Whilst aerodynamics has always been at the forefront of our product development, a fast wheel is immediately made redundant as soon as you’re forced out of the aerobars or into a more upright position to control the bike in a crosswind. Equally, if riding in a group, poor response to crosswinds can be dangerous to both yourself and riders nearby. Handling performance therefore must also be considered when evaluating a rim profile.
This is of particular relevance for the front wheel. Only the front wheel is free to rotate on its axis (through the handlebars) and will therefore have a far greater impact on bike handling. Furthermore, in most riding positions, a greater proportion of the overall system weight is on the rear wheel.
The Chrono has always been our fastest front wheel option. As part of the evolution in rim design, we wanted to respond to rider feedback regarding not just the speed of the wheel, but also the handling and stability. We set out to develop an all-new front wheel-specific rim profile that would at a minimum maintain the well-established aerodynamics of the existing Chrono, whilst decreasing side force in a crosswind, thereby improving stability.
So that the new profile could be applied to both our rim brake and disc brake wheels, we began by introducing a number of design constraints around rim width at the brake track. Once finalised, we could then apply the same tyre/rim optimisation as on our existing disc brake range.
In partnership with our manufacturing engineers, we used CFD simulations to iterate our rim design, looking for the optimal rim depth to deliver the required performance. The results of the simulations showed that a 77mm rim depth could maintain the aerodynamic performance, with the toroidal bulge (i.e. the point of maximum rim width) moving closer to the edge of the rim to maintain airflow attachment.
The prototype rim was tested at the A2 Wind Tunnel, with the results shown in the chart below.
On a translational drag basis, the new Chrono front wheel tested within 0.1W of the existing model. However, when looking at the side force comparison, the new rim profile really stood out. At lower yaw angles (<10°), the reduction in side force was just under 15%. As the yaw angle increased, so too did the improvement in handling – precisely the point at which a more stable wheel is required. When we applied the same weighted average yaw angle to the data, it showed an average reduction of 15.8%.
For comparison, this puts the new 77mm Chrono front wheel at the mid-point between the prior generation 86mm Chrono front wheel and 56mm Passista front wheel in terms of crosswind performance.
When developing the #thinkwider rim profiles used for the Strade and Ronde wheelsets, we were able to prioritise handling as a key characteristic for the front rim design process. The larger radius of a wider (28mm) tyre allows for airflow to remain attached for longer when running across the side profile of the wheel. If the rim design can be optimised to recapture this airflow, handling performance will be improved. With the scope for far wider rim profiles (versus a rim brake wheelset), the #thinkwider rim shape was developed to deliver stable handling in the windiest of conditions.
The Strade front wheel outperformed the prior generation 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 with 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 (25mm tyre).
The shallower Ronde front wheel improves on this further, with 17% less sideforce than the Grimpeur Disc and 25mm tyre setup. When compared with the same 28mm tyre, the reduction increases to 21%.