Originally Posted by
Andrew R Stewart
"Then after fully welding on the dropouts I extend the dummy axle by a few mm (it's just a threaded rod and some locknuts) when putting in bridge tubes." guy153
Not sure I agree with this process. We know that when bridges are attached the rear end will want to close up so why not plan of this from the beginning. When you spread the rear end with your all thread but don't cold set it so I believe you induce more stress at the bridge. Now there will be compressive forces acting on the stays inner sides at the bridge as well as the stays taking on a bow due to the heat being focused for longer on the insides. I recently repaired a Soma (the current version of this decades old brand) steel frame whose seat stay had cracked right at the bridge. I could tell that the bridge had been too long as the stays had a distortion at the ends of the bridge, the bridge had "indented" the stay. Andy
If the rear spacing starts off correct, is then pushed out (elastically) while the bridge tube brazed in, and then provided that when the dummy axle is removed again the rear spacing is correct then in theory you just accounted for the shrinkage of the braze, and there is now no residual stress (well not much).
But if you make the rear spacing slightly too wide to start with then at the end of it all the bridge tube is now pulling it elastically back into the right place. So you have some residual stress, with a bit of tension at the ends of the bridge tube.
Yes if you overdo it then the bridge tube will be pushing the stays too wide and you will actually have to bend them back, and the joints at the ends of the bridge tube will have some residual compression. But this should actually reduce fatigue.
The aim is to get it just right. I don't think the residual stresses make too much difference and may not have contributed to the failure on the Soma-- it could have been something else like a defect in the weld. My main reason for doing it like this is to preserve the dropout alignment.