The rim that tamed the stones.
We know you're here to check out Zipp's latest batch of speed weaponry, the 303 NSW Carbon Clincher Road Wheel, but tarry for a bit and follow us back in time. The year is 2010, and the two greatest cobblestone riders of this generation are preparing to lock horns in yet another season of pavé monuments. Flanders was first, and when one of the protagonists, Cancellara, rolled to the start line with 45mm carbon rims, we scoffed into our Duvel. Having been witness to the failure of carbon on cobblestones year after year, we thought that there was no way he'd survive to Ninove without those wheels failing. And we were wrong. Resplendent in the Swiss champion's jersey, Spartacus rode away in spectacular fashion on the Kapelmuur and never looked back.
Zipp's 303 platform is the wheel that made the carbon/cobbles marriage a reality, and the event is immortalized by images of Cancellara rolling down the finishing straight while defiantly waving a tiny Swiss flag against a backdrop of countless Flandrian and Belgian flags. The combination of aerodynamics and reliability meant that carbon no longer had to fear the stones, and the 303 NSW represents a major update. It's more aerodynamic, brakes better, and is even more reliable than before. And that's just the rim. For the first time in its lifespan, the 303's new hubs are every bit as impressive as its rims.
We admit that we haven't always associated Zipp with hub innovation, but the 303 NSW's Cognition rear hub may be the most exciting technology in factory wheels that we've seen since Zipp first introduced its bulging sidewalls. The freehub is centered on Zipp's Axial Clutch mechanism, which features two Metal Injection Molding (MIM) ratcheting rings — one mated to the freehub body and one mated to the hub body. The two rings are machined like a ratchet, so they ramp off of each other while freewheeling but engage each other during pedal input.
Compared to a standard pawl design, which actually resembles a drum brake, the Axial Clutch's MIM rings engage laterally to reduce friction while freewheeling, so coasting doesn't negate watts already spent. We're familiar with this model, as we've seen similar mechanisms on the designer hubs featured in top-end custom builds for years now, but Zipp is the first to replace the usual tensioning agent, steel springs, with magnets. This substitution further reduces friction between the ratcheting rings, making for what may be the smoothest freewheel on any mass-produced hub set. It's a design that we suspect will eventually be the norm, but — once again — Zipp is leading the charge.
Shiny new hubs aside, we now return to the subject you probably expected to be the headline for a new Zipp wheel. The brand's signature Aerodynamic Boundary Layer Control (ABLC) dimpling returns with a new designation. It's called SawTooth Technology, and it consists of ABLC dimples applied in a single pattern repeated in 12 discrete swathes (or "nodes") across the rim's surface, looking much like the strokes of a paint brush.
Zipp's description of SawTooth's effect is a bit, how shall we say, erudite, with the company claiming a 34% reduction in side wind forces compared previous models by "reducing the laminar bubble effect on the aerodynamically shielded side of the rim's profile." Translated into saddle speak, that means while rolling at a Cancellara-approved speed of 20mph and above, the rim's dimpling is designed to dispel the negative, drag-inducing force on the rim's non-wind side.
SawTooth's premise stems from 42 computational fluid dynamics studies and testing that spilled out of the wind tunnel onto — gasp! — actual roads. In actual wind. You know, the conditions that we actual cyclists face every time we throw a leg over the top tube either side of trainer season.
To help understand how SawTooth works, consider an aerodynamic NACA shape in the pristine wind tunnel conditions of zero-degrees yaw. When air resistance is perfectly head-on, drag only exists in a parallel plane with an object, so a teardrop NACA cross section reduces drag to its absolute minimum by reducing the wake. Turn the yaw angle upwards of 15 or 20 degrees, though, and a true NACA shape begins to experience drag from the side opposite cross wind pressure as well. This lateral drag is Zipp's "laminar bubble," and it's the phenomenon responsible for the violent instability that leaves our triceps and lats as sore as our legs after a workout on gusty days.
Though Zipp was one of the shape's early adopters, the industry is now replete with rounded, bullet-shaped rim designs that aim to lessen destabilizing drag from cross wind forces; however, if Zipp's numbers are any indication, the SawTooth nodes take that reduction to a whole new level by better controlling cross winds as they detach from the rim's opposite face. While deep rims will always be more vulnerable to lateral forces than shallow box clinchers, the Firecrest DSW's rim shape and SawTooth Technology contribute to a ride that requires less correction in windy conditions, and the corrections themselves are more subtle and intuitive than with yesterday's NACA profiles.
The rims are finished with Zipp's ImPress graphical treatment, which involves directly printing the aesthetic embellishments onto the rim rather than applying decals. This leaves the SawTooth nodes of ABLC dimpling uninterrupted and unimpeded. Finally, it's only appropriate that we end the way every good ride does: with the brakes. The rims are equipped with an updated version of Zipp's Showstopper brake tracks.
Showstopper enjoyed an impressive debut on Zipp's Firestrike 404, but it's already been improved for the NSW line. It's still imbued with the same silicon carbide particles, but the direction-specific grooves have increased in number and feature an altered depth. The original Showstopper netted some pretty impressive stopping across all conditions; while we haven't tested this new brake track, Zipp assures us that it's even better. The upshot is you can hold speed longer while approaching a corner, braking later and gaining tiny increments of time with each bend or switchback.