We’ve all done it — been riding along, minding our own business, when out of nowhere a rock or tree stump rears its ugly head and bam! The derailleur is hanging down all limp and flaccid. And even if it doesn’t break off, a good thump can bend the cage, the hanger, or worse, the frame.
Most modern mtb frames are made from aluminium and because this material doesn’t like being bent back after a whack, the majority of manufacturers fit them with replaceable hangers. On some bikes these are made from low grade alloy or have tiny bolts holding them in place so are actually not that strong. We actually think more bikes should come with modular dropouts (includes both the mech hanger and the dropout) like on the Santa Cruz bikes we tested recently or go back to a solid dropout, which would be stronger.
In most cases the derailleur and hanger gets bent in towards the wheel and this misaligns the whole derailleur system. Then when you shift to the smallest gear (the largest sprocket) the chain goes over the top of the cassette and down into the spokes. Like a chainsaw it then nibbles a bit out of each spoke, which then snap one by one.
Fixing a busted mech on the trail depends on the severity of the impact. If it’s just a slight tap you can just tweak the hanger straight, but if it’s sticking out at a 90 degree angle then you could try bending it straight, but you’ll probably need to remove it. If it’s totally snapped off (usually the knuckle breaks) the only option is remove the offending item, shorten the chain and ride singlespeed.
If you do have a derailleur failure, for the sake of safety make sure to get your bike checked out afterwards by a qualified dealer. It should have the proper tools to make sure the frame and hanger are both straight before you go riding again.
TOOLS FOR THE JOB:
Allen keys / Park chain tool / Park derailleur hanger alignment tool (shop only) / Park frame alignment gauges (shop only) / Loctite / Cable cutters
1 If you’ve bashed your rear derailleur on a root or rock, the first thing to do is bend it back. Grip it as shown and apply a firm but even pressure.
2 If it won’t bend back or snaps off replace it. You can see from this picture that the derailleur hanger is slightly wonky.
3 But to really see whether the hanger is out of alignment, hold a straight one next to it.
4 To repair a bent hanger get a bike shop to use a derailleur hanger alignment tool on your frame. This is particularly useful for steel frames without replaceable hangers.
5 While you’re there, get the shop to check the dropout alignment with a set of frame alignment gauges. Both should meet perfectly in the centre.
6 Sometimes only the actual cage is bent. Grip it as shown and apply a firm, even pressure. Check the alignment from behind; make sure the cage is parallel to the frame and sprockets.
7 Check the fixing bolt is properly tightened. A jumping chain or errant shifting is a sure sign the hanger is loose.
8 If your hanger often comes loose, a small dab of Blue Threadlocker in the threads should be enough to hold it in place.
9 If you’re in the middle of nowhere and you’ve ripped the hanger/derailleur off completely, these tips will show you how to get home. It doesn’t work on full-suspension bikes though. First remove the chain and broken derailleur.
10 Unbolt the rear gear cable and coil it round the seat tube or swingarm, out of harms way.
11 Place the chain in the middle sprocket of the cassette cluster and middle chainring at the front. Sight down the chain — you’re aiming for a straight line.
12 Overlap the chain, remove the excess and join both halves to make a singlespeed. Nurse the bike home slowly.