One of the worst experiences when riding is the realisation that your rear derailleur has just inserted itself into your rear wheel. The ensuing carnage of spokes, derailleur, hanger and potentially frame will echo through your mind for months to come. So to prevent this happening to you (again or ever), it’s important to understand how it can occur.
These are the possible causes of such an incident:
1. Bent Hanger - The rear derailleur hanger (the small piece of aluminium that joins the rear derailleur to the frame) was bent due to some sort of impact on the rear derailleur. This impact may have been a crash, leaning the rear derailleur against a wall, a shed door or a car boot being pushed onto the rear derailleur, etc. The rear derailleur hanger is designed to bend/snap in preference to the rear derailleur itself; the hanger is a $30-$40 part where as, for example, the Ultegra Di2 rear derailleur is a $449 part. If the hanger is bent inward, the rear derailleur will become too close to the spokes in 1st gear. If shifted into 1st gear with a bent hanger, either the chain will fall off the back of the cassette into the spokes, or the rear derailleur cage will catch on the spokes. Either way, there will be significant damage.
2. Incorrect adjustment of the rear derailleur lower limit screw - If the lower limit screw is not adjusted correctly, the shift from 2nd to 1st gear can result in the chain moving off the back of the cassette into the spokes. The same damage as detailed above will occur.
How to prevent this carnage:
1. Don’t ride with a bent hanger
a. It’s a good idea to regularly visually inspect the derailleur and hanger. The cage of the rear derailleur should be aligned parallel with your cassette cogs.
b. If your gears aren’t shifting smoothly, there’s a good chance your hanger is bent. One symptom of a bent hanger is that your gears will work smoothly at one end of the cassette, but not at the other end. This is because the incorrect angle is exaggerated toward the easier gears as the derailleur parallelogram extends.
c. If the hanger is bent, avoid shifting into the easiest 2 gears, take the bike to a bike shop and get the hanger straightened or replaced and gears re-adjusted.
2. Ensure correct limit adjustments
a. If the rear derailleur can be pushed toward the spokes past it’s ideal alignment in 1 st gear, then while in 1 st gear, screw the lower limit screw (marked with L) inward until it contacts the stop. Check that the derailleur can shift to 2 nd and back to 1 st . If it can’t get back to 1 st , the limit screw is in too far. When in 1 st gear, there should be no more possible movement of the rear derailleur toward the spokes
3. Run a spoke protector behind your cassette
a. Yes, you’ll never see the pro’s using them. Yes, the main reason they come on your bike is to satisfy Australian Standards. However, they’re also really good at preventing the rear
derailleur eating wheel syndrome.