Rear Mech broke off hanger and spun to inside of frame

starbuck

Was on a ride recently and a steep slope was coming up so I changed to small ring (34) on front and largest (28) cog on back, suddenly I heard this clattering noise and my rear wheel locked (I was only going quite slowly - 5mph ish at the time so was able to stop the bike without coming off when my wheel locked). My rear mech had snapped off it's hanger and was now sitting on the inside of the frame. I was able to detach it (love masterlink chains) which allowed me to walk the 3 miles home wheeling the bike.

Luckily there was no damage to my frame or wheel, but the part of the cage which holds the jockey wheels was quite twisted.

I have a Giant Defy 1 2013. Rear mech was 105. Cassette was originally a tiagra 12-28 and when I replaced the cassette through wear, I put an 11-28 on (front rings are std 50x34). Chain was not overly stretched (had checked it with a chain checker a week or so ago and would have probably needed replacing in next couple of months but end of checker did not go into links which suggested chain was ok). When I have replaced the chain over time, I've used the bigring/big cog calculation (allowing a full link overlap) as I've always found this easier.

Any idea why my mech may have snapped (and how I can avoid this in the future)? It probably wasn't the best idea to try and change front and back gears at near enough the same time, but as I was also going to small ring/large cog I wouldn't have thought this would have overstretched the chain or any parts.

Apart from getting a new hanger, I obviously have to get a new rear mech (along with new gear cable etc). For the gear range I've got, would I need a short cage or a medium cage (prob looking at another 105 due to cost)?

I need to try and make sure it doesn't happen again as I'm doing the London/Surrey 100 ride in July, so will be putting in a lot of mileage between now and then.

arlowood

Sounds like the low limit setting on your rear derailleur was wrong. When you switched to the 34 chainring and the 28 rear cog the derailleur cage probably moved too far inboard and caught one of the spokes on your wheel with the resultant torsional force snapping the mech hanger.

You need to make sure that the low limit is set cotrrectly when you set up any new rear mech. The cage should set so that the jockey wheels are directly below the bottom 28 tooth sprocket.

veronese68

You say you changed before the steep bit so presumably not under heavy load at the time. Trying to force a change when pushing hard on the pedals stresses everything more. As mentioned above check the limit carefully when fitting the new mech, that could have been it. Look at the spokes for damage caused by the mech hitting them. Could have been that everything was in order and it just baulked when shifting with dramatic consequences. Check the hanger is straight after fitting, a new hanger is no guarantee of straightness.

noodleman

Very much doubt your hanger snapped first. As others said, it's much more likely your limit screws were set incorrectly or your hanger had somehow got bent inwards slightly, resulting in your mech cage moving into the spokes when you moved into the big sprocket.
Worth making a habit of checking the clearance of the rear cage every so often as hangers are very easily bent.