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Derailleur Help

Old 03-16-18, 06:32 AM
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ad18
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Derailleur Help

My bike mechanic skills are limited at best, but trying to learn. I have a Trek FX3 that came with factory installed Shimano derailleurs. Back is Alivio and front is Acera. Rear one shifts very crisply and cleanly during up and down gear changes. Front shifts up great, but when shifting down has a tendency to do what I call chatter or drag from larger gear to middle gear. If I hit the gear shift lever rather aggressively it shifts better, but if I apply normal pressure it happens much more frequently. I had the bike shop where I bought it adjust the setting on both the shifter and derailleur and it helped a little, but is definitely still there. Does it just need further adjustment or would replacing the front with a better quality derailleur be the best option? Will add info to my growing mechanic skills Thanks in advance for advice.
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Old 03-16-18, 06:43 AM
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The problem is as likely to be caused by cable friction as anything else. Before changing the derailleur, you should lubricate the cable and the shifter to see if that is the problem
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Old 03-16-18, 07:58 AM
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Originally Posted by ad18 View Post
Does it just need further adjustment or would replacing the front with a better quality derailleur be the best option?
Even Shimano's lower-end Acera/Alivio/Altus derailleurs shift well. You shouldn't need to replace yours. What you typically get with a higher-priced derailleur is lower weight, better materials in strategic locations, and a nicer finish.

Adjusting front derailleurs can be a bit finicky, in part because there are other factors at play besides the position of the front derailleur. For example, the gear you're in on the rear can have an effect since it alters the angle the chain passes through the front derailleur. Also, front shifting occurs on the top loop of the chain, which is under tension from the crankset. The more pressure you're applying on the pedals, the rougher your shifts are going to be. Make sure you're easing up on the pedals when you shift the front.

Those things aside, you may still find that your front shifting needs a little tweak. Park Tool has a good setup & adjustment guide on their web site, linked below. You can check the basic setup stuff like derailleur height and angle, then check your indexing.

https://www.parktool.com/blog/repair...eur-adjustment
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Old 03-16-18, 08:18 AM
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ad18
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Thank you all for your help. Great information to try and see what happens. Appreciate it
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Old 03-16-18, 09:12 AM
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maybe it needs an alignment tweak? sometimes the front derailer on my road bike slips down on the seat tub & I have to slide it back up. hasn't happened in years tho. wonder if I finally got it tight enough. another bike I had the front derailer needed a rotational twist left or right on the seat tube for it to be perfect
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Old 03-16-18, 09:13 AM
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good photo illustrations!
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Old 03-16-18, 09:13 AM
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Front derailers push on the tension side of the chain and pretty much always shift slower than rear derailers. There's a limit on how good you can get it.
Assuming all parts are in good working order, you're likely to see more change from easing up on pedal pressure while shifting than from replacing the derailer.
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Old 03-16-18, 09:43 AM
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The chain tension diagnosis may be a key factor. Definitely noticed problem going up any type of incline. Another great tip. Thanks.
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Old 03-17-18, 07:51 AM
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Follow the Park procedure from the beginning, without skipping any steps. Keep your adjustments very fine, 1/8 turn or less, lest you overshoot the sweet spot. And don't be afraid to back off an adjustment and do it again if it is just not going right. Park's rear derailleur instructions are also good.
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