Based on the shift in tyre and braking technologies, at the outset of the Strade design project we set out to develop a wheel that is:
- Aerodynamically optimised for a 28mm tyre
- Disc brake-specific
We would use the current Passista Disc (56mm rim depth) wheelset as the design benchmark, with a goal of delivering a wheel that, when fitted with a 28mm tyre, exceeds the aerodynamic performance of a Passista Disc fitted with a 25mm tyre.
In addition, we wanted to benchmark against both the weight and handling of the Passista Disc, to provide comparable performance.
Tyre width optimisation
We began by building a database of real-world tyre width measurements across a range of internal rim widths. To allow the safe running of a 25mm tyre, the internal rim width for the Strade was set at a maximum of 22.5mm. This gave an indication of the required rim width to maintain the "Rule of 105%" for aerodynamics:
Internal rim width: 22.5mm
Implied measured width for a Continental GP5000TL 28mm tyre: 30.3mm
Implied 105% outer rim width: 31.8mm
We also used the depth ratio of the existing Chrono wheelset (77mm front / 86mm rear) to guide the rim depths for the Strade.
Differential rim profiles
As part of our technical partnership with the Sports Engineering department at Nottingham Trent University, we have been conducting an analysis of real-world wind conditions that would impact wheel design. Specifically, this has involved collecting wind angle data from sensors located at both front & rear wheels, across real-world riding conditions as well as controlled (wind tunnel) conditions.
The most interesting finding of this study to date has been the difference in observed wind conditions between front and rear wheels. The average yaw angle at the front wheel is consistently higher than at the rear. Initial suggestions point to the airflow at the rear wheel being disturbed by the front of the bike and rider interaction (predominantly legs, feet and pedals).
In order to maximise the aerodynamics of the front wheel (versus rear), it would need to perform better at higher observed yaw angles, whereas the rear would need to be optimised for lower yaw conditions. Prior analysis has shown that a more “blunt” U-shaped rim is more suited to higher yaw, whilst a “sharper” V-shaped rim is better at lower yaw. In addition, the crosswind performance at the rear wheel is significantly less important given the wheel is not free to move on its axis for steering.
Following an iterative CFD design process, 4 prototype rims were moulded for final testing and validation in the wind tunnel:
Parcours Strade prototype front wheel A (49mm depth / 32.0mm width)
Parcours Strade prototype front wheel B (50mm depth / 31.5mm width)
Parcours Strade prototype rear wheel A (54mm / 30.5mm width)
Parcours Strade prototype rear wheel B (54mm / 31.8mm width)
Test One: Aerodynamic performance
Learn more about how to interpret a drag chart here
Prototypes were tested (and benchmarked against existing Passista Disc wheelset) at the A2 Wind Tunnel in Mooresville, North Carolina in October 2019.
Time savings (Prototype A)
Learn more about how time savings are calculated here
Note: all time savings are taken versus the baseline Fulcrum Racing 5 wheelset
Prototype A outperformed the Passista Disc with both a 25mm and 28mm tyre. The most exciting result was that the Prototype wheelset and wider tyre actually outperformed the existing design even with a narrower tyre, meaning a rider can switch to the 28mm tyre, whilst still reducing their overall drag.
Test Two: Handling performance
Having also set out to benchmark crosswind performance against the existing Passista Disc, we next compared the sideforce generated at yaw.
Again, the Strade front wheel outperformed the Passista Disc by a wide margin, showing a 15% reduction in sideforce when compared to the Passista Disc fitted with the same 28mm tyre, and a 12% reduction with the Passista Disc fitted to a narrower 25mm tyre. In fact, the new Strade front wheel is so stable that it produces over 3% less sideforce than we measured in a previous test of the Grimpeur Disc.
The production version of the Strade wheelset (based on Prototype A) has been shown to outperform the existing Passista Disc in all respects.
Wind tunnel data shows that the Strade wheelset, fitted with a 28mm tyre, represents the new benchmark for a mid-depth disc brake wheelset. This setup has been proven to be faster than our existing Passista Disc wheelset even if fitted with a narrower 25mm tyre. It will also provide more stable handling than the shallower Grimpeur Disc (40mm) wheelset.
Having taken the insight gained to date from our collaboration with Nottingham Trent University on observed yaw angles and combined this with our existing development of rim profiles for the new Chrono front wheel, we now have a proven front/rear rim profile relationship which can be applied to future wheel development.
Further analysis of the observed yaw angle data will ultimately also enable us to apply an updated average yaw angle weighting to front and rear wheel test data.