Introducing The Next Generation Of Deep Section Aero Rim Brake Wheels

Luisa Grappone, Chris Colenso, Peter Marchment, Tom Marchment

HUNT BIKE WHEELS (ITS Cycling Limited), West Sussex, United Kingdom

Wind tunnel operated by Ernst Pfeiffer

GST Windkanal, Immenstaad, Germany

WARNING: Tyre Brands, models and widths used for testing purposes are not a validation or approval for fitment on a Hunt wheel. We share this data in an open format so you the Rider can see how we have worked through the process to bring the wheels to market. Please always check tyre/rim compatibility with the tyre manufacturer. In addition, always read your wheel's user-guide. Failure to do so could result in catastrophic failure and serious injury or death.

Abstract

When reading about the next innovative HUNT wheelset you might not expect this white paper to be about brand-new aero rim brake wheels. The engineering team at Hunt Bike Wheels exists to design world leading aero performance wheels for all riders. The authors understood that from road racers to time trialists and triathletes there was an opportunity to engineer high performance rim brake specific wheels to serve these riders.

The authors have created three highly aerodynamic rim brake specific wheelsets optimised around a 25c tyre which offers the end user an excellent balance between aerodynamics, comfort and grip. The development takes engineering knowledge gained from HUNT’s world leading disc brake specific 48 Limitless Aero Disc project and transfers it to optimise the aerodynamic performance of more strictly constrained rim brake specific wheels.

This paper introduces the 52, 62 and 82 Carbon Aerodynamicist deep section wheels. The engineering development is discussed, from aerodynamically focussed design to wind tunnel testing against a range of the best deep section competitor wheelsets. The results reveal that the HUNT 52 and 62 challenge the industry’s best wheelsets with the HUNT 82 out-performing all tested competitors.

1. Introduction

HUNT’s recent paper [1] introduced the world leading 48 Limitless Aero Disc concept born from the principles of a wider rim providing better aerodynamics in the global system. This was only possible because of the design clearance brought with a disc brake specific frame. And with the increased uptake of bike manufacturers producing disc brake only bicycles, it is perhaps an unexpected move for HUNT to be introducing rim brake specific wheels.

Being a company of riders HUNT understood that many riders have invested heavily in their equipment on a dream bike or when considering time trialling and triathlon, in a specific bike that is perhaps ridden only a few hundred kilometres a year. It is not unexpected that these are a long-term purchase and are rim brake specific. The authors saw the opportunity to transfer their engineering knowledge gained from the 48 Limitless Aero Disc project to three offerings of aerodynamic rim brake specific wheels, delivering new technology from engineering development to enhance their experience and performance.

HUNT’s latest project aims to provide those riders with wide profile, deep wheelset options that are extremely competitive aerodynamically in the market whilst offering all round riding performance. Being wider than historical rim widths the wheels are optimised for a 25c tyre, ensuring better flow transition between tyre and rim as well as offering well documented benefits to ride comfort, rolling resistance and grip.

1.1 The purpose of the wheels

The HUNT team established a design brief for the project:

Three rim brake specific wheelsets at different depths for mass start Road, TT, and Triathlon racing.
Optimised for the best aerodynamic performance with a 25c tyre.
Use of a hooked rim design to ensure compatibility with tubeless and non-tubeless clincher tyres.
A tyre bed design compliant with the latest ETRTO tubeless standards.

Key dimensions for the wheelset were determined:

The three rim depths settled on were 52, 62 and 82 mm, a range of depths to service the disciplines outlined in the design brief. All are aimed at providing a choice of deep section rims based on their intended use, all with exceptional aerodynamic performance whilst maintaining the handling, stability and low mass required for professional, elite and amateur road racing.
The internal rim width was set at 19mm – providing a stable tyre profile with a wider than standard base whilst still ensuring a safe and secure tyre fit.
The rim profiles were developed to maximise the design space given the geometrical constraints of rim brake framesets.

1.2 Tyre Selection

A common talking point among engineers, industry professionals and riders alike are the various benefits associated with running wider tyres with lower pressures. [2][3] The advantages gained in grip and handling are widely accepted, but recent testing has also shown that wider tyres with lower pressures can also reduce rolling resistance. In certain cases, the market leading performance tubeless tyres offer lower rolling resistance than tubular tyres, which have been traditionally been favoured by professional road racers. [4] Tubeless tyres are also becoming more popular in the professional peloton because of rolling resistance benefits and the fact that a tubeless set up can prevent punctures that might otherwise cost a rider a race victory. [5]

When considering the intended use of the Aerodynamicist 52, 62 and 82 wheels, the authors were aware that typically (at least up to c. 2012) they would be equipped with a 23c or even 21c tyre. In order to achieve the benefits expressed above HUNT decided to optimise for a 25c tyre whilst still ensuring excellent performance should the rider wish to run a 23c or 28c tyre, perhaps more suited to the riding surface. Finally, knowing the benefit of an aerodynamically optimised global system, the final profiles were designed with consideration of the profile the tyre and rim form.

