The answer:
1. Notes
I will use the terms "outer" and "inner" instead of the "leftmost" and "rightmost" sprockets and chainrings.
The answer assumes that the bicycle uses a correct chainline (
article explaining the bicycle chainline):
2. Triple shifting pattern
The two pictures below show the general idea, but I'll explain it in more detail.
Cross chaining (
what we want to avoid):
General triple cranks recommended shifting pattern:
6-speed freewheels (
freewheel vs freehub/cassette explained) start with the smallest sprocket of 14 teeth, and for them, the picture above shows an ideal shifting pattern. 7-speed freewheels usually start with 13 teeth, and for them the picture is also spot on.
However, with cassettes, even the 7-speed ones usually start with as little as 11 teeth. The same goes for 8, 9, 10 etc. speed cassettes. 12 teeth is as high as you can get on the smallest sprocket, unless you manage to source an uber-expensive "junior racing" cassette.
When the chain is wrapped around a sprocket with 13 teeth or fewer, it's bent around a very "sharp" angle, and the drivetrain is not very efficient. That's why the
drivetrain is more efficient and has reduced wear when we use 14+ teeth cassette sprockets. Of course, we also
don't want to do severe cross-chaining, as that too increases wear and mechanical losses.
How do we achieve that?
It boils down to gear ratios. Here is what that looks like with a typical triple crank (48-38-28 teeth chainrings) and a typical 8-speed cassette (11-13-15-18-21-24-28-32 teeth sprockets):
To display this, I've used the Gear calculator (awesome little tool). I've set it to use:
- A 622-32 sized tyre for the calculations (28 or 29 inches as marketing calls it - bicycle tyre sizing standards explained).
- A cadence of 80 revolutions per minute - that is not fast, but not too slow either.
- The speed is shown in km/h (give me the metric system, or give me death!).
- The chain angle is set very conservatively (well below 2 degrees, so even with short chainstays it should not be problematic).
Combinations that are grey have the chain crossed over the set limit in degrees, and the speed in km/h is printed above each combination (with 622-32 tyre and an 80 rpm pedalling cadence).
As you can see from the graph, we can use the largest chainring for speeds above 20 km/h, without much cross-chaining. So, for example, if we use the largest (outer) chainring like 48-21 combination (front-rear), the drivetrain will be more efficient and the wear might even be slightly reduced compared to using a similar gear ration with the middle chainring (that would be the 38-18 or 15 combo - the closest ones).
With the smallest (inner) chainring (28-13 or 28-11), it would result in severe cross-chaining, increased wear, and low efficiency.
Of course, ideally, on flats, one could start with the middle chainring, say 38-21 combination, and then, when up to speed, shift to the largest chainring, though I often start right from the large chainring on flats with a bit of pedalling while standing until I get up to speed. If I'm near an incline, then I would start from the middle chainring.
The overlapping gears of the triple provide those nice options. They also allow for fine-tuning of gear ratios for those who insist on that. Even with "only" 8 cassette sprockets, I can fine-tune the gear ratio. For example: the "jump" between 48-18 and 48-15 combination is relatively large, but we have the 38-13 combination right in between.
I hope I've answered the question.
Relja
