94BCD, bolt circle, bolt circle diameter, chainring, circle diameter, compact double, gear inches, Sheldon Brown, Shimano, shimano xt, Suntour, suntour xc, tread, XC Pro
I like to minimize my front derailleur throw to two chainrings, and with modern cassettes like the Shimano HG61 12-36t, you don’t need a triple anymore unless your are really slogging up 20% grades or running a hundred pound cargo/touring bike. I don’t find myself in these situations, and my knees are still fairly stable. As such, I wanted to try a double that was lower than the XD2 crank I was using. Since it only had one bolt circle diameter (110BCD), this resulted in a minimum chainring of 33t. Here’s a handy chart from the venerable Sheldon Brown pages. This was a bit too much for the really steep hills. I want something that is more “granny” for the Seattle pitches I find myself climbing on the daily commute.
First things first. I had two older cranks – a Suntour XC Pro 175mm from my old Stumpjumper Pro circa 1992. This has 94 and 56 Bold Circle Diameter options. The 56 is fairly hard to find these days (Peter White sells them), and there are really 20-24t rings for this. That’s a bit too small. However, with 94 BCD, I can go down to 29t. This was one option. I also have an ’88 Shimano XT 175mm Crankset that sports 110/74 BCD. This would allow a small ring down to 24 on the inside ring.
Because I’m partial to the XC Pro (it was my first and favorite mtb), and I would be able to use the outside ring bolts for both rings, I opted for this setup. My current bottom bracket is a 107mm SKB, and it results in a fairly narrow tread. By putting my rings on the outer bolt, it results in a better chain line for this setup.
As you can see, I’ve left the inner ring empty:
In using a 32t chainring with my new cassette’s 36t big ring, I have a 24 gear inch setup for the low end. This goes up to 103.5 gear inches for my 46t-12t big chainring small sprocket combo. That’s as wide as most triples these days, and allows me plenty of gearing for most anything I’m riding.