# Bicycle Mechanics - Figuring out gear ratio

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View Full Version : Figuring out gear ratio

wlevey
11-22-02, 11:37 AM
I ride a Specalized Sirrus Comp with Schimano Deore LX derrailures. The gears are 11-32 in the rear and 49/38/28 in front.

All the training stuff I read is for Road gearing. Does anyone know how I can figure the propper gears to be in when it calls for a 53xsoethng or a 39x something??

I just ordered a Tacx BASIC trainer and I want to make use of the trainging schedules they have on their web site.

RegularGuy
11-22-02, 11:48 AM
Would an online Gear Ratio Calculator (http://www.panix.com/~jbarrm/cycal/cycal.30f.html) help?

Here's one at Sheldon Brown's (http://www.sheldonbrown.com/gears/) invaluable website.

And yet

http://www.pipemedia.net/~sar/gears/gears.html.

Figure out the gear inches each combination of sprockets and rings on your bike renders, make a chart. Figure out a model road bike's gears and compare the two.

Have fun and good luck!

P. B. Walker
11-22-02, 11:56 AM
It's pretty easy actually. Figure out the size of your wheel in inches (i.e., the roll out). I know mine in millimeters since I had to measure that for my cyclocomputer. So then I just convert it. Once you have that number (for my MB it is 1988 mm, or 78.27 inches) then you just find the gear ratio.

The gear ratio is the chainring size divided by the cog size. So if you are in your largest chainring and smallest cog you'd divide 49/11 = 4.45. Multiply that number by your tire size and you get your gear inches. 4.45 x 78.27 (in my case) = 348.30 gear inches. Which means that my bicycle travels 348.3 inches for every full revolution of my pedals.

EDIT: Sorry, 348.3 inches is the distance per pedal revolution. Your gear inches is the gear ratio multiplied by the diameter of your wheel. So if you have a 26 inch wheel, and your gear ratio is 4.45, then your gear inches would be 4.45 x 26 = 115.7 gear inches. Sorry for the confusion.

From that you can expand to find your speed at a certain RPM (80 pedal turns per min, etc).

If you just want to figure out how to translate from what they say (53x11) to what you have on your bike, skip the wheel size and just compute the gear ratio. Obviously, you will not be able to match a 53x11, since your biggest gear is a 49x11. But in another example, say the book says 39x17 and you need to find a similar gear on your bike. Take 39/17 = 2.29 gear ratio.

You will need to figure out all of your gear ratios... perferably in a table format. So you'll need to know all the cog sizes in your 11-32 cogset. You'll start with your first chainring (49 tooth) and divide it by each of the cogs (11 tooth, then 12 tooth, etc., etc) and put that in one column. Then move to the next chainring and fill that column and so on and so on. Since you have a 38 tooth chainring, you'll need a cog with between 16 and 17 teeth to match that ration above (2.29) (38/16 = 2.375 gear ratio and 38/17 = 2.235 gear ratio). Or you can try your 49 tooth chainring and put it in a cog with about 21 teeth (49/21 = 2.33 gear ratio). Or you can try your 28 tooth chainring and put it into your 12 tooth cog (28/12 = 2.33 gear ratio).

As you can see, you can get really close to their gear ratio using different chainring/cogset combinations.

I don't think most people tend to work all this out before a ride. At least I don't. I just ride in the gear that lets me keep my cadence (RPM) at what I'm trying to ride. Somedays I like to do a high cadence (100 is high for me) and other days I like to just ride at my normal cadence (the cadence that I naturally do when I'm not thinking about cadence) which is 78. And sometimes, like on hills when I'm trying to work my legs more, I'll bring my cadence way down (65 to 70) and go up in my gears.

good luck,

PBW

John E
11-23-02, 02:38 PM
PBW is right -- learn to use your gears in a way which permits you to cruise at a good (90-100 rpm, in my opinion) cadence. You can judge the effectiveness of your training regimen from your average speed over a given course. You can simplify your shift pattern by using your middle chainring most of the time. When riding my mountain bike on-road, I avoid using the inner chainring; when riding it offroad, I never use the outer ring.

If you frequently spin out of your top gear (unlikely), consider getting a larger chainring. If you struggle up hills in your lowest gear, consider getting a larger low cog. If you never use your top or bottom gear, consider converting to a closer-spaced set of ratios.

MichaelW
11-26-02, 07:40 AM
Originally posted by wlevey
All the training stuff I read is for Road gearing. Does anyone know how I can figure the propper gears to be in when it calls for a 53xsoethng or a 39x something??

The "proper" gear to be in is the one you feel comfortable with, so you can ride at your chosen pedalling cadence, and your chosen power output.
Unless you are a competing athlete, your gear setup is far more useful and sensible than racing gears, and is pretty similar to the one I use.
Use "gear-inches" to compare any combination of gears.

Bandit
11-27-02, 12:32 PM
i know this about calculating gear ratio:

1) if it is too hard to pedal, i calculate it is time to shift to an easier gear.
2) if i start spinning too fast, calculate it is time to shift to a harder gear.

jmlee
11-27-02, 01:32 PM
If you are comfortable using an Excel spreadsheet, and if you have a cyclometer that gives you your speed, check out the tables I made, which are available at www.uni-bonn.de/~jmlee

Make sure to download the sheet appropriate to your gearing set up and please make sure to read the instructions. Hopefully, they will be of some use to you.

Cheers,
Jamie