Gravel: Does Aero Matter?

 

“As riders & racers of gravel, we’ve increasingly been discussing (and asked by our customers) whether or not aero is particularly relevant within the discipline. Working with Ernst at the GST Wind Tunnel and doing lots of number crunching, we wanted to find out if gravel speeds and wide tyres negated aero considerations.”

Ollie Gray, HUNT Product Communications Co-Ordinator

 

Gravel; the all-rounder, the all-day adventure, the ultimate hybrid of Road, CX and MTB. It’s quickly become the hot topic for amateur & professional riders alike, and it’s continuing to grow in popularity. The cycling industry has seen exponential growth in the gravel sector over the past few years, with more people choosing gravel (or road-plus) bikes over conventional drop-bar road bikes. Whether for versatility of usage, comfort, more exploratory commuting, hitting those local off-road trails with friends, there’s a reason to pick up a gravel bike for almost everyone. As well as this increase in ‘casual’ riding styles, there has also been a proliferation in gravel racing. The growth of single-day events such as Dirty Reiver or Battle of the Beach (in the UK) and Dirty Kanza, SBT, and the Belgian Waffle Ride over in the USA is evidence of its success and breadth. In addition, gravel has long been a surface raced across by ultra-endurance cyclists in events like the Tour Divide, Italy Divide and Atlas Mountain Race.

As you may know; HUNT are just riders excited by new ways to explore by bike and serve riders with wheels. In 2016 they launched the first dedicated gravel wheelset in the world, the 4Season Gravel Disc. As early adopters of tubeless, pioneers in aerodynamic disc-brake wheels, and supporters (and employers!) of exceptional athletes such as Josh Ibbett (2015 Trans-Continental Winner, and also HUNT Beyond Brand Manager), they live to serve riders to the best of their abilities - gravel racing and riding has been at the heart of so much of their riding and product development.

The authors would therefore like the opportunity to offer the information gained from their extensive real-world research and wind tunnel testing, to answer a question which more and more riders ask: “For Gravel, Does Aero Matter?” – well the simple answer is yes. This research white paper will discuss the testing and results HUNT have yielded in understanding how the rim profile (depth, width), and gravel tyre options, (tread, width) affect the aerodynamics when considering the specific conditions appropriate to gravel racing seen at an event like Dirty Kanza. It will present how and why aerodynamic performance is an extremely important factor in the conditions and quantify those performance benefits of an aerodynamically designed rim compared to a normal rim designed without aerodynamics in mind.

 

TESTING THE THEORY

• Luisa Grappone, Product Engineering Manager (MSc Aerospace Engineering)
• Chris Colenso, Development Engineer (MEng Mechanical Engineering)
• Peter Marchment, HUNT Co-Founder (MSc Materials Science)
• HUNT BIKE WHEELS (ITS Cycling Limited), West Sussex, United Kingdom

1 - Introduction

HUNT have followed the growth of gravel riding closely, always thinking about how to push the boundaries of performance. Their initial question surrounded, broadly, how to make a gravel rim as fast as possible. Inevitably, with the context of aerodynamic development in road performance, this led to another question: “Does Aero Matter? If not, why not? And if yes, what proportion of overall performance can be attributed to aerodynamic capability?”. With HUNT’s access to the GST WindKanal, they had the opportunity to answer this question, believing that it to be something riders want to (and should) know, and to aid them in answering the first question.

Many household cycling brands have previously suggested that aerodynamics is unimportant in gravel riding and racing, because the addition of a knobbly/wide tyre will ruin the smooth transition of airflow to the rim, negating the benefits of any aerodynamic profile.

Having developed aerodynamic wheels primarily in the disc-brake road performance area, some of the findings yielded during the Limitless Research Project meant that HUNT wanted to explore this further. Taking the increasing speed of gravel riding and racing alone, the evidence for the importance of aerodynamics was clear. Colin Strickland’s 2019 Dirty Kanza success came with an average speed of 20.19mph, and at that speed aerodynamics is very important. (Remember: the component of velocity when calculating drag is always squared).

With this in mind, HUNT wanted to understand whether with the addition of a knobbly, large tyre, the hard work to optimise the aerodynamics of a rim would be rendered useless (because the smooth transition between the tyre and rim was no longer there). 

 

1.1 - Research Purpose

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.

2 - Testing method

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)

 

3 -Results and Analysis

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

at 32 km/h	Av Pwr	ΔP tot or power loss	Average Force Fd [g]	ΔF tot [g] or drag loss	*Required force to overcome the Drag Force	Total time [s] required to cover 321km at 32 km/h	Time loss [s]
ENVE 4.5 AR SES	30.55	0.00	141.12	0.00	3635.685	36226.9618	0.00
HUNT 48 Limitless	30.64	0.09	141.53	0.41	3636.098	36231.0768	4.11
ENVE 3.4 AR SES	32.24	1.69	148.93	7.81	3643.495	36304.7836	77.82
HUNT Non-Aero Gravel	39.58	9.03	182.83	41.71	3677.395	36642.575	415.61

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.

 

4 - Conclusions

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.

 

 

Acknowledgements 

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.