What Causes the Front Derailleur to Scrape Against the Chain?
2 Answers
Front derailleur scraping against the chain most commonly occurs due to aging and elongation of the front derailleur cable or loose front derailleur mounting screws. Below is additional information: Derailleur: The role of the derailleur on a bicycle is not to increase speed. Its function is to alter the chain's position via the rear and front derailleurs, thereby changing the bicycle's gear ratio, not to accelerate the bike. Function: The derailleur enables the bicycle to adapt to different road conditions by providing more suitable gear ratios for specific terrains. This allows riders to better overcome challenges posed by varying road conditions during cycling.
I encountered this issue while repairing a car the other day. The front derailleur scraping the chain is mainly due to incorrect alignment. The L and H limit screws control the left and right movement range of the derailleur. If adjusted too inward, it will rub against the chain. Additionally, if the shift cable is too loose, the derailleur won't pull enough when shifting to the large chainring, causing the chain to scrape the edge. It could also be that the derailleur cage was bent in a crash—last week I worked on a bike that got hit by a rock. Moreover, if the chain is worn and stretched beyond 0.75%, it will scrape, like my neighbor's old bike that hadn't had its chain replaced in three years. Lastly, check if the mounting bolts are loose, as the entire derailleur shifting can cause scraping. When adjusting, turn the crank and fine-tune the screws simultaneously until there's no friction sound for accurate alignment.
