Hi All - Leah's "Getting Over My Head" thread seems to have evolved into a gearing thread. I retired from a career in data mining and statistical modeling, so you can just imagine how much time I've wasted on "optimizing" bicycle gearing.
Here's the TL;DR version: it's impossible to optimize gearing, so stop sweating it. The slightly longer version is that we are locked into integer tooth counts; when the optimal cog is 14.5 teeth, that ain't an option! So, we are forced into compromising or, better yet, satisficing. There are essentially three key points we are trying to fix with gears: the lowest low, the highest high, and the steps in between. Given those, we then try create a system that reliably and easily shifts among the gears. For me, a 2x system using components available today provides the best combination of low-low, high-high, steps in between, simplicity, consistency, and reliability. But that depends highly on the chain, the chainrings, and the front derailer playing nicely together. It also works for me because I use it as 2 gearing ranges, one for flats and downhills, the other for long, steeper uphills. The lowest low and the highest high are pretty straightforward, and plenty of ink has been spilled on how to choose those, so there's no point in elaborating on that. It's the "steps in between" part that drives us wild. In theory, we think, we'd like to have perfectly even steps between the high and low. In practice, though, 1) that's simply not possible with a cog-and-chain drivetrain, and 2) it might not even be that desirable. A lot of riders, myself included, find that we prefer smaller steps between gears in the range in which we normally ride, and larger steps out in the extremes. What I definitely don't like is having a big difference between 3 adjacent cogs in the middle of my cruising range. For example, a 1-tooth difference one way and a 2-tooth difference the other. Unfortunately, this is a common occurrence in large cog count cassettes with tiny small cogs - they go from a 1-tooth difference to a 2-tooth difference near the middle of the cluster. That's twice the amount of reduction/increase in effort. So, when I'm looking at cassettes, I'm looking for ones where that 1-to-2 transition occurs as close to the small cog as I can get it. A major factor here is the movement to smaller smallest cogs, which has gone from 14 to 10 in my time. Starting from 11 (or, God forbid, 10!) you use a lot of cogs to get to the point where 2-tooth steps start to make sense. On the other hand, those small smallest cogs mean we can use small outer chainrings, and that's something of a boon if your front derailer can handle it, because it means we can also use smaller inner rings on a 2x to get sufficiently low gearing. The difference between chainrings is worth examining a bit. Typical road double front derailers have a 16-tooth max difference specification, which derives from the standard "compact double" 50-34. That's a 39% difference, which is a pretty big jump, roughly 3.3x the average jump on the cassettes often paired with those chainrings. So, shifting up front is the equivalent of around 3 1/3 cogs in back. On my Waterford I use a 42-tooth large ring, and a 26-tooth small ring is 16t smaller, but that's a whopping 48% difference, which is 5x the average jump on my cassette. Piaw mentioned going with smaller tooth differences up front, and there's a good example of why. When I built my Breadwinner I went with 44x32 up front, a 32% difference that is 3.1x the average difference on the cassette. I find that to be a much less disruptive change than on my Waterford. The front chainring difference as a multiple of the average in back turned out to be a significant factor for me. Sure, it means I have more overlap in gears, but that's less important to me than the change in cadence caused by shifting. One last consideration as I'm designing a drivetrain is that I want my preferred cruising gear near the center of the rear cogs, maybe a little closer to the small end, so that I can be on the large ring for most of my riding around home. That's around a 5.0 gain ratio for me now, which is the 42x17 on my Waterford. And, as noted, I want a consistent difference above and below that gear, which means, for me, a 2-tooth change on each side of it. Ted Durant Milwaukee, WI USA -- You received this message because you are subscribed to the Google Groups "RBW Owners Bunch" group. To unsubscribe from this group and stop receiving emails from it, send an email to rbw-owners-bunch+unsubscr...@googlegroups.com. To view this discussion on the web visit https://groups.google.com/d/msgid/rbw-owners-bunch/abd254f4-42f4-4be7-bed3-326b6b03d574n%40googlegroups.com.
