Zipp leads the way in aerodynamic crank
design with the new Vuma Chrono carbon fiber aerodynamic crankset.
If you’ve optimized your position,
your bike frame aerodynamics, your wheels, you are wearing
a snug fitting one piece suit and an aero helmet it’s
time to start shaving seconds by attending to the details.
Zipp has built an industry of attending
to the aerodynamic details of going fast on a bicycle.
An American company within earshot of the Indianapolis
Motor Speedway, Zipp holds the high ground in performance
bicycle component design and domestic manufacture. Zipp
wheels have won more Tour de France time trials and Ironman
Triathlons than any other brand over the past decade.
With this resume a foray into components
makes sense. Zipp has a firm foundation in component design
and manufacture that includes the interesting and still
advanced bike frame, the Zipp 2001, that is no longer
in production.
Zipp has since found a niche in aero
components with handlebars and now cranks with their new
Vuma Chrono aerodynamic crankset.
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A narrow profile with smooth
transitions reduces drag and increases stiffness.
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Unique crank are profile optimize
aerodynamics.
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The arms use the same width to
depth ratio as the Zipp 303 race wheel but with a unique
airfoil shape optimized for crank speed rotating forward
into the boundary layer of air surrounding the bike as
it moves forward.
These are also the stiffest cranks
available according to Zipp. |
The Vuma Chrono features uniquely profiled,
wind tunnel tested crank arms, crank “spider” (center
section of the crank) chainrings and even assembly hardware.
The section of the arms is optimized to reduce drag, especially
coming forward at higher speed across the top of the pedal stroke.
According to Zipp the crank arms on an aerodynamic bicycle have
the smallest profile but are among the fastest moving parts
through the boundary layer as the bike moves forward. Remember
that the crank is encountering the air at a slightly higher
speed than the ground speed of the bicycle as the crank arm
rotates over the top of the pedal stroke.
The unique arm profile is
optimized for forward pedaling.
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Zipp spent
2 years on prototypes and pre-production examples of the
Vuma Chrono crankset, tweaking arm profile and shape of
the crank spider. The idea was to develop an arm depth
and shape that smoothly transferred to the outer circumference
of the chainring. While there are other disk-shaped aerodynamic
cranks, none of them feature this smooth transition so
important to good aerodynamics.
The width to depth ratio of the crank arm is nearly identical
to the depth of Zipp’s 303 aerodynamic race wheels.
The cross section of the arm is an asymmetrical airfoil
with a longer trailing edge than leading edge. These refinements
reduce drag when the arms come over the top of the pedal
stroke into the fast moving boundary layer of air. At
this point the arms move forward into the air faster than
the bike itself is moving through the air so aerodynamics
here are important.
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The arms transition smoothly onto the dome
shaped crank “spider” that is a solid carbon fiber
disk. It is bulged outward smoothly to mate with the crank arms
and deflect cross wind pressures at all yaw angles. There is
no transition from the crank disk to the special chainrings,
further reducing drag.
A proprietary 9 bolt pattern
fixes the chainring to crank dome (it isn’t a “spider”
in the traditional sense) using very small bolts to present
a smoother profile for air moving across its surface. The chainrings
are custom made at a machine shop in Florida and available in
55, 54 and 53 teeth on the big ring and 39 and 42 teeth for
the inner ring. Miniature plugs are provided with the production
cranks to close the opening in the bolts to further lower drag.
No detail is missing. Even the chainring fixing bolts on the
back of the small chainring have miniature aerodynamic covers.
Sold with a variety of bottom bracket
configurations, including ceramic bearing equipped versions,
the Vuma Chrono ranges in price from $1349.99 including bottom
bracket to $1559.99 with a ceramic bearing bottom bracket.
While the aerodynamic appearance of the
crank is attention getting another performance advantage comes
from the crank’s stiffness. Zipp claims it is the stiffest
crankset available. Their testing indicates the new Shimano
Dura-Ace 7900 series road crank for 2009 will deflect 17/1000th
under a given load and theta the Zipp Vuma Chrono will only
deflect 1.7/1000th under the same load. Riders who have used
the new Vuma Chrono at Ironman Hawaii said it “Felt like
a solid drive shaft” compared to a normal crank.
The stiffness of the crankset also augments
front shift quality according to riders who have used the Vuma
Chrono in race conditions.
Available with 55, 54 and 53 tooth large rings and 42 or 39
tooth small rings and in a variety of cranks lengths the new
Vuma Chrono is an aerodynamic and mechanical upgrade to any
time trial or triathlon bike.
The crankset is sold with a variety of bottom
bracket configurations for different format bottom brackets.
Total crank weight is not published yet but is “competitive
with other time trial specific cranks” according to Zipp.
Four of the Zipp Vuma Chronos were used
at Ironman Hawaii in 2008 with professional T.J. Tollakson,
a full time engineer and elite level Ironman, joining in the
evaluation.
