Out of round…

RotationPhasesB

One of the fundamental problems with ‘a rotational, opposing cranks, transmission input system’, what we cyclists call a chainset, is that it produces power output that ranges from maximum to minimum with each revolution, like constantly pressing hard and releasing the accelerator in a car; very inefficient.

The diagram above shows that as your pedal reaches around 60 degrees from the horizontal it enters the power phase where your leg muscles have optimum strength potential.

Your power input increases dramatically, reaching its peak about 15 degrees above the horizontal, an approximation based on the average angle of your saddle behind the vertical.

Then, as your pedal passes about 15 degrees below the horizontal, power diminishes considerably. It is possible to gain a small amount of power by literally pulling-up on the pedal, to contribute to the power phase of the opposing pedal. During these power phases you are easily capable of pushing a higher gear.

 

PowerPhaseB

 

During the lower and upper phases you probably produce no measurable power input at all and therefore you will benefit considerably by driving a much lower gear.

UpperLowerPhaseB


Posted by ajantom

The elliptical chainrings, added to the low-geared hub gears (so you can change whilst putting on the power) feel odd, but mean you have power in the pedal stroke exactly when it’s needed.

I rode all the way up a hill that I could never manage on any geared bike I’ve ever owned. Probably a 30/35% gradient on rutted, wet grass.


An elliptical chainring provides a higher gear during the power and pull phases, with a nice progressive gear reduction easing you rapidly through the upper and lower phases, getting you back into the power phase much more quickly, resulting in a very efficient transfer of your effort into forward motion.

It not only maximises your strength and minimises your weak spots, it also reduces spin-out, improving traction because power is more even, giving you greater control.

At higher cadences, whizzing along a smooth pre-prepared mountain bike trail, you usually have momentum to spare, this variation in power is hardly worth worrying about.

Winching yourself out of a quagmire, with a cadence around one revolution per second, it’s a killer.

Varatio

Elliptical chainrings, provided they’re set-up correctly, make hard-pedalling situations, particularly climbing hills, feel much easier and more comfortable.

Assuming you are determined to ride as fast as possible, in all situations, you can thus be fooled into pushing a gear-ratio higher than is good for your legs, making your ride feel sluggish and potentially causing long-term knee injury. As far as I can discover, this is the only reason experts can come up with to support their claims that elliptical chainrings are completely impractical.

Since I’m not an expert, and in no particular hurry, I just use a lower gear-ratio, as could you.

When the forces of gravity and momentum are in your favour, on the level and downhill, you can happily push a slightly higher gear and thus get along a bit quicker.

If you’re in a hurry.

Click here to go to the beginning of the whole story.

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About gmacleland

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