My last article drew the following comment that gave me food for thought:
I have seen a frame fail at the downtube shifter braze-on. The down tube was installed backwards, the short butt was at the top so this braze-on was installed in the thin center part of the tube. It weakened it enough that it buckled there.
First let me explain that the down tube in this case was not necessarily installed “Upside Down.” Double butted tubes are so-called because they are butted at either end. The butt at one end is longer. That is the end that is cut shorter when building smaller frames.
So yes, if the framebuilder was building a large frame and using the full length of the tube, he could put the long butt at the top, but this would not necessarily be normal practice.
In a production setting, which included small batch production like the Fuso, the down tubes would be pre-mitered at the top end, because that angle was the same across a range of sizes. The long butt was left at the bottom bracket end so it could be cut later to accommodate whatever size frame I was building.
Incidentally, a simple way to tell which end has the long butt, is to balance the tube on a finger in the center. The longer butted end is obviously heavier. Tubes are also usually marked with the maker’s name or trade mark on the end that is not cut, but these marks can get stamped on the wrong end, so best to double check.
The short butt is usually 3 or 4 inches of the heaviest wall thickness, then it tapers down gradually for 3 or 4 more inches to the thinnest part in the middle of the tube. So the gear lever boss would not normally be on the absolute thinnest part of the tube, and even if it was, under normal circumstances it shouldn’t fail.
The commenter mentioned that the tube buckled. This usually means the bike had a front end shunt at some point, and it does not have to be a serious crash. I remember one time in the UK, I built myself a brand new cyclo-cross frame. The first time out I dropped my front wheel in a mud hole, and did a spectacular vertical stand on my front wheel.
I did not go over the top, but simply fell over sideways. Later I noticed the down tube was buckled right behind the bottom head lug. Once a tube is rippled, it will crack and eventually fail.
Barring such accidents a good steel frame will last fifty years or more. Ridden hard enough and long enough metal fatigue will eventually cause it to fail. But how many frames are ridden that hard and that long? Although sometimes a tiny crack can happen during building.
Metal like wood has a grain. Actually nothing like wood, but the only reason I draw that parallel is to remind me that wood will crack or split along the grain, whereas metal will usually crack across the grain.
When metal cools from its molten state, it forms a crystalline structure. Steel is then often cold rolled into bars or sheets. Wire and tubes are drawn though dies. Either process crushes and elongates the crystals in the metal forming a grain that runs along the length of the bar or tube. This actually strengthens the metal. (See above picture.)
I found from experience that damage can be done to the very thin bicycle tubes, not only by overheating, but more often than not, by heating cold metal too quickly. Overheating while brazing causes the brass to flow in between the crystals of the steel, thus weakening it greatly. However, for this to occur the metal would have to be white hot and in the verge of melting.
More common is heating too quickly and this often happens when tacking a frame together. The metal is cold, and the framebuilder comes in with a small, hot flame to put a little blob of brass to hold the tubes in a lug, or a part like a brake or chainstay bridge.
Metal expands when it is heated, but if the metal is heated in one tiny spot, the surrounding cold metal will not expand and a minute crack can form, often so tiny it cannot be seen with the naked eye. The crack can fill with brass and may not fail until many years down the road.
My advice. Preheat the area first, and always tack at a point where the grain in the tube is a 90 degrees to the component part you are tacking. Not parallel with the grain. (See picture above.) Follow this simple rule and there will be less chance of a tube cracking.
This really applies to the initial tacks when the metal is cold. After two or three tacks and the metal warms up, others can be safely added. And don’t forget when fully brazing later and the frame is cold again. Start in a safe place at right angles to the grain, although not necessarily the same place or you will melt the original tack.
Earlier I mentioned a front end shunt, or crash. When this happens either the down tube ripples, or the front fork bends, occasionally both will happen. If the down tube ripples, it will break eventually, and so needs replacing. It will not fail suddenly, a crack will appear first.
If the front fork gets bent, don’t replace it unless the fork blades are rippled. It can be safely straightened. Let’s face it, the fork blade was first rolled into a round tube. This was done while the tube was in a cold state.
Next it was rolled into a taper and during this operation the wall thickness increased at the thin end. The excess metal has to go somewhere, right. The top end was pressed from a round to an oval shape. All these operations where done while the tube was in a cold state, no heat was required. Cold working actually strengthened the steel.
Finally the framebuilder cold bends the fork blade into a curve. So if the fork is bent slightly in a front end crash, and re-straightened (Cold.) by a skilled person with the right tools and know-how. Why should that compromise the integrity of the fork?
Of course I am not advocating you bend and re-straighten a fork more than once, but that is the beauty of steel. It will rarely fail suddenly, and when it does it can be fixed quite easily.