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Bottom bracket spacing/ front derailleur

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Bottom bracket spacing/ front derailleur

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Old 10-09-15, 09:23 AM
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Mjoshualong
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Bottom bracket spacing/ front derailleur

I'm having an issue with a build I am doing. This is the first time I have built a bike and I have it all together, but I am having issues with tech front derailleur. I can not get it to adjust out far enough to shift into the large chainring. I have a 3x9 shimano setup. I am wondering if I need to move the spacers on the bottom bracket from one side to the other to allow the derailleur to reach all the way out to the large chainring. The derailleur I bought is a deore and the crank is a xtr. Any help would be appreciated.
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Old 10-09-15, 09:30 AM
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What model crankset, bottom bracket and derailleur are you using? Can you move the derailleur far enough by hand?
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Old 10-09-15, 09:33 AM
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Measure very carefully from the center line of your frame (probably the middle of your seat tube will work best) to the center of the middle ring and let us know what that number is.

It should be around 47.5mm, but if it is not, then that will help know how to proceed.

The exact model of your crank and FD will help. It will stamped on somewhere FC-XXXX and FD-XXXX.
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Old 10-09-15, 10:02 AM
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the seat tube diameter relocates the FD as well , over size seat tubes change that. outward
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Old 10-09-15, 10:09 AM
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You've tried adjusting the limit screws?
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Old 10-09-15, 10:49 AM
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I will look at the part numbers tonight and measure the distance to the middle ring. Pulling the derailleur out by hand and setting the limit screws all the way out, still does not get me out far enough.
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Old 10-09-15, 11:23 AM
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Your dilemma is a prime example of what happens when you "put together" a bike without understanding the relationship between interacting parts, as well as between parts and the frame. The problem is not likely the front derailleur but rather the frame/BB setup. The first step you need to do is determine the rear chainline. The next step is to match the front chainline as close as possible to the rear, taking into account which gears you will tend to favor (lower or higher). Make sure that the BB you use is matched to the crank arms. Once you have done that the likelihood is that the front derailleur will work.
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Old 10-10-15, 01:50 PM
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Ok, the parts list for what I have is as follows,
shimano sm-bb90 bottom bracket
fc-m970 crank
fd-m590-10 front derailleur
I also measured the spacing and it at about 53.75 mm. I am guessing that is where my problem is. Right now I have two spacers on the crank side, is switching the spacers to the other side the appropriate solution to change the spacing? Thanks for all the input!
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Old 10-10-15, 01:52 PM
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Originally Posted by cny-bikeman View Post
Your dilemma is a prime example of what happens when you "put together" a bike without understanding the relationship between interacting parts, as well as between parts and the frame. The problem is not likely the front derailleur but rather the frame/BB setup. The first step you need to do is determine the rear chainline. The next step is to match the front chainline as close as possible to the rear, taking into account which gears you will tend to favor (lower or higher). Make sure that the BB you use is matched to the crank arms. Once you have done that the likelihood is that the front derailleur will work.
Forgot to ask, how do I check the rear chain line?
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Old 10-10-15, 02:04 PM
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The word "chainline" in my post is a link to Sheldon's explanation, including how chainline is measured. No sense in redoing it here. Another page discusses chainline specific to multi-speed bikes.
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Old 10-10-15, 02:05 PM
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Originally Posted by Mjoshualong View Post
Right now I have two spacers on the crank side, is switching the spacers to the other side the appropriate solution to change the spacing?
Depends on what the width of the spacers is and the difference in chainline you need to make. It's simple math, addition/subtraction.
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Don't tell me what "should" be - either it is, it isn't, or do something about it.

If you think I'm being blunt take it as a compliment - if I thought you were too weak to handle the truth or a strong opinion I would not bother.

Please respect others by taking the time to post clearly so we can answer quickly. All lowercase and multiple typos makes for a hard read. Thanks!
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Old 10-10-15, 02:13 PM
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Ok I see that now. Thanks.
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