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 Post subject: 2X9 setup --cranks?
PostPosted: Wed Nov 26, 2003 11:42 pm 
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Joined: Wed Nov 05, 2003 1:09 am
Posts: 40
Location: Dartmouth, NS, CANADA
I was thinking of running a 2X9 setup on my bike, but it has shimano LX cranks(4 arm). Was wondering if I could still run the 4 bolt for a 2X9 setup or do I have to go to 5 arm? if 5 arm what cranks? I would REALLY like to stick with shimano...

thanks


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 Post subject:
PostPosted: Wed Nov 26, 2003 11:49 pm 
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Joined: Wed Nov 05, 2003 8:52 pm
Posts: 126
Sure if you can find 4 bolt rings. you only really need a 28t inner or so depending on your terrain of course. 28 x 34 is fine for me and a buddy rides same. Dont really save much weight tho just by removing inner ring.

Ideally buy double chainring way lighter than equiv triple.


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 Post subject:
Posted: Wed Nov 26, 2003 11:49 pm 


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 Post subject:
PostPosted: Thu Nov 27, 2003 1:25 am 
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Joined: Wed Nov 05, 2003 1:09 am
Posts: 40
Location: Dartmouth, NS, CANADA
I should have mentioned i wanted to run 29t and 44t/or 42t


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 Post subject:
PostPosted: Thu Nov 27, 2003 1:54 am 
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Joined: Fri Oct 31, 2003 10:30 pm
Posts: 225
From: http://www.light-bikes.com/articles/drivetrain.asp

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

Square taper
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.

Cassette choice
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...

Chainring choice

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.


Standard 2x9
Marathon racing/trailbike
XC only

Chainrings
44 t, 29t
44t, 29t
42t, 29t

Cassette option (SRAM)
-
9.0 11-34t
9.0 11-32t

Cassette option (Shimano)
XT 12-34t, XTR 12-34t
XT 11-34t
XT 11-32t


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.


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 Post subject:
PostPosted: Thu Nov 27, 2003 2:05 am 
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Joined: Fri Aug 29, 2003 9:01 pm
Posts: 2477
Location: Colorado
SPRUNG, Did you just totally space the topic thats posted below you? Theres no need for two of the same topics.

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 Post subject:
PostPosted: Thu Nov 27, 2003 3:05 pm 
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Joined: Wed Nov 05, 2003 1:09 am
Posts: 40
Location: Dartmouth, NS, CANADA
Maybe if you read my question, you will notice that it is completly different. I am asking can I use 4 arm cranks or do I have to go to 5 arm? If it bothers you that much, I am sure I can have the thread deleted. But I don't think its that big of a deal.

Cheers

-mark


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 Post subject:
Posted: Thu Nov 27, 2003 3:05 pm 


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 Post subject:
PostPosted: Thu Nov 27, 2003 6:55 pm 
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Joined: Mon Jun 02, 2003 8:17 am
Posts: 6709
Location: Drenthe, Holland
Hi, I've ridden with the 2 x9 setup for two years with shim. lx 4 arm. and it woked out very good. I drove with a 30 32 in smallest gear and a 44 11 in the big ring!

Thats all!

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