There are plenty of good, rational words about bicycle gearing out in the world. Dave Hood's page at:
covers the subject quite well and, as usual with all things related to bicycles, Sheldon Brown generously wrote a wealth of wise words on the subject here:
And, for those of you who love history and like seeing words on paper pages and not just computer screens, Frank Berto and some other very smart folks wrote this book:
While the folks I've mentioned above cover the subject quite thoroughly, the one thing I'd like to add to the discussion is something of a rant. While I'm not as skilled at ranting as some of the pros like Lewis Black or Dennis Miller, my kids will tell you I can still get up a good head of steam now and then, so hang on folks, I'm just getting rolling here.
When I was a kid, we had bikes. You get on it, you turn the pedals, you go. You want to go faster? You pedal faster. That was the story.
Now the fancy bikes had speeds. What we called "English Racers" had three speeds and shifted with a little trigger or a twist grip like a motorcycle throttle. The gearing was built into the rear hub and the wonderfully clock-like mechanism was sealed away from the weather and mostly wasn't something you'd think about. One was the easy gear, Two was harder, Three was hardest. You'd use One when going up a hill, Two when cruising around and Three when you were hauling butt. That was the English Racer story.
But any kid would tell you that the really fancy bikes, the really fast bikes, were ten-speeds. Five gears in the back, two gears up front and you shifted the gears with these things called derailleurs. Since there were ten different ways the derailleurs could connect the chain between the gears (five times two is ten) the bikes had ten gear ratios, but everybody just called these ratios "speeds". In practice some of the ratios might work out to be very similar or identical and it was best to avoid cross-chaining, where the chain has to go at an extreme angle connecting the small ring up front with the smallest cog in back or the big ring up front with the biggest cog in back. Real gear geeks would know all their ratios and serious bike shops would have a board with different cogs for your freewheel. Dick Marr, wrote the standard book at the time, a slim tome called Bicycle Gearing: A Practical Guide.
Around the time I got into bicycle racing (the late 1970s), ten-speeds were being supplanted by twelve-speeds. If five gears in the back were good, six would be better. The folks who made bicycles figured out a way to squeeze an extra cog in the back. Oh and if you're into touring, they put another chain-ring up front. That set-up was called a triple. So you could have 18 gears! (three times six is eighteen). And before Jan and other historians jump in with "oh but the French had X speed in 19XX") I'm talking about the general perception of things as I saw them as a kid growing up in the Midwestern United States. I know now that all kinds of bicycling technology existed many years before, but in terms of a bicycle you'd see in your local shop, it was at the end of the 70s when six cogs in the back became common. And racers rode double rings up front.
Hmm, if six is good, seven must be better. It's around this time that the folks who make bikes decided to increase the width of the rear triangle from 126 mm to 130 mm. That let them squeeze that 7th cog in. Racers got 14 speeds, tourists got 21. Moving the bearings out caused more strain on the rear axle and it was a while later that the cassette hub replaced the old freewheel mechanism. Sheldon explains the difference here:
By the way, the rear spacing standard for Mountain Bikes is now 135 mm. Track hubs and BMX are 120 mm. But in the road bike world, going from 126 to 130 mm gave that extra four mm to let that 7th cog fit in nicely.
Now here's where I kind of start ranting. When I worked in the software business, I learned about what we call feature creep. Microsoft Word is a classic example of a product that has suffered feature creep. Back when Microsoft and Word Perfect were duking it out, product reviews would always point out if one product had a feature the other was lacking. So as users we got a thesaurus and auto-capitalization and annoying animated paperclips that would pop up and say "it looks like you're trying to write a death threat to Bill Gates, would you like to use Helvetica for your font?" and other such dubious features. And now Word is packed with all kinds of things that very few people need, use or even want. But that's the way feature creep works.
Well, I'm sure many people will disagree with me, but I think around the time bicycle and component manufacturers slipped that 8th cog on the back of bikes, we were well into the feature creep side of things. Because if seven is good, eight must be better. Except when they kept packing more cogs into the same space, they started making things thinner. And things didn't stop at eight cogs.
Nine speed cassettes and chains became the norm. And then ten. Yes, now when we talk about ten speed drive-trains, we mean ten cogs in the back. Your bike with a double ring up front would be called a twenty-speed if we still used the old nomenclature. And your triple-equipped bike? It's a thirty-speed.
Unless you have the latest drive-train from Campagnolo. Campy goes to eleven. No, I'm not making this up. See:
And if you want all those gear combinations, go for it. I'm not going to stop you. But this "progress" comes at a price. Narrower chains and narrower cogs don't hold up as well. In the damp part of the world where I ride, I see nine and ten speed chains and cassettes wear faster than the older stuff. And that newer stuff costs more, sometimes a lot more. I can replace a seven speed cassette for twenty bucks and a SRAM 830 chain (which will work fine on five, six, seven or eight speed cogs) sells for $13 at my shop. Nine and ten speed stuff is more expensive and wears out faster. And the Campy eleven speed stuff? Well, you can check out the prices here.
Now admittedly, my shop sells refurbished used bikes. We're a non-profit and our stock is all bikes that have been donated to us that we've refurbished. All our proceeds go into our kid's programs. So most of what we sell are bikes from the pre-nine speed era. But as near as I can tell, everything past seven cogs in the back is not a real gain for most folks.
I tell people that you need a low enough gear to climb what you want to climb, a high enough gear to go as fast as you want and enough gears in between that you don't feel that something is really missing. For some people, that's just one gear! Others will be happy with a three speed. Or a bike with seven gears in the back and three up front. And I'm sure there are some folks who really need a bike that goes to eleven. Maybe pro racers or the guys in Spinal Tap.