Are all gear ratios on a 21 speed bike completely different?

Question

For each gear (3) in the front there are 8 in the back. That's twenty four positions. 1.1, 1.2, 1.3 etc. all the way to 3.8. Curious.

Answer 1

Yes, although some might be pretty close ratio.

Hence it's a 21 "speed" not 21 "gear" bike. Really there are only 7 gears, and 3 mains sprockets (could also be called gears, I guess :)).

Because 3 x 7 = 21 ;)

Answer 2

I thought there were 7 in the back on a 21 gear bike.

Answer 3

That depends on the design of the gearing of that particular bike.  So, do some science, and exercise your curiosity.  Figure out how to calculate each gear ratio based on the number of teeth on the driving cog and each driven cog.  Graph the ratios by range.  Y-axis is ratio.  X-axis is gear (1-7), a separate line for each of the 3 ranges.  I'm sure you'll find some overlap, because it's much easier and quicker to go back and forth between 2.6 and 2.7 than from 2.7 to 3.1.