Now Part II - How to shift

If we understand the human biomechanics and optimal range for performance from the post above, the next step is to understand how to shift. And this is more dependent on the gearing that the bike has and the ratios it has. For any set of front chain rings, and rear cluster, we can make a chart of gears that looks like the specs found here: http://www.sheldonbrown.com/bridgest...ne-1991-33.htm

The table explains the ratio and distance traveled for each combination of gears for mtn bikes with 3 chain rings front and 6 - 7 speeds in the back.

Notice we have overlap. That is, if we use just the big chain ring and move through the cluster of rear cogs, there are some ratios of (e.g. big ring : big cog) that are a lower gear ratio than the middle ring (e.g. mid ring: small cog).

In an ideal world, we would memorize the exact ratio and switch linearly by ratio, shifting front and rear derailleur to optimize the ratio we are in. But we don't do this in practice. Instead, we leverage the fact that we humans do have some range in optimal power output. We can sustainably output power at the pedals with some difference in RPM between 60 - 90. We can also vary our pedaling force by 0 - 100% depending on how short of a duration we need to do it. So if we encounter a short hill, we could just power up it in the same gear and then return back to a sustainable pace and try to recover. But in general, if we have a change in terrain or environment that lasts more than a minute, we may want to pop out by 1 or 2 gear cog(s) in the back to find a more optimal cadence. And we continue shifting just the rear until we get to the cog adjacent to (but not the actual) smallest or largest gear, and then we shift to the next chain ring.

An example: 12 speed with 52,40 front rings, and rear cogs 13, 14, 15, 17, 19, 22T.
We were riding flat out with slight 1% downhill and tail wind, in top gear 52/13.
Road flattens out so we switch to 52/14.
Wind calms so we pop first one gear and try to push 52/15
Can't sustain that so we slow down and push 52/17
We turn a corner and now head into a slight head wind and push 52/19.
A slight grade uphill is ahead. We pop the front now, and push 40/19.
The uphill flattens out. We pop into 40/17, then 40/15
...

The scheme above is a basic approach at avoid "cross-chaining" where the rider would make the chain pull at extreme angles.

Some thoughts:

1. Choose gearing ratios and bicycle types to fit the type of riding and terrain. For example, a commuter bike might be only over relatively flat terrain. Many times, a single speed bike is simpler, lighter, and perhaps more reliable than a multi-gear bike for this purpose. OTOH, if you plan to do a 1000 mile tour through the mountains and plan to pack everything on bike racks/panniers, having a wide range gearing is perhaps the most important. And if planning to compete in an individual time trial or triathlon in mildly rolling paved roads, you may only need single 48T crank up front, and a close range "corn cob" rear cluster like a 12 - 18 straight block and tight short arm RDR.

2. Training and fitness can help determine shifting patterns for a given terrain/conditions and make shifting second nature or instinctive. Too often, lack of riding results in folks getting lazy and using just two gears - the highest gear, and the lowest. A seasoned rider knows his own fitness and knows to anticipate a shift the moment the terrain starts to exceed a sustainable rate, or even to know he doesn't need to shift because it will only be a short duration and he can power through without shifting, or perhaps, he can power through near the beginning and then settle down and shift with confidence, even under load when some mild fatigue appears.

In short: Figure out what kind of gears you'll need, then make sure you know how to use them.