In the Wind Tunnel
We tested all five sets of wheels on a complete bike, in an attempt to create the same environment in which they are expected to perform in real life. While this may seem like an obvious choice, it is in fact much less common than one would expect. Why? Just as rims and tires interact in unique and often unpredictable ways, the relationship between wheels and frames is also highly variable. That means that the fastest wheels in one frame may not be the fastest in another.
That said, a wheel that tests very well without any frame at all may not, in fact, be fastest in any frame. We feel that our method, while not absolute, provides enhanced insight into how these wheels perform in the real world. Specifically, our protocol places additional emphasis on the aerodynamics of the front wheel, just as the real world does.
We tested all five sets across a full yaw angle sweep from -20° to 20°. The negative yaw angles represent wind hitting the drive side of the bike, and positive yaw the non-drive side. As the data show, the various components on the drive side can have an impact on wheel aerodynamics, so we felt it was important to gain drag figures from both positive and negative yaw angles.
Rotational inertia is the rotational equivalent of mass. Because this calculation offers insight into the amount of energy required to accelerate (through pedaling) and decelerate (through braking), a wheel with a lower inertia will perform better on both fronts. We can measure the mass of the wheel easily enough, but it is not necessarily the case that the lightest wheel will have the lowest moment of inertia, or vice versa. So, we used a torsional pendulum setup created by Lennard Zinn to measure a wheel’s rotational inertia.
Just as subtle changes in the shape of each rim’s spoke bed proved vital to both overall aerodynamics and crosswind performance, tire selection can have a dramatic impact on the same. Vittoria provided us with 21, 23 and 25mm Corsa CX tires to help us find the best combination for each wheel. Rim and tire are a system, and one impacts the other. In fact, we saw changes of as much as 97 grams of drag, or nearly 10 watts at 30mph, with the same wheel simply by swapping from a 21 to 25mm tire.
On average, a 21mm was the fastest option for all the wheels tested. But the impact of tire size was not consistent across all of them. On the Psimets, for example, tire size had very little effect — less than 10 grams difference at most yaw angles. On the Bontragers, a 21mm tire saw a sharp drop down to 953 grams of drag at -10° yaw, while a 25mm tire saw a sharp spike up to 1050 grams. In other words, the wheels were stalling out at -10° with a 25mm tire, but not with a 21mm tire. The 23mm tire was right in the middle.
Of course, numerous other factors need to be taken into account regarding tire selection. The rolling resistance associated with a 25mm tire will usually be lower than that of a 21mm, and both 23mm and 25mm will corner and handle much better than their skinny sibling. Racing a technical criterium on a 21mm tire for the sake of a few grams of drag isn’t a great idea, but throwing 21mm tires on for a time trial will likely save a few seconds. As with most equipment selection, there is no right answer for every situation.
More Important Than Weight — Rotational Inertia
In time trials, the moment of inertia may matter very little, due to the steady speeds and minimal braking involved. Heavy wheels with lots of mass around their edges will take more energy to get started, but once in motion, they will not require any more energy to keep moving than a lower-inertia wheel, in the absence of frictional forces. However, wind is a big frictional force on a bike, and if a wheel is more aerodynamic at the expense of higher rotational inertia and higher mass, you still come out ahead if there is no braking to be done on the course. If there are lots of turns that require hard braking, though, the mass and rotational inertia both matter considerably more, and at a certain point, a less aero wheel with lower rotational inertia will outperform a more aero one with higher rotational inertia. When climbing, a rider benefits from lower wheel mass, and as long as he or she does not use the brakes, it does not matter how the mass is distributed (as long as it is uniform) around the wheel.
— LENNARD ZINN
Wide rims are here to stay, and we couldn’t be happier about it. For those with the funds, deep carbon wheels can now be seriously considered as everyday wheels — perhaps not the tubulars in these pages, but the same technology applies to clincher options from most of the top brands. Crosswinds are not the issue they once were, and impact resistance has increased dramatically. Versatility is the hallmark of the latest generation.
Enve System 6.7: 81 points
Bontrager Aeolus 5 D3: 78 points
Hed Stinger 6 Flamme Rouge: 75 points
Rolf Prima TDF 60: 69 points
Psimet/White Industries Custom: 62 points