Ever thought why they say a bike is 18, 21, 27 speed? I think it is incorrect. Well the mathematically it is…. 2X9=18, 3X7=21, ect. But in reality we have something called “wrong gear”. Eventually we can’t use all the gears provided and so it’s not 18, 21 or 27 speed bike anymore. Am I right or I’m missing something out?

Hmmm.....in road lingo we use 7-spd, 8-spd, 9-spd, & 10-spd.....simply referring to the number of rear cogs......& it doesn't matter what you have up front, a double or triple.

George

Yes true, we don't really have 18,24,27 ect. speeds, because there are un-usable gears. (except on road bikes with double front chainrings). What is more realistic is to think of the range of gear ratios provided by a specific drivetrain. Presumably a "27 speed" will provide a wider range of gear ratios, accessible in smoother steps than a "24-speed".

There are some gears that are duplicated at times, or are very close to the same thing when you have double/tripple up front. You could think of the range it gives you, but this would be missleading. Reason for this would be a megarange cluster 12-34 would sound better then a 11-24 cassette. There is no perfect way unlesss you want to show the gear inches of every single gear and put that along the bike. That would be a royal pain to describe though.

For a 3x8 you might only have a dozen or 15 gears that are a) useable and b) any amount significantly different from other gears

Well, back in the days when 10 speeds was standard. You know 5 gear freewheel and 2 chain rings? People used to write long and learned articles about exactly which gears and chain rings to have and what shifting patterns to use so that they had 10 useable gears with no big jumps between gears. Back then a good bike shop could give you a freewheel with the gears to your specifications. Even so, achieving shifting nirvana (every gear a functional gear with an easy shifting scheme) was tough even with just 10 speeds.

I have not even seen a discussion like that in years. With the new 9 speed rear clusters on two chain rings you have 2 gears that your should not use because of cross chaining (big ring, big cog and small ring, small cog) and you certainly have a substantial amount of gear duplication. But I really don't think anyone cares.

These days you can have a 10 speed system with only 1 chainwheel. These young'uns don't know how lucky they are.

OK........I'll make it easy on all of ya... I'm building up a touring bike right now with a gear inch range of 17" to 92".

There are two things lumped together.

1. Ideal gears

There are, as you said, "wrong gears": gears which should be avoided because the chain is very oblique. There is no hard rule on that, but on a "27-speed" bike, there are either 2 or 6 gears you should avoid. Nothing wrong in using them occasionnaly, especially if you don't apply too much pressure at the same time, but using the large-large combination, for example, is noisy and will result in premature wear of the drivetrain.


2. Duplication

Back when we had only 10 or 12 gears, bike makers made sure the gears fell in between each other. Now that we have 27 gears, it seems that they are perfectly spaced for racers (compute all ratios for 52-42-30 rings and a 12-23 or 12-25 cassette and you'll see there is little duplication).
But keep the same chainrings (or a MTB compact crankset) with a wide-range cassette (12-32), and you end up with 7 or 8 duplicate gears.

By the way, some duplication or overlap between the range covered by each chainring is good to avoid double shifts.

Now, it is possible to avoid some duplication, either by replacing rings with much wider-spaced ones (say 24-38-52 on a touring bike) or by customizing one's cassette (my 12-14-15-16-17-19-21-25-32 cassette gives me close ratios and minimize duplication, and I still have 18.6 to 99 gear-inches).

"How many gears are there?"

As I get older, never enough........