As a general rule, when comparing two aerodynamically profiled rims, deeper shapes will provide greater aerodynamic benefits. When studying equipment choices at premier gravel events, it was apparent that riders generally favoured shallower rims, presumably to achieve light weight, compliant ride characteristics and crosswind stability; all at the expense of substantive aerodynamic gains. The vibrational losses associated with deep rim profiles, along with the energy lost fighting hours of crosswinds, can cause fatigue over long racing distances. After testing a number of shapes and depths, HUNT identified the opportunity to develop the ideal solution: an extra-wide, aerodynamically optimised 42mm profile with outstanding crosswind performance and low relative weight.

Understanding Crosswind Stability

During testing, we monitor various elements of aerodynamic performance, including side forces that act on the rims, and the effect those forces have on the steering moment (a tool for measuring crosswind stability). As with our previous projects, our wheels are tested through a range of effective yaw angles between negative 20 degrees and positive 20 degrees.

In real world riding conditions, wind does not distribute forces evenly. The frequency of effective yaw angles experienced by a rider follows Gaussian distribution (bell curve), based on the variables of wind direction and speed, as well as rider direction and speed. Simply stated, low yaw angles have a greater probability of occurring in real world riding conditions than high yaw angles. According to well-referenced data from scientific and industry sources, "a vehicle will spend at least 70% of the time at yaw angles below 10 degrees." (Cooper, 2003). In the context of riding, the rider will spend approximately 42 minutes experiencing yaw angles between negative and positive 10 degrees over the course of an average one-hour ride.

In the chart below, the steering moment is tracked on the Y-axis, while the Yaw Angle is tracked on the X-axis. A flatter curve across the X-axis represents less impact on the rider's steering and stability. We are able to deduce from these results that the 42 Limitless Gravel Disc has the smoothest steering moment relative to the other wheels tested. Whether descending or going flat out on unsheltered roads, the rider will spend less energy fighting against crosswinds, leading to a faster, more comfortable, and more efficient ride.

Wind Tunnel Data

The wind tunnel results below depict the performance of HUNT 42 Limitless Gravel Disc wheels among a range of world class aero wheels commonly used in competitive gravel racing. The data suggests that the HUNT 42 Limitless Gravel Disc is the most aerodynamic gravel wheelset, offering the rider a small power gain of 0.05W, or 6 seconds of advantage when compared to the next best ranked offering over the course of a 200-mile race averaging a speed of 32 km/h and power output of 317W (Note: the 3T Discus 45|40 was not available to test, as it was not released to the public when testing occurred).

With the use of patented Limitless construction, HUNT was able to create a wheelset that offers an extremely wide rim profile with a truncated edge (blunted spoke bed) to help airflow remain attached to the system, while maintaining low overall weight and providing day-in and day-out performance and durability on the harshest gravel roads.

The wind tunnel data HUNT recorded is shown against the full range of reputable aero wheels in the chart below. The results below depict the performance of the 42 Limitless Gravel Disc wheelset among the range.

Using the aerodynamic force data from wind tunnel testing sessions, it is possible to calculate the average aerodynamic power to derive the time needed to cover a certain distance. In this case, the authors have calculated the time loss based on 200-mile race.

When it comes to quantifying performance, the use of Power (Watts) has become the norm, calculated in its simplest form, this is:

𝑃 = 𝐹𝑣 (1)
Where,
F = force acting against the forward motion
v = velocity of object

The Aerodynamic Force to overcome drag obtained from the wind tunnel being:
𝐹d = (p * cd * v2 * A) / (2)
p = density of fluid
Cd = coefficient of drag
A = reference Area (often frontal area)
However, the power required to maintain a rider’s speed is dependent on many other forces:
𝑇𝑜𝑡𝑎𝑙 𝑃𝑜𝑤𝑒𝑟 = 𝑃! (𝑑𝑟𝑎𝑔) + 𝑃) (𝑟𝑜𝑙𝑙𝑖𝑛𝑔 𝑟𝑒𝑠𝑖𝑠𝑡𝑎𝑛𝑐𝑒) + 𝑃* (𝑓𝑟𝑖𝑐𝑡𝑖𝑜𝑛) + 𝑃+ (𝑠𝑙𝑜𝑝𝑒) + 𝑃, (𝑎𝑐𝑐𝑒𝑙𝑒𝑟𝑎𝑡𝑖𝑜𝑛) (3)

This is now a complicated equation that is very dependent on the conditions and not possible to solve from wind tunnel data alone.

Calculation of Power and Time Loss
Hyp: a 32km/h (8.88 m/s in S.I. units) speed is considered and a professional rider producing an average power of 317W over the 200 mile (321 km in S.I. Units) Dirty Kanza Course.
Therefore, the propulsive force on the pedals is:
𝑃=317 𝑊
𝐹= 𝑃𝑣= 3178.88=35.7𝑁 (1)
In terms of grams of force 1g = 0.00981 N, so:

36 𝑁=3638,96 𝑔
Assuming aerodynamic drag (Fd from wind tunnel results) as the only resistive force, the propulsive force (Ftot) to maintain the rider’s speed can be calculated using:

Ftot = ΔFd + F (4)
The values of F and P are now known. Therefore, by manipulating equation (1) it is possible to calculate the time it would take to complete 321 km:

𝑃=𝐹 𝑣=𝐹𝑡𝑜𝑡∗ 𝑠 / 𝑡

𝑡= (𝐹𝑡𝑜𝑡∗𝑠) / 𝑃
Where s is the distance (m)