Cassette & crank combo for commuting

Question

Okay so I've been reading about gear ratios and what not all morning and I can't make any sense of it. Some of it is registering but it's still kind of over my head.

I'm ordering a Tiagra group set for my commuter bike. It will be around 10-15 miles a day (5 days a week) carrying approximately 35 pounds on panniers. The commute will be mostly flat, couple small hills and some easy elevation increases.

I used to cycle quite a lot (200 mi/wk) but haven't ridden in a year due to time constraints.

So my options for the cassette are:

• 11-25
• 12-28
• 12-30

From my understanding I would want to go with either an 11-25 or 12-28. The 11-25 would be more for speed/power and 12-28 would offer a wider range of gears (i.e. lower gears for climbing hills)?

Crankset options:

• 39/52
• 34/50 (Compact)

Crankset I have no idea about, this is where I keep getting lost with everything.

Any advice or recommendations would be very much appreciated.

Answer 1

50x12 lets you do about 55km/h (35mph) pedalling at 100rpm. Since you say you're commuting, mostly on the flat, with a non-trivial load, you're not going to get any real use out of a bigger gear than that, so an 11t cog or 52t chain ring won't have any advantage. However, having a small ring of 39t or a biggest cog of 25t will hurt you if you have any steepish hills (say, 5% or more).

That points towards a compact crankset and either the 12-28 or 12-30. Honestly, there's not much difference between the two for the terrain you describe. The 12-30 gives you an extra big gear, which is nice if you're going up any big hills but you say you're not. The 12-28 has slightly shorter gaps between the cogs, which makes it slightly more likely that you'll have that perfect gear at any given time.

You might also want to consider secondary uses of the bike. If you think you might start going for rides at the weekend, the presence or absence of hills on those rides might push you towards the 12-30 or the 12-28.

Answer 2

If you have a bike you can ride now, I'd suggest putting its gear configuration into an online gear calculator (I'm kind of fond of this) so that you can see the ratios and spacings of a bike you've ridden and compare them with the options you're considering for your new bike.

For me, I'm most interested in a gear that feels good for cruising on the flat and a good hill climbing gear (for me this means one where I can spin pretty close to my "normal" cadence). From the bike that you know, think about what you'd like to improve – say a higher climbing gear, or maybe a couple of options for a cruising gear (especially if it is windy where you are).

I also really like Jasper's analysis of the step between the chain rings, that seems like a helpful way to think about cranksets.

Answer 3

Some math: the bold numbers are turns of the rear wheel per turn of the crank.

39/52 crankset:

with 11-25 cassette: from 39/25 = 1.56 to 2/11 = 4.73

with 12-28 cassette: from 39/28 = 1.39 to 52/12 = 4.33

with 12-30 cassette: from 39/30 = 1.3 to 52/12 = 4.33

---

34/50 crankset:

with 11-25 cassette: from 34/25 = 1.36 to 50/11 = 4.55

with 12-28 cassette:from 34/28 = 1.21 to 50/12 = 4.17

with 12-30 cassette: from 34/30 = 1.13 to 50/12 = 4.17

In general, the 34/50 crankset will give you a wider range, because 50/34 = 1.47 > 1.33 = 52/39, i.e. with the 34/50 crankset, the larger chain ring has 1.47 times more teeth than the small one vs. 1.33 times more teeth with the 52/39 crankset.

As @Batman states, you'll probably not be using the smallest cog (11t) that much, so we're left with 12-28 or 12-30, and I don't think that there's much of a difference between them.