HollowGram Wheels D.R.A.F.T Technology D.R.A.F.T Technology D.R.A.F.T Technology

D.R.A.F.T. and HollowGram Wheels

Aerodynamic performance is at the core of the HollowGram road wheel lineup. Starting with the HollowGram R64 wheel, the R64 models are deep, fast, and beg to slice through the wind and keep the miles coming. R50 series wheels combine the aerodynamic prowess of the R64, but in a lighter package that aims to be a terrific all-around race wheel. R45 brings aerodynamic expertise to a lower price point, letting riders Rise to Speed easier than ever before.

The aerodynamics of road wheels are complex and require a nuanced understanding of various fluid mechanics principles. That’s why we apply D.R.A.F.T. (or Drag Reducing Aerodynamic Flow Technology) philosophy to all HollowGram roads wheels. D.R.A.F.T. is HollowGram’s approach to aerodynamic design and delivers optimal aerodynamic performance across the range of on-road conditions that riders will experience. By combining the variation of aerodynamic drag as a function of yaw angle and environmental wind data, we ensure that every wheel in the HollowGram road lineup is delivering maximum performance on every ride.

Rims and Tires

The HollowGram R64, 50 and 45 road wheels have a 21mm internal width and 32mm external width, to deliver the best performance with the most common road tire sizes. This is crucial because the tire is a key element in wheel aerodynamics. Especially in the case of the front wheel, the tire is the first part of the bike to meet the air, so the interaction between the tire and the rim drives the movement of air over the rim, which impacts how much drag the wheel creates. In aerodynamic terms, a bicycle tire is typically not an efficient shape, and so the rim needs to work with the tire to minimize separation and maintain flow attachment. For a rim to lower drag, it needs to encourage the air to reattach to the rim after separating from the tire and then manage its movement over the surface. The combination of width and shape makes HollowGram rims less sensitive to both tire construction and tire size; with minimal aerodynamic penalties for using larger width road tires.

What is D.R.A.F.T?

D.R.A.F.T. is the name given to HollowGram’s encompassing aerodynamic development philosophy. At the core of the work is an in-house developed and peer-reviewed process for evaluating aerodynamic performance in on-road wind conditions, called Yaw Weighted Drag. This ensures that designs are optimized to maximize riders’ speed in real world wind conditions.

What is yaw weighted drag and why is it important?

When developing and testing products, we evaluate drag at a range of yaw angles (the angle between the direction of travel and the effective wind vector). Figure 1 (below) shows a plot of drag against yaw angle. This is a typical output from wind tunnel testing. . Note the significant variation in drag as a function of yaw angle. In this plot, the drag of the wheel decreases as yaw angle increases up to an inflection point where it rapidly increases. This behavior is typical for a high-performance wheel.

The variation in drag with yaw angle raises the question; which yaw angle(s) should be used to evaluate on road performance? This is particularly important in the case when comparing two configurations with intersecting curves. (e.g. Figure 2 below)

Take as an example the HollowGram R50 vs. the Roval Rapide CLX, tested with the same tires. In this case, both wheels can claim to have the lowest drag at different points across the yaw spectrum.

Which yaw angles are most important?

If yaw angles experienced by a rider were evenly distributed, then a simple numeric average of these results would yield an appropriate average drag number. However, in practice, yaw angles are not equally distributed, and so we need to account for this effect.

Keep in mind that yaw angle and wind angle are not the same. When a rider is in motion they have a forward component of velocity, which combined with the atmospheric wind, creates the resultant yaw angle. Thus, yaw angle is a function of wind speed, wind direction and road speed. We used statistical functions of wind speed and direction, combined with the geometric effect of forward motion to determine an analytical model of yaw angle distribution. The result is a bell curve centered on a yaw angle of 0 degrees (see figure 3 below).

Using this weighting function, we transform the wind tunnel results to create a plot in which the magnitude of drag is scaled proportionally to the probability of a rider seeing that yaw angle on the road. Yaw Weighted Drag is then calculated by taking a weighted average of the weighted drag plot.

When seen through the context of yaw weighted drag, we see that there is little separating the aerodynamic performance of these two wheels. This method provides two clear advantages for analysis. Firstly, it simplifies the analysis process by combining large subsets of data into a single value for each configuration that encompasses both drag as a function of yaw angle and the likelihood of those yaw angles on the road. Secondly, it facilitates the objective comparison of on road performance without cherry-picking data from wind tunnel results.

We use yaw weighted drag when developing all of our performance products to ensure that they are fully optimized across the range of riding conditions our riders will experience.

For full details on the derivation of yaw weighted drag, read the paper here:

doi:10.3390/proceedings2060211

How does road speed and wind speed affect this approach?

This case study is applied using 40 km/h for road speed and 11 km/h for wind speed. But the model is not rigid and can be applied to different roads and wind speeds. Increasing the road speed or decreasing the wind speed will tighten the yaw distribution, increasing the emphasis on the low yaw angles. Decreasing the road speed or increasing the wind speed will flatten the yaw distribution, decreasing the emphasis on low yaw angles. We use 40 km/h road speed as it is representative of average racing speed and is a speed that is achievable for amateurs in shorter bursts. As such it is a great representation of fast road riding. The 11 km/h value for average wind speed is derived from experimental data and mirrors the SAE in their similar approach for evaluating road vehicles.