ABLC™ (Aerodynamic Boundary Layer Control)
Zipp ABLC technology maximizes the advantages of Zipp's patented Firecrest and Firestrike rim profiles. The idea is that these mathematically designed rim shapes can only take advantage of the airflow if the air adheres to the rim surface. With a V shaped rim or flat-sided rim, the airflow becomes separated from the leeward side of the rim as soon as the rim begins to face airflow more than 1 or 2 degrees off axis. This separated airflow creates a vacuous region behind the rim, which is the primary source of pressure drag on the wheel itself. With a curved section, we are able to keep the airflow on the rim surface out to 7 or 8 degrees of yaw, but eventually the flow begins to separate or 'stall' on the backside. With ABLC dimples, we are forcing the airflow into a higher energy state, forming a turbulent boundary layer near the surface of the rim, which allows the air to remain attached to the rim even at higher angles.
The tradeoff here is that we are creating a slightly higher skin friction drag on the rim, but since this is some 10 times lower than the pressure drag, we find a shaped rim to be remarkably analogous to a golf ball in that the pressure drag reduction is many times greater than the total skin friction drag. Whereas most aerodynamic rims show minimum drag at between 5 and 10 degrees in the wind tunnel (conditions only seen 20-25% of the time in the real world) and then show increasing drag through higher wind angles, ABLC allows the minimum drag to occur between 10 and 20 degrees and be much lower than smooth sided rims.
This technology has allowed for rims that demonstrate lower drag values at every single measured data point than previous rim designs, but more importantly have minimum drag in conditions that riders experience the most often. The concept is quite simple, roughly 50% of real world riding occurs with effective wind angles between 10 and 20 degrees, at angles between 0 and 5 degrees, the drag is primarily affected by tire choice, and at angles from 25 degrees and higher, the surface area of the wheel becomes the primary player. This new technology allows us to shift the sweet spot of the rim into the ideal zone (10-20deg.) while still reducing drag at all other values. The result is a wheel that is not just faster in one condition, but faster through the range of conditions you will experience the majority of the time.
ABLC and Firestrike
Equally important to controlling how airflow attaches to the Firestrike rim profile, is controlling how predictably the flow detaches. The new Firestrike ABLC dimple pattern was designed to work in conjunction with the rim profile resulting in a combination that is both low drag, yet very stable and predictable even in the windiest of conditions. The new wavy dimple pattern is designed to utilize 18 nodes that are specifically clocked to start shearing at a rate of 66hz at a rider speed of 17mph. The wave pattern accomplishes this by inducing small sheet vortices that shed at this low magnitude but at a higher natural frequency, decreasing the laminar bubble effect on the leeward of the profile. This high frequency shedding is less perceivable to the rider, creating less disruption, and inspiring greater confidence to hold more aero position in worse conditions.