In a recent local triathlon, the bike course was mostly flat, but with two substantial hills that couldn't be seen ahead of time. For many riders new to the course, they were already having a hard time pedaling up an increasing incline before the hill came fully into view and they realized they needed to shift.And then they would try to shift.And the bike would make lots of noise, but not shift into an easier gear.Some riders had to stop, and then had an almost impossible time getting started again on the incline. Another rider fell over when she couldn't turn the pedals over anymore in the hard gear she was stuck in.So why does that happen? Why won't your bike shift when you need it to the most?Well, once you are heading up a steep hill, there is a lot of pressure on the chain and shifters. That makes it hard for anything to move around smoothly. Even if your bike is fresh from a tune-up, you might find you can't shift from the big chainring into the small one under a lot of strain. That's perfectly normal. In addition, your pedals and chain need to make a few circles before the shift is complete, and when you are straining for every inch, it takes a long time to turn the crank around enough to complete the shift.Newer bikes with electronic shifters sometimes create additional problems because you can't "baby" the shifters and slowly move the front derailleur, waiting for the chain to catch on the other chainring. It's just a button. You are either shifting, or not. Clunk. Clunk. Keep in mind, you don't want to have a ton of pressure on the chain at the moment of shifting, especially shifting the front chainring. You don't need to back off for several seconds, killing your momentum. Do back off the pressure for half a second while the chain is actually moving from one ring to the other. If you are standing up to climb the hill, sit down for the second when you are shifting, and then stand up again if needed. Keep the pedals turning so the chain can catch onto the next chainring, but ease up briefly. We want the shift to sound like "click" not "CHUNK."
Here are some tips that can help eliminate common problems:
If you ride a smaller bike (52 cm frame or smaller) keep in mind that the distance from the front chainring to the rear cogs is shorter than it is on bigger bikes. This means the chain forms a more aggressive angle when it is in the smallest or largest gear in the back. If you look at the rear cog in the photo above, you can see the chain is currently using the smallest of the available cogs ... the one on the far outside. That means the chain is pulling at an angle against the front chainring that moves with the cranks and pedals. This is important to know because it helps explain why a smaller bike, under a lot of pressure when you are pedaling your hardest, is more likely to have the chain fall off. The chain is being pulled not just forward and backward, but also slightly off to the side. If you find this is a frequent problem, you can compensate by waiting to shift the front chainring until you are sure you are in a mid-range gear in the back. This will make sure there isn't a lot of force yanking the chain to one side or the other while the front derailleur is trying to do its job.
Here's a helpful video that gives you a closeup view of gears and cassette and derailleurs, in case you're having trouble picturing how it all works.