Harvey M Sachs wrote:
> The reason I ask is that the metallurgy of the silver brazing materials
> I'm used to doesn't favor this application. They are extremely strong
> and appropriate for filling very small gaps (<0.035", if I recall
> correctly), where the loads "look like" shear. This would describe
> bonding lug to tube, for example. On the other hand, as the gap
> widened, strength decreased rapidly. In addition, the silver I've worked
> with gets very liquid just above the start of melting, so it is hard to
> build up a fillet.
>
> Brass complements this well: strong gap-filler (fork ends to tubes, for
> example), has a good viscous range for building fillet radii, etc. I
> always thought that the rational custom frame makers typically boasted
> of using silver for the lugged joints, and never claimed to use silver
> for fork ends, seat stay eyes, etc.
All you say is true, and most builders do only use silver for lugs and braze-ons, but at Trek at least our 753 frames used silver everywhere, including dropouts and fork ends. The results are more than strong enough (never had one of them fail at the dropout/fork end, IIRC), but you use almost as much silver filling those gaps as you do in the rest of the frame all together. That gets expensive, and I suspect that is a significant reason not to use silver there unless required to by the tubing properties.
FWIW, only the early Trek 753 frames used inordinate amounts of silver; the later production used cast, socketed dropouts/fork ends that only required a tiny amount of silver.
--
John (john@os2.dhs.org)
Appleton WI USA