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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.

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    You're probably not going to be spinning in the 11t or 12t cog for long while commuting. I'd probably opt for the 12-30 and 34/50, though they're all pretty reasonable. – Batman Jul 17 '15 at 19:32
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    Another chainset option which I think works really well for commuting is 36/46 - this gives you a smaller jump between chainrings, and really, how often are you going to use 50x11 on a loaded commuting bike? Whilst you don't get this option with the Tiagra groupset, I believe that Shimano do some non-series chainsets in this configuration. – John M Feb 22 at 15:05
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    @JohnM Based on my experience, rarely. I've only used the 11t/12t cogs coming down mountains/steep hills and on interstates/motorways (which is allowed in parts of the US). Doing 35mph with a 50 pound touring load down a winding mountain road is scary af though... – Michael Hampton Feb 22 at 15:27
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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.

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    Now in 2018 we have even more options - tiagra now has a 34t cassette which I would pick if carrying 35lbs of panniers. Unless racing, i'd nearly always pick smaller ratios. In general manufacturers still haven't quite got there in terms of producing gear ratios for people with 'normal' power outputs rather than highly trained athletes. – Andy P Feb 22 at 14:41
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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.

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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.

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    While the math is correct, it ignores whether you want the gears you get. This promotes the 11 tooth gear because it extends the range. If it extends it in a direction you won't use, that doesn't help. Carrying that load, I would want low gears, but I spin high even on an unloaded road bike with a 50/34 and 12/30. If I could change one thing on bike I would lower the gearing even more. Since there probably won't be many hills commuting, you can get by without the low gears, but the finer spacing doesn't seem that worthwhile to me (said by one who hasn't tried it). – Ross Millikan Jul 18 '15 at 4:31
  • I dont think that I'm promoting the 11t. The 12/30 cassette has a wider range than the 11/25 one. – Jasper Jul 18 '15 at 7:16

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