It’s July, that means hours streaming the Tour de France through some pirated grainy feeds…AND a lot of bogus Aerodynamic information, including an ESPECIALLY ugly looking Giro helmet:
Obviously drafting a car gives you a huge advantage, but a car drafting you? It turns out you can get a SIGNIFICANT advantage from a follow vehicle. Current UCI regulation is 10 meter minimum distance.
Bert Blocken has a great course on coursera.com about mostly cycling and some city/urban aerodynamics. You can check it out here.
He also recently published a research paper about the effect of follow cars in a TT situation.
When a body is moving subsonically through a fluid, an area of high pressure is created in front of the object as the air rushes to get out of the way of the moving body. Behind a body is an area of low pressure, this is basically the mechanism that creates pressure drag.
So how does a car help a rider from behind? The high pressure area in front of a vehicle (or even other rider) for that matter, acts to fill the low pressure region behind a cyclist. This acts almost as a fairing, increasing the pressure on the back side of the cyclist, reducing drag. A picture from the above mentioned paper gives a better example.
The high (red) pressure area acts to fill the area behind the cyclist (which is pretty clear to see when the car gets real close). The effect is even noticeable at smaller distances:As you can see here, the benefits get pretty drastic below 8 meters (26 feet). I took a quick look at the Tour de France prologue using a simple F=1/2 Cd A rho v^2 calculation for force then converting to power. Based on this calculation, Dennis put out 492 watts (neglecting drive-train/elevation losses) for almost 15 minutes over 13.8 km’s.
A car at 10 meters (UCI legal limit) give’s you a 0.23% advantage. Let’s say his car was only 2 meters closer, giving him double the car draft advantage of 0.45% (Thus a 0.22% difference). I’ll factor this in as a reduction of the CdA term and keep the wattage for Dennis constant for the calculation…
Given this ONLY 0.22% reduction in drag, Dennis would’ve saved a whopping 10 SECONDS!!!!!!
In a short TT decided by only 5 seconds across the top 3 riders…any team manager in their right mind would get FINED TO DEATH in order to drive as close to their rider as possible.
This isn’t even taking into account video moto’s…which somehow are above UCI rule and can drive as close to riders as they please (in front or behind).
So pay attention at the next short TT in this year’s tour to see if each respective DS is doing their jobs correctly (rider safety be damned!!!).
-This analysis is quick and dirty, CdA values are VERY approximated
-TOTALLY neglecting acceleration
-Drive train losses neglected
-However the actual wattage produced by Dennis may be different, the benefit should be roughly the same since we are not looking at that side of the speed equation