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Every new bike in my price range comes with a compact crankset, which I want for climbing, but I race and I need to keep the same top speed that I've had for sprinting and descending (I live in the mountains). So I'm just wondering what rear cassette I would need to use with my compact cranks to equal the same gearing as using a normal crankset (53/39) with a 11-28 cassete.

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1  
That's a bit unusual for a compact crankset... Sure you don't mean 50/34? –  freiheit Jul 10 '13 at 22:28
    
You could just swap a mid-compact crankset on the new bike running 53/36 or 52/36 chainrings. This is an option for Shimano 105, Specialized S-Works and other brands. It wouldn't be super cheap with race level components. Adjust the rear cassette size to get the range you need for climbing. –  Benzo Jul 11 '13 at 3:07
    
@freiheit not so unusual, I'm using one from sram. –  imel96 Jul 11 '13 at 22:18

4 Answers 4

It's not possible. You'd gain lower gears, but lose your very highest gear. If you want to keep that highest gear, maybe you should look at a wider-range cassette to get there, instead?

If you were switching from a 53/39,12-28 setup to a 50/34,11-28 setup, it would work out well.

Switching your largest chainring from 53 to 50 basically shifts all your high gears about 1 notch lower.

11 teeth is the smallest cog you can get on a road bike cassette, currently. I've seen BMX bikes with a smaller gear on the back, but they're not cassette based.

You can compare gear ratios on a given bike (where wheel diameter, tire width and crank length can all be assumed to be a constant with simple division.

Assuming your 11-28 is 11-12-13-14-15-17-19-21-24-28, or similar:

53:11 = 4.8
53:12 = 4.4
53:13 = 4.1
...
39:21 = 1.9
39:24 = 1.6
39:28 = 1.4   

50:11 = 4.5
50:12 = 4.2
50:13 = 3.8
...
36:21 = 1.7
36:24 = 1.5
36:28 = 1.3
... or if it's the more common 50/34:
34:21 = 1.6
34:24 = 1.4
34:28 = 1.2
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Use bikecalc to calculate the ratios or gear inches for yourself. Then you can figure out range equivalent setups for any combination of chainrings / cogs.

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Keep in mind, you'll never get as fast of a ratio using a crankset with a 50t big ring vs a 53t big ring. You just can't get a rear cog any smaller than a 11 tooth on standard 9 or 10 speed cassette that will match the 53x11 since your fastest possible combination will be 50x11. –  Benzo Jul 17 '13 at 13:00

Since you are interested in top speed I assume that means you really want to replicate the 53/11 combo. One way of finding a similar combination is by looking at a ratio of the front and back gear.

A 53/11 gear ratio will give you a gain of 4.81. A 50/11 gear ratio will give you a gain of 4.54.

To recover the .27 difference you would need to run a 10.4 (or 10) tooth top gear.

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2  
I don't think I've seen too many 10 tooth cassettes. It would probably be too small to fit on the freehub body. 50/11 is faster than 53/12, so you're right it's only that very bottom gear that matters. I think that you really have to analyze how often you are in the 53/11 gear, and if you're really maxing out the RPMs in that gear. I think you could probably go just as fast in both gears, you'd just need to spin slightly faster. –  Kibbee Jul 10 '13 at 12:31
    
I've never seen a ten tooth either. That wasn't really the question though. Figuring out speed, gear inches, and anything else is really a trivial exercise for excess detail. –  Ritch Melton Jul 10 '13 at 16:25

11-26.

You can't get higher gear combination than 53/11 with a 50, 11 is the smallest cog available. And 53/28 is about equal to 50/26. Hopefully it will be more comfortable because the tighter gear ratios.

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The Shimano Capreo system goes down to a 9 tooth cog, but it only works with the Capreo hub, and it's designed for small wheeled bikes like folders, not normal road bikes. sheldonbrown.com/capreo/index.html cf. pinkbike.com/news/9-36-cassette-prototype-2011.html (9 tooth MTB cog, prototype only), moultonbicycles.co.uk/images/models/Pdfs/… (Moulton hub with 10 tooth cog). –  armb Jul 11 '13 at 9:39

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