Tuesday, 24 November 2015

Fix a Cracked Steel Bike Frame

Fix a Cracked Steel Bike Frame - Intro and Disclaimer

This is the first time I have done this. I'm just throwing it out there (USA) offering it up for scrutiny (UK).

It seems to have worked OK but then it may fail and kill me so please consider this before you choose to use any of the below information. That is your choice.

Fix a Cracked Steel Bike Frame - Causes

Having think about the cause is important.

The crack may have been caused by corrosion of the tubing, or a crash or large impact. Perhaps a seat post extended too far out of the seat tube, perhaps an overloaded rack.

A cracked steel frame - Crack is around 8mm in length.

You need to figure out which one an acute trauma will likley mend up OK, if not too serious. But chronic problems such as corrosion may not respond well after fixing.

The crack I have is located under the crimped portion of the chain stay. Apparently this is an inherently weak part of the frame. So I am attempting a repair.

Fix a Cracked Steel Bike Frame - Method

I have a welder, but have heard that heating up chromoly beyond a certain point weakens it. So I am opting for silver solder. This is not a soilder really but a type of low temperatur brazing which can be down with a normal household blow torch running propane or MAP gas.

I have asked the good citizens of google plus for advice on this one and had some good answers. Post is here. Thanks to +Scott Anderson  +Andy Tong , +Derek Haggerty +David Saul +Seb K and +simon russell-roberts for your answers.

So a 55% silver solder is quite expensive costing around £7 / $12 for  two small rods, buy from ebay with the correct flux. The flux has be be the right stuff.

You will need to clean up the crack and the area around it very well, I used a spinning wire brush thing on a drill. Remember the metal on the frame will be thin so don'y go mad.

Apparently you can use acid based cleaners to get the metal really clean, although some say its OK just to brush.

Once the crack and area around it are clean, prepare the flux. Mix it up like toothpaste and then apply it on the area where you want solder (filler) to go. If the flux is messy the filler will be messy.

For a patch I used the link from a bicycle chain. This (I think) is quite good as it allows you to feed the rod in to the center of the patch (through the two holes). And I should imagine that the plate from a chain has a very high tensile strength. Others may know better (please comment).

Holding the patch in place is tricky but once the flux has been melted it should hold it in place fairly well. Heat up the work, until it is glowing a little, then apply the silver braze / solder rod to the hot metal. It should melt and run in to the joint and beneath the patch very quickly.

If the filler is not taking to the areas where you want it too dip the end of the rod in the powdered flux and try again, it may be that you need more flux.

When finished there should be no gaps, and the filler should be ramped up around  the edge of the patch, and the inside of the holes (if you have any).

Results below. Sorry about blurry photos.

Fix a Cracked Steel Bike Frame - Result

Well time will tell . . . Update: 3 Years Later: Yep still going strong. 

I have painted over this with basic metal paint. I think the soldering will hold, I have tested the strength of silver solder on other objects and it is very high. The only questions are whether the patch is big enough, and whether the frame is not internally corroded.

If this crack were in the main triangle or the (gulp) head tube, I would not attempt a mend.

Environmental Consultants London


  1. My dad brazed one of the two wheel-holding plates back onto the forks of my 531 Falcon (main tubing only, so the forks were just regular steel). Looked a bit ugly, but it worked. The plate came off in about '85, but the bike (once fixed) was regularly ridden both by myself and my 18 stone brother-in-law for the next ten years :-)
    That's one of the beauties of steel - repair ability!

    1. You are quite right. I like the thought of a steel frame for that exact reason. This one holding up well, ridden off road, loaded with 120kg. Can't say I am noticing the "steel is real" feel despite this being a fairly high grade frame.