Originally Posted by
trail_monkey
I converted my mid 80s road bike from 6 speed freewheel rear (126 spacing) to a 7 speed free hub rear with cassette. The hub came from a 700c road bike rear wheel of a late 80s early 90s Trek. It is a Shimano Exage hub. I measured it and it is 130 on the spacing. I cold set my rear drop outs and all is good. I just recently stumbled across some info that says 6 and 7 speed road bike rear spacing is 126 and 7 speed mtb spacing is 130. But this hub was from a road bike. What am I missing here?
There's no Hub Spacing Police. Those numbers are merely reflections of how most manufacturers spec'd their bikes. Bridgestone stuck with 126mm 7s on their mountain bikes for a while, then resisted the move to 135mm a few years later, and got Shimano to make them some special 130mm Deore hubs.
There are two reasons I can think of for the difference.
One, mountain bikes used triple cranks, which require the cranks be more outboard from the frame, especially on non-ferrous frames. Moving the cassette farther outboard keeps the chainline good.
Two, mountain bike rear wheels are more likely to be subjected to lateral loads, so the wider spacing decreases dish and equalizes spoke tension side-to-side.