Thinking of getting a used wheel, which I've been told is a couple of mil off perfectly round. I've never had any great success with correcting 'ovalness', as against the usual trueing - does it actually matter? Would I notice any difference in the ride (assuming I couldn't fix it)? Would it get worse over time?
You will feel 2mm if you glide along on a very smooth surface like a perfect garage floor or down a supermarket aisle. This is unlikely to happen so, you won't really feel it. However, you will always know it and see it. That's worse than feeling it if you are anywhere near (ab)normal like the rest of us.
Here is your chance to develop a new skill. Buy the wheel and fix it by trial and error. Here's how:
1) See the bump as a mathematical normal curve. You know, one of those symmetrical bell curves.
2) Identify the centre point of the curve i.e. the highest point of the bump.
3) Identify the two outliers.
4) Mark these three points with something on the rim - a permanent marker is good.
5) Count the number of spokes inside the curve. It could be an even or uneven number, it doesn't matter, as long as you stick with your number.
6) Measure the size of the bump at the highest point. In your case that is 2mm. Do some maths. There are 56 threads per inch on a spoke. That's 2.3 threads per millimeter. If you have a 2mm bump, you have to turn the (centre) nipple 2 x 2.3 times to advance the nipple 2mm down the spoke. This is a crude calculation because a) the spoke will stretch and therefore you will have to turn the nipple a bit more and b) the spoke follows a triangular line from rim to hub and you have to "trig" a bit of extra turn to compensate for the extra length. Make a decision and come to a conclusion that you want the spoke to travel 2.5mm, not 2. Redo your math and go 2.5 x 2.3 = 5.75 turns.
7) Now draw your normal curve on a piece of paper, draw your "involved" spoke in and assign numbers to each spoke according to the above calculation. The centre spoke/2 spokes will get the brunt of the 5.7 turns but as you move away from that spoke/s on either side you taper down. Work from the centre and turn the nipples as planned and keep your wits about you. You don't have to make all the turns in one bold move. Start small but keep account of how much.
8) If this was a front wheel on a road bike, if your measurements were accurate to start with and if you kept your head with the numbers, your wheel is now true. Obviously it won't, a bit of fine-tuning and you are there.
9) However, if it were a back wheel or a disc brake front wheel, all things are not equal. The spoke angles are not the same on the left and on the right. Those at a slacker angle will need more turns to travel the extra distance and those on the side with the straighter angles will stretch more. Conundrum and calculus!
Keep calm, proceed as above for a symmetrical wheel and afterwards simply do a lateral true. In summary for this wheel, you do the match, turn the spokes and then true laterally.
There is only one rule: Don't loosen spokes, just tighten them. Because the bump is smaller than the normal area of the wheel, you have to pull it in, not relax the entire wheel to bump level.
Off you go. Report back here how it went.