Morning,

I'm 5'9-5'10 and have 170mm crank arms. I normally ride with about 700-1200ft of climb. I was told that I need to have 11-28 cassette with compact 50/34. I think the 11-28 is fine, however am I hurting my speed with the compact crankw/ 170mm arms?

Unless you race you'll be fine with a compact crank. You have to be pretty strong to our spin a 50 and 11.

The crankarms don't really factor into it. You can go plenty fast with 50x11. Heck, you can go plenty fast with 50x12. The real question is, how does your gearing feel to you?

That is the gearing I ride/race with and have no troubles. I could see it being an issue if the race finish was downhill or very fast sprint. Also, I climb lots and my cadence is 95+.

At 90 rpm cadence the calculated speed is...

50/11, 32.4 mph

53/11, 34.3 mph

I ride a 7-day amateur stage race in the mountains in France every year. Four cols per day, with usually about 3000-4000m of climbing every day. Every year somebody shows up trying to be a hero with his 53/39. By the end of the second day that guy is totally baked. It doesn't matter what your speed on the flats is if the road isn't flat. Having a 50/34 will help a lot more than it will hurt if you have to climb a lot. I only run 53/39 on my TT bikes, FWIW. I've never missed those extra teeth on my road bike, I even won a crit on flat ground last year with a compact. I run 170mm cranks too.

So unless your surname is Contador or Froome, I would stick with the compact if you're going to do a lot of serious climbing. Especially if your username "learning2ride" means anything.

thanks

You and I are the same height, ride about the same elevation and have the same crank/ring set up as you except I have the 11-32 cassette. I can't imagine riding some of the hills around here without the compact crank.
That said I have some very flat century rides later this summer that I'll be using a mid compact on but other than that the compact stays put.

I'd just swap the cassette to a 12-25 for those flat centuries. Much cheaper, and you'll appreciate the tight gear spaces while riding flats with a group.

Have you asked them why you need this ratio?

+1.

Who told him this to begin with

I'd go 12-28 even. 16 > 11 for almost all circumstances. If you are a hardcore racer in non-beginner categories, you may find use for an 11, but 99% of the time the only I ever use an 11 (or 12 for that matter) is when I want to bomb a descent and not coast.

What does the crank arm length have to do with this question? Sounds short to me, but it shouldn't affect your gearing to a great extent.