What a wobble is and how to spot it
Okay. New module. Brakes are done, they're clean and aligned and not grabbing on one side anymore, hopefully. Now we're moving to wheels, and specifically wheels that wobble.
Here's the thing. A lot of people think a wobbly wheel means a bent wheel, like the whole thing's taco'd and you need a new one. Usually not the case. Usually it's a handful of spokes that are a little loose or a little tight relative to their neighbors, and the rim is just following wherever the tension pulls it. That's fixable. That's most of what this module is.
What a wobble actually is
A wheel is round because the spokes are all pulling the rim toward the hub with roughly even tension, all the way around. When one spoke or a few spokes get looser than the rest, the rim drifts away from true in that spot. When you spin the wheel, that spot swings side to side or up and down more than the rest of the rim. That swing is the wobble.
There are two kinds, and it matters which one you've got.
Side-to-side wobble is the rim swaying left and right as it spins, like it's waving at you. This is what usually causes brake rub, because the rim is drifting into the pad on one side of its rotation and away from it on the other.
Up-and-down wobble, sometimes called a hop, is different. That's the rim rising and falling as it spins, not swaying side to side. This one's less about spoke tension and more likely something's actually bent, or the tire's not seated evenly, or in rare cases the rim itself has a flat spot. Worth knowing the difference before you start turning spoke nipples, because truing fixes side-to-side wobble. It does basically nothing for a hop.
How to actually spot it
Flip the bike over so it's resting on the seat and handlebars, or get it up on a stand if you've got one. Spin the wheel and watch it relative to something fixed, not just staring at the rim in isolation. Your eye needs a reference point.
This is where I do the zip-tie thing. Take a zip-tie, loop it loosely around the fork blade or the seat stay, right next to the rim, and snug it until the tail is pointing at the rim with just a hair of clearance. Spin the wheel slowly. Watch where the rim passes closest to the zip-tie tail and where it swings furthest away. That gap opening and closing as the wheel spins, that's your wobble, and now you can actually see it instead of guessing.
I don't own a fancy truing stand for most of my life, I do this off my own fork with a zip-tie as the reference. It's not precise, but it's honest, and it'll get you most of the way there for a bike you're riding around the neighborhood, not racing.
Spin it slow, not fast. Fast spinning blurs the rim and you can't pinpoint where the problem is. Slow enough that you can call out "there, right there" when the gap changes.
Marking where it's bad
Once you can see the wobble, mark it. Chalk, a strip of tape, whatever you've got. Mark the spot on the rim where it swings furthest from your zip-tie reference on the side-to-side wobble. You're going to come back to that mark next lesson when we actually adjust spoke tension, so don't skip this part just because it feels like busywork. It isn't. Knowing exactly where the problem lives saves you from turning spokes randomly and making it worse.
While you're at it, spin the wheel again and just watch the brake pads, if you've got rim brakes. Watch for the rim ticking the pad at the same spot every rotation. That's confirmation you're looking at the same wobble from a different angle.
A quick story on this
Jocelyn's brakes were grabbing on one side a while back, and I walked her through the whole barrel adjuster explanation, all the theory, why one side rubs. She just nodded along. Next time it happened she fixed it herself without asking me. Didn't even mention it. I noticed a few days later the bike wasn't dragging anymore and asked her about it, and she'd just done it.
That's the goal with wobbles too, honestly. I want you looking at your own wheel, spotting where it swings, marking it, without needing me standing there. It's not hard once you know what you're looking at. It's just that nobody ever shows you what "looking at it" means.
One caution here
Don't try to true a wheel that has a tire and tube still fully aired up sitting right against your face while you're spinning it fast to check. Not dangerous exactly, just annoying, and you'll get grease and road grit near your eyes for no reason. Spin it slow, use your reference point, and you'll see everything you need to see without drama.
Before next time
Get your bike flipped over, rig up a zip-tie reference on the fork, and spend ten minutes just watching your wheel spin. Figure out if you've got a side-to-side wobble, an up-and-down hop, or honestly maybe nothing at all. Mark the worst spot if you find one. We'll fix it together next lesson.