Originally Posted by
DCB0
You are exactly right. Multi-chainring (front gear) derailleur equipped bikes have tonnes of overlap between the gears.
Usually. Compact cranks and tight cogsets don't especially when you've yet to upgrade to 10 or 11 cogs in back and that's not a good thing - you really want the repeated gears for faster smoother shifting and less chain noise.
I currently ride 50-34x13-14-15-16-17-18-19-21-23. Disregarding the fully cross-chained 50x23 and 34x13 combinations (which are noisier and can lead to chain rub issues on the derailleur or big ring depending on equipment and setup) there's only one overlapping gear ratio provided by 50x21 and 34x14.
At a sustainable hard pace that means the small ring is only good for about 19 MPH and big ring down to 17. Going slower reasonable limits are about 18 and 15 MPH respectively. The net effect is that the wrong terrain, wind, fatigue, and rest day combinations there's a lot of shifting between rings which means the next gear is on the other ring five cogs away (ex - 34x15 is one notch easier than 50x21). That's more of a hassle and slower than one click on the rear shifter by itself.
It also means that in a lot of situations there's a lot more chain noise than with more overlap, as with 50-40-30x13-14-15-16-17-18-19-21 which provides the same overall range but would let me ride 40x16 or 40x17 in the middle of the cassette instead of the extreme combinations necessary with the wide range front and one end of the cassette.
If you wanted to go from easiest to hardest gears in the smallest possible steps then you would have to find a complicated series fo shifts and memorize it. Luckily, there is no requirement to always go to the closest gear in the smallest step possible.
Disregarding half-step setups people generally use the rings to select ranges, like on a 4x4 with a two speed transfer case. Steep climbs always call for the smallest ring; steep descents and big tail winds on flat ground always use the big ring. In between you might choose a ring to get tighter spacing (ex - with 53-39 x 12-13-14-15-17-19-21-23 you might ride 39x14 instead of 53x19 because the next higher and lower gears are closer), to minimize the shifting (you might go over a highway overpass using 53x21 instead of the 39x15 or 39x16 you'd use for a sustained grade of the same steepness because you'll be back at a fast pace in thirty seconds), or just get a better and quieter chainline (with 50-40-30x13-14-15-16-17-18-19-21-23 I'd take 40x17 in the middle of the cassette over 50x21)