My understanding is that it has to do with form drag – aka pressure drag – which results in vortices forming in the “separation region” directly behind an airfoil. Or in this case, a rider. Essentially, the swirling of air behind the rider is turbulent – which is why a hoodie might flop all over the place – and that causes energy to be lost.
By providing a smooth surface for air to “cling” to, where it would otherwise form vortices in the separation region, should reduce form drag, although it will cause additional induced drag (aka friction with the new surface). But induced drag scales with speed and at cycling speeds, that’s less a problem than it would be at airplane speeds.
Here’s one site that has info on skirts (under the trailer to reduce side flow) and tails at the back. There’s an image of a wind tunnel view that gives a good idea what it helps with. Those vortexes at the back of a flat trailer suck a lot of power.
I’d much rather have my shirt going all floppy flappy in between the vortices and keeping my back cool than shaving 2 seconds off my relaxation cruises.
I know some people who would jump on this 100% and I should probably send them the article…
My understanding is that it has to do with form drag – aka pressure drag – which results in vortices forming in the “separation region” directly behind an airfoil. Or in this case, a rider. Essentially, the swirling of air behind the rider is turbulent – which is why a hoodie might flop all over the place – and that causes energy to be lost.
This video on Nebula (and YT as well) describes pressure drag at about the 02m30s mark for a sphere. But this graphic from Skybrary also shows the problem:
By providing a smooth surface for air to “cling” to, where it would otherwise form vortices in the separation region, should reduce form drag, although it will cause additional induced drag (aka friction with the new surface). But induced drag scales with speed and at cycling speeds, that’s less a problem than it would be at airplane speeds.
A related drag-reducing device has been used for semi-truck trailers, and those have really been proven to reduce fuel consumption. Although the Wikipedia article does not describe in detail the aerodynamic principles at play.
So would this also reduce the benefits of “catching drift” behind another rider?
Here’s one site that has info on skirts (under the trailer to reduce side flow) and tails at the back. There’s an image of a wind tunnel view that gives a good idea what it helps with. Those vortexes at the back of a flat trailer suck a lot of power.
I’d much rather have my shirt going all floppy flappy in between the vortices and keeping my back cool than shaving 2 seconds off my relaxation cruises.
I know some people who would jump on this 100% and I should probably send them the article…
My genuine thanks for taking the time to craft your lovely reply.