Originally Posted by Mark McM
I've been looking into this quite a bit recently, so just to add a bit more:
As noted, the 120/80 spider normally is the one to use for 42-28 chainrings. However, there are a number of manufacturers who make 104/64 BCD chainrings, and there are some 42-28 chainrings in this BCD.
Aluminum vs. carbon crank spiders: SRAM's Carbon cranks have a larger profile than their aluminum cranks, so the relief cutout in the spider for the carbon crank-arm is larger than for aluminum cranks. Aluminum cranks will work with the "carbon" spiders, resulting in gap in the relief cutout around the crank; but carbon cranks will not work with "aluminum" spiders because the relief cutout is too small to fit the carbon crankarm. SRAM road and MTB cranks have the same profiles, so a "carbon" MTB spider mates perfectly with a carbon road cranks (vice versa), and an "aluminum" MTB spider mates perfectly with aluminum road cranks (and vice versa). There is a caveat for the 104/64 SRAM MTB spiders, however: On these spiders there are two small raised nubs on the face that mates with the crank where the 64mm threaded holes are. MTB cranks have corresponding divots to accommodate these nubs, but road cranks do not. However, the nubs can be ground down to mate with road cranks, with no affect on function. It should be added here that North Shore Billet makes aftermarket SRAM compatible spiders for 104/64 that do not have these nubs, and mates perfectly with road cranks without modification. Another feature feature of the North Shore Billet crank is that it uses a standard diameter for the chainring "lands", so that any 104mm chainrings will fit - SRAM spiders use a larger diameter for the chainring "lands", so only SRAM (or SRAM-specific aftermarket) chainrings will fit. North Shore Billet 104/64 SRAM compatible spiders come in different versions for GXP and BB30 (see below), and different versions for carbon and aluminum crankarms (labeled "X0" and "X9", respectively). for my SRAM Force 22 cranks, the GXP XO version is the correct version.
GXP vs. BB30 spiders: SRAM's BB30 MTB cranks are narrower at the spindle than their GXP MTB cranks, so the spider mounting surface is more inboard. Consequently, in order to maintain the same chainline, the BB30 spiders have offset the chainrings a few millimeters more outboard of the spider mating surface than the GXP spiders. SRAM road cranks use the same width between cranks for both GXP and BB30, so the road (130mm and 110mm BCD) spiders have the same chainring offset for GXP and BB30 cranks. For double chainring spiders, the GXP version of the MTB (120/80 and 104/64) spiders have almost exactly same chainring offset as the road spiders, so substituting an MTB spider on a road crank will give nearly the same chainline.
11 speed: SRAM MTB spiders are available in a 2x10 version, with a C-C dimension between chainrings of about 8 mm. SRAM Road 2x11 spacing is about 7.5mm C-C between chainrings. In most cases, this will be close enough.
Some caveats: Like most cranks, SRAM cranks splay outward from the spindle to the pedal. This means that as the front derailleur is lowered (for smaller chainrings), there will be less clearance between the derailleur cage and the back of the crank arm. When lowered for the smallest chainrings, the derailleur may actually scrape the crank. To mitigate this, chainring spacers may be used to shift the chainrings inboard. The practicality of this solution may vary, depending on frame clearances, front derailleur movement range, affects on chainline, etc. I have tried this on one particular crank, chainring, frame and front derailleur combination with some amount of success, but I can't guarantee that it will work with all chainring, frame, drivetrain and derailleur combinations. I should also note that I am using Campagnolo Ultra-Shift shifters, which are micro-indexing and adaptable to wide range of front derailleurs - with other shifters, you may be stuck with one specific type of front derailleur, which may not be compatible with subcompact chainrings.
For more information on some of the dimensions and offsets of SRAM cranks and spiders, see the SRAM Frame Fitting Specifications documents. These can be found on the SRAM website, or by googling "SRAM Frame Fitting Specifications".
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