CROSSFIT = PRODUCING POWER
Great Read! Enjoy!
Chronicles of Science by Chris Slaughter (CS)^3: Issue 1
So one of the big criteria of CrossFit is measurable, observable, and repeatable results. That is why we time workouts. That is why we record what weights we use. Both time and weight are measurable, observable, and repeatable to a very small degree of error. When time goes down, we have gotten faster, when weight goes up, we have gotten stronger. Plus we feel awesome when that happens! These criteria are very closely related to the scientific method, and so we’re going to explore that a little bit more, and my hope is that some of the reasons behind the movements of CrossFit will have new light shed on them from a physical basis. Who’s ready to CrossFit NERD OUT!? This post is long but will read quick.
Today we’re going to break down the rower (also known to some as the portal to hell) and explain the damper setting and how it works. To do this we’ll break the system of rowing into two parts: #1 – You, #2 – The machine. If you just want the conclusion scroll down, important parts are in bold!
Lets define a couple of physics terms:
Mass, Velocity, Acceleration, Force, Work, Power, and Kinetic Energy. See the bottom od post for definitions. (math? I thought we were working out!).
Whew that’s done! But how do these apply to us on the rower? When we extend our legs, hips, and pull with our back and arms, we are generating a force. This force is transferred to the handle, through the chain, and to the machine. By generating this force we cause the mass of the flywheel (that whizzing thing in the machine) to accelerate. Or in other words an increase in the flywheel’s velocity (rate of spinning). The harder and faster we pull, the bigger increase in the velocity of the flywheel we will make. Now we’re going to translate these physics terms into a spinning object (which is basically adding the radius of the spinning object into the equation). See below for definitions.
Ok now we’ll translate what the body does to the rower.
Lets go back to the force we generate: F = m x a
And starts substituting in for how this translates to the flywheel. Simplify and Torque becomes equal to the moment of inertia (the rotating mass of the flywheel times the angular acceleration). T = I x b
Point 1: The force we generate becomes torque and speeds up the flywheel.
Well now lets look back at that equation for power. P = T x w
Point 2: The more torque (force we apply) or the faster the flywheel spins (angular velocity) the more power we generate.
WHOA did I just blow your mind? No not really, makes sense right. But there are two factors of this equation, the force applied, and the speed of the flywheel. If you halve one and double the other, you end up at the same power (keep this in mind for the damper setting). But what does it all mean batman?
We’re almost there hang tight! This is the answer to the question of “What do I put the damper at?” Lets look at the kinetic energy of the flywheel, and the power it dissipates (loses) due to the air it moves. Power equals Energy over time.
Point 3: The last piece of the puzzle is the mass of air the flywheel moves is in direct relation to its angular velocity, multiplied by the time it is spinning, multiplied by the “drag factor”(z) – or the damper setting! m(air) = w x z x t
Put these together and viola! The power becomes.
Point 4: When you change the damper setting (z) you alter the relationship between power and flywheel speed. The damper setting has nothing to do with how much power you can generate.
The power dissipated by the machine must be equal to the power put into the machine (Law of Conservation of Energy). This means that the power we put into the rower is not dependent on the damper setting! It is directly related to the cube of the velocity of the flywheel, and the amount of drag we set it at. The power we put in correlates directly to speed, but we now have two ways to generate the power.
Point 5: The Concept 2 rower is constantly measuring only 1 thing: the velocity of the spinning flywheel. From that it can find how quickly it slows down and can calculate the “drag factor” (z). It knows the resistance of the mechanical systems (y). From there it knows the power, and in another derivation of exactly the same factors; the Concept 2 calculates speed of the rower (in meters per second).
As we come forward on the rower in the recovery phase, the flywheel slows down, and the difference between the maximum and minimum speeds of the flywheel is the difference in power of the flywheel, and therefore the amount of power we put into the machine, and therefore the speed and distance we traveled on the rower.
Point 6: The Concept 2 constantly takes measurements, and measures the change in velocity of the flywheel during the deceleration (forward movement or recovery) of the flywheel to find power input, which is directly related to distance.
- The body generates power (force per time) which we transfer into the rower.
- It is the goal in CrossFit to increase our power output (more force in less time).
- We can transfer that power to the machine via one stroke in a given time. If we halve the time it takes us to do one stroke, we only have to generate half the force to meet the same power.
- The higher the damper setting; the slower maximum speed of the flywheel, and the faster it will decelerate. The lower the damper setting; the higher the maximum speed of the flywheel, and the slower it will decelerate.
- The rower measures power based only on the design of the flywheel’s inertia factor(y), the drag due to air, and the angular speed of the flywheel. Power = 1/2y x z x w(cubed).
- The fastest way to row is dependent on how much force your body can generate. If your capable of high force (ex: a 400 lb dead lift) you might row faster at a higher damper setting, if your capable of half that force (ex: a 200 lb dead lift) you will probably row faster at a lower damper setting.
Definition Relating to the Body
- Mass – it’s your mass. Duh! (kilograms)
- Velocity is the distance traveled in a given time: v = d / t.
- Acceleration is the rate of change in velocity. a = change in v/change in t.
- Force (gravity, friction, or what humans generate to move) = to mass (kilograms) times acceleration. F = m x a.
- Work (the force done over a distance = Force times distance. W = F x d.
- Power (amount of work in a given time) = Work divided by time P = W / t.
- Kinetic Energy (the energy of a moving object) = half of it’s mass times velocity squared. E = 1/2m x v (squared).
Definition Relating to the Flywheel of the Rower
- Mass relates to moment of inertia (this has to do with spinning objects) I = m x r (squared).
- Velocity becomes angular velocity (amount of spin divided by time): w = (angular velocity) / time.
- Acceleration becomes angular acceleration. a = change in v/change in t.
- Force relates to torque. (Force times radius of the flywheel) T = F x r.
- Work becomes Torque times the angle the flywheel spins).
- Power is still Work divided by time, but becomes Torque times angular velocity.
- Kinetic Energy (the energy of a moving object) now becomes in relation to a spinning object with an inertia factor y (gears, grease, and bearings) E = 1/2y x m x w(squared).