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Saturday, August 6, 2011

Shifting Bicycle Gears Explained or at least Discussed

8/6/11 #226 0 miles
Did not ride today; other obligations.

8/6/11
Week 30
this week:
216 miles
[3rd quarter: 825 miles]   
2011: 2560 miles


The Embarrassing Complexity of Bicycle Gears

The bicycle industry sells geared bicycles but doesn't provide the buyer with an explanation of how to use the gears. It's not obvious, it's not intuitive, and it sets the rider up for a frustrating experience. This nonchalant sale of unintelligible complexity is unmatched with the possible exception of Windows computers.

I found myself wanting to explain bicycle gears and thought I would set it down here in case it might be of use. This image below attempts to depict the names of the various pieces of the drivetrain. A "low gear" is used to do a lot of work at slow speed, such as climbing. A "high gear" is used to move at high speed.



In this image, there are three "front gears" or chain rings; low gear is small/inside, and high gear is big/outside.

On the rear wheel, the "back gears" are rings that make up the "cassette". Low gear is big/inside, and high gear is small/outside.

UGMT (usually generally most of the time) you should stick with the front/rear matchings suggested by the colors above. If the front chainring is small/closest to the frame, the read cassette should be in one of the three largest rear rings, also closest to the frame.


The goal is to keep the line of the chain parallel to the bicycle frame. Avoid "cross-chaining", or putting your chain on a diagonal combination.

How many gears does your bike have?

Say you have a triple-chainring on the front, and a nine-gear cassette on the back. How many gears does your bike have? Simple, 3x9 = 27 gears, right? That's not quite right.

There are 27 possible combinations of gears, but there are really only about a dozen useful, practical combinations of gears. Bicyclists follow a gear shift pattern that leads them efficiently through all the effective gears, as shown in this image:



In this simplified depiction of a gear shift pattern, the rider starts in the lowest usable gear, the granny gear, low on the front and low on the back. This is the gear for climbing steep hills carrying supplies at low speeds. (low gear=low speed)

For the next gear we move the rear cassette over one ring, and for third gear we move the back again, but then we face a question - how do we move to the next gear? To avoid a double-shift (changing both the chainrings and cassette) we use a transition shift to the fourth ring in the back.

For fifth gear the rider moves the front chainring into the middle. Now the bicycle is in the "middle range" of gears. The shift pattern progresses through sixth and seventh by shifting the rear cassette, and the eighth gear is accomplished by another transition shift.

For ninth gear the rider moves the front chainring onto the big outside ring. Tenth and Eleventh gear are configured by shifting the rear cassette into its final two positions.


Calculation of an actual gear-shift pattern involves knowing (or counting) the number of gears on each of the front chainrings and the rear cassette, doing some math to calculate the gear-inches of every combination, and then choosing a path that provides the optimal coverage between low-gear and high-gear.

The chart on the right depicts my shifting pattern for my Surly Long Haul Trucker (LHT), which as a touring bike skews toward delivering low gears for climbing with baggage.

And that's not all: you're not looking for a consistent incremental change in gear-inches, you're looking for a consistent incremental percentage change in gear-inches, so it's a logarithmic function.

  • Each wheel size / front chainring / rear cassette combination has a unique gear shifting pattern.
  • Why don't bike manufacturers provide this information to bike buyers? Wouldn't it be cool to know that one hybrid has a low gear of 16 gear-inches, while the other hybrid has a low gear of 28 gear-inches?
  • Gear shifting has been a manual/analog process involving cables and springs. Manufacturers are now introducing electric shifting mechanisms - nothing but wires between the handlebars and the derailleurs. The next big step in bike gear management will be when the gear combinations are managed by software, and the rider just chooses "higher" or "lower" and the software picks the correct combination.

2 comments:

  1. A VERY good representation and explanation of gearing and shifting basics! Thanks for your effort in making this post!

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  2. You obviously understand the gears better than my 'bikeability' instructor. But he was trying to teach me how to cycle on the road and I do have a basic touring bike. This explanation explains why when it was suggested to keep it simple by only using my back gears I could go no lower than 3 and no higher than 5

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