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I have a Cannondale Synapse SE 105. Whenever I go downhill I tend to shift into combination of top (smallest ring) gears in back and large gear in front. My bike just makes so much amount of noise every time I try to pedal in these combinations (I think the chain is rubbing on front derailleur). Can anyone please help me on how to eliminate this sound?

  • By top 3 gears in the rear, do you mean the biggest 3 cogs? Experienced cyclists would usually say the bottom 3 gears, since those would be the lowest gear ratios. Also, do you actually mean the bike is creaking, or do you mean the chain is rubbing on the front derailleur? – Weiwen Ng Jan 23 at 18:24
  • Well, I am new to cycling. I meant, the bottom 3 gears and the chain is rubbing on front derailleur. – MV Sai Kiran Jan 23 at 18:28
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    The smallest gears in the rear are the top or high gear ratios. I think this is what you mean as you say you change to these gears when going downhill, where you would need the highest gear ratios. – Argenti Apparatus Jan 23 at 18:53
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    You are "cross-chaining" . Either use your FD's trim (a half-click) or change chainring. – Criggie Jan 23 at 23:24
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    @Criggie the OP says the FD is rubbing in big ring/small cogs. Trimming the FD would move the cage away from the chainline, which should make the rubbing worse. Trimming will reduce noise in big ring/big cogs. I am assuming the OP described the problem correctly, though. – Weiwen Ng Jan 24 at 18:05
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If you think your chain is rubbing on the front derailleur I think it's safe to say that the noise is coming from the drivetrain.

Front derailleur rub tends to occur when the chain is at the most inboard or outboard positions, either small chaining / largest sprocket or large chaining / smallest sprocket. I would not expect to get rub on the 2 or more of the smallest sprockets, unless the derailleur rub is very bad.

In any case, the first thing to do is to check the front and rear derailleur adjustment. (You may be getting some noise from bad rear indexing.)

Detailed written instructions would be far too long and difficult to follow, but there are many instruction article and videos available on the Web. The best by far I have found are those from Park Tool.

https://www.parktool.com/blog/repair-help/rear-derailleur-adjustment

https://www.parktool.com/blog/repair-help/front-derailleur-adjustment

Run through those adjustment procedures and see if that fixes the problem.

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    @MV Sai Kiran Just wanted to mention that one should first have the rear drivetrain properly adjusted (limit screws set and shifting indexed) before moving on to the front. Watching the Park videos and completing their procrdure in the order Argenti has outlined above would cover this detail, but I wanted to emphasize the order. – Jeff Jan 27 at 7:35

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