Hi,
as when brazing in the SS bridge pulls dropouts together - I’ve heard, that some people put additional spacers to the axle, so it’s not eg 142 but more so after the whole operation everything is in place. And the question is how thick spacers? Of course the answer is “it depends”, but maybe someone has any comment on that?
1 Like
Yes, you’re correct, that usually happens. Depends on the wall thickness of the seatstays and chainstays. I usually get about 2mm “shrinkage”. I’ve been adding a couple of mm to the spacing beforehand to accommodate this. You can minimize the shrinkage by “witch wanding” ie heating up the outside of the seatstay for a few seconds after the bridge is brazed, this equalizes the heating and prevents some of the metal movement.
3 Likes
I’ve experienced it, but never measured. Thanks for your input. I thought about adding 2mm spacers each side. And eventually changed my mind and will not be adding SS bridge at all to this frame 
2 Likes
You can also just put your foot in there afterward and fix her up.
4 Likes
That’s moreless the way I’ve dealt with it before
The old joke is that you adjust it with a custom tool that is so delicate and valuable you keep it in a leather case.
-Walt
9 Likes
Maybe I should provide a more useful answer:
-If your bridge miters are really tight, expect about 1mm on each side of shrinkage.
-If miters aren’t very tight (which is fine for a bridge in general, so long as you’re not mounting a brake to it) you can end up with 2 or even 3mm on each side.
-Walt
6 Likes
Is that where Surly’s 132.5mm spacing came from? 
2 Likes
Thanks for that, I think I’ve got miters pretty tight, but as the frame is disc brake I thought I don’t need the SS bridge.
Yes, you do not need one. I only do them because customers get upset if there isn’t one.
-Walt
2 Likes
can I reanimated this thread? did the spacers work? I’m getting at least 2mm either when adding bridges, would be nice to be able to account for this.
cheers
Dan
As Mark said, it will greatly minimize it if you heat the outside after finishing brazing the bridge in. I usually put flux all the way around the tube, braze in the bridge, then bring the flux on the outside up to activation temperature (goes glassy). this way inside and outside cool together. I don’t think there’s a standard you can count on and have that transfer from one builder to another reliably. It depends on how fast you braze, whether you’re using silver or brass or nickel silver etc. etc.
PMW dummy axles came in ‘exact’ and wide spacing to account for this. The exact was, for example, 130mm and the wider spacing was 132.5mm (or whatever).
I’ll give that a go next time, thanks. I’ve been alternating between each side of the bridge to keep the temperature down a little, but in hindsight that might be making to issue worse from mini heating/cooling cycles maybe?
One thing to consider is that if you start with a longer than nominal dummy axle, or use some spacers before brazing, then when the frame closes up, the dropouts won’t be parallel anymore. If you start with the correct spacing and tweak the frame after the brazing, your dropouts will be back to being parallel again.
Someone with more time than me, will probably work out that with a 2mm spacer added, the dropouts will deviate from parallelism by 0.00067º
All that to say, it probably doesn’t matter.
3 Likes
Yeah, i guess as long as there’s no twist all will be fine. I’ve got a Ti frame where you have to pry the dropouts apart to get the well in. One bonus is it works fine with 142 or 148 wheels
Yes. I think it’s best to always fixture everything up at the correct spacing, and then bend it back to where it needs to be. But if the stays are already fully welded or brazed on then you can put a slightly extended dummy axle in before doing the SS bridge tube. It should then only pull them back to where they’re supposed to be and the dropouts should still be parallel.
Hi,
Additional 2mm spacing is perfect for me. My sequence is to do dropouts and tack ss/cs with an exact axle to keep them in place/parallel. Than - weld/braze them and also the bridge with additional 2mm.
That won’t work unless you actually ‘set’ the dropouts further apart. And by that I mean bend them past their point of elasticity. If you just open up the stays by a couple of mm to squeeze in a longer dummy axle, that’ll do nothing when it comes to pulling the stays back to the correct width. It’ll still pull them in tighter than you want. If you’re going to cold set the spacing, just do it once after everything’s brazed.