1.3 Design Constraints and Aerodynamic Principles

The 48 Limitless project showed HUNT that in order to achieve optimal aerodynamic performance across a range of yaw angles the rim profile should be wide with a truncated edge to help flow stay attached. This was very achievable with a disc brake specific rim. Rim brake specific bikes offered the challenge of a very constrained design area due to the brake callipers and resulting fork design meaning a profile as wide as that for disc brake specific rims is not achievable.

The solution:

A rim profile engineered to make the most of the available design space, resulting in a wider profile lending itself to improved aerodynamics.
Using aerodynamic principles to create a more blunted spoke bed profile than has been seen before. The benefits of blunted spoke bed areas revolve primarily around the predictable airflow, ensuring the air flow stays attached for as long as possible as it passes over the rim (as opposed to older style V-shaped rims, which would create turbulence in the form of stalls).
A tubeless ready, hooked rim to ensure very secure fitting of the tyre and compatibility with ETRTO standards.

Below are the finalised rim profiles which were brought forward to wind tunnel testing. Each rim is designed with aerodynamic principles using HUNT’s expansive in-house aerodynamic knowledge built from years of research and their successful Limitless project.

HUNT Carbon Aerodynamicist Rim Profile

Figure 1 – Cross sections of the 52, 62, and 82 Carbon Aerodynamicist rim profiles

2. Testing method

Three primary tools are currently used for measuring aerodynamic drag when developing bicycle components:

Wind tunnel testing – widely accepted as the industry standard for testing completed products. It generates reproducible and reliable results and allows testing over a range of wind yaw angles.
GPS based track testing – Uses a GPS locator combined with power data to measure aerodynamic drag. Cannot be used to measure drag at non-zero yaw angles and relies on consistent rider position to measure component performance.
Computational fluid dynamics (CFD) – A computerised virtual wind tunnel is used to analyse the airflow around a computer-generated model of the wheel. CFD has its benefits and limitations, HUNT aims to utilise the benefits of CFD to aid the future development of aerodynamic wheels.

For this project HUNT returned to the industry trusted testing ground, the GST wind tunnel 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 testing facility in the cycling industry.

2.1 Bicycle and component set up

HUNT tested the wheels with the new Continental GP5000 TL. The authors wanted to provide riders with as much detail as to the effect of tyre widths on the aerodynamic performance so that they can make an informed decision based on the other factors as to what rim and tyre combination would be optimal.

Testing was conducted on:

Continental GP5000 23c
Continental GP5000 25c
Continental GP5000 28c

*only 23 & 25c on HUNT 82

Each tyre had the mould flashings removed with sandpaper before testing, and the same individual tyre was used to test all the wheelsets.

2.2 Testing procedure

The bicycle was mounted on a rotating table, fitted with front and rear rollers.

For each run the wheels were driven by the rollers at 45 km/h and air was passed through the tunnel at a constant speed of 45km/h. The turntable was then rotated continuously through yaw angles between -20 ° and +20° to the oncoming airflow.

Figure 2 - Photograph of the wind tunnel test setup

Before each test, tyres were inflated to 100psi and aligned in the wind tunnel.

3. Analysis of results

3.1 Yaw angles

A key factor in aerodynamic performance of bicycle wheels is how they perform not only when travelling straight on into the wind, but their performance when riding into a cross wind. When riding in real world conditions the wind approaches the rider from a given angle, α.

When the vector of the bicycle velocity, Vb, is added to the vector of the wind, w, the rider experiences wind at the yaw angle, β.

YAW Angles

Figure 3 - Diagram showing the relationship between rider velocity, Vb, wind velocity, w, wind angle, α, and yaw angle, β.

When running the test in the wind tunnel, the bike is held stationary so the yaw angle β is simply the angle the bike makes to the oncoming airflow.

3.2 Wind averaged power/wind averaged drag

Methods for making an absolute ranking of the aerodynamic performance of bicycle wheels are an area of debate in the industry, however it is widely accepted that the performance of wheels should be considered at a range of wind yaw angles. To do so quantitatively requires calculation of a weighted average of drag or power based on the relative time a cyclist may experience wind at a particular yaw angle while riding. This process is referred to as calculating a wind averaged power or wind averaged drag.

In order to allow the best comparison of our data with those of other wheel companies yaw angle weightings have been calculated using the ‘ponderation law’ proposed by Mavic [6]. This was produced using a bike mounted sensor on a time trial bike in a variety of locations.

