Originally Posted by
Bob Dopolina
My understanding of the process may be in error (I haven't welding anything for more than 20 years and it's probably a good thing, too, based on my initial efforts!) but don't you choose a rod based on the material you are welding? Doesn't this rod then become part of the liquid metal but is such that it is consistent with the material being welded? In the case of a rim, wouldn't this additional material then be machined away leaving behind it metal that is indistinguishable from what you started with? Let me know if I'm wrong here.
I know that Mavic runs a current through the rims and they are fused together without the use of a rod or (I believe) any inert gas. They even cut the hoops a little over-sized and calculate how much of the material will be used up during the process, resulting in a rim that has the correct BSD. I also believe that this process was designed specifically to not add any material to the join. It is hard to be 100% here as this is proprietary knowledge that they don't go into in great detail. With machining the excess material should be removed and the composition of the aluminum should be consistent throughout the rim, non?
This should be the big advantage over pinning a rim even though it is a more expensive process. I can't imaging going through all the trouble to make special alloys if your going to introduce a weak spot (or at least a section of rim with different properties than the rest) through the production process. It just don make no dad blern sense is all.
I would be interested to hear from some welders about this. (We know braze adds material so let's just skip that for now).
Yes, welding rod is chosen specific to the type of material being welded, as well as considerations as to specs, strength, etc. When the item is welded, the welding rod and molten metal become one. Very often (I'm hedging here, as there are always exceptions), a well executed weld is stronger than the non-welded wrought metal. Differences in weight b/w the rod and the metal would be negligible, so yes, assuming a perfect grind, the resulting seam could be the same, or even lighter (assuming the weld was ground down to below the original surface level), than the rest of the rim. I was only taking issue with your characterization of welding not adding any additional material. On most frame welds, for example, there is definitely material added at all the welds, even for ground welds, such as on Cannondale frames.
Taking a quick look at my Mavic Open Pros, the weld is not ground down perfectly, but pretty well. Since it is a boxed rim, I am wondering if there is weld material internally. Clearly the internals couldn't be ground down, and would lend credence to the other poster's assertion that the welded seam has higher weight than other parts of the rim.