Hey, I've searched and read up, but I'm still having trouble figuring out how this rear spacing works.

I bought my bike off of craigslist from a guy that used to own a bike shop and is now selling his old frames as singlespeeds because "they sell quicker". The chainline is straight and it rides great. I'm looking to convert it to a fixed gear bike so I'm gonna get a new wheel. My current rear wheel looks like this:


So what I would want it would be a 120mm hub that can be spaced out to 126mm, (like the formulas i think?) and then some spacers which just screw on to the axel and space it out the same way that I had before. I'll need to have the wheel trued to the left because of the uneven spacing, right?

Anything else I should know? This website (& sheldon's of course) has been a treasure trove of information.

It's actually a lot simpler than that. All you need is a normal track hub (with 120mm spacing) and two 3mm spacers. Put one spacer on each side of the hub, and you have a 126 spaced hub. The wheel is built centered and with zero dish. You don't need uneven spacers with a real track hub, the reason yours is off center is because it is a converted multispeed wheel.

IRO sells formula hubs (and built wheels) and will space them to 126 for you if you wish. But almost any track hub and 2 M9 washers from the hardware store will work too.

Would there be any chainline issues when using a 120mm hub in 126mm rear spaced frame. I too am in the process of converting an old road bike.

Or if your frame is made out of steel you could simply install the 120mm spaced wheel into the 126mm spaced drop outs and tighten evenly on both sides. You now have 120mm dropouts. They may even cold set to around 123mm.

Some frame manufacturers use 132.5 spacing so that cuctomers have the choice between 130 & 135mm spacing. Steel flexes. In short there really is no problem using a 120mm spaced hub in 126mm spaced dropouts. I've even used 126mm hubs in 120mm dropouts no problem.

awesome diagram, +1. i don't see any problem with chainline when you're installing a 120mm hub in a 126mm dropouts. chainline is determined by the hub and bb (and how the cranks sit on the bb spindle), and is independent of how the rear dropouts are spaced.