The Mavic distribution is shown below. The 22.5 ° and 25° points have been omitted in our calculation because the wind tunnel turntable allows data collection only up to 20°.

Ponderation Law

Figure 4 - Yaw angle distribution proposed by Mavic, after carrying out measurements on a time trial bike with a bike mounted wind sensor.

4. Testing results

Wind tunnel testing was conducted in September 2019. The following section details the full testing results presented in the tables and charts which are separated by wheel depths for ease of comparison.

4.1 HUNT 52 Carbon Aerodynamicist Wheelset

4.1.1 Tests conducted with Continental GP 5000 23c tyre

HUNT 52 Carbon Aerodynamicist v Competitors

Figure 5 - Drag [g] vs yaw angle [°] from wind tunnel test showing HUNT 52 CA against competitor wheelsets using a Continental GP 5000 23c tyre

Configuration Continental GP 5000 23c	Mavic calc WAD Power [Watt] @45km/h	Power difference [Watt]
Roval CL 50	71.155	0.000
ENVE 4.5 SES	71.927	0.772
HUNT 52 Carbon Aerodynamicist	72.392	1.236
ZIPP 303 NSW	74.553	3.398
DT Swiss ARC 1400 Dicut 48	75.422	4.266

4.1.2 Tests conducted with Continental GP 5000 25c tyre

HUNT 52 Carbon Aerodynamicist v Competitors

Figure 6 - Drag [g] vs yaw angle [°] from wind tunnel test showing HUNT 52 Carbon Aerodynamicist against competitor wheelsets using a Continental GP 5000 25c tyre

Configuration Continental GP 5000 25c	Mavic calc WAD Power [Watt] @45km/h	Power difference [Watt]
Roval CL 50	71.153	0.000
ENVE 4.5 SES	73.074	1.920
HUNT 52 Carbon Aerodynamicist	73.386	2.232
ZIPP 303 NSW	74.548	3.395
DT Swiss ARC 1400 Dicut 48	76.163	5.010

4.2 HUNT 62 Carbon Aerodynamicist Wheelset

4.2.1 Tests conducted with Continental GP 5000 23c tyre

HUNT 62 Carbon Aerodynamicist v Competitors

Figure 8 - Drag [g] vs yaw angle [°] from wind tunnel test showing HUNT 62 Carbon Aerodynamicist against competitor wheelsets using a Continental GP 5000 23c tyre

Configuration Continental GP 5000 23c	Mavic calc WAD Power [Watt] @45km/h	Power difference [Watt]
Roval CLX 64	68.435	0.000
ENVE 5.6 SES	69.890	1.455
HUNT 62 Carbon Aerodynamicist	71.117	2.682
ZIPP 404 NSW	71.399	2.965
DT Swiss ARC 1400 Dicut 62	71.408	2.974

4.2.2 Tests conducted with Continental GP 5000 25c tyre

HUNT 62 Carbon Aerodynamicist v Competitors

Figure 9 - Drag [g] vs yaw angle [°] from wind tunnel test showing HUNT 62 Carbon Aerodynamicist against competitor wheelsets using a Continental GP 5000 25c tyre

Configuration Continental GP 5000 25c	Mavic calc WAD Power [Watt] @45km/h	Power difference [Watt]
Roval CLX 64	67.239	0.000
ENVE 5.6 SES	70.589	3.350
HUNT 62 Carbon Aerodynamicist	70.888	3.649
DT Swiss ARC 1400 Dicut 62	71.396	4.157
ZIPP 404 NSW	72.531	5.291

4.3 HUNT 82 Carbon Aerodynamicist Wheelset

4.3.1 Tests conducted with Continental GP 5000 23c tyre

HUNT 82 Carbon Aerodynamicist v Competitors

Figure 10 - Drag [g] vs yaw angle [°] from wind tunnel test showing HUNT 82 Carbon Aerodynamicist against competitor wheelsets using a Continental GP 5000 23c tyre

Configuration Continental GP 5000 23c	Mavic calc WAD Power [Watt] @45km/h	Power difference [Watt]
HUNT 82 Carbon Aerodynamicist	67.000	0.000
ZIPP 808 NSW	68.728	1.728
ENVE 7.8 SES	68.903	1.902
DT Swiss ARC 1400 Dicut 80	69.339	2.339

4.4 Comparative Testing between HUNT 52, 62 & 82 Aerodynamicist Wheelsets

Reflecting on the above results, HUNT can say that their wheels are very competitive against class-leading competitors with the HUNT 82 Carbon Aerodynamicist performance exceeding those in test.

The authors were keen to present data on the comparisons between tyre and depth combinations believing that more data would really benefit our riders enabling them to make an informed decision as to which wheelset would be most suitable for their desired discipline.

