Hub sizing

In my experience, same hub, just spacers added for wider drop outs.

etnerual said:

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 physcially 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?

AFAIK (which could be incorrect), the flanges on SURLY's hubs are indeed set further apart on the hubs which have a wider "spacing" ...

Consquently, if you're building a Single Speed MTB (for example), then you would probably want a SURLY-or-comparable hub whose flange spacing is indeed wider.

If you have a frame with 126mm spacing, then it is probably steel, and you can simply respace the rear triangle to 120mm.

You CAN add spacers/washers (between the cone & nut) to a 120mm hub to use it in a 126mm spaced frame, of course ... and, of course, to the FRONT hub (if you buy a hubset with a 120mm rear hub).