Crownring
Versus Round Ring and Oval Ring.
Leverage is the only reason a rotary crank system works.
On the common adult bicycle leverage runs from zero to about 7
inches. These are your drive gear choices. Each of these have
individual characteristics on how they utilize leverage.
The round ring has a fixed radius. Leverage acts on this
with a symmetrical ratio of high, low, high. The highs average a
ratio of 1:1. Low is 2:1. Center stroke is twice as easy as top
and bottom stroke. Center is your power. Top and bottom is your
speed.
The oval ring has a profile of dual radii. Leverage acts
on it with a linear high, high, high. The entire stroke is a 1:1
ratio. Its claim is continuous force input is more efficient,
resulting in a power output for the entire stroke. What is not
mentioned is the constant Time Under Tension (TUT) is exhausting.
Nor does it provide a power zone. Hills are laborious. Ovals are
great for short, flat sprints, but they'll deplete endurance on
lengthy runs.
The osymetric ring is similar to the oval as it raises
radius for the greater length of the stroke, has the same radius
top and bottom, and limits the power for uphill climbs. What is
different is the osymetric chainring has a longer duration of high
radius, and a more aggressive interchange between high and low
radii.
Before revealing the dynamics of the crowned chainring lets look
at the biomechanics of a pedal stroke. Everyone has experienced
standing from a too low chair. It is difficult at first but after
getting half way up it gets easier. The straight leg is stronger
than the bent leg.
The round ring makes no concessions for the difference in leg
posture. Its constant radius demands as much from the bent leg as
it does from the straight leg. Although the ratio is the same, the
top of the stroke is much harder than the bottom of the stroke. It
siphons a rider's energy at the top, but fails to cultivate the
greater strength at the bottom.
The oval ring also demands as much from the bent leg as it does
the straight leg. As the rotation progresses the radius increases
to maintain the continuation of the 1:1 ratio in contrast to the
increased leverage. As rotation continues the ratio remains at
1:1. The leg posture has provided more strength, but the ratio
fails to cultivate it.
Neither the round chainring nor the oval makes any use of the
straight leg strength. A rider pedals along struggling through the
first half stroke then limps through the second half stroke very
easily but without relative power output.
In addition to taking the power of the center stroke out of the
race, when each are limited to a 48T chain draw, the oval profile
lowers radius of the top and bottom of the stroke as compared to
the round ring. It reduces the ratio at the point of greatest
strength limiting the speed a rider can accomplish.
The Crownring is different from both. At 48T it has a
lower radius throughout the first half of the stroke providing an
easier start, much like the oval does. But Crownring holds on to
the lower radius through the center of the stroke thus not only
keeping the power zone intact but its center stroke radius is
lower than the round chainring's center stroke radius, and very
much lower than the oval's. Center stroke with Crownring is more
powerful than either of the others.
What happens next is the incline to the peak. Crownring raises the
radius in the second half of the stroke thus making use of the
rise in straight leg strength. This increase in ratio adds speed
to the travel which increases kinetic energy which reduces the
resistance that would be incurred on the other chainrings.
Two points about the pointy peak of the Crownring: the peak is
active in a bell curve over the entire second half of the stroke.
While it might look like a pointy sharp strain on the stroke, it
has a natural feel that balances with the rise in leg strength
over the course of its engagement.
But most important, where the round ring and the oval ring end
their strokes with infinite ratio rise, the Crownring's declining
radius matches the decline in leverage, resulting in a smooth,
longer application of the ratio derived at peak. It is those odd,
flat, straight, ramps that gives Crownring its superb efficiency.
If you want to look at it as effort, the round ring and the oval
ring get harder to apply force in the last degrees of their
strokes until it results in the dreaded Dead Zone. Crownring
maintains the peak ratio through the last degrees of the stroke.
It does not get infinitely harder.
Crownring provides an easier start, more uphill power, a boost of
speed when strength, leverage, and pedal trajectory are optimal,
and it holds that speed deep into the dead zone.
Because of the bio-mechanical balance that Crownring provides on
any stroke through any terrain Crownring is easier, and that
translates to more endurance.
Don't take my word for it. Let's go for a ride.
Text updated January 12, 2025
