If this has ever happened to you - you're riding along and you shift your front derailleur only to find that the chain jumps off the chain ring and then wedges down between your frame and crank arm behind your chain rings - you should think about getting one of these handy little devices. A chain drop protector attaches to your front derailleur housing - either braze-on or clamp - and it works like the chain ring pin on the outside of your chain ring - the one that is located UNDER your crank arm - this prevents the chain from coming off the big chain ring and getting caught and stuck between your chain ring and crank arm. They run around $15-$20 and take about 5 minutes to install. And they are a MUST if you have a carbon-fiber frame. _________________ There are two screws on the top of your front derailleur - these are the low and high gear limit point screws. They set the absolute position of the derailleur when in either high or low gear. In time things change on your bicycle. Cables stretch, your chain links will stretch, and cold-hot weather can affect the amount of contraction or expansion of the metal parts of your bike (cables and chains included). It is completely normal to require slight adjustments to your cable tension and/or the limit points on your derailleur. It is also reasonable to expect that in time your chain will stretch and wear. When these things happen you become at risk for having your chain jump off your front chain rings. Most modern bikes have outer chain drop protectors - especially those with carbon-fiber cranks. This is a little pin on the outside of the chain ring that rests under the crank arm to prevent the chain from wedging under the crank arm (see the second image in this card). The Inside Chain Drop Protector physically blocks the chain from jumping off your inner chain ring where it can wedge between your crank and frame. WHAT IS CHAIN SUCK? Chain suck is another phenomenon that can cause your chain to jump the rings. This occurs when you try to downshift both your front and rear derailleurs at the same time. It's a loss of chain tension. Chain tension is what keeps the chain from jumping off the selected gear - kept in the proper line or placement of the derailleur for said selected gear. The position of the derailleurs and the tension of the chain keep everything where it belongs. When you are shifting gears there is a moment in a downshift where when between gears there is a tension drop. If you attempt to downshift with both derailleurs at the same time - especially if your limit points are not properly set, your chain will want to jump off your inner front chain ring. DAMAGE FROM DROPPED CHAINS CAN BE VERY BAD! Your bicycle chain can become quite the weapon against your carbon frame if it jumps off the rings while you are pedaling. Metal frames will not suffer more than cosmetic (paint) damage usually - but you will probably bend or snap your chain. A carbon frame is made of carbon fiber sheets bonded by an epoxy resin - basically plastic. In a fight between steel and plastic - steel is going to win. You can slice into your carbon frame and do serious damage (See Image 3). By adding one of these simple devices - you can protect your investment. It's a smart frame insurance policy. Lastly, many high-end carbon frame makers are now including these devices as part of their factory builds.