Endurance Skating

Training, Racing & LCHF Fueling For Skating And Endurance Sports

Aerodynamics, Power & Pace-Lining

nsim-1998The study of fluid dynamic concerns itself with forces acting on a body as it moves through a liquid or gaseous continuum. Aerodynamics is the sub-area of fluid dynamics looking at form and energy of an object moving through the medium of air.

Aero 101

All good Rocket Scientists know that there are 4 fundamental aerodynamic forces that we must consider: Lift, Weight, Thrust & Drag.

– Lift is the vertical force created when a fluid moves over a solid object – negligible if you’re not talking about an aerofoil, and sadly, as wingless skaters, we are not generating any lift. 🙂

– Weight is the force of gravity acting on your midriff, and in the absence of Lift, is a cost borne by your body’s musculoskeletal system.

– Your Thrust is a product of Engine x Chassis – how much power your are able to put out and how much of that is converted to forward motion via your form and technique.

– Lastly, Drag is the forces of the medium that we are moving through (ie air), continually acting against forward movement, working to slow you down and bring you to a stop. It is this force that we are mostly going to concern ourselves with in more detail.

In a complete vacuum there is no drag at work, hence why an object such as a satellite or meteor can travel in space forever even with no propulsion other than gravitational pull of other celestial bodies. However, for a body moving through a dense medium such as air, continuous energy is required in order to literally push the molecules that form the medium aside in order to maintain forward velocity.

Drag Queen

Drag can be divided into two distinct forms:

Pressure Drag

Pressure drag is caused by an object’s frontal area moving through the air. The bulkier the object, the more air resistance it encounters as a result of more air molecules in front of the object needing to be pushed aside, and also more air molecules moving in to fill the low pressure area immediately behind the object. This is also commonly referred to as Form Drag.

form drag

Form drag is created by displaced air

Friction Drag

Frictional Drag is drag force created by air moving across the surface of an object, and is influenced by the properties of the surface material of the object. Generally speaking, the smoother the surface then the less frictional drag is generated, although this is not perfectly true (the reason why a golf ball is not perfectly smooth).

Friction drag is created by a rough surface as air moves over it

Friction drag is created by a rough surface as air moves over it

The Coefficient Of Drag & Overall Drag Force

Combined together, Pressure Drag and Friction Drag make up what is known as the coefficient of drag, which can be thought of as the overall aerodynamic efficiency of an object, determined by it’s shape and surface properties.

As well as the coefficient of drag, there are other factors which determine the overall Force of Drag acting on an object. These can be expressed by the equation:

drag equation

So in other words: The drag force (Fd) is the product of several factors: density of the fluid (p) x the square of the velocity (v^2), x the Coefficient of Drag (Cd) x frontal area (A).

Density of fluid (air) – You move faster in less a less dense medium. That is why the same power will propel you faster at altitude, and slower in a very dense medium such as water. For skaters, there is either nothing we can do about this if you are solo… or a lot you can do about it if you can find a pack to work with 😉

The square of velocity The Theoretical Square Law states that the resistance a body creates as it moves through a fluid varies with the square of it’s velocity. That’s just physics.

The Coefficient of Drag we already talked about, is a numerical expression of the object’s Form Drag and Friction Drag – it’s aerodynamic efficiency – how streamlined it is and therefore how easily can manage to slip through the air.

coefficient

Drag coefficient of various shapes – all with the same frontal area

Different shapes have different coefficient of drag vectors – note the aerofoil creates only 1/28th of the drag as a flat plate with the same frontal area.. at least according to NASA. Wait a minute, who said “this isn’t Rocket Science?”

Frontal area – regardless of how streamlined a shape is, the absolute size of the object’s face also determines how much air it needs to push aside. Bigger parachutes are more effective, right?

Single skater

Gumbie

Power, More Power!

Intuitively we all know that you must work harder (ie generate more power) in order to go faster, all other things being equal. But when it comes to speed, not all work has the same payoff.

In physics, power is the product of the force x velocity.

Power (P) = Force (F) x Velocity (v)

But we know that the Force in question (Fd, ie aerodynamic drag) is already a product of velocity squared (v^2) from the previous calculation of Drag Force (Fd)… Therefore, Power is the product of velocity cubed (v^3)…

power equation

The implication of this is that a 10% increase in speed comes at a 21% (1.1 x 1.1) increase in drag, which requires a 33% (1.1 x 1.1 x 1.1) increase in power to overcome. So if you are expending 600kcal/hr to hold 20km/hr (2:06 marathon pace) at your aerobic threshold, then an increase to 22km/hr (2hr flat marathon pace) would require an increase to 798kcal/hr and likely be pushing you higher into your anaerobic zones. A 1:45 marathon would need 1037kcal/hr and a 1:30 marathon a whopping 1646kcal/hr.

Very quickly we can see how going faster becomes exponentially harder if we rely on increased muscle power alone… or to put it in Star Trek terms, “The engines cannae take any more, cap’n!”

Bugatti Engine + Prius Body = 290MPH?

Lessons From The Pool

So far it’s a pretty bleak picture for the aspiring speed skater who is trying to go faster by muscle power alone. Thanks to this theoretical square law, the greater the absolute speed, the less effect an increase in power will have in relative terms.

However, as we are biomechanically flexible humans and not mannequins on wheels, rather than trying to overpower resistance with more power, the smart speed skater willl instead seek to minimize the forces of drag acting on him.

