The shape of the arms provides low Q factor with enough clearance at the stays to fit many cross and most road style bikes, as well as a Rambler

The two rings are centered properly in 3d space to work well and be centered with the cassette body on a 130mm spaced rear. No fiddling with square taper bottom bracket lengths or fine tuning with spacers or a Phil Unit. And I am saying that as a guy who loves his square taper bottom brackets dearly, with a stash of sizes.

Third the placement of the inner ring is such that the 110 and 74 mm circles are in the same plane, the ring mounts to the arm with bolts and spacers. that is the magic that allows for the above two to happen.

The smooth shifting in front is facilitated by the ramps, pins and tooth profiles. The lack of a crunchy rear is due to the centering of the two rings. You end up with an amazing range of useful gears, and little cross chaining.

The cost and benefit really shines on a bike that you want a bit more performance out of. A modified triple come double on a long BB spindle will get you there. Some people even like the feel of really wide Q or you cranks being asymmetric in space while you shifted things to the right to get better alignment. It all comes down to what you really want out of the total package.

There are some other ways to get to the same place with cranks like the TA offerings, vintage 94bcd cranks will let you run a 30 tooth ring on the inside, and of course the White Industries VBC cranks arms.

That said, you bring up a good and interesting point about fudging a triple via BB spindle length to get it centered. I haven’t tried that personally so I can’t comment on how well it works. It SHOULD work and shouldn’t be a problem with friction shifters.

However, I wanted something that would work with STIs specifically. I don’t know if ring spacing changes bw a triple or double to make it less compatible with STIs. Something to try at some point.

R

http://www.rivbike.com/product-p/cr7.htm

As a dedicated double and not a modified triple, you get access to most of the gears with little redundancy.

Russ

Curtis.
You can get what you want in an outer chainring but the Shimano option is a Dura-Ace cyclocross ring(spendy but worth it). I suggest looking at TA Specialties for an excellent selection of chainring sizes. You could do a 46T or 48T outer.
As for your 30T swap it should be possible on a 74BCD which both Shimano & Campagnolo have been using since the 90′s on road triples . Lowest would be a 24T.

Additionally with the popularity of cyclocross 46T chainrings are now readily available in the 110BCD and to a lesser extent 130BCD. The industry has settled on 110BCD as the default for CX rings so there will always be smaller outer rings.
There are many smaller after market brands that make really nice chainrings. A search of CXMag or CyclocrossWorld.com will cover most of the bases.

Shimano will say to use only their equipment but another brand will work just as well especially since the ring is smaller.
Good luck in your search.

JimW

I was thinking it might have a better Q-factor, but since it uses the same method of mounting the smallest ring, I don’t think it would. Even if that were so, I’m not sure it would justify the cost.

I find that using standard 110/74 touring/MTB cranks offer plenty of customization for my needs. My road bike uses 49/34 and my touring bike is 46/36/26. One doesn’t need to spend hundreds to avoid “road compacts” or “road triples.”

http://www.compasscycle.com/cranks_rh_double.html

I am currently using an old Sugino AT triple crankset as a double crankset on my touring bike. I have a chainguard in the outer position, a 48t ring in the middle and a 26t ring on the innter 74bcd position. I’m using friction bar end shifters and it shifts pretty well with an old Shimano Deore (deerhead) front derailleur up front and a 12-32t 7spd freewheel with a Shimano Deore XT (9speed era) rear derailleur. With the 22t jump between the small and large ring I can’t use the smaller 4 cogs on my freewheel without the chain rubbing on the larger chainring. If I do some jockeying with BB length and chainring spacers I might be able to make it work better with less rub. My 48t/12t gearing is plenty large enough for spirited riding by this relatively “bike fit” 35 year old. I can’t imagine trying to make use of the 53t/11t setup that many bikes ship with.