar bike at my local second hand store but while I was waiting for the price to drop someone purchased it. love these older bikes there such a joy to ride.

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Bio: I build, I teach, I learn. Happiest when covered in saw dust, sweat and machine grease. Visit CobyUngerDesign.com for more projects and info.
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