Bicycle Mechanics - Help With Installing Front Derailleur
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11-09-10, 07:07 PM
I'm having problems tweaking my Front Derailleur. I purchased a Mercier Galaxy SC3 online (bikesdirect) and therefore received the bike partly assembled. Having put together a BD bike in the past, this was a piece of cake...until I encountered the shifting with the Front Derailleur. I can't seem to get the front derailleur in just the right position to allow shifting in ALL 24 gears WITHOUT the front derailleur touching the chain. I tried moving the derailleur up and down the tube, tightening/loosening the derailleur but only succeeded in cutting my finger and practically shredding the metal cable. Might the problem be that the derailleur can't accommodate 3 rings in the front and 8 in the back as the range is too large?
Any help or suggestions will be greatly appreciated, thanks!
Derailleurs: Shimano Sora RD3400GS Triple rear, Shimano FD-2303 front
You may be cross chaining, which is when you're in two extreme gears.
For example, you're in the biggest charining up front, and the lowest gear in the back. If you look down the chain line, it will be slightly diagonal, which is bad for the chain. You should try to avoid gears like this... If you need that gear combination, shift to the lower-middle of the rear cassette and use the middle chainring. It's the same for when you're in your granny gear (smallest chainring) and the highest gear in the back.
The FD will often rub while cross chaining (especially in triples). Even double crankset setups have some chain rub while cross chaining.
If you need help setting it up, Park Tool is a good source.
11-09-10, 07:34 PM
The front derailleur will almost always need to be moved slightly (trimmed) to avoid rubbing as you move across the entire rear cog range . That is normal, and the STI levers have trim built into them. If you push on the lever slightly the derailleur will move to a trim position without shifting. If the derailleur were made wide enough to accomodate the entire range of rear gears it would not be able to shift properly.
11-10-10, 02:30 PM
Now that you mention it, the FD ONLY rubbed in the extreme gears while cross chaining, while every other chain combination worked fine. That makes a lot of sense... And about the trimming, I do believe I did some of that, but maybe not quite enough. That makes a lot of sense...
Well, thanks a lot guys for the advice!
11-10-10, 05:05 PM
Also go to this site: Sheldon Brown Gear Calculator (http://www.sheldonbrown.com/gears/), and enter in the actual gearing you have. Then develop a shifting-pattern to avoids cross-chainring and duplicate gears. Even though you have a 24-spd, most of those gears are duplicated and you may have only 12 unique gear-ratios. Such as this sample bike (18-spd with 10 unique gears):
11-12-10, 02:29 PM
Hey thanks! I'll check it out.
11-12-10, 02:38 PM
there is an old trick that you can do if you want - modify the shape of the outer part of the FD cage. But I would do this as a last resource. I wanted two more gears without trimming when in the lower chain ring on my Kestrel Evoke with SRAM Rival and I did it (I remembered we used to do this on our friction shifting bikes in the early 80s)... worked like a charm. It only rubs in the 10th gear, and you're not supposed to cross chain like that anyway (and I don't).
Here's a link to what I posted: http://www.bikeforums.net/showthread.php?596252-09-SRAM-Rival-10-speed-fine-tuning-help-please.
11-15-10, 04:56 PM
Alright thanks! I'll keep that in mind as a last resort...
Most Shimano triple road shifters have two positions for each chainring to allow trimming. I don't know if your shifters have this many positions. Avoid cross-chaining, it adds friction to the chain and reduces efficiency. Cross-chaining is not needed because there are similar gear ratios available from other gear combinations.
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