Yesterday I put together a custom rear cassette for myself.
The background to this is that I have an 8-speed rear, and the oem 11-32 cassette (11,13,15,17,20,23,26,32) had an awkward jump from 20t to 17t that didn't suit me. After some research, and a friendly commuter at work (who's as nerdy and anally retentive as I am) gave me a copy of a gear ratio spreadsheet he'd put together, I set to work.
After working out what ratios suited me (actually it's MUCH simpler to think of this in terms of cog teeth), I first replaced the 11-32 with an 11-28 (11,13,15,17,19,21,24,28) which suited me much better.
Now if I had any sense, I would have stopped at this stage, and been happy.
The problem is, I have no sense but I have a spreadsheet. What the spreadsheet told me, in conjunction with knowing what gear I like to cycle in, was that I have a comfort zone. My comfort zone is ratios of between 2.5:1 and 3:1, or in teeth terms between 19 and 16 teeth. It's not all I use, but it's what I like to use. My original 11-32 gave me just one gear (17t) in my comfort zone, my new 11-28 gave me two. (17t & 19t).
But what if I had a cassette that gave me every possible gear in my comfort zone? Now I appreciate if you have 9 or 10 cog cassettes, this is pretty easy to achieve. But on an 8-cog, it's more problematic. So I then started a hunt for the perfect rear cassette.
Anyway, without out trying your patience any more, I didn't find one, so I made one myself. I took a 13-23 (13,14,15,16,17,19,21,23) and a discontinued MTB 11-32 (11,13,15,18,21,24,28,32) and mixed and matched the cogs to give myself my perfect 8-cog cassette. (13,15,16,17,18,19,21,23). I had a nice ride out today and it performed very well. It's not perfect, Shimano HG is very clever and if you monkey about with it you lose some of the smoothness of operation, but it shifts well and is a joy (for me) to ride.
Best of all, I learned loads about my bike gears on this little project.