Subcompact gearing - 42-28 possible with T47 BB?

[Mark McM]

Originally Posted by donevwil

All exacerbated if riding 177.5s and 180s. I fortunately found a used set of White Industries 180s a few years ago and have now decided a second set is warranted, but with used being so scarce and both new WI and Rene Herse solutions being so expensive I'm attempting a 177.5 SRAM S900 retrofit. If any one part (crank, spider, chainrings) has to be sourced new that'll pretty much blow the financial viability. Side benefit being the reasonable Q factor of these SRAM cranks vs WI.

In your first post, you mentioned an outer chainring of 44 or 42, and inner chainring of 28. Currently on ebay there is a new SRAM X0 120/80 GXP spider and 42/28 chainrings (plus chainring bolts) for $69 (+ $7 shipping). If you've already got the cranks and BB, this is a pretty cheap way to convert to a sub-compact.

As far as the whether the big component makers will introduce sub-compact cranks, that's probably an inevitability. But they probably won't be quite the same as the these home brew setups. Any new sub-compact cranks will likely be aimed at bikes with fattish tires and disc brakes, and will likely use wider chainlines (and the accompanying wider Q factors), and not the narrow chainline (and Q factor) of road cranks.

I first started down this road for very niche usage - hillclimb road races. I was looking for a crank that had the combination of the narrow chainline/q-factor and light weight of a road crank, combined with the smaller chainrings of an MTB crank. In the search, I stumbled upon discussions of the fortunate happenstance that some brands made road cranks and MTB cranks with removable spiders which shared the spider mounting interfaces. This appears to offer the combination of features I was looking for.