Author Topic: Rear Mech Hanger - Repeated Failure  (Read 391 times)

Rear Mech Hanger - Repeated Failure
« on: January 28, 2020, 10:47:07 pm »
A friend recently brought me his Cannondale Caadx for repair after the rear mech hanger suddenly broke resulting in several broken spokes and a slightly bent derailleur.  I replaced the hanger, rebuilt the wheel and managed to straighten the derailleur.  Less than 200 miles later the new hanger has broken again but fortunately with no other damage to the bike.  Any ideas what might be happening here? Here are some pictures of the first failure.
Most of the stuff I say is true because I saw it in a dream and I don't have the presence of mind to make up lies when I'm asleep.   Bryan Andreas

Re: Rear Mech Hanger - Repeated Failure
« Reply #1 on: January 29, 2020, 08:50:20 am »
It seems that problem is being caused off the bike as the hanger I replaced has been bent before it fractured.  It seems that a double decker bike storage rack may be to blame.

Most of the stuff I say is true because I saw it in a dream and I don't have the presence of mind to make up lies when I'm asleep.   Bryan Andreas

Re: Rear Mech Hanger - Repeated Failure
« Reply #2 on: January 29, 2020, 11:46:46 am »
If the chain snags in the derailleur and you keep pedalling it’ll pull the whole lot into the spokes. Learning to back off when you feel something isn’t right could be part of the solution, as can less aggressive shifting technique.

(Or it might be that you have a bad chain link or bent mech)

Re: Rear Mech Hanger - Repeated Failure
« Reply #3 on: January 29, 2020, 02:46:14 pm »
modern 'road' 11s setups often have a very small clearance between the rear mech and the spokes. It only takes a small nudge to bend the hanger, or a tiny bit of wear on the rear mech stop screws to have the mech tangling with the spokes in low gear. Once that has happened, it is anyone's guess whether the chicken preceded the egg or not.

Some things that don't help;

- most modern shimano mechs no longer have a curved piece on the inner cage plate that kisses the spokes first; they could, for the most part  hardly be better designed to go into the spokes than they are
- modern chains want to derail off the sprockets, including the biggest sprocket. If the chain derails off the big sprocket inwards, it usually drags the RD into the spokes with it.

Some things that do help;

- using aero spokes on the DS (this gives 1 to 1.25mm more clearance)
- using a spoke protector disc
- checking the L stop screw on a regular basis (they often wear or move)
- checking that the hanger isn't bent ( preferably every time you ride the bike)

cheers