To protect the seat stay, wrap it with some tape.
Attach a piece of wire coat hanger with several zip-ties to this protected stay.
Bend the wire so it almost touches the top most edge of the sheave.
Turn the wheel slowly and see how far the wire gets from the edge of the sheave.
You may have to adjust the wire several times to get it to where it will just touch then leave a gap 180 degrees later.
I do believe the sheave must be removed inorder to get it recentered again...If it is found not to be concentric to the hub.
Yes! The hub..not the wheel, because the wheel may not be concentric to the hub.
The outter circle is the rim and its distance from the hub can vary as the wheel turns. Poor wheel lacing would permit this, so I will never use it as a guide for a sheave install, unless the rim is found to be very true. The rim could be used for a good rough quess-ta-ment otherwise.
The inner circle is the sheave, it must have the same dimention or distance from the hub/axel as the wheel turns.
If the rim is not concentric to the hub two things will happen.
1-The wheel assembly will be way out of balance. Fine, it can have weight added to bring it into balance. No big deal, most will say.
I don't like weights. They look bad and indicate poor wheels or lesser than tires...in my mind. (i have pre-balanced cast motorcycle wheels by bonding lead inside the outter edge of the hub before the tires are mounted and generally no further weights are needed)
2-The tire will run hotter than one running true because the tire must flex more to make up for being out-of-round. This too is no big deal if one is not running long distances, on hot days, carrying a heavy load(my 240#), plus pulling a trailer. Hot tires have shorter lives and are the ones that blow out.
If the sheave is not concentric to the hub
1-The belt can bounce 'n slip. It does not have teeth all the way around for a reason. And I think that reason is to permit a degree of poor installation or out of round. If it had teeth all the way around, there just might be a chance it would eat some of the rubber teeth off the belt if the belt bounced off the sheave just enough for slippage.
2-Rubber hates heat and this out of round will cause more heat build up in the belt drive than is necessary. Belt life will suffer.
I think the FAQ section over at GEBE talks about how much the tensioner handle can move and still be OK. It is not much. You might consider going over there for some reading.
I did last month.
See where it says radial truing? That's what we are addressing. For both the rim and the sheave. The sheave being the more important of the two.
http://www.parktool.com/repair/readhowto.asp?id=81