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A friend was just showing me the 2010 Cervélo S2 specs, as well as the 2011 S2 specs, and the Ultegra line, at least, changes from a 53/39 chainring set in 2010 to a 50/34 set in 2011. This seems like a pretty steep difference, especially given that the more expensive Red and Dura-Ace lines for 2011 keep the larger chainring sizes.

I can't imagine someone spending $4,000 on a bike being that put off by a larger chainring set than someone spending $5,000 enough to cause customer demand to change the size.

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Did the cassette also shrink? If so, it's called 'compact' but the gear range might be similar –  David Jan 15 '11 at 7:08
    
I always assumed compact cranks were just double-ring cranks, which both the 2010 and 2011 are, though I'm not positive. Anyway, no, the cassettes on both years are 12-25, so literally the bike is just slower (or easier to pedal, depending how you look at it :) ). –  Marc Jan 15 '11 at 7:18
    
This may have changed, since the question was asked, but the current S2 specs recommend a 53/39 crankset. –  Paul Wagland Jan 31 '11 at 3:23
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Compact is actually referring to the chain ring bolt circle diameter, which is 110mm for compact, and 130mm for standard. The smaller BCD of a compact allows a smaller tooth count on the chain ring, which allows better climbing gears. Combined with a smaller cog in the rear, you can maintain the same top speed, but better climbing gears. –  zenbike Mar 18 '12 at 15:06
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5 Answers

up vote 9 down vote accepted

Well, in 2010 there's the S1, S2 and S3. S1 is lower-end and has 50/34.

In 2011, they dropped the S1 and there's only the S2 and S3. (based on what's up on their website right now)

The lowest end of the line is probably considered something like "entry level" or at least "not for racers", so it gets the "slower" chainring setup. They've kept the chainring size on the cheapest bikes in the line the same, they just moved the labels for what the cheapest bike is.

And it's probably all about marketing and whatnot, nothing to do with any real solid technical reasons.

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That was pretty much my impression, but I was basing it entirely on conjecture. Thanks for the background! –  Marc Jan 13 '11 at 22:19
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They still seem to have the S1 in 2011: cervelo.com/en_us/bikes/2011/S1 –  Paul Wagland Jan 31 '11 at 3:16
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Rather a late addition to this but I see a lot of people referring to 53/39 as racing sets and in fact they used to be so called, the definitive feature of these chainrings is that the diameter between the bolts is 130mm hence 130 BCD. The 'compact' has a diameter of 110mm or 110BCD. Difference? 130BCD has typical 53/39 set up; 110BCD 50/34 - but you can also get 50/36 or 50/39. I run a 50/39 110BCD compact on my Cervélo P2C Triathlon bike sometimes I change the rings to 51/39. On my road bike I run a 50/34 but then have an 11-23 cassette so the 34/23 is almost equivalent to a 36/35.

There is an awful lot of macho posting about chainrings and crank lengths and a lot of people simply have too high a gearing and wrong crank length. I run a 165 crank as I have diddy legs.

Most new riders - and many experienced - are best suited to a 110BCD compact. As mentioned a 50/11 is more readily achieved than a 53/11 and many riders will drop down a cog to a 53/12 which is actually a smaller gear in terms of gear inches than a 53/12.

With a 130BCD set up you will also find that there is a longer crank length on as standard, if the crank length is too long the seat height drops to accommodate the longer travel to the bottom of the stroke, at the top of the stroke the hip angle becomes compressed result in discomfort and less power.

So IMHO a 50/36 is ideal for most, 50/34 sweeps up everyone and 50/39 great for racing. So get the 50/34 110BDC Compact and get a 36 and 39 rings and you are rocking whatever the terrain.

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As I also added in a comment on the question, it would appear that the premise behind the original question no longer the holds true. That is the Cervélo S2 2011 specs now include a 53/39 crankset, the same as the S3. The three pre-configured configurations on the S2 come with:

  1. SRAM Rival (53/39)
  2. Shimano Ultegra (53/39)
  3. Rotor 3D (Round 53/39)

Edit: Curiously, I seem getting getting different results to other people. I can view the S1, I just checked the link again, and it works for me, it also gives prices in AUD. It also works for me from a Dutch host, and gives prices in EUR. Perhaps it it is not sold in your region and that is why you get the 404? Using googlecache I can also see the S1.

As to the S2, again, using the googlecache link I can see that the US has both a 50/34 and a 53/39 setup. The four configurations from that link are:

  1. SRAM Rival (50/34)
  2. Shimano Ultegra (50/34)
  3. SRAM Red/Rotor 3D (53/39)
  4. Shimano Dura-ace/Rotor 3D (53/39)

So the same link seems to serve up different content depending on which country you are viewing it from, not only the price in the local currency, but also different sets of configurations.

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I'm still seeing the Rival and Ultegra packages listed as 50/34 at the above link. Is your region for the site en_us? –  Marc Jan 31 '11 at 6:01
    
@ Marc - I have updated my answer, no my region is not en_us, although that is the link that I use. It appears that the same link delivers different content, depending on where you view it from :-\ –  Paul Wagland Jan 31 '11 at 10:33
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FWIW, both chainrings and cassettes can be changed out fairly easily. Most shops that you buy a higher end bike from will work with you to select the cassette and chainring setup that works best for you.

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50/34 makes more and more sense for most people. Especially as many cassettes come as 11-25 rather than 12-25. A 50x11 is the same ratio as 53x12. The 50/34 combination therefore offers great flexibility in gear ratio even for top level racers.

I know of one rider whom finished regularly in the top 50 at the Etape du Tour and competes at Premier Calendar level in the UK who uses a 50x34 with an 11-25 cassette. Unless you are Andy Schleck 50x34 is the way to go for most riders/racers.

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Makes sense. I should have promoted this from my comment to the actual question, though: both the bikes in question have a 12-25 cassette. –  Marc Jan 26 '11 at 15:35
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