Set out to do a 35 mile ride today, but only made it 31. I was about four miles from home when something went wrong. I was going up a slight grade, and shifted from high to low range on the front derailleur. In a split second, the chain made a loud grinding sound, and a split second later, there was a loud BANG from the rear derailleur. The bike ground to a halt. The rear derailleur broke off the hanger and hung in the rotating wheel. The wheel was jerked out of true, and the shift cable was jerked so hard it broke through its housing. And the chain slightly damaged the frame in a couple of places. I wonder what else is out of alignment, or otherwise screwed up!

It's off to the bike shop tomorrow to test that warranty! Has anyone seen anything like this, and if so, what might have gone wrong.

This happens when the high limit of the rear derailleur isnt set right. The limit screws are what control how far in/out the derailleur will shift on the bike. If it is not adjusted right you can shift your rear derailleur right into the wheel.

Ive seen it happen and its not pretty. At the least you are looking at a new rear derailleur or at least lower cage of the derailleur, New chain, new derailleur hanger, and again hopefully the wheel can be saved.

Whoever set up the rear derailleur did not do it properly. Good luck with the warranty.

I can see how the chain being driven into the wheel could cause this, but there are two problems with that scenario. First, I'm sure the rear derailleur was set up properly. I've been keeping my own bikes in alignment for decades without a problem and this bike had 400 trouble free miles on the clock, and had been shifting perfectly. But more importantly, I was in third or fourth gear when I shifted from high to low range up front. This is a new one for me.

I saw this happen to a brand new bike out on a test ride in front of the shop. Even watching it was impossible to determine the sequence of events out of two possibilities (the RD did not go into the spokes).

1- either a weak o brittle hanger picked that moment to fail and the chain carried the RD back
2- something on the chain snagged the RD, or the RD upper pulley snagged on a sprocket (through the chain) pushing it back and snapping the hanger.

I suspect the latter, especially if it happened during an FD shift. Possibly some chain suck pulled it forward and snapped the hanger, then it all got pulled pack, or the B-screw was adjusted too high and there was pulley/s[rocket interaction. This is especially likely with a Shimano RD because the upper pulley can rise when when downshifting the front. If you read the Shimano tech-docs for RD B-screw adjustment, there's a (often ignored) reminder to confirm the adjustment, and pulley clearance in all gear combinations.

Could someone have altered the bike adjustments without your knowledge?

I once threw a chain off the inner chain ring during a shift to low. The chain swung into the spokes and pulled the Mirage RD into the spokes. I'd been trying to adjust that occasional over-shift out without success for a while. I installed a "Dog Fang," straightened the derailleur hanger, and bought a new RD on eBay.

I've seen this happen to a nearly new bicycle and I think it's probably what happened to you. "Chain suck" happens when the chain climbs up the backside of the chain ring due to a tight link in the chain. If the tight link does not flatten out and when it reaches the rear derailleur it cannot travel through because it catches on a tab that the chain normally passes under. The load that you've put on the chain can rip the derailleur right off the bike. This is not normally covered by the warranty unless you can prove that the tight link was caused by the manufacturer or dealer. Tight links are very common and are often due to a lack of chain maintenance or poor installation technique. The tight link is often the link the chain was joined on.
In the case that I mentioned the dropout plate was ripped out of the frame. This required sending the frame back to the factory for repair.

No. I ride alone, and store the bike in my house.

I'm going to fight for warranty coverage, as the bike is only two months old.

Certainly worth a try.

Seems fair if you haven't tinkered with the bike, and the parts/service warranty is still in force. Even if the shop won't warranty it, they might offer a compromise, offering to fix it at a greatly reduced price. Back when I was in retail, "compassion pricing on gray area issues or just plain bad luck was our SOP because we didn't like to profit from someone's misfortune+.

This is my first thought. Shifting under pressure. The rider will almost always say they didn't. Are there scratches on the chain stay underside near the rings? If I saw the tell tale signs of this on a bike coming into the shop I'd bristle at a warranty claim but likely put one through anyway. But I would also be willing to be honest with the warranty guy over the phone. Something like "we think it was from jam shifting but can't say for sure. Any consideration you can offer will help us keep our customer happy and devoted to your brand". Likely both the brand and the shop will take a hit. Andy.

I once owned a computer store, and did quite well, because I didn't look for excuses NOT to cover my work. I always sided with the customer - it's the best advertising in the world.

I've been riding for 35 years. I always lighten up when I shift, and I release all pressure when shifting the FD. It's been habit for a long time now. I this one was just bad luck, or some odd mechanical failure.

I need you to talk to my wife and explain your shifting technique under load. She won't listen to me since I'm a "know it all".

Ha! Been there...

it is not just adjustment that can position the rd too close to the spokes
but also a fairly minor impact to the drive side of the bike
will bend the hanger in
possibly enough to put it in peril

The OP has already clarified that the RD was over the middle cogs, so limit screws are not a factor.

How would chain suck- chain jamming and being pulled up into chainstay/FD- break the RD, since it stops crank rotation, but doesn't load the chain?

I've had this happen- fortunately without damage, a couple of times. Usually when shifting from large to small chainring, but once without any shift at all, suddenly pedaling motion stops. Slight back pedaling & back to normal.

My theory is that the front shift flaps the chain sideways, causing it to jam between the jockey wheel & it's cage. Possibly it is the quick link that jams, this combination occurs rarely, & would tend to happen on newer, less worn in equipment. I you're pedaling hard enough, the RD will be pulled up & back, causing the mayhem.

Any of a number of things could have have happened, such as kicking up a branch large enough to do the deed but small enough to have missed... A spoke protector would have prevented it.

People keep assuming the RD went into the wheel. The OP says clearly that he was in a mid range gear, so it's extremely unlikely that the RD went into the wheel before the hanger broke. Something jammed the RD or chain, and the RD was pulled back by the rider, snapping the hanger.

One telling bit of evidence may be the hanger itself. If the the break is a ductile failure, showing some bending before it snapped, then we know the jammed RD broke it. OTOH- if it's a brittle failure (clean fracture line, with no hint of prior bending) then it's possible that the hanger broke first and that was the cause.

Says.

A spoke protector may have kept it out of the spokes in any case. Better a non-broken wheel even if a broken derailleur and hanger.

The timing suggests it is related to the chain going slack during the front shift. The chain must have snagged the rear derailleur. Quick link came loose? Stiff link wouldn't pass?

A dork disk is not likely to make much difference if the RD is broken off entirely,

& I take the OP at his word that he has been riding 33 years, & knows what gear range he was in.


I had a minor crash a while back, which caused my buddy behind me to crash also. There didn't seem to be much damage besides some minor road rash but soon after he started off again, his RD hanger snapped. No damage to the wheel, but it turned out that the seat stay had been cracked by the RD whipping around. (Ruckus Composites in Portland did a great repair)

Maybe those things should be a little stronger.