First, let's talk 'Gear Inches'.
'Gear Inches' is the equivalent diameter of the drive wheel on a high-wheel bicycle. When chain-drive 'safety' bikes came into being, the same system was used, multiplying the drive wheel diameter by the sprocket ratio. The formula is:
[(number of teeth on chainring) / (number of teeth on cog)] * (diameter of drive wheel in inches)
For the sake or simplicity, we'll call the diameter of most 700c & 27" wheels '27"'. So, if you had a 42-tooth chainring and an 18-tooth cog, your gear would be:
[42 / 18] * 27 = 63"
Okay, so now you know how to figure out how big your gear is, what combo do you use...? Good question. Keep in mind that since you can't coast on a fixed gear bicycle, you'll need a gear that will allow you to not only climb most hills but also to DESCEND most hills. If you have never ridden a fixed gear bicycle, this latter issue will be a bit hard to understand until you actually get out there and have to go down something steep. All of a sudden it's a whole new ball game!
Experience has taught me that the above combination of chainring and rear cog is pretty darn versatile for riding around town. Remember, NO gear will be 'perfect' for the flats AND the climbs AND the descents. That's part of the fun of riding a fixed gear bicycle. A 42x18 will allow you to ride comfortably at 18-21 mph on the flats, climb most hills without busting a gut and once you get a few skills, descend most hills without grabbing a handful of brakes.
ASIDE - I like this gear because it's not too big. It's a great change of pace in the off-season. This size gear is like a governor of sorts in that it prevents you from going too fast. No mashing/hammering allowed; it just isn't possible. It's also pretty hilly where I live so if I were to go much bigger, I'd grind to a halt on the climbs and tip over. No fun that. I know some Randonneurs that use larger gears for long rides and if long is what you want then I recommend a 42x16; I rode this combo on a three day, 170 mile ride over three mountain passes and it was perfect. As perfect as an imperfect gear can be anyway.
So, take my advice and get a 42-tooth chainring and an 18-tooth rear cog. If, after riding this for a while you decide to switch, go for it. But, part of the benefit of riding a fixed gear bicycle will only be realized with a 'medium' sized gear like this. See the training page for more on this topic.
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