[bigbike]

I was looking online for a flip flop hub and noticed that there are typically options for a 120mm, 126mm, and 130mm. I know that the 120mm is the standard track hub. I was wondering if the 126/130mm hubs are physically bigger than the 120mm or are they the same size but with longer axles and spacers? If the former can I assume that the chainline will be further out by 3 and 5mm respectively?

[BigDumbBear]

I can't remember wich is wich, but one fits older road bikes, one fits newer road bikes, and one fits mtn. bikes....I think. The best bet is to go measure your spacing on your frame and fork because they could even be different. On some steel and a few aluminum frames, you can sometimes get away with spreading the frame out to allow for a wider hub.

[tajcrews]

Normal road bike rear spacing is 130 while MTB's are 135mm. The different width hubs are designed for different frames (road/mtn/track/etc) and they are specifically designed to run a better chainline. For example, the Surly New SS hub is for 135mm spacing and is designed to run a 52mm chainline which is what works best for a MTB. Hopefully that makes some sense. To answer your original question, the hub itself is normally bigger but it can depend on the manufacturer. Check out http://www.sheldonbrown.com/harris/fixed-hubs.html for some additional info. This may help or it may just confuse you a little more. You could always talk to the guys at your LBS for some additional and visual help. What kind of bike are you gonna try this one?