Rear derailleur got caught in the spokes, what could be a root cause

Question

I have several problems on this bike. Here are the elements that have failed and other pieces of information:

• the chain has derailed past the largest sprocket
• the rear derailleur has rotated 180 degrees, and is caught in a spoke (between the frame of the derailleur and one of its sprockets)
• half of the derailleur is now bent 45 degrees from its frame (derailleur on picture 2 should be U-shaped, not V-shaped)
• sprockets showed some signs of wear, but not to the point where it causes the chain to skip when load is applied
• the threaded hole to which the rear derailleur is screwed shows some sign of wear ( this is a modular part which is itself screwed to the frame)
• rear axle is broken in half right in the middle. I don't have the tools right now to open it, I believe this requires a punch and a hammer to remove the cog set.. Right now I have already lost some bearing balls after I moved the axle from the dropouts.

Now I'm wondering if there is a single failure that could explain what happened.

Thanks to the answers, I inspected the derailleur closely and found this: There is a missing chip exactly where the L screw is supposed to hit the frame. Apart from that and the fact that it's bent beyond repair (IMO) the derailleur is actually brand new..

Here is a picture of the broken axle I knew something were really wrong when bearing balls started falling on the ground...

Answer 1

If the low limiter screw on the rear derailleur was very badly adjusted, it would be possible to move the derailleur far too far past the end of the cassette by shifting into the lowest gear (i.e. shifting onto the largest cog on the cassette). This would move the chain off the top of the biggest cog and could force the derailleur into the spokes which, in turn, would bend it.

There's a lot of information on setting derailleur limiter screws on the internet - here's a fairly good example

Answer 2

(in addition to Diado's answer) I would suspect a bent derailleur hanger. Usually the derailleur cage should be perpendicular to the wheel's axis. When the derailleur hanger is bent (it's said that happens when the bike falls over or when squeezing it into parking positions) it will often be bent towards the wheel. I've seen bikes where this goes so far that the cage touches the spokes on the smallest gear, even when the limiter screws and cable tension are adjusted correctly (i.e. the small wheel in the derailleur is spot on at the largest cog, and screwing in the limiter would prevent reaching the smallest gear).

One of these bikes experienced the same damage as yours when shifting into the smallest gear at high speed. Never happened at slow hill climbing, we suspect vibrations at higher speed pushed the derailleur just that little bit further in to get caught in the spokes.

Answer 3

As for repairs...

Axle - you're up for a new axle. Check the condition of the cone nut and their bearing surfaces - they may be okay but any dents or pits and they're better-off being replaced.

Wheel - Inspect the wheel's cups too - they are much more expensive to replace so minor damage to bearing surface you may choose to accept.

Bearings - Do just replace all the bearings in the wheel too - even if you saved them all they will be worn. Ball bearings are cheap.

Derailleur - needs more inspection but I'd replace it. The bent bits may be straightened, but the missing limit stop for the large cog is going to be fiddly. You could braze or solder something in to act as a stop, but any repair could fail repeating this whole problem - not recommended.

Transmission - I suspect the chain and cassette will be okay, but do clean and inspect them for damage.

Spokes - again inspect. Its possible for the Drive Side spokes to get munched at the bend where they enter the hub, leading to premature spoke failure. I'd remove the cassette and any plastic protector, clean down and look closely at the elbows. Decide if damage is minor scratches or significant gouges, and replace the worst spokes first.

Daniel rightly points out that a spoke protector disk can be installed between the cassette and the wheel, which will save your spokes if the chain jumps over the top in the future. They're cheap and a good idea for this kind of bike.

Frame - your derailleur hanger looks bent in the photos, but hard to know. Its a replaceable part so consider just buying a new one. They're not cheap, but they are much cheaper than a replacement frame.

You can do this yourself with time and effort, and its quite rewarding to fix it yourself. Or you could involve the borrower to help, depending on the social situation.

Once the repairs are done, you will need to fine-tune the rear gears and limit screws to ensure they're in the right place to stop this happening in the future.