A wise old man once told me: If it separates you from the ground treat it seriously.
Here's what I'd do. You will need 4-5 small zip ties, and something to trim the zip tie like a Leatherman.
- Weigh down the back of the bike so you can rotate the front wheel
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Inspect the rim using a "poor man's truing stand": Get your two zip ties and gently zip tie them to the front fender struts so the tails are facing forward (do not trim yet). They shouldn't be so loose they fall off, but not so tight you can't rotate them.
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- Rotate each zip tie until they just barely touch the top of the rim (where a bead would hook the rim). Trim as needed. They don't have to be even or the same length, they just need to touch the rim at the same place on the rim on both the left and right sides. Example given here using a bicycle but it works with the fenders as well.
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- You have created a poor man's truing stand by doing this. The zip ties represent a fixed point in space relative to the center of rotation.
- Spin the wheel. Any rim imperfections will be visible relative to the zip ties. You will be able to see alignment issues (rim doesn't touch one of the zip ties evenly as you rotate it) and radial alignment aka "hops" (rim will push the zip ties up or down from the hub).
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Inspect the tire: You might need to adjust the zip ties for this, trim them more, etc. but you can use the same process to repeat the inspection process, lined up with the radial line above the bead on most tires. I'd assume you can see this visually but some people don't notice it without the guides. Rotate the zip ties to align with the "line" on the tire that follows the bead.
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- Repeat the same process as you did with the rim. You're now checking the bead radial accuracy relative to the hub. This time, you're looking for the line disappearing into the rim and then reappearing, on one or both sides. That would be a sign that your tire is improperly seated. If it increases in distance (hops outwards) that could indicate tire casing damage between that line and the bead, either a manufacturing or installation defect.
- Finally, if neither of those work, place a brick under the front wheel so it just barely touches the tire as it spins and rotate it, checking for "hops" in the tire casing itself.
I've included photos for where I would be inspecting against and zip tying my fender. If you need more of a guide, something Youtubey, I can probably wrangle something up in a bit.
Also I'd seriously consider finding a new mechanic.