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    Compact Gear Ratios

    I'm confused ... comparing compact and standard cranks, is the difference between the small gears (36.7 v. 42.1) bigger or smaller than the difference in the big gears (112.5 v. 119.3)? I know the difference in gear inches for the small chainring is about 5 and for the big about 7 gear inches, but I was wondering if the 5 gear inch difference on the small is more noticeable (maybe a greater percentage of something?) than the 7 gear inch difference on the large. I am thoroughly confused! Thanks!

    Gear in chart for a compact and standard with a 12-25 cassette ...
    Attached Images Attached Images

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    AEO
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    try it in mph@90rpm
    Food for thought: if you aren't dead by 2050, you and your entire family will be within a few years from starvation. Now that is a cruel gift to leave for your offspring. ;)
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    Quote Originally Posted by trail-rider View Post
    I'm confused ... comparing compact and standard cranks, is the difference between the small gears (36.7 v. 42.1) bigger or smaller than the difference in the big gears (112.5 v. 119.3)? I know the difference in gear inches for the small chainring is about 5 and for the big about 7 gear inches, but I was wondering if the 5 gear inch difference on the small is more noticeable (maybe a greater percentage of something?) than the 7 gear inch difference on the large. I am thoroughly confused! Thanks!
    Yes, 5 is smaller than 7. The numbers on the chart are gear inches not ratios. Gear inches are linear values that are used for comparative purposes only. They don't tell you anything else. This chart assumes that the compact crankset chainrings are 50 and 34 teeth, and the standard crankset chainrings are 53 and 39 teeth. It also assumes a working wheel diameter of 27 inches (50/12 X 27 = 112.5). If you multiply "gear inches" by pi you get "development" or how far the bike travels for each full crank rotation (112.5 X 3.1416 = 353.4 inches).

    Al

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    Thanks! The link saved me a lot of time ...

    I was in the process of converting each combination to mph. Here's the output for my extreme gears

    I guess what I'm really trying to figure out is if the lower gears you get with a compact are worth sacrificing the higher gears you get with a standard crank. But I know there are TONS of posts about this, so no need to respond, I'm just trying to understand the issues by looking at the numbers.
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    Quote Originally Posted by Al1943 View Post
    Gear inches are linear values that are used for comparative purposes only. They don't tell you anything else.
    Thanks! This is exactly what I was wondering ... gear inches ARE linear values. I think I was getting confused with the proportional nature of the rings on the cassette.

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    don't try this at home. rm -rf's Avatar
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    Comparing different large chainring sizes for the same cassette?
    Mike Sherman's Gear Calculator can help.

    Load your typical cassette settings, then set the chainrings to 34, 50 and 53.
    Scroll down to the Speed at X RPM section. You can change the rpms to see how the different chainrings would affect your speed at that cadence. Both the 50 and 53 speeds will show in the chart.

    I have a 50-13 as my biggest gear ( a 13-26 cassette) and I spin out at 32-33 mph (about 120 rpm). Any faster on a downhill, and I'll just coast.
    Last edited by rm -rf; 12-17-09 at 08:31 PM.

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    Thanks for all your help. I'm finally getting a grasp of how bicycle gearing works. Now I just need to go out and ride some different setups to feel the difference.

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    Think in terms of percentages, not gear inches. At the top end, a shift from an 11T cog to the 12T cog is 8% or 10 gear-inches. At the low end, a shift from the 23 to 25 is the same 8%, but less than 5 gear-inches. Percentages make the most sense, when you consider that an average cadence is in the 100 rpm area. Each shift would create the same change in cadence or the same increase in available torque.

    My other favorite formula is what I call "equivalent gears". When comparing different crank setups, a simple formula allows an easy comparison. For example, let's say you have a 53/39 with a 12-27 and want to know what setup with a compact provides about the same low gear. 27/39 x 34 = 23.5 That tells you that a 34/23 is not quite as low, but a 34/24 would be lower. 27/34 x39 = 31 That tells you that a 39/31 would be needed to match the low gear of a 34/27. The same forumla works nicely to compare triple cranks to a compact.

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    For most purposes, Dave's "equivalent gear" formula is all you need to know. I use that method all the time for assessing gearing changes or comparing two bikes (keep in mind that this formula ignores wheel/tire size which can be a factor in gearing too). Only when I want to dig into the specifics do I bother with anything else. At that point, I'm looking at mph at 90rpm. Much more useful than gear inches.

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    When I was considering a compact I worked out all the new gear inches and saw that the old-way 1st gear was about the same as the new-way 2nd gear. So for me, it was like adding one lower gear and losing the topmost one. Realising that helped me sort it all out. (BTW, it made a big difference for me, most worth it!)

    Some have said the way they plan gearing is to get the lowest gear they need for the terrain they ride and let the top gear fall where it will, and also "I never need more than 90 for the top gear". Both seem like a good idea to me.
    Biking isn't a sport because anybody can do it. I can bike, you can bike. For goodness sakes, my mother can bike! You don't see her on the cover of Sports Illustrated, do you?

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