Sunday, May 01, 2005

Bottom Bracket Follow Up

Well, after Freddy pointed out to me that Allstate 11 isn't meant to be used in a socketed joint I asked if I could heat up the BB and pull the tube out. He responded that #11 has "....Eutectic qualities..." and wouldn't likely come apart.

I had to look up Eutectic. couldn't help so I went to Wikipedia. I will summarize here my layman's understanding - so if you're an engineer or scientist, don't hold me to a precision definition.

Eutectics appear to generally be made of two substances - in the case of metals they are a type of alloy. One characteristic is that in their liquid state, the parts blend but in their solid state they tend to separate. If there is too much of one element, it will start to harden and sort of precipitate out as the liquid cools. If there is too much of the other element, then this one is the one to first harden come out of suspension. However, there is a balance point between the elements where solidification can maintain the blend of elements. Further, at this blend, the melting point is lower than for either of the elements individually.

So, what may happen is that in the course of brazing some separation of elements can occur. If so, this should mean that re-melting the filler requires a significantly higher temperature - which would be difficult and could damage the tube and BB.

Because of all these factors, I decided to cut the tube out of the BB. I'll reheat the headtube joint and remove the tube which will be shortened and used for the seattube. Meanwhile, the blank from which I was going to make the seattube will be now used to provide the downtube. Net of this, I'm only sacrificing the BB.

I've cut it up and found that the #11 did flow through the joint well. Because the BB is of pressed steel, it has large radii where the spigots join the cylinder of the BB. The #11 didn't fill these (which is as I expected), but did a great job of filling all of the normal gaps. Fred says it is possible to use #11 this way, but the characteristics that allow it to be controlled for a fillet (that it can work in a plastic state that isn't totally liquid) make it less than ideal for the socketed joints.

I'm pleased that the #11 drew through the joint so well. It makes me increasingly confident of my ability to make a sound joint. And the setback of time gives me the opportunity to try better changing the DT angle on another BB.

Some pictures will be posted, probably later in the week.

Meanwhile, I'm waiting for some of Freddy's special aerospace rod. Its supposed to work much like #11 but at lower temperatures. Should be fun.

By for now.

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