Why "Skin is Slow"

 

Wow! You can't un-see that, huh? Our mantra here at ERO is something many of you've heard around the internet forums, "Skin is Slow." Well, it is, so let's find out why.

 

"Skin is Slow" is a simple phrase rings very true - if you cover your skin, you'll reduce your aerodynamic drag and go faster. The genesis of that phrase, or at least our use of it, goes back to November 2012 and the initial introduction of the Alphamantis Track Aero System. Our test rider had completed his baseline run in a sleeveless tri suit, and had begun his next run in a short sleeved skin suit. When the CdA appeared on the screen, everyone was amazed at the reduction in drag. Everyone, except one person - Paul Harder from Trek. Unimpressed and without emotion, he simply said, "Yeah, skin is slow." It stuck, and sleeved tri suits have been a growing trend in Triathlon ever since. For Time Trials on the road, skin suits are nothing new, but the attention the aerodynamic benefits they bring has been magnified as well, so the R&D going into these suits has risen dramatically over the last few years.

Why is skin slow? Why is it a poor aerodynamic surface? The bottom line is this: skin surface is too smooth. On a blunt shape — a shape not optimised for efficient aerodynamics — the surface of an upper arm, for example, will create a smooth stream of air close to the surface. This smooth stream is not able to “make the turn” when going past the arm and will detach from the surface. This will create a large vortex region of negative pressure behind the arm. This is called Pressure Drag. Pressure Drag is bad. (see photo below)

On a rougher blunt shape, the boundary layer is turbulent, which means that it is better mixed, almost like a foamy layer of air. This turbulent layer adheres better to the surface and detaches much later than the smooth layer, thereby creating a smaller vortex behind. This reduces the drag from the negative pressure region behind the arm.

However, while skin may be slow, we have found that WRINKLES are often worse.  So before you go buy your aero, long – or short – sleeveed suit, make sure it fits tight, with little or no wrinkles when you’re in race position.

Thanks to Andy Froncioni of Alphamantis Technologies for his contribution to this article. He's my Aero Hero.