Originally Posted by
Miller2
I currently run a 50/34 with a 12-27. I've had the opportunity to demo a Scott Foil and Fuji Altamira both with a standard crank during my usual weekly group ride. Now sure, there could be a placebo effect in place but I felt as though I could keep up much better with the group on the compact. My question is, can I simulate the gear ratios of a standard crank by changing the cassette while using the compact
You can match the range but that's not the only issue.
With the same cassette a standard crank provides a high gear about one tooth smaller (50x11 is like 53x12 or 50x12 like 53x13) and low gear about one size smaller (34x23 is like 39x26). You can often compensate for that.
The more significant difference is in where the small to big ring transition happens. The same cog acts like one two teeth smaller (34x13 is like 39x15). With the power to overcome aerodynamic drag proportional to the cube of velocity that's a _HUGE_ difference in when you need to shift up from the small ring which can lead to front shifting like you have ADHD.
At 100 RPM 34x13 is 20.5 MPH and about 170W for a hypothetical cyclist like in Gibertini and Grassi's paper with .4m^2 Sd and .760 Cd riding on the brake hoods, 75kg weight, and .004 Crr on flat ground. 39x13 is 23.5 MPH and 245W. Upping the RPMs a bit as on VO2max intervals 39x13 at 110 rpm nets 25.8 MPH and 315W. For a small ring sprint you could hit 33 MPH at 635W at 140 RPM.
For a small guy with ~240W of threshold power this is the difference between the small ring only being usable at a fast endurance pace and it working for most solo rides on flat ground.
This is why so many anecdotal reports comparing compact crank use indicate riders spending much less time on the small ring, shifting the front much more often, or riding nearly cross-chained (34x14 and 50x21 instead much less time on the small ring, shifting the front much more often, or riding nearly cross-chained (34x14 and 50x21 instead much less time on the small ring, shifting the front much more often, or riding nearly cross-chained (34x14 and 50x21 instead much less time on the small ring, shifting the front much more often, or riding nearly cross-chained (34x14 and 50x21 instead of 39x16).
Personally where my weight/fitness/terrain choice precludes spinning a 39x21, 23, or 26 (depending on whether I prefer an 11, 12, or 13 starting cog assuming 10 speeds; you can adjust that up a size with 11 cogs or down with 9) up nearly all my climbs and being manageable elsewhere I run a triple with a 39 or 42 middle ring, cassette with one tooth jumps to the 19 cog, and whatever small ring it takes to make that work.