I am not sure if this is appropriate for the frame builder forum, but I think it is somewhat relevant. Please let know if I am in the wrong forum and I will remove this thread. Thanks.

I recently purchased a 3rensho frame in decent condition. As I started handing components on the frame, I noticed that the middle section of the top tube seemed to be sagging a little bit. I put a straight edge on the top tube, and, sure enough, the middle of the top tube was about 2 mm lower than the end points of the top tube. It was essentially a very gradual and gentle curve in the top tube. The frame seemed otherwise straight with no sign of being previously crashed. Since it is a petite frame, I was even thinking that the original builder was trying to create a lower standover height this way. On the other hand, 2 mm contributes nothing to standover height for all intents and purposes.

My questions is: Is this something that I need to worry about? If it is, is there a simple way to fix this other than a complete tube replacement. Thanks a lot for you insight.

It's not likely that the builder curved the top tube or purpose in this application and that small amount. I would first look inside the frame to see if the tubes are fully inserted in the lugs and that the geometry looks correct. If all that looks good, I would look very closely for other curved tubes like at the bottom of the seat tubes. I would check the alighnment for other clues of what caused the issue.

If the only thing that pops up is a warped top tube, it will need replacement. I have a machine built specifically to remove subtle bends but not in a hardened condition. I would also try to locate the exact location of the bend, approx radii and if any distortion in the tube that took place during the process of warping it.

Thanks for the advice. I believe the top tube is filet brazed to the head and seat tube.
Do you mean that even if the slightly curved top tuve is the only problem I can find with the frame, you would not recommend riding the frame?

My first inclination is to believe the top tube suffered an impact from something soft. Like maybe someones sensitive bits.
Is there anything wrong with putting your feet on the BB/downtube and giving a good pull to the top tube? A couple of MM is not much. I've done this with chainstays and head tubes that were rocked a bit to the side.

I'd imagine the sensitive bits would sustain more damage than the frame. I installed a stem, handlebar, and a saddle and put the bike up side down. I then gave the top tube a good push, but it didn't deform at all. Maybe I am just weak....

I would monitor the head tube joints for cracks. This is always a good idea anyway, no matter what the construction method. My best guess is that the angle between the tt and dt shrank during brazing, and when the front half of the frame was put together with the seat tube, the tt was forced outward so the seat tube length would be correct. This puts the head tube joints under more stress, and it might lead to cracks in the joints.

This makes a reasonable hypothesis. If it is true, so much for the legendary build quality of 3renshos, eh?
So your suggestion is to ride it and closely monitor it?

actually, now that I think of it, it could be that the tube was shipped with that curve from the factory. Steel tubes are not perfectly straight, it's just a fact. It also is possible that it moved that much because of the cable guide braze ons. When you heat a tube on just one side, that side becomes shorter. This can be used to the builder's advantage, that's called "witch wanding."

From high school physics, I thought the heated side elongates and supposedly return to its original length when it cools down.

I don't understand the process at all, but it's well known among steel builders. High school physics would never tell you about such a thing. I'm sure it would surprise most working physicists, but they might be able to figure out a reason for it. You don't even have to get the tube fully up to red to have a fairly dramatic movement.

Interesting!
Thanks for your insights. I sleep better at night now. I will build it up, ride it and monitor it closely.