Just got my new Rotor rings! Delivered on Saturday. The observant amongst you will notice that they look like they've got a bit squashed in the post. Look a bit mis-shapen don't they? Well, that's because they are oval - not round.
Weird I hear you say. What's that all about?
The Principle
When you pedal you are alternately pressing down on one pedal, then on the other, then the other, then the other. Least that's what you are doing when using basic pedaling technique. So the powerful part of the pedal stroke is when the pedal passes through 3 o'clock - you've got the most leverage at that point. And the least powerful bit is at 12 o'clock with the pedal right at the top - hard to see how any force is being applied at 12.
Here's some pictures to illustrate this:
So there are different amounts of force being applied at different parts of the pedal stroke. At 3 you are applying lots of force, and at 12 you aren't applying much - again this is if you are using what I call a basic pedaling technique. Some people have power meters which can measure the amount of force they are applying whilst pedaling and they can look at graphs their meter produces which show the rise and fall of their power as they go through the pedal stroke. Such a graph might look like this:
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| At 3 o'clock there is high power but at 12 there is low power |
This means that there are weak parts in the pedal stroke. There a bits where you can't apply much force. The pedal stroke isn't a constant application of power, instead it is pulses of high and low power.
To take advantage of this phenomenon engineers have come up with the oval chainring. Because it is oval the gearing changes as it turns - sometimes it is hard and sometimes softer. With a normal, circular chainring you can say that it has the same gearing all the way around - so when you have maximum mechanical advantage (at 3 o'clock) you have the same gearing as at minimum mechanical advantage (at 12 o'clock).
But with an oval you have some allowance made - when you can push hard at 3 o'clock the chainring is at the widest part of the oval, and when you are struggling to apply much force at 12 o'clock then the chainring is much smaller to take account of the limited power that is available.
That's the principle. How does it work in practice?
The Practice
I'll get them fitted to my bike soon and then start reporting on what happens here - more later...
Diary #2 - what happened when I got going on them.