Originally Posted by
Andrew R Stewart
The OP seems to be assuming that both stays will bend symmetrically. Often they don't because of reasons like drive side chain stays can have different indents for chain ring clearance that the LH side doesn't. This is the main reason why I do my rear triangle spreading one side at a time. I block the BB shell (what, you mean I have to do more work just to realign a frame...) in a bench vise and work on one set of stays, then the other. Besides controlling where you end more easily you also do a better job at maintaining the frame's chain line and tracking.
Yes this is a good point. On all the frames I've made they were symmetrical and I use the threaded rod when both sides need to move the same. Sometimes one side does need to move more than the other and in that case I just put the BB in a vice and pull one of the dropouts towards me.
In any case OP should rig up a bit of string and keep an eye on things. Maybe the frame wasn't perfectly aligned to start with and this might be an opportunity to correct it.