When your derailleur goes into your spokes it may not only destroy your frame, but it may end up destroying your wheel, derailleur, and derailleur cable and housing. In really bad cases it will destroy your chain as well.
So why does it happen and what causes it?
The answer is rarely maladjustment. Unless you or someone has been fooling around with your hi and low adjustment screws on the rear derailleur.
The most likely cause is a bent derailleur hanger. The derailleur hanger is the piece of the frame that the derailleur screws into.
Sometimes the hanger is removable and replaceable, but not on all bicycles.
When the derailleur hanger gets bent it misaligns the whole derailleur system. This is usually exhibited by sudden poor shifting behavior. In most cases the derailleur hanger and the derailleur gets bent in towards the wheel.
The problem may not seem serious until you shift into your lowest gears with the rear derailleur. If the derailleur hanger is bent in, this shift will drive the lower pulley of the rear derailleur right into the spokes of the rear wheel followed by a lot of bent metal and sometimes expensive repair.
So why does the derailleur hanger get bent?
When you crash or lay your bike down roughly on the drive side (right side) of the bike it can easily hit the derailleur and bend the hanger.
So how do you stop this from happening?
First, try not to lay the bike down on the drive side. Second, if you crash or if your shifting is suddenly behaving poorly, slowly shift your bike into the low gears near the spokes while not on the bike. Check to make sure the lower derailleur pulley is not too close to the spokes. If it is really close it may go in over a bump or under harder pedaling.
If your hanger is bent, you will need to go to a shop to get it bent back into place or replaced. For bicycles that do not have a replaceable hanger, you’ll need to replace your frame.
Since the damage is usually a result of an impact, this would not be considered a manufacturing defect (a deficiency in materials or workmanship).