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Old 10-21-11, 09:41 AM   #1
I_like_cereal
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Short Cage Rear D with a 118mm BB

Initially I put a 118mm BB on my bike because I needed a triple. I have since graduated to a just using the 53/39 and removing the 30.

I have a long cage rear d for the triple, but I would like to go down to a short cage if I could.

I have read articles from people that use short cages on triples and they limit the amount of cross chain so that the rear d will shift. So for setting the maximum throw you would put the chain on the 53 and 23/24 or one down from the biggest rear cog. This would then prevent the rear d from shifting to places that it cannot handle due to maximum chain wrap.

This makes sense, but can this be done with a 118mm BB? My fear is that the chain line will be all screwy.

On a similar note can I adjust the front d to not shift to the inside most gear by adjusting the limit screws?
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Old 10-21-11, 11:55 AM   #2
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Cage length doesn't have anything to do with BB width or chain line. Cage length just determines how much gear range you can handle. If weight isn't a problem there is no reason not to keep the longer cage. Even though it isn't advisable to cross your chain to the extremes (large sprocket to large or small to small), it is good to be able to do so. That way if you accidentally down shift too far while still on the large chain ring, you don't twist your derailleur off. Also when climbing a hill you may need to downshift for that last 100 feet and may not want to shift to the small chain ring and then up shift the rear a gear or 2. For a 100 feet once in a while it won't hurt. I have a 10 speed compact crank with almost as much range as a triple and I use a medium cage derailleur. Your requirements will depend on your total gear range.

As for chain line you may be OK. Ideally your chain line should be correct, but if it's a little off to one side you will have more chain wear when crossing against the chain line. If your crank is too close to the BB you may also drag the chain on the large chain ring when crossing from small to small sprockets, it may do this even with a correct chain line. In your case the double crank will probably be too far out and that won't be a problem. The good news is that you will have less chain wear when you don't cross (lol). You will probably be off a couple of mm. See Sheldon Browns page on chain line: http://sheldonbrown.com/chainline.html. If you don't often cross to far against the chain line or can live with the added chain wear, I wouldn't worry too much. Otherwise get a new BB, unless you have something special they aren't that expensive.

Your front derailleur adjustment will depend on the length of your adjusting screws. It shouldn't be a problem, depending on the specific derailleur. A bigger problem may be in your 3 speed shifter (I'm assuming you have some sort of indexed shifting). My Ulterga 10 speed double STI shifter has 1 half click, A friend's 9 speed shifter has 2 half clicks and I suspect it may really be a triple shifter, however it seems to work on his double crank.

I suggest that you try to adjust your front derailleur on your triple crank to just use the 2 larger chain rings. This will be similar to having a double crank and should give you the information you need. You will probably run into any problems that you will have going to a double.

good luck
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Old 10-21-11, 12:13 PM   #3
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Originally Posted by I_like_cereal View Post
I have read articles from people that use short cages on triples and they limit the amount of cross chain so that the rear d will shift. So for setting the maximum throw you would put the chain on the 53 and 23/24 or one down from the biggest rear cog. This would then prevent the rear d from shifting to places that it cannot handle due to maximum chain wrap.
This is not what you are doing. You will only have a [faked] double up front. Unless you're still using a big cog in back, you won't exceed the chain wrap capacity. 53/39 + 12/25 is short cage friendly.
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Old 10-21-11, 12:56 PM   #4
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So I could go short cage and shave a few grams.

He waits for the weight weenie attacks

I figure if I do not need the extra weight why have it. I have an ISIS up front and I replaced it about 6 months ago. So I am not in the market to remove and reinstall.

As for shifting I have DT Friction so I will try to adjust the limit screw and see what that gets me.

Thanks
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Old 10-21-11, 01:09 PM   #5
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if you just take off the granny gear the chainline will be too far to the right.

where the middle ring is , should be the space between the 2 remaining chainrings.
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Old 10-21-11, 01:38 PM   #6
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I figure if I do not need the extra weight why have it. I have an ISIS up front and I replaced it about 6 months ago. So I am not in the market to remove and reinstall.
Not a weight weenie issue

I always like to save a few grams when possible, but not at the cost of something not working correctly. The best reason to keep the long cage is cost and the effort to change it. Reducing weight just takes some money and effort. If it's a choice between having the correct BB or a short cage derailleur I would go for the BB (of course this depends on how far off your chain line is). You can get a decent ISIS BB on eBay for about $25. Depending on the quality of your derailleur the cost could be anywhere from zero to $$$. If you need the BB and really want the short cage, change both.

Ex Prez is correct, a short cage should handle a 53/39 + 12/25, but you haven't stated what your gearing is. Just be sure the cage is large enough to handle both your extreme gear ratios with the same chain length. If you get a used derailleur, be sure the springs are good.

The friction shifters should make everything a lot easier.

good luck
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Old 10-21-11, 01:42 PM   #7
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if you just take off the granny gear the chainline will be too far to the right.

where the middle ring is , should be the space between the 2 remaining chainrings.
True, but not any worse than if he keeps the granny gear but never uses it. For that matter, we don't know that the chain line is correct with the triple.

Last edited by deermouse; 10-21-11 at 01:50 PM.
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Old 10-21-11, 04:13 PM   #8
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If you're trying to save grams, get a short cage rear derailleur and a shorter BB spindle so that your makeshift double crank has its chainrings line up to about where a double should.
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Old 10-21-11, 04:27 PM   #9
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Running a 53/39 and 12/25.

My chainline looks pretty good for a triple using the middle ring. It is not that far off.

Thanks for the input.
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Old 10-21-11, 05:19 PM   #10
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Running a 53/39 and 12/25.

My chainline looks pretty good for a triple using the middle ring. It is not that far off.

Thanks for the input.
But think of the gram(s) you could save in by shortening the spindle! Probably more than going to a shorter RD cage since the spindle is thick steel and the cage is flat aluminum.
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Old 10-21-11, 05:30 PM   #11
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A new bb would shave about 119 grams off plus the 40 grams for the rear d. 159 grams = 5.61 ounces. Or $62 for the BB and another $60 for the rear d = $122/5.61 ounces = $21.39 per ounce.

SHOCKING!!

Granted that is a near top of the line BB and a microShift Arsis rear d which weighs less than Dura-Ace and costs about as much as Ultegra.
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Old 10-21-11, 07:01 PM   #12
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You're the one counting grams man. If it's that comical, then just stick with what you already have, which works.
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