The HUNT team established a design brief for the project:

• Test two (leading) aerodynamically developed gravel rims from Enve, one non-aero optimised HUNT wheel (4 Season Gravel X-Wide) as well as their 48 Limitless Aero Disc.
• Conduct the test using the Schwalbe G-One 38mm tyre – a popular gravel tyre.
• A simulated moving speed of 32km/h – chosen to represent the speeds achievable in the Dirty Kanza 200-mile race (and also many others).

With the overall purpose of offering riders detailed research and results into the aerodynamic performance of different rims.

1.2 - Gravel Tyres and Aerodynamics

The Limitless Research Project (resulting in the 48 Limitless Aero Disc) showed HUNT that in order to achieve optimal aerodynamic performance across a range of yaw angles, that the rim profile should be very wide with a truncated edge (blunted spoke bed) to help airflow stay attached. This assumed the flow is able to flow without significant obstacle around the tyre and rim profile.

The conditions encountered in gravel riding necessitate larger volume tyres, with well protected and knobbly treads for puncture resistance and good grip. This results in a cross-section profile with a much less ‘smooth’ (to be clear a road tyre does have tread, and this is critical to flow attachment) transition between tyre and rim, resulting in the previously stated assumption that the turbulence created from the knobbles would cause early separation, adversely affecting the aerodynamic performance of the wheel and tyre working together as a global system.

The following profile characteristics are important factors when considering the purpose of the wheel whilst trying to extract the maximum aerodynamic performance.

1.2.1 - Rim Depth

As a general rule (considering road performance wheels) when considering two aerodynamically profiled rims; the deeper the rim, the better that rim will perform aerodynamically. This notion is also true for gravel, as our results below confirm.

However, shallower rims are conventionally used more often in gravel riding, for reasons of comfort/compliance, and suitability for the terrain. Hence, these additional needs of the rider must balance against aerodynamics when arguing for overall performance. When gravel riding, the rim is subject to a much harsher ride and environment than would generally (pothole dependant) be seen road riding, hence the profile and depth must not become susceptible to rock strikes, nor can they be too deep (thus significantly compromising the ride feel and control for longer days & multi-day rides or races).

1.2.2 - Rim Width

The challenges with rim width are much the same as rim depth; namely that a compromise must be made between the aerodynamic advantages of a wider profile rim, and the suitability to the terrain and purpose. In simple terms, it’s not much use having the most aerodynamic rim, if they’re so much wider than your tyre that a rock strike could easily end your adventure or your race.

The primary method to provide accurate data is to use wind tunnels to test, therefore HUNT returned to the industry-trusted testing ground, the GST WindKanal in Immenstaad, Germany.

GST is an open wind tunnel, constructed in 1986 for use by Airbus Defence and Space. It is now independently operated, and as a low-speed tunnel it is well suited for bicycle testing, making it a popular facility within the cycling industry. 

The test method is based around single front wheel (wheel only) tests, thus reducing the number of influencing variables and giving clearer aerodynamic results for comparison (as the data is focussed on the wheel drag alone).

2.1 - Wheel Setup & Testing procedure

The front wheels were set up using the specific single wheel rig, having been fitted with the same Schwalbe G-One 38mm tyre. Before each run, tyres were inflated to 100psi and aligned in the wind tunnel. Naturally, you would run the tyres at much lower pressures than this, but the higher pressure ensured consistency in terms of the tyre’s profile across the runs.

For each run, the wheel was driven by the rollers at 32 km/h and air was passed through the tunnel at a constant matching speed. The turntable was then rotated continuously through yaw angles between -20° and +20° to the oncoming airflow. 

 2.2 Aerodynamic Performance

The results obtained from the wind tunnel are processed to produce the recognised drag v yaw angle plot, which enables a visual representation of how the wheels perform against each other but does not give an idea of the real advantage in terms of Power Loss or Time Loss in a scenario.

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

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

Where,

F = force acting against the forward motion

v = velocity of object

The Aerodynamic Force to overcome drag obtained from the wind tunnel being:

Where,

p = density of fluid

C_d = 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:

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

Instead as mentioned we will only consider aerodynamic forces. The following equations detail how the authors calculated the Power and Time Loss presented in the Results section.

For this study a 32 km/h (8.88 m/s in S.I. units) speed is considered and it is assumed a professional rider (Colin Strickland, 2019) produces an average power of 317W over the 200 mile (321 km in S.I. Units) Dirty Kanza Course. Therefore, using equation (1), the propulsive force on the pedals is:

In terms of grams of force 1g = 0.00981 N, so:

Assuming aerodynamic drag (F_d from wind tunnel results) as the only resistive force, the propulsive force (F_tot) to maintain the riders speed can be calculated using:

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 the 200-mile Dirty Kanza Race:

Where s = distance (m)

Wind tunnel testing was conducted in February 2020. The following section details the full testing results presented in the tables and charts.

All tabulated power loss data has been calculated using the industry-accepted Mavic WAD distribution

3.1 - 32 km/h - Tests conducted with Schwalbe G-One 38mm tyre

Figure 2 Drag [g] vs yaw angle [°] from wind tunnel test showing 2 leading competitors at 32km/h

Table 1 – Calculated Power and Time Loss data at 32 km/h based on WAD

These results show that at 32 km/h the drag curve for the non-aero optimised wheel is displaced to a higher magnitude than the 3 other wheels, equating to a significant power loss of 9.03 Watts. In terms of time difference using an aero-optimised gravel wheel, a rider could expect to gain over 6 minutes during the Dirty Kanza 200-mile race.

Looking more closely at the drag curves, the 3 aero optimised wheels generally exhibit a reduction in drag at yaw angles after ± 8 degrees, whereas the non-aero optimised wheel does not and in fact trends up in increasing drag, another factor in the performance gap.

The data suggests that the HUNT 48 Limitless Aero Disc has a very small power loss of 0.09 W when compared to the 49 mm deep ENVE 4.5 AR SES, it is therefore clear that the ENVE is a wheel that performs (aerodynamically) very strongly when paired with this tyre.

A clear conclusion can be drawn from the wind tunnel results, what we are certain of is that Aerodynamics is a very important factor in gravel riding performance.

The aerodynamic benefit of an aero-optimised wheel in terms of power loss is large (over 9 Watts), representing a significant amount that will make a tangible impact to the performance. Further, it is clear that other industry-leading brands such as Enve are already making wheels that perform very well indeed with wider tyres. Therefore, if considerations are made to optimise a rim to a gravel tyre, then the gains could be reasonably significant. 

When quantifying this aerodynamic impact as time saved (using an aero-developed rim compared to a non-aero for an event such as Dirty Kanza), a rider might expect to save as much as 6 minutes.

The authors would like to thank Ernst Pfeiffer at GST for all of his patience, hard work and good humour on this project and over the years working together.

Thank you to all the staff at The Rider Firm, every one of whom contribute hugely daily so that we can keep serving our riders with exceptional products.

And finally thank you to all those HUNT riders who believe in what HUNT are trying to achieve, who are constantly providing invaluable feedback and support and who ultimately are the purpose in researching and developing highly competitive wheels.