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I have a conventional old-ish mountain bike front wheel - 26", 32 hole, disc brake, 100 mm over locknuts. I've been using it for road riding for years and have never trued it, but recently borrowed a wheel truing stand, so thought i would give it a look.

It appears to be reasonably true laterally and radially - a millimetre or less, i would say. But when i align the stand's indicator with the non-drive-side rim (with the brake rotor), then flip the wheel round so it points at the drive side, there is a 7 mm gap. If i understand correctly, that means the wheel is 3.5 mm out of dish (or however you say it).

Could it have come from the factory like that? If not, could it have drifted that amount in use, without losing its true in other ways? Is it a problem that it's that far out? Or have i just misunderstood how wheels should work?

I am tempted to correct it, now i have it in the stand (and while i am waiting for some new inner tubes to arrive). Would i be wasting my time?

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Taking what you've said: you've been using it for years, it's probably fine left unaltered.

How does it look in the fork? is that 3.5mm obvious? is the tyre closer to one side than the other? The wheel centre obviously not at the fork centre?

A disc brake front wheel is dished, and has shorter, tighter spokes (generally) on the disc side.

The fact that the rim has drifted away from the disc over time may not be much of a surprise to most of us for a basic OEM wheel.

You could add half a turn to each of your disc-side spokes, if the nipples aren't bound up with corrosion. If you feel the need to get the wheel more central, winning back 2mm would be great. But only do it if you have the time.

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    And only use a good spoke wrench - a cheap one will fit very loosely on the nipples, and given the age and usage at least some of the nipples are probably going to be hard to turn. A cheap spoke wrench is going to round off a lot of the nipples... Sep 26 at 18:53
  • @AndrewHenle Even using my brand new Park Tool spoke key, i managed to break the end of one nipple off entirely. But these are double square nipples, which you're probably not really supposed to use a key on at all, whoops. Oct 11 at 12:31
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    @TomAnderson the park spoke keys are crap. one that grips 4 sides (3 corners), such as spokey pro (by R&K) or the VAR one, is much superior and not necessarily more expensive
    – Noise
    Oct 11 at 16:53
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It could be that the wheel is off-center. But you would notice that when you put it in the frame. When your wheel is mounted, check the distances to the chain stays and the seat stays, or to the fork for a front wheel. If both are approximately the same and symmetrical, your wheel should be correct.

So what's wrong could be the truing stand, or the person using it. I have two truing stands, both not too expensive, and what each of them thinks is the center simply isn't. But it could also be that you're not inserting the axle in the slots correctly, or that the "dropout-equivalent" of the truing stand is not mounted correctly: For one of mine that is a bit of plastic pushed into a hole. if that bit of plastic is not fully inserted on one side, or if there's dirt or such in the slot, your readings are off.

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    The way i measured the off-centreness was by aligning the stand's indicator on one side, then flipping the wheel round and comparing that caliper to the flipped rim, which i think is immune to alignment problems in the stand. The stand's own indication of centre is indeed miles off! Oct 11 at 12:30
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That is quite a substantial amount for a conventional wheel to be out of dish. Measuring the way you describe, 0.5mm would be considered about a normal tolerance for a good quality build, and 1mm would be around the max for a sloppy tolerance that would more or less work in practice. (Measuring and expressing dish error is a little bit of a rabbit hole of a topic - you're doing it the most logical way, but a less thought-through way is to site a gap dimension number obtained with a dish stick, which works by one side contacting the rim while gap on the other side is measured, so it amplifies the error by 2x).

It's possible for a wheel to get like this if it ever had extensive truing/tensioning work done, especially if a radial issue needed correcting, and someone missed the step of checking dish afterward. The dynamic here is that for a given rotation of the spoke nipple, left and right side spokes create about an equal difference in radial "movement" of the rim, but the right side ones (on a disc front wheel) will produce much more lateral movement. Correcting the radial blips can then introduce lateral ones, and when when correcting those it's possible for things to drift over if one doesn't understand what's going on or isn't checking the dish.

Spokes can lose tension from "settling in" at the hub, and a given amount of this will create more movement to one side just as above. But, it doesn't seem likely that's what happened here since the offset you have now is pretty extreme, and that much tension loss wouldn't typically result in the wheel still being laterally true.

I would correct it since handling is likely being impacted now if nothing else.

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