Alternative setup: 2x9
This setup is common among cross country racers and comes stock on a small number of bikes. There are several places where you save weight:
No more (steel) granny ring! -40g
No more bolts for the granny ring: lose 15-20g
Middle ring reduced from 32t to 29t: 7-15g
Shorter BB: 5-10g
Next to saving weight, there’s another advantage: you’ll do a lot less shifting up front, where drivetrain mishaps often occur. The downsides of this system are the increased difference in gear ratios, and the slightly reduced granny gear. Effectively, you’ll be missing the last one or two cogs off your current setup. In practice, however, most of us climb just as well in the 28t cog as we do in the 32t cog! The question is, could you do without the 28?
The most common 2x9 setup consists of 44 and 29 tooth chainrings combined a 12-34t cassette. To convert to 2x9, your bike has to meet several requirements:
Your rear derailleur should be able to shift to a 34t granny cog. Practically all derailleurs will have “max. 32t” or “max. 34t” printed on the back of the cage. Check to make sure your derailleur will work! Also, forget using a road rear derailleur: Shimano’s road derailleurs cannot shift to a cog higher than 27t.
Secondly, a 29t chainring will not fit on standard (110mm bolt circle diameter) 5-arm cranks. Have a look and see what type of cranks you have – it may be time for an upgrade. While you’re at it, get something lighter.
Bottom bracket length
In many cases you’ll need to buy a shorter BB-unit, to compensate for the difference in chain line. Why? With the common triple setup, the three chainrings are centered compared to the cassette. When you go to a double set of chainrings up front, the chain makes a bigger angle when in granny gear. Using a shorter BB axle will re-center the crankset, reducing chain rub on the front derailleur.
Generally, the new axle should be about 3-5mm shorter than your current setup. Here’s an overview of what’s commonly available:
BB-unit for triple crankset
For 2x9, switch to:
Shimano XTR splined 112.5 mm
Shimano Dura Ace / Ultegra splined 109.5 mm
ISIS splined 118 mm
ISIS splined 113 mm
ISIS splined 113 mm
ISIS splined 108 mm
Just subtract 3mm off current size (they come in 3mm increments)!
A note about Shimano Deore/LX/XT cranks: their BB-unit is not compatible with any other. Practically all bikes with these cranks are equipped with a 113mm BB-unit, and you CANNOT get a smaller size. In other words, stick with these cranks or you have to get both a new crank and BB unit.
The obvious choice from a weight weenie standpoint is Shimano’s 12-34t XTR cassette. It is the lightest cassette available with a 34 tooth granny, weighing in at 265g. As the cost may be somewhat prohibitive, there is also an XT version (300g).
Using the above cassette leaves you with a 44x12 top gear. This may not be enough for some people who regularly ride fast fire road downhill and the like. Luckily, both Shimano and SRAM make 11-34t cassettes. The spread of the gearing between these two cassettes is quite different, however. The Shimano cassette (290g) has a more gradual spread compared to the SRAM 9.0 (300g).
Finally, some XC racers among us may have absolutely no need for the low granny gears. People riding fast, flat courses have been known to run 11-32t or 12-27t cassettes. This is only practical in a few cases, and I wouldn’t suggest is at it leaves you without bail-out gears for when you suddenly have to slow down for a muddy or sandy section, or when somebody dismounts right in front of you. Unless you like walking, of course...
Generally, a 44t and 29t chainring will be the best option. For practical reasons, going lower than 29 teeth isn’t possible, and going higher only reduces your already high granny gear. XC racers can opt for a 42t outer chainring, which will save a couple of grams more, but it leaves you without a high top gear for fast descents. The choice is yours.
Regardless, the list of manufacturers that make 44t rings is endless. The two lightweight alternatives to Shimano are Spécialiés TA and SR Suntour. Their rings are the lightest available, and shifting performance is marginally less than Shimano.
Far fewer companies make 29t rings, however. Cannondale, FSA and Spécialités TA make them, with TA being the lightest. A word about the last manufacturer: their chainrings are slightly thinner than most. In many cases, to get proper shifting, you need to place thin washers (0.2-0.5mm) around the chainring bolts, between the middle ring (inner on 2x9) and the crank. This is a must if you don’t also shorten your BB!
Putting it all together
So there are a couple of options. Which one is right for you depends on the terrain you’ll be riding in, and how strong a climber/how fast a descender you are.
44 t, 29t
Cassette option (SRAM)
Cassette option (Shimano)
XT 12-34t, XTR 12-34t
When you’ve put it all on the bike, there are two things to pay attention to:
Make sure the chain is the correct length. Shift to the big-big cross gear see if you need to add or remove links. The chain from rear derailleur to chainring should be as tight as possible, but not so tight that the rear derailleur gets ‘stuck’.
Turn the inner stop screw of the front derailleur to the point where the chain rubs slightly on the derailleur cage when in granny gear. Now turn it back out ¼ turn and presto, everything should be fine.