I got this used wheel for a bike I am building that has a 12 - 20 7 speed Dura Ace freewheel.
What was the use for a 20t for the largest cog? It's going with a 42-52 crank.
I can deal with my 42-52 that has a 23t for the large cog, that seems reasonable but this combo will rule out any steep hills.

I am trying for a low budget build so I don't really want to start replacing parts. It's more of a curiosity question.
Thanks

Maybe it was for a Crit bike where the terrain would be flat?

Flatland or TT specific cassette for flatter courses.

Or maybe he was running it with a 39T inner chainring? Running a 39T/20T low is about the same as a 42T/22T combination but weighs slightly less. A 42T/21T was my lowest gear when I was younger.

I have a 12-21 on my 1985 Pinarello. It's a bit of a challenge on big hills, but so what. Get out of the saddle and climb.

My '84 Trek 760 came stock 52/42 with a 12/21 7s freewheel. Just makes some things more interesting But boy it shifted smooooothly with that close ratio.

When I lived in New Orleans I ran a 52/44 - 13-21 and that was plenty. The only hills were the occasional overpass.

You should see RobbieTunes's 12-19 8s straight block. We did 54.5 miles today and he dropped me on most climbs, and I have a 13-23 7s. Then again he's in substantially better shape than I am.
It really depends on the rider and the terrain, though not always the latter. Racers generally use narrow ratio blocks on most any terrain.
-Gene-

i used to race with a 13-21 6-spd FW & 52/42 crankset for road races here in the pacific northwest. that was considered pretty normal, general purpose gearing, even for mountainous terrain. here's pics from approx 1983 on a casual 2 day, 3 mountain pass ride with my girlfriend over the north cascades- each pass with over 3000 ft of vertical elevation gain. note that i'm running a 13-21 FW and 53/48 on the front-cranks are 167.5's.

my partner (mrs cat) completed the ride with a 52/39(Shimano) and a 13-24 FW.

13-21 would have been my training setup back in the day. You don't need anything smaller until you get dropped. I lived in the mountains of Southwest Virginia, and that was pretty much it. I remember when I had to go buy a 24, it was a sad day

i run the same "corncob" cassette on my long distance wheels. It works well for keeping your cadence in check while maintaing a fast pace.

Interesting, I've never heard that comment before. Why do you need to keep your cadence in check? I can't imagine a problem with that unless I were barreling downhill on a fixed gear with no brakes.

more a matter of training, keeping my cadence at 90 even though the landscape changes.