Bike Cranks for Dummies

Mechanical advantage of different chain ring:sprocket combinations

There are times when our staffers just look at a freelance post and shrug: "What was this bozo thinking, anyway?" is pretty much the reaction. That's especially true when the writer claims to have a background that would lead to the right answer, but somehow manages to miss the point. Want an example? Sure: consider the SportsRec.com question, "What Is Better: Big or Small Front Sprocket on a BMX Bike?" It was addressed (although not answered) by one Jonathan D. Septer, who touted himself as, "A professional bicycle mechanic with more than ten years experience," although Demand Media allowed him to contribute because of his BA in English.

Septer never bothered to explain to the OQ that sprockets are on the back and chain rings are on the front. That, however, was the least of his shortcomings... Jonathan went into gory detail about such minutia as, "Common Gear Combinations," "Sprocket Size Hub Limitations," and "Brake Mount Limitations." We won't argue Jon's conclusions there, but we would like to point out that those metrics aren't "what is better?" Instead, they're "what works?" When push came to shove, Septer hemmed and hawed, finally barfing up:

• "Big Sprocket Advantages: ...a BMX bicycle with a larger sprocket will experience less broken chains or rear cog teeth than a BMX bike with smaller gearing. Large sprocket teeth also contact the chain surface less frequently and usually wear longer than small sprockets."
• "Small Sprocket Advantages: small sprockets are lighter than large sprockets, and weight is a major concern with most cyclists and other operators of human-powered vehicles, as a lighter ride result in less effort from the rider."

None of that's wrong. What's wrong is that Jonny-boy wasn't taking into consideration the purpose of a BMX bike; instead thinking about road and mountain bikes. That's why he skipped entirely over the physics of a bike drivetrain (see image above).
What is better in a bike drivetrain depends on the purpose of the bike. For bikes that must accelerate quickly and climb steep hills, a small chainring is preferable. For bikes that will be ridden long distances at high speeds, a large chainring is preferable. That playoff is why road bikes have two chainrings and most mountain bikes have three: a smaller chainring for quick acceleration and steep climbs, a larger chainring for fast flats. With a relatively small chainring you have a greater mechanical advantage, although it becomes difficult to maintain high speeds: it's a trade-off.

Oh, and confidential to our Dumbass of the Day: the weight difference between a 36- and 44-tooth chainring is on the order of ten grams, about a third of an ounce.

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