Ask a Mechanic: Front Derailleur Chain Rub

Question: When I’m in the smallest two gears in the cassette and in the small chainring I get chain rub on the front derailleur. To me it looks like the chain-angle is too steep and if I put a spacer on the crankset the angle would decrease, but I don’t know what other ramifications would occur if I did so. Is there a way to determine the ideal spacing of the crankset? From: Mark

Answer: The derailleur rub problem is either a derailleur adjustment issue, a chain-line issue, a trim issue, or a limitation of the component group. If it is an adjustment problem the derailleur could be out of alignment, mounted too high, or the inner limit is set improperly.

The chain line issue is typically a frame alignment problem, but you need professional level frame alignment tools to assess this and chances are this isn’t the cause of your problem (it’s rare). If this was the problem, spacers would be the solution, but it is tricky to do it right without a lot of experience because Shimano cranks are made to work without spacers, meaning this is technically a modification.

On the topic of trim, Shimano groups from a few years ago had a trim function where there is a half shift to move the front derailleur over slightly for cross chain situations like yours (small/small). You need to dial in your cable tension in order for this function to work correctly.

With all this said, I suspect that there is nothing you can do about the problem and you are merely experiencing a limitation of the group you have. Older groups were never intended to work rub free when cross chained (small/small or bib/big gear selections). Many new groups are marketed as capable of working cross chained but I’ve found that this is rarely the case in the real world. Short chainstays, minor frame alignment issues and chainring spacing on some crank sizes all conspire to make rub free cross chain performance very difficult to achieve, especially for an inexperienced mechanic. At the end of the day, cross chaining isn’t a great solution anyway because you have a duplicate gear that you can get to in the other chainring that is not being used. Selecting this duplicate gear will keep your chain straighter which will save your components from excessive wear while providing you with improved efficiency (less wasted wattage).

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