Originally Posted by
spclark
My venerable '72 MBGR came with 52/40 chainrings & 14/24 FW in back. It was perfectly fine when I lived and rode in Flatland (Illinois) until we moved up into SW WI late in 2013.
Or I should say until I started riding it again earlier this year, after a 12 year hiatus.
There are hills here. Some're HILLS. Even a few HILLS... it't ain't California or Colorado though so my question is:
With 120 OLD geometry in back, and a choice of cogs from 14t to 34t, what's a useful 2nd - 4th combo for recreational rides < 20 miles in length?
Assuming a (very common) 130mm BCD (or 128 for Nervar), your spider can accommodate 38T, reducing your lower-gear ratios by 5 percent. Good start.
Most derailleurs are good for 26T, so now you are at 38/26 instead of your current 40/24, and your low gear ratio has dropped by more than 12 percent, which is quite noticeable.
As indicated earlier in the thread, a short-cage Campag. NR rear derailleur
may be able to handle 28 teeth in back -- definitely not true with my Bianchi frame. The best 2x6 I could devise for it was 46-38 / 13-15-17-20-23-26, which makes hills noticeably easier than the 50-42/14-16-18-20-23-26 I had run for almost four decades prior. A very popular 1.5-step 2x5 setup back in the day was 52-40 / 14-17-20-24-28, which is a very standard, easily obtained freewheel combination. You already have the ringset for that.
You mentioned gear-inches, which I find a very convenient way to describe bicycle gear ratios with wheel sizes, i.e., overall cadence and climbing effort. For years, manufacturers had settled on a 52/14 high gear ratio, which works out to 100.3 gear-inches with 27" wheels. Then one can simply regard all other gear-inch calculations of percentages of nominal top (100%) gear. Personally, I find a top gear in the mid-90s to be perfectly adequate, and have no desire to waste valuable ring-sprocket combinations on anything higher than that. I strive for about a 6 or 7 percent ratiometric progression between gears and decide whether I want half-step or, more commonly for me, 1.5-step. The double-shifts are a non-issue, because most of the time you just shift up two steps (one freewheel/cassette cog at a time), unless fine-tuning for optimal cruising on a steady grade.