4.4.1 HUNT 52 Carbon Aerodynamicist with Continental GP 5000 23, 25 & 28c tyres

HUNT 52 Carbon Aerodynamicist - 23, 25 & 28c Tyre

Figure 11 - Drag [g] vs yaw angle [°] from wind tunnel test showing HUNT 52 Carbon Aerodynamicist with Continental GP 5000 23, 25 & 28c tyres

Configuration Continental GP 5000	Mavic calc WAD Power [Watt] @45km/h	Power difference [Watt]
HUNT 52 Carbon Aerodynamicist, Conti GP 5000 23c	72.392	0.000
HUNT 52 Carbon Aerodynamicist, Conti GP 5000 25c	73.386	0.994
HUNT 52 Carbon Aerodynamicist, Conti GP 5000 28c	75.696	3.304

4.4.2 HUNT 62 Carbon Aerodynamicist with Continental GP 5000 23, 25 & 28c tyres

HUNT 62 Carbon Aerodynamicist - 23, 25 & 28c Tyre

Figure 12 - Drag [g] vs yaw angle [°] from wind tunnel test showing HUNT 62 Carbon Aerodynamicist with Continental GP 5000 23, 25 & 28c tyres

Configuration Continental GP 5000	Mavic calc WAD Power [Watt] @45km/h	Power difference [Watt]
HUNT 62 Carbon Aerodynamicist, Conti GP 5000 23c	71.117	0.000
HUNT 62 Carbon Aerodynamicist, Conti GP 5000 25c	71.396	0.280
HUNT 62 Carbon Aerodynamicist, Conti GP 5000 28c	72.694	1.578

4.4.3 HUNT 82 Carbon Aerodynamicist with Continental GP 5000 23 & 25c tyres

The HUNT 82 Carbon Aerodynamicist has been tested with 23 and 25c tyres only as for this depth wheels a 28c tyre would be extremely unlikely.

Figure 13 - Drag [g] vs yaw angle [°] from wind tunnel test showing HUNT 82 Carbon Aerodynamicist with Continental GP 5000 23 and 25c tyres

Configuration

Continental GP 5000

Mavic calc WAD Power [Watt] @45km/h

Power difference [Watt]

HUNT 82 Carbon Aerodynamicist, Conti GP 5000 23c

67.000

0.000

HUNT 82 Carbon Aerodynamicist, Conti GP 5000 25c

67.499

0.499

4.4.4 HUNT 52, 62 & 82 Carbon Aerodynamicist with Continental GP 5000 23c tyre

HUNT Carbon Aerodynamicist Range - 23c Tyre

Figure 14 - Drag [g] vs yaw angle [°] from wind tunnel test showing HUNT 52, 62 & 82 with Continental GP 5000 23c tyre

Configuration Continental GP 5000 23	Mavic calc WAD Power [Watt] @45km/h	Power difference [Watt]
HUNT 82	67.000	0.000
HUNT 62	71.117	4.116
HUNT 52	72.392	5.391

4.4.5 HUNT 52, 62 & 82 Carbon Aerodynamicist with Continental GP 5000 25c tyre

HUNT Carbon Aerodynamicist Range - 25c Tyre

Figure 15 - Drag [g] vs yaw angle [°] from wind tunnel test showing HUNT 52, 62 & 82 with Continental GP 5000 25c tyre

Configuration Continental GP 5000 25	Mavic calc WAD Power [Watt] @45km/h	Power difference [Watt]
HUNT 82	67.499	0.000
HUNT 62	71.396	3.897
HUNT 52	73.386	5.887

5. Conclusions

The HUNT Rim Brake Road Series now has three new wide and deep additions utilising all the available geometrical design space to deliver the 52, 62 and 82 Carbon Aerodynamicist wheelsets available in a combination perfect for every road racer, time trialist and athlete out there. The extensive wind tunnel testing discussed in this paper shows that all wheelsets are up there battling with the world leading aerodynamic wheelsets and that the HUNT 82 Carbon Aerodynamicist leads all those tested with a Continental GP 5000 23c tyre.

The development of these rim brake wheelsets has been a result of the knowledge and principles taken from the 48 Limitless project. This showed the particular benefit of a wider than traditional rim profile, where the external width of the rim marginally exceeds the width of the tyre, especially at higher yaw angles which are experienced more frequently by drop bar road racers and riders in real world conditions These principles have been successfully transferred to maximise the aerodynamic performance despite the constraints of a rim brake wheelset.

6. 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 Canyon Bicycles UK for loaning the Aeroad CF SLX used for the wind tunnel testing.

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.

7. References

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Introducing The Next Generation Of Deep Section Aero Rim Brake Specific Wheels: The 52, 62 and 82 Carbon Aerodynamicist Wheelsets