New crankset alignment - FD or BB?

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New crankset alignment - FD or BB?

Postby m@ » Thu Nov 06, 2008 9:46 am

So I've just installed a new crankset on my mtb (Shimano M442 170mm). The largest chainring is a 44 as opposed to 42 on the original but the diameter is quite a bit larger, and the chainrings also seem to sit a bit further out from the BB. The problem is that the FD doesn't quite have the reach to reach this chainring, and also doesn't appear to be high enough to clear it sufficiently. Can't move it up the seat tube as it's bottom-bracket mounted :evil:

Does it sound like I need a BB with a shorter axle length? Or am I going to need a new FD? I notice that the FD moves down as it stretches, so bringing the chainrings closer to the frame should increase the clearance...

The photos below will hopefully show what I'm talking about; the FD isn't quite at full reach here...

Image Image
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by BNA » Thu Nov 06, 2008 10:10 am

BNA
 

Postby Kalgrm » Thu Nov 06, 2008 10:10 am

With a larger chain ring, you need to raise the FD a few mm. In the short term, you can "fix" the problem by putting your 42t chain ring back onto this new crank set.

Adjusting the "H" screw should help you get the FD to move out far enough, but it sounds like you're going to need a new BB with a shorter spindle too. (You will also need to adjust the "L" screw with this crank set - everything should have moved outwards by a few mm, so both screws need to be adjusted.)

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Postby mountain tamer » Thu Nov 06, 2008 10:51 am

m@,

I had the exact same problem! Did you go from truvativ to shimano? thats what happened to me, the shimano design was much further away from the frame. From those pics, mine was a lot closer than yours though. I would try adjusting the screws.

I would agree with kalgrm, on my one I had to adjust the low screw because it was now further away, that gave me enough tension to get out far enough.

good luck,

MT
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Postby m@ » Thu Nov 06, 2008 12:31 pm

Thanks for the replies, must admit I hadn't even thought to check the limit screws :oops: Wonder whether the DR could be mounted on the outside of the chain guard instead of the inside; that would move it out my ~2mm?

Unfortunately the old crankset is a one-piece (which is why I replaced the cranks as well as the rings), so can't use the old 42 chainring. The thought of running the old cranks did occur to me, but they're in pretty poor nick (abusive previous owner) and I don't want to chew up my shiny new chain... I can probably get away with using the inner chainrings for the time being though.

The old crankset was actually also Shimano, though very low-end, which is why I assumed I wouldn't have any compatibility issues... Ah well, will start looking for a replacement derailleur - or maybe just change the big ring back to a 42. I guess that's the risk of saving cash by DIY-ing when you don't really know what you're doing - all a learning experience though ;)
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Postby mountain tamer » Thu Nov 06, 2008 12:56 pm

did you change the screws?
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Postby m@ » Thu Nov 06, 2008 2:36 pm

Nope, though now that I think about it I do have a sneaking suspicion that I'd already set the H screw at its outer limit due to shifting problems with the original crankset (it's been quite a while since I've ridden this bike). Not too familiar with BB-mounted FD's but perhaps the mount is bent? Now that I think about it, on the innermost setting the derailleur body was hitting the seat tube (you can see the mark/paint chip level with the top FD pivot in the second photo).

Ended up overhauling both shifters and derailleurs and replacing the cables as well as the chain, crankset and cassette... so a new FD isn't too much of a problem. Just wish I'd included it in the Wiggle order to take advantage of the free shipping - I'll have to think of a way to spend another $200 or so now ;)
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Postby RobS » Thu Nov 06, 2008 5:05 pm

You should check your chainline. If the BB is too long it will show up there.

http://www.sheldonbrown.com/chainline.html

If the chainline is way off you might actually have trouble getting any FD to work.

If you stick to the shimano chainline specs things rarely misbehave.
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Postby m@ » Fri Nov 07, 2008 3:09 pm

Thanks RobS - I'll definitely check the chainline. Sheldon's site really should've been my first port of call I guess ;)

Good grief FDs are complicated beasties - there appear to be about five different Deore FDs that are compatible with my bikes' seat tube diameter, cable routing etc...
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