  1. Reduce Friction Drag – wearing a skinsuit or other similar garment will lower surface drag and reduce your drag coefficient compared to loose, baggy clothing. Shaved legs? Yeah, it’ll make you faster.
  2. Skate lower – OK, you saw this one coming from a long way away. Bend more at the waist so that your torso is less upright and more parallel to the ground.  However, it’s crucial to point out that we can never just look at one thing in isolation – more waist bend necessitates more knee & ankle bend also – ie, you must adopt an altogether “lower” profile to maintain balance and biomechanical efficiency. By adopting a lower profile we make ourselves faster in 3 ways:
    1. We present a longer body to the direction of travel and reduce our Drag coefficient. Remember the aerofoil is 28 times more aerodynamically efficient than the flat plate?  A long, tapered object is inherently more streamlined than shorter/oblique objects with the same frontal area, creating less turbulence and hence less Pressure Drag behind it.
    2. A lower skating profile with appropriate ankle, knee & waist bend also reduces the frontal area presented as the skater slices through the air. Remember that frontal area (A) is itself an independent factor in the Force of Drag (Fd) calculation.
    3. We also improve biomechanical efficiency – lowering our center of gravity, allowing us to better harness gravity during the “fall” phase of the stride, and increasing stride width due to the increase flexion in lower body, all of which increase the power displacement during skating.

I’ve always thought that speed skaters could learn much from swimming, where they understand the importance of perfect form to minimize drag. Terry Laughlin, write about this very elegantly in the essay”Longer Boats Are Faster”: http://www.breaststroke.info/longboat.htm

Of course, getting lower is much easier said than done – it requires strength and stamina from a very defined set of muscles and joints to hold a good low position. It was that easy to skate lower then we all would do so! Nonetheless, all good skaters know the truism that you can never skate too low!

Lessons From The Petrolheads

It’s no secret that drafting (aka slipstreaming) in many sports allows someone to follow in the low pressure area created behind another competitor while the sucker at the very front of the line has no such luxury and is working the hardest of anyone to overcome the drag caused by the higher air pressure directly in front of them.

However… we must remember the lesson that “Longer Boats Are Faster” within a pack dynamic – two bodies close together moving together in a close harmony present themselves almost as one longer singular body – what does this mean? By virtue of the increased length of two bodies moving in harmony presented almost as a single longer body, the drag coefficient is lowered overall – so while the following body is benefitting greatly from low pressure area created by the body in front, the body in front is also benefitting a little from having their low-pressure wake filled by the presence of the following body.

Nowhere do they know this better than in the world of NASCAR racing.

Utterly distinct from single-seat, open wheel racing such as that seen in F1 where aerodynamics always work to the overall detriment of a following car, in NASCAR two or more cars working together can both travel 15-20mph faster in the draft during races than in (non-draft) qualifying.. or single cars.

NASCAR races are a highly and fluid and evolving game theory scenario for the teams and drivers, where in order to move ahead toward the front of the race a driver must work cooperatively with their competitors, while continually swapping between them from lap to lap.

See? And you thought oval racing was just a bunch of cars brainlessly going round in circles for 500 miles…

Takeaways for Skaters

Back to skating, and the practical tips for what we now know about the effect of aerodynamic drag should be a lot clearer:

  • Work on minimizing your drag and thus your power requirement by improving your form – ie practice the skill of skating. As will all skill-based sports, power and strength are rarely the the best way to improve. The very best skaters not only have a very high level of fitness and technique to deliver the most propulsive power onto the tarmac, but also are most skilful at creating the least disturbance as they pass through the air.
  • Race in a skinsuit or other suitable attire. A 1% advantage is still an advantage.
  • The importance of working together with other skaters in a paceline cannot be overstated. Given what we know know about aero in a pack
    • The front of the line is the worst position
    • The back of the line is the 2nd worst position. As well as having no one behind you to fill the low pressure area you are creating, it’s also certain that you will be left behind with the slower group if the line splits up.
    • High up the line, 2nd or 3rd position is ideal… of course that could quickly turn into the worst position if and when the leader peels off and you’re left at the front 😉
    • The more coordinated the paceline, the easier it is and the faster it can move. Here is a great video of the 1998 NSIM Marathon where the 3 person strong Rollerblade Team (spearheaded by Chad Hedrick) are working together in perfect harmony while the chase pack (led by Eddy Matzgar) in contrast is an uncoordinated mess, unable to get it together with mismatched strides and large gaps constantly developing… even though they have many more skaters they are well beaten and finish way back.
multi skaters

Very much not gumbies

Pacelining is a whole topic in itself, and like with any skill, the only real way to get comfortable and good at it is to practice it so that you get more experience, but suffice to say for now that under race conditions all experienced speed skaters know that they stand the best chance of doing well by maximizing the advantage of cooperative pacelining for as long as possible.

At the top level things get very complicated as the action is characterised by periods of breakaway attacks followed by regroup/rest periods, all with the purpose of breaking up the paceline and dropping the weakest skaters as the race progresses. Again, there is a high degree of game theory in operation, and it is a distinct advantage to be working cooperatively with teammates or other skaters you can ally yourself with.  However, we are now talking advanced race tactics, not aerodynamics. Maybe I’ll delve into that when I’m finally racing with the big boys. 🙂

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This entry was posted on June 16, 2015 by in Endurance, Science, Skating and tagged .

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