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Tuesday, April 6, 2010

Fixing a Bent Derailleur Tab

Got a bent derailleur? Its probably a Shimano, the most popular derailleur sold on the market.

Well you're in luck, they're usually pretty easy to fix... Usually its not the derailleur that is bent anyway, but the tab (also called a hanger) the derailleur attaches to the frame has become bent. (Its actually a very COMMON problem for bicycle mechanics.)

The derailleur itself is usually made from a very strong metal and the parallelogram design makes it highly unlikely to get damaged. The weakest point therefore is the tab and if the derailleur gets bashed hard the tab will usually bend inwards. If it gets bent really far your derailleur will end up rubbing or jutting into your rear wheel spokes. Even a minor bash to your derailleur can muck up your alignment if the tab is weak.

You can roughly check the alignment with your eyes, if you look at the derailer from the rear. The derailleur pulleys should be lined up vertically with no sideways slant.

Is the tab Steel or Alloy?

If the tab is steel it can be easily bent back into position and realigned.

If its alloy metal you may be able to bend it back into position, but the next time it gets damaged it will be even worse. If it gets damaged again and again eventually the tab will snap right off. (A bit like twisting the tab off a pop can.)

To do a really good job of fixing the tab you will need the proper tool. Bicycle mechanics usually have this special tool in their shops so its not something a person would normally have in their garage.

Because many new bicycles now have narrow chains, Hyperglide, 7-9 speed cog sets proper derailleur alignment becomes really important to shift normally and smoothly. You want the derailleur to be PERFECTLY aligned to the front gears and the chain.

So what you want is a Park Derailleur Hanger Alignment Gauge, which costs about $60 US to buy unfortunately... without it you will just have to make do with whatever tools you have.

STEP ONE: Place the chain on the smallest gear.

STEP TWO: Remove the derailleur from the derailleur tab, with a 5mm allen wrench.

STEP THREE: Remove the derailleur and chain.

STEP FOUR: Use a Park Derailleur Hanger Alignment Gauge to realign the derailleur tab. (The tool uses the rear wheel as a guide so make sure your rear wheel is on perfectly straight!) If you don't have that tool you will have to bend the tab using whatever tools you have available. If its an alloy tab try to be gentle with it.

DO NOT JUST YANK ON IT. You don't want to overcompensate and bend the wrong part by accident or break it off entirely.

STEP FIVE: Reinstall the chain and derailleur.

STEP: SIX: Check and modify your derailleur and derailleur screws to make sure your derailleur is functioning perfectly.

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