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I have a 21 speed bike, and I'm thinking of purchasing a 1 speed. Does anyone know what gear on the 21 would be for a 1? I'm trying to see how well my bike does on that gear in my area. Thanks!

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    You're testing different gear ratios on a multispeed bike to see what it would be like to ride that gearing on a singlespeed ? That's really good thinking - excellent idea. Do please test it both up and down the steepest grade you're likely to ride. – Criggie Jul 31 '16 at 9:30
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    Learn how to count teeth on your sprockets and do the math. – Daniel R Hicks Jul 31 '16 at 11:44
  • Single speeds don't have a single ratio. There is a variety of chainring and cog sizes that can provide different ratios to suit different riders and terrain. – whatsisname Jul 31 '16 at 20:20
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We can't tell without more information about your gears.

However, you can figure it out for yourself quite easily, by looking at the ratio of chainring teeth to rear sprocket teeth on the single speed bike (the most common I've seen is 44/18 = 2.44).

Then you just need to figure out the size of your current bikes chainrings, the size of the sprockets on your current bike, and calculate all 21 gear ratios, and find which is closest to the single speed ratio.

If I had to guess, I would say it would most likely be equivalent to your middle chainring (39 teeth?) and somewhere near the third smallest rear sprocket (16 teeth?) because 39/16 = 44/18.

Of course, you can almost always pick and choose the gearing on a single speed bike, so realistically you yourself get to pick "what gear on the 21 would be for a 1".

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  • Don't forget that the wheel size affects the effective gearing as well. If one bike has 700C/28"/29"and the other has 26" the difference is significant. Tyre width makes much less difference and can probably be ignored unless you're going very fat<->skinny. – Chris H Aug 1 '16 at 5:48
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There are numerous gear ratio calculation tools out there. Once you know which gears work best in your area, you can use these tools to find the ration you need. Something like this for example: http://www.bikecalc.com/gear_ratios

Or you can do it on paper :)

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    Well on the website you get a nice table with all the gears as well. But yeah – Giannis Jul 31 '16 at 13:49
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    Yeah. Actually, I guess the point is that you can compute the gear ratio of the single-speed bike by dividing the two numbers, and then look in the table to find the closest match on the multi-speed bike, without having to calculate all the ratios that might be close. So I'm gonna nuke my comment and give you a +1 instead. :-) – David Richerby Jul 31 '16 at 13:54

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