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I think it might have to do with the valve hole, or the valve itself if it is a complete wheel with tube and tyres. (That is assuming it's not a badly built wheel)
Edit: out of interest, I googled it a bit too. I found this:
The wheel can be unbalanced due to asymmetries in material density and concentration in the rim, the tire, and the tube. To locate the imbalance, install the wheels on the bike while it is clamped in a bike stand. Install the rear wheel with the chain off so that the freewheel doesn’t slow the rotation of the wheel (you don’t have to remove the chain; just bypass it and leave it to hang below the cogs). Spin the wheel and let it spin down to a stop. Repeat. If it repeatedly stops with the same part of the rim at the bottom, that is the heavy spot, and the light spot is the one that is always on top when it stops.
[...]
Aluminum rims have either a splint or a weld where the rim is joined opposite the valve, and this extra-dense area is heavy enough that, even though the valve is the heaviest part of the inner tube, the light spot of the entire wheel is generally at the valve. If the spin-down test indicates that the valve is at the light spot, balancing the wheel is easy; you can thread extra collars onto the (threaded) valve stem, and/or put on a metal valve cap. If that fixes it, you’re done. Check that the imbalance is gone by once again letting the wheel spin down, and if it doesn’t favor any particular spot at which to stop, the wheel is balanced sufficiently well.
So... kinda. I think with carbon rims which do not have any kind of 'joint', but generally even longer valve stems, the valve location will probably be the heavy spot? I'd have to test that with my front wheel.
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SwissChap
@Pasty_Spumante
Why sometimes, even with brand new wheels, does there appear to be a slightly uneven rotational force (ie at one point of the revolution there's a pull forward)?