The Chrono has always been our fastest front wheel option. As part of the 2019 product range development, we wanted to respond to rider feedback regarding not just the speed of the wheel, but also the handling and stability. After all, a fast wheel is immediately made redundant as soon as you’re forced out of the aerobars to control the bike in a crosswind.
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 alongside our full 2019 product range, with the results shown in the charts 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 2018 model 86mm Chrono front wheel and 56mm Passista front wheel in terms of crosswind performance.
Side force comparison
- Front wheels only were tested
- Each wheel was tested from 0-20 degrees of yaw, at 2.5 degree increments
- Positive yaw angles cover non-drive side (i.e. brake rotor, where relevant, exposed to the wind)
- Each test sweep was conducted twice, with results averaged
- Test wind velocity at 30mph
- The same Schwalbe One 23mm tyre was used throughout, inflated to 100psi (consistent with previous testing)
Note: we did not remove tare (i.e. subtract the drag from the wheel clamp) for two reasons:
- In real-world riding, the wheel will have the fork supporting it
- As the wheel is rotated into the wind at higher yaw angles, one of the clamp posts will become increasingly “hidden” from the wind. Subtracting a simple tare value could therefore be misleading at higher yaw angles
The wheels we tested were:
- Rim brake
- Parcours Chrono 86mm tubeless-ready
- Parcours Chrono 77mm tubeless-ready