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Zipp Vuma Chrono Crankset.
By Tom Demerly.
Read this first about our reviews


Zipp Vuma Chrono Crankset
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.

Zipp Vuma Chrono Crankset
A narrow profile with smooth transitions reduces drag and increases stiffness.

Zipp Vuma Chrono Crankset
Unique crank are profile optimize aerodynamics.

Zipp Vuma Chrono Crankset
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.

Zipp Vuma Chrono Crankset
The unique arm profile is optimized for forward pedaling.

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.

 

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.

Zipp Vuma Chrono Crankset
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.

Zipp Vuma Chrono Crankset
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.


 

 

 

© Tom Demerly, Bikesport Inc.
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