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Old 05-14-12, 11:43 AM   #1
Aquakitty
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No happy medium with front derailleur on this bike!

This is driving me crazy.

I have a dj style bike with only the bottom and middle front ring in use. The top ring is a chainring guard. I had to replace the bottom bracket and the crankset. The old BB was an Octalink 118 mm with matching crankset. The new setup is a square taper crank 113mm with matching crankset. The chainline seems fine, the middle of the rear 8 speed cassette lines up with the middle chainring, plus I did a bunch of calculations to make sure it was correct.

Anyways, I have tried 2 different dérailleurs and I cannot get the chain to not rub on the last 3cogs of the cassette when in the small ring in the front, or vice versa. I would expect some rub on the small/small but it does this on the 6/7 gear as well. If I adjust it any more, it will rub on the top cogs. The current dérailleur is a 9 speed front. There is just no way I can set it up any nicer, it is perfectly in line and as low as it can go without smashing the chainring guard.

Any idea what could be up here?
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Old 05-14-12, 11:53 AM   #2
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Inner 2 of 3 you actually need a longer BB rather than a shorter.
the middle space between the 2 chainrings is chainline center on a double.

you may need someone who can see 1st hand what you are talking about.

I ran a 2 of 3 double in the 80's , no issues .. 52-36
chainring guard just a bit bigger than the outside chainring..

so FD height was about the same as if I had the 52t on the outside
rather than the middle.

Last edited by fietsbob; 05-14-12 at 11:59 AM.
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Old 05-14-12, 12:03 PM   #3
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Well the front chainrings are tiny MTB rings. All I should have to do in theory is stop it from shifting to the top ring.
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Old 05-14-12, 05:20 PM   #4
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Quote:
Originally Posted by fietsbob View Post
Inner 2 of 3 you actually need a longer BB rather than a shorter.
the middle space between the 2 chainrings is chainline center on a double.
This could be the problem, although I have run a triple as a double (inner-middle-bash) without changing the bottom bracket, and it was fine. Can you post photos?
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Old 05-14-12, 06:06 PM   #5
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How many speeds is the cassette and what speed is the chain?
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Old 05-15-12, 04:35 AM   #6
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Sounds like your BB is a bit short.

If you don't want to buy a longer one, you could try tweaking your derailleur cage (not easy to do right).
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