We spoke to Dov at Parcours about the wind tunnel testing they do on their wheels and the results:
We recently spent some time testing our entire wheel range in the A2 Wind Tunnel, where we learned some really interesting things.
1. Increasing rim depth reduces drag
The benefits of a deeper rim actually come as the wind yaw angle increases. On our drag chart, you'll see the deeper Passista and Chrono rims start to really differentiate themselves around the 7.5-10 degree mark.
One point to consider is that the deeper the rim, the more sideforce generated as the yaw angle increases. What was interesting to note was that the increase in rim depth wasn't quite proportional to the increase in measured sideforce, so you actually don't pay quite as much of a penalty for a deep rim as you might expect. Reading the chart below, you'll see that the measured sideforce lines actually tilt slightly downwards at the "magic" 7.5-10 degree yaw mark.
2. A wider, shallow rim shows less drag at zero yaw than a narrower, deeper rim
This one surprised us as it appears to go against #1 above. However, looking deeper into what's happening, it would appear that the rim width is the key driver here - potentially by allowing the tyre to spread wider. Certainly one for further investigation.
3. Rim shape is more important than rim depth
The difference in drag between a standard box rim (24mm) and a low profile aero-optimised rim (38mm) is substantially larger than any jump in the range.
So by switching to the optimised rim shape delivers a greater benefit than increasing depth. Although doing both is clearly where it's really at!
4. A solid disc rear wheel is always faster
Again, referring back to the drag chart shows that the disc wheel is faster than all our other wheels across every tested yaw angle. However, this comes with two caveats.
Firstly, this will come at the cost of increased side force. If you're going to ride your disc, you need to be comfortable on it, in particular staying in your aero position - don't save it for race days only!
Secondly, the benefits will depend on the wind conditions - despite the first point above, the windier the day, the more you'll save.
5. It's not always necessary to tape the valve hole on your disc wheel
This was something we'd always wondered. Does it actually make a difference if you tape over the valve hole on your disc wheel. Well, after sifting through the data, the difference is very small (c.1g of drag or 0.5s over 40km). The good news is, the marginally quicker option was not to tape your valve hole.
We should note that this was tested on the Parcours disc, which has a double-sided valve hole. We simulated a single sided valve hole by covering one side on our disc and here there was a measurable difference. Here, the untaped single-sided hole was noticeably slower (up to 2s over 40km), so you'll need to get your tape out.