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It seems that I'm old (my last good bike had a 5-speed freewheel and shifters mounted on down tube braze-ons - they were indexed, though!), and I've been out of cycling for quite some time.

I've seen several references to a "compact crankset" and questions like this one asking about replacing the whole crankset just to change gearing. Back in my day (get off my lawn you meddlin' kids!) I would have (and, occasionally did) simply changed a single chain ring to one with a more or fewer teeth to change the gearing.

Why would one now change the complete crankset instead of buying a single new chain ring? Does it bring specific advantages to the table, or are replacement/different size chain rings simply no longer manufactured? (The cynic in me says that means more profit for the manufacturers.)

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Getting though all the hype and testosterone - Compact cranks allow a smaller small chain ring as they have a smaller BCD. Modern tech means you can have a wider gap between chain rings, giving larger spread.

This means modern compact cranks with wide range 11 or 12 speed cassettes can get the same gear range and stepping as a triple 7 or 8 speed with just a double.

Where you would switch to a compact is if you have decided you need a smaller chairing than will fit on a standard crank. However, you may still need a new Front derailleur and possibly a shifter.

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  • So, essentially, swapping chain rings for a 1-2 tooth change is probably still the best bet, but if you need to go further than that, it simply won't fit, thus, new hardware? – FreeMan Jun 24 '16 at 20:21
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    Some of the double ring road cranks in the 1940's had much larger differences than today (i.e., 28/48 were common on randonneuring bikes) so I think the increasing gaps we see today are as much the result of market demand as technology. – Rider_X Jun 24 '16 at 20:33
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    @Rider_X it's mostly about people today expecting shifting to be easy and reliable. I've had a bike with a ~18T jump, and while it was definitely possible to shift that much it was a definite effort to do so, and I periodically dropped the chain (or nearly, most of the time I'd be looking down and see it, back pedal, twiddle the shifter, and try again). 14T isn't as bad, but modern 10/12T are the standard because it's as big as you get with easy shifting. – Móż Jun 26 '16 at 10:39
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Using Sheldon Brown's gear calculator at http://sheldonbrown.com/gear-calc.html and staying consistent with these values

Using a triple of 53/39/30 teeth

A 11-28 7-speed cassette will return 9.6-3.8 7.1-2.8 and 5.5-2.1

A 11-28 11-speed cassette returns exactly the same, but with smaller steps.

Using a compact double of 50/34 teeth

A 11-28 7-speed cassette will return 9.6-3.6 and 6.2-2.4

But technology has moved on and we can now have 11 speed cassettes with 30,32,36,40 and 42 teeth.

8 and 9 speed maximum tooth count is 34 10 speed can come in 34/36/40/42 maximum teeth provided your rear deraileur can handle it.

Using a compact double of 50/34 teeth

A 11-32 11-speed cassette will return 9.1-3.1 and 6.2-2.1 equal to the triple mentioned above

A 11-36 11-speed cassette will return 9.1-2.8 and 6.2-1.9 lower low than the triple

A 11-40 11-speed cassette will return 9.1-2.5 and 6.2-1.7 lower low than the triple

A 11-42 11-speed cassette will return 9.1-2.4 and 6.2-1.6 lower low than the triple

Using a standard of 53/39 teeth

A 11-32 11-speed cassette will return 9.6-3.3 and 7.1-2.4

A 11-36 11-speed cassette will return 9.6-2.9 and 7.1-2.2 almost

A 11-40 11-speed cassette will return 9.6-2.7 and 7.1-2.0 lower low than the triple

A 11-42 11-speed cassette will return 9.6-2.5 and 7.1-1.9 lower low than the triple

Note: these numbers are development ratio, so one turn of the cranks results in that many turns of the rear wheel.

ANSWER A compact double with an 11-36 rear cassette has a lower-low than the older triples.

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    It is instructive that all the possibilities cited in this answer start with an 11 tooth cog for high gear. For many riders, 53/11 is too high a gear, but many (most?) road bikes come an 11. With the smaller bolt circle, you can get 50/34 on the front (respecting the received wisdom of 16 tooth differential maximum). There are few cassettes without the 11, so the compact makes sense for many riders. – Ross Millikan Jun 25 '16 at 3:23
  • @RossMillikan Excellent observation. I was simply trying to be consistent and not distract by varying top gears. On visual inspection, 11 seems to be the most common small cog, followed by 12, with a small number of 13 and 14s, and a couple of cassettes with 16 tooth little gears. Finally there is one SRAM cassette with a 10 tooth little gear. Outliers like the shimano with 9 tooth little gear is ignored because it only fits folders with funny measurements, – Criggie♦ Jun 25 '16 at 7:54
  • @RossMillikan As a personal response, I'm more annoyed to run out of gears at the top than at the bottom of the cassette. So if I'm carting about a small cog that gets relatively little time, that's okay. If I run out of big cog then push the pedal harder, which is achievable whereas "increase cadence" I find much harder. – Criggie♦ Jun 25 '16 at 7:56
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    I like your answer. It shows well how we have additional range available in the back, which makes range in the front less valuable. What range people want varies a lot depending on fitness, terrain, and riding style. I run a compact coupled to 12/30, never miss the 11, and if I changed anything I would lower it a bit more. – Ross Millikan Jun 25 '16 at 13:53
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    Very useful info, @Criggie, but it really didn't address my original question of "whole new crank set vs 1 new chain ring". It's more of a "compact vs triple". – FreeMan Jun 28 '16 at 17:20

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