Rim shot? Goes badly out of true at medium tension I am pretty sure this rim is shot, but wanted confirmation and/or a better understanding of what's up. I'm fixing up a donor bike that was pretty apparently stored outside for years. Despite that, many of the parts are salvageable -- so good in fact that I decided to just overhaul the bike rather than part it.

Except the wheels? Rear wheel was pretty badly out of true. So I put it in my stand, tested the spoke tension, it was all over the place on the drive side, from 80-140 kgf. Non-drive similar, but not such high tensions. I began backing off the spokes and trying to get drive side to about 105 kgf, non-drive about 65, and the thing got worse. 

I backed off all the spokes until they were slack. The rim (a 23mm wide, 32-spoke aluminum with plenty of brake track, BTW) looked fairly straight. I had expected it to show evidence of being badly bent, but it looked no worse than plenty of wheels I've gotten nice and straight.

So, I began tightening again, moving up slowly. Probably 5-10 kgf per side. (It was tedious.) As soon as I got the drive side up to about 55 kgf (non drive would be about 17 at this point I guess) The thing started to pringle badly again. That's with tensions nearly perfectly even on each side. The rim's easily 1cm out of true.

I'm convinced this thing is a goner (not surprised given its history) but I've never seen a rim behave like this. Maybe all that weathering and corrosion has just made it completely and randomly unstable? I was thinking maybe the eyelets were failing in some spots, but of course, if I had eyelets failing or weak spots near holes, I'd see the move reflected on the tension. I don't. I get even tension and a really warped rim.

Anyone seen this on an old rim?
Yes.

Once the rim has taken a set (ie it is really bent), you can only true it so far with tensioning spokes.  Too much tension will strip nipples, or can even snap spokes outright.

You did right by backing off all spokes till they're all loose.  Then use brute force to "straighten" it once you've identified the areas of the rim that are out.  A large flat surface helps, or another rim that is known to be true.  Flip things over and over, use elimination to identify the bad spot/s.
The straighten the curves.  This is sometimes done using a street drainage grate as a lever or plain old stomping, like this:



As long as you can pressure the high and low spots and make it "flat" then it could be trued from scratch.

If its really dead, then its dead.



However you say the rim looks fairly flat with the spoke tension all slacked off.  That's puzzling, and I'd recommend a close look at the rim nipple holes.  Could be some of them are a bit softer metal than you might want, and they're moving under pressure.  Also check the hub end of the spoke to see if there's any damage to the flange.

Unless there's something really good about the wheel, it can suck up a lot of time and still come out with a sub-par wheel.

I have about 4 spare wheels for various bikes - as you come across good ones, then keep them.  



I have a seriously dodgy wheel where one spoke has no tension at all.  It goes out of true if I so much as finger tighten that one spoke.  Its perfectly ride-worthy for me, but I'd not put it on someone else's bike without fully sharing the problem.   As a rim brake wheel, true is more important than spoke tension.

Separately, I've wasted 4 hours trying to true up a front wheel for a give away.  In the end I swapped the hub and spokes to another rim, and got the assembly and truing all done perfectly in 30 minutes.