Very slight high spot in a rim affecting braking - can/should I do anything about it?

Question

One bit of my rim (less than the distance between 2 spokes) is 0.2--0.3mm wider than the rest. It causes a pulse of increased braking once per revolution, which is annoying. As the rim isn't worn and there are no signs of thinning, I'm inclined not to be too concerned, but I'd like a 2nd opinion - and even better if I could make it less annoying.

This is a fairly lightly used aluminium rim - I bought it new to build up a dynamo wheel for commuting a couple of years ago. That bike died, but I've put the wheel on another bike. I noticed this issue on the previous bike, but didn't solve it (I assumed it was the join).

The rim has a spot that's a tiny bit wider than the rest. I can feel the increase in the braking (V-brakes) at typical everyday braking forces. Having assumed it was the join, or a rough patch, I looked a bit more closely, cleaned and felt all round the brake track. No issues detectable by sight or touch.

Walking the bike with very light pressure on the brake allowed me to find the spot where it catches and get the Vernier calipers out. It's neither near the join or the valve hole. The wear indicator groove in the brake track is as good as new, and the brake tracks aren't shiny enough to see if a reflection is distorted on one side or the other.

Measurements with no tyre showed variation on a scale of 0.1-0.2mm inside and 0.1mm outside except this spot that was 0.3mm above the minimum, with no correlation inside to outside. In practice there was also a measuring error on a similar 0.1mm scale even taking the lowest of 3 measurements in almost the same spot (probably due to minor scratches/dings, or just to not squaring up the calipers perfectly). Without the weight of the tyre+tube, it was harder to locate the problem region by spinning the wheel and taking up the slack in the brakes, but I didn't have time to fight the tyre on, off, and on again over lunch and it needs to be rideable this afternoon. It does seem to just be variation in the extrusion.

Unfortunately the bike is now the one that never goes home, so I'm limited in what I can do to it.

Answer 1

Next time you get the tyre off, check the inside of the rim at that point. If the rim wall is bowed out at that point but has a consistent thickness, this will be where the rim eventually fails. Could be decades away. I suspect this is most likely.
I've tried re-bending an aluminium rim wall back inward and while it succeeded initially, I saw cracks developing so it was time to shop for a new one.

If the rim gets thicker here (ie, the inner wall is straight without deviation) then it may be feasible to thin-down the braking track and then polish the braking track.
This is unlikely though not impossible, because rims are made as an extruded coil which is then snipped and one "turn" becomes a rim. So a thicker rim wall is improbable.

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If it were a thickness in the wall, like a callus, the machining process to fix that would be some combination of difficult, sketchy, and expensive. But if the rim is unuseable otherwise, what have you got to loose?

Spinning the entire assembled wheel on a lathe would need an enormous lathe that only the largest machine shops would have.

Mounting the wheel in some kind of jig that allows it to rotate under an endmill spinning in a milling machine, might work though would be janky and need a lot of weird fixturing.

Grinding it down by hand is going to be error prone, though a finger sander might be a good solution if you go slowly and can test often.

My approach considering the bike is remote, you might get away with some 120~150 grit sandpaper. Wet and dry would be best, not cheap garnet paper. Start by somehow marking where the high spot is, with sharpie or paint marker or a strap of masking tape around the two nearest spokes. Then ~5 strokes with a small piece of sandpaper right on the crest, and retest. You can do this with the tyre in place too. Just be careful to not overshoot. When its close step up to ~400 grit, and scuff the whole rim for a consistent finish to prevent surprise grabbing on the new toothier surface.

Consider that any improvement is good, and you will likely never make it perfect. Personally in the short term, I'd just live with it and be aware this bike will use brake pads a little faster than normal.