Zipp didn’t just port over the SawTooth rim shape from the 454s onto a deeper-section rim—the 858s feature a new rim profile that is more pointed, and thus more aerodynamic than that of the 454s. The 858s also feature a new HexFin dimple pattern (HexFin refers to the hexagonal shaped dimples introduced on the 454s) that improve the wheel’s ability to hang onto air and create a turbulent boundary layer. Both these changes enable the new wheel to garner a 7.5 percent aerodynamic savings compared to the 808s, says Zipp.
The 858 also feature the major improvements Zipp brought to the 454s a year ago, like the Cognition hub, which uses magnets to disengage the ratchet mechanism of the driver when you’re coasting. Zipp says this reduces rotational drag by more than half compared to competing hubs, providing the rider with “free speed” that is particularly noticeable on downhills. The rim-brake version of the 858s also feature Zipp’s Showstopper one-directional brake track, which improves braking performance in wet weather.
The wheels are also lighter and stiffer than 808s, says Zipp. The 858 NSW disc-brake wheelset weighs in at 1,834 grams total claimed (850g front; 984g rear) while the rim-brake version tips scales at 1,750g total, claimed (808g front; 942g rear). The 858 NSW sports an internal width of 17mm.
How They Ride
I got a chance to ride the new 858s on a 30-mile, out-and-back test ride from Kailua-Kona onto the IRONMAN World Championships course on the Queen Kamehameha Highway.
My first impression of the wheels actually had nothing to do with stability or aerodynamics—I was struck by the ride quality. The 858s rode really, really nice. With 25mm tires inflated to 90psi, the combination of the wheels and the bike (I was on a Specialized Venge) smoothed the road and, on choppy sections and even rumble strip, soaked up chatter and made the bike feel planted and calm. In terms of compliance, if I didn’t know better, I would have guessed I was…