I'm mystified by this and hope someone has some ideas to help me out. Here's the scoop.

Bike in question: 2003 era Marin Highway One, a flat bar road bike with mid-grade Shimano components (actually had Ultegra RD).

Sold it to my friend who had a crash a couple weeks later. She is positive there was a loud noise from the rear, after which the crash occurred. The damage included a mangled and destroyed RD (missing pulleys and half the cage) and RD hanger. I replaced them both (went with used but nice condition 105 and new hanger).

Shortly thereafter she complained of shifting problems. Turns out one of the chain links had been bent in the previous crash. Installed a new chain. Everything is working fine.

A week later, another problem. This time, it's another mangled RD and hanger. Pulleys gone, cage half missing.
According to the rider, she was on a relatively flat, paved path, in the middle chainring, and shifting to a smaller cog in the rear when it went out. She said she was "maybe a 5" on an exertion scale of 1 to 10, so not pegging the power meter for sure.

Rear spokes show no signs of damage or misalignment. Rear wheel is true.

I've never seen anyone tear up a rear derailleur like these, and now it's happened twice. What in the world am I missing here? The first time I thought "maybe the jockey pulley screw backed out, the pulley flew out and BANG!" But twice in a row?

Without knowing more and examining the broken parts for clues, I'm just speculating about possibilities, rather than naming a cause.

A common cause of mysterious hanger breaks is something jamming in the chain and RD, so as you pedal, the jammed chain takes the RD back with it and snaps the hanger.

Another is a B screw adjusted improperly, allowing the jockey wheel riding too close, and engaging a sprocket through the chain in certain gear combinations. This can stress the TD and push it back with the turning of the cassette, and snapping the hanger.

There are others, but not being in low eliminates the most common cause, snagging a spoke.

You might try "reading the changer" which is usually ductile enough to show the direction of the break by way of the last bit bending rather than breaking clean. Once you know which way it snapped -- inward, backward, or at an angle -- that might give you clues to the sequence of events.

Maybe a pre-ride issue.

Either bad installation (for example, maybe dropouts aren't straight, and the hangers aren't mounted straight) or poor storage practice (for example, laying bike down on the mech side or in a place where the mech gets banged up)?

Is it possible that the chainrings are overly worn and the new chain led to chain-suck and the pedaling effort hyper extended and ripped up the RD. That would be my first guess, especially after the chain replacement. The two failures are likely unrelated due to the known cause of the first (the crash), and the fact that the second didn't involve one.

-Jeremy

ha s she been dropping the bike on the Drive Side, by chance? "shifted to a smaller cog" makes me suspect that the der. mount was bent, and the der. cage got caught in the spokes.... maybe...

Shifting to a SMALLER cog would be going AWAY from the spokes.

The only time I've ever seen a derailleur mangled as much as you describe is when I got a stick caught in my MTN bike's derailleur.

I split it into 3 pieces.

Maybe she's back pedaling when cross chained and ripping the derailleur off? But that wouldn't explain the missing pulleys.

That was my thought, too. That's theory 1.

My only experiences with losing an RD was when I worked in the bike shop and didn't set the low gear RD limit screw correctly. The buyer shifted the RD into the spokes and that was that. This also aligns with Corrado33's experience. Theory 2 would be low gear RD limit misadjusted.

Theory 3 would be the high gear RD limit screw misadjusted and the chain getting caught between small cog and frame.

Theory 2 doesn't jibe with the claim that the breakage occurred whilst shifting to the small cog. As for theory 3, I can't figure out how that would result in cage and jockeywheel loss.

Maybe she just likes visiting with you

Too long or short of chain? Or too short of cage? Does it do OK with big-big and small-small?

Otherwise perhaps chain suck as mentioned above. Can the derailleur flatten out 100%?

A new matched set of chainrings can help with chain suck.

Remind her that if she feels a crunch and added tension, they stop pedalling immediately.

second hand account, and sometimes people say "smaller cog" when they actually shift to a LARGER rear gear.... I've heard that many, many times... and quite frankly, the damage level talked about could only come from the der. getting snagged in the spokes, or an overshift/tangle( i hate it when that happens)... and then, there's that earlier wreck to consider... bent der. hanger would be a result of the wreck, perhaps...

and since rider claimed to have been in the middle chain ring, with no reference to whether the front, or rear der. was shifted from the OP author, my assumption was of improper terminology as to the gear change......

more info, from the rider, is needed? Or a good inspection in person, which is not an option on these forums, eh?

and to the OP... i'd be looking at the spokes BEHIND the rear der...... overshifts cause hidden spoke damage that may only be visible once the cassette is removed... and inspect the frame for overshift off the small side, too.... once again... more info is needed.

This is a second hand JRA story on a 13 year old bike.

I'd have to see the bike.

The hanger got bent in the initial crash, you didn't align it when you installed the 105.