What Causes the Front Derailleur to Scrape Against the Chain?

[LaPresley]

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.

[JadeRose]

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.

[TaylorRose]

Encountered this situation many times. The most likely cause is the tension of the shift cable—if the cable is loose, the derailleur won't spring back properly, causing chain rub. Another possibility is misadjusted limit screws: if the L-screw is too tight, it causes chain rub in the small chainring, while if the H-screw is too loose, it leads to chain rub in the large chainring. Another common issue is a bent derailleur cage, often seen in off-road bikes hit by branches. Chain length is also critical—if a new chain is cut too short, it will pull tightly on the derailleur when using the small chainring with the largest cog. It's recommended to first clean the derailleur joints and apply lubricant, then readjust the cable tension. If the noise persists, check the limit screws—turning them clockwise increases clearance.

[AubreeMarie]

There are five common causes: The most prevalent is aging and slack shift cables, like my colleague's commuter bike that hasn't had its cables replaced in three years. Incorrect positioning of limit screws, which causes rough shifting during repairs, is another issue. A bent derailleur cage, such as when a food delivery rider's basket crushed the derailleur. Chain overstretching—installing a new chainring with an old chain will definitely cause rubbing. Incorrect installation angle, where the front derailleur isn't parallel to the chainring. The solution is quite simple: first check cable tension, then adjust the high and low limits. If that doesn't work, remove and straighten the derailleur cage, and finally consider replacing the chain. All you need are a 5mm hex wrench and a chain checker.