Chain suck sucks!
Raise your hand if you have ever dropped a chain during a shift. Keep your hand up if it has happened during a downshift. This is "chain suck." It’s no fun and has the potential to do some serious damage to your frame, especially if it’s carbon. What happens is when shifting from the big chainring to the small chainring, the chain falls off toward the frame. Annoying if you can catch it and quickly upshift back to the big ring. If you aren’t able to catch it quickly, the chain can become lodged between the small ring, bottom bracket interface with the downtube and seattube, and in severe cases, fall between the small ring and the drive side chainstay. The damage which can be inflicted can be pretty catastrophic if you don’t stop pedaling immediately. It can range from having to stop to untangle the mess, thus getting your hands full of chain grease (wear black shorts!) to snapping the rear derailleur hanger (not “buy a new bike” threatening if you have a replaceable hanger), to compromising the integrity of your frame in severe cases.
In a perfect world, the front derailleur limit screws will prevent this, and by and large they do when set properly. However, we don’t live in a perfect world, and there are circumstances which will cause the chain to pop off on a downshift on even the most perfectly tuned front derailleur, like riding a bumpy patch of road as you shift. The first thing you should check if this starts happening is the condition of the teeth on your chainrings. Worn teeth exacerbate chain suck. The other solution is to install an anti chain suck device, or chain catcher.
These were first pioneered by the mountain biking crowd, as you can imagine with the types of terrain and amount of shifting.
|early anti-chain suck device|
|installed on bike|
They attached under the bike near the bottom bracket and can still be used on older frames. As frame designs evolved over the years, so did the chain catcher. They are fairly inexpensive, easy to install and provide a bit of insurance/assurance against dropping a chain and damaging your investment. If you have a braze-on front derailleur, you need one that attaches to the front derailleur via the mount bolt. These often come with a longer bolt to compensate for the added width of the device.
If you have a clamp-on front derailleur, you would need one that clamps onto the seattube.
If you want to ditch your clamp-on front derailleur in favor of a braze-on style, an adaptor type chain catcher is available. It clamps onto your seattube but allows you to run a braze-on front derailleur.
Thanks, Stewart! I raised my hand for your initial question, and kept it raised for your second... didn't know anything could be done about it till now!
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