Welcome back to the bike shed. I wanted to do a quick video about derailleur capacity while I've got a derailleur I can use as an example. Right now, I've got the largest gears selected in both the front and back, and as you can see, the derailleur is almost completely extended. The chain is currently the absolute shortest it can be and still fit over both the large gears. Now, as I start to shift to smaller gears, you can see the derailleur starts to retract. On smaller gears, the chain doesn't need to be as long, so it has more slack, and the derailleur's tensioner takes that extra slack out of the chain. The problem comes as I continue to shift to smaller and smaller gears. Eventually, the derailleur collapses all the way down, and the chain starts making contact with itself. The derailleur can't handle the difference between the longest the chain needs to be, and the shortest the chain needs to be. It doesn't have enough capacity. To put that in numeric terms, capacity is the difference between the size of the largest possible gears, and the smallest possible gears, measured in teeth. In this case, the crank has 43 teeth on the largest gear and 23 teeth on the smallest gear. The cassette has 34 teeth on the largest gear and 11 teeth on the smallest gear. Plugging that in, we get 43 plus 34 minus 23 plus 11, and that equals 43. So when I replace this derailleur, I need to make sure that it has a capacity of at least 43 teeth.