I apologise if this topic has already been covered. I did a search on here and did not find the answers I was looking for. Please can someone explain to me why brazing in a brake bridge and/or a chainstay bridge brings the rear dropouts closer together. This has not come as a surprise to me, I read about the phenomenon before I built my first frame and took steps to compensate with fair success on a few frames. My latest frame, with an intended rear dropout width (spacing) of 130mm has ended up 129.6mm. Somehow this irritates me more than if it had settled at 130.4mm. The latest frame has longer (44.5mm) chainstays than my other frames so I accept that I have under-compensated but I wish I understood the physics better.

Paul Jacobs.