How much can you safely change the rear spacing of a steel frame by cold setting? I have a frame with a 120mm spacing that I'd like to put a 5 speed drum brake hub in. The hubs I've found all have an over the locknut dimension around 130mm. Is it safe to go up that many sizes by cold setting, or should I settle for a 3 speed that just fits?
Your wheel may be off a mm or more to the one side. the seatpost is about halfway between the front of the bike and the seatpost so a .5mm difference at the seatpost will be a mm or more at the back. If that isn't a big deal to you then don't worry about it. Personally, a wheel that is that far off would bother me.
But before you go nuts with the 2x4 put a wheel in there just to check. Sometimes it is hard to get a good measurement with the string test. Get the wheel as centered as you can get it and measure from each of the stays in the rear triangle until you find out what that looks like and if it is lined up with the rest of the bike.
Personally, a tiny bit of misalignment is really evident to me but I have worked as an electrician for many years bending pipe and my eye is pretty good with that sort of thing when it comes to alignments of things.
I'm like Siskle and Ebert when I'm at Critical Mass and looking at all the bikes. I can spot a cobbled-up frame or one that has been bent from fifty feet behind it. I'm picky about stuff like that.
Thanks again. With the wheel mounted I was surprised at how much off a 1mm center tube difference made in the alignment. I am not like Siskle and Ebart, but even I could see it.
I put the all thread piece back in, bolted it to the spacing I wanted (135), and then bent the whole rear triangle together. Worked like a charm. I'm riding no hands.
Now if only I could get the derailleur to hang a bit straighter.....
If you get an old axle and cut it in half you can straighten the dropouts by eye like it shows in the Sheldon link.
Once you have the dropouts straight put the wheel back in and get it where you want it. A cheap plastic Harbor-Freight $2.99 vernier caliper is very helpful with this and is accurate to 1/20th of a mm (believe it or not -but it is.)
If you get lucky with your used axle that you cut in half it will be the same thread as the drop-out bolt. You can buy a 3-foot section of aluminum 1/2"x3/4" angle stock at Home Depot or Menards for under $5. Drill a hole in the middle of the angle stock and use the axle to bolt it to the drop-out and you have a drop-out alignment gauge. You can then measure from the rim to the angle stock and see how far out of alignment the hanger is. You can rotate the stock around and get different measurements to a degree to measure front/back and sort of top/bottom although it's limited in its rotation by the axle of the wheel. Tweak the dropout by grabbing the axle bolt with something like the end of a box-wrench or a small piece of pipe. Keep tweaking the Dropout until you get it so that the measurement from the aluminum angle to the rim is equal on both sides regardless of how you spin the "gauge." If you can get it to within .5mm from side to side that is close enough and just as close as you can get it with the professional gauge and closer than you will be able to line up the wheel again once you take it out unless you have dropout alignment bolts that will make sure the wheel is in the same place every time you put it in.
i'd take a picture of this but I'm too lazy to yank a derailleur off one of my bikes and dig out my own home-made gauges to show you want I'm talking about right now. I should take pictures the next time I'm using it.