Drag factor, damper, and speedring, Oh My!

[shu246]

I've been rolling this whole 'drag factor' and 'damper setting' thing around in my head the last couple of days, looking for a clear way to explain it all to myself. Best way I found is to approach the subject from both ends; damper setting and then drag factor. Perhaps others will find the following useful.

What follows concerns just from the handle and chain forward. Energy accounting for moving the rower's body (and seat and bungee) is a whole other subject.

DAMPER SETTING. JUST ONE PART OF HOW THE STROKE FEELS.

As the handle and chain are pulled on the power stroke the flywheel turns. The flywheel passes some of that energy to the air and keeps some (kinetic energy) as momentum. On the recovery stroke the flywheel slows as it continues to pass energy to the air.

If somehow less energy is passed to the air, the flywheel will be easier to spin up on the power stroke and will retain more of its energy during the recovery stroke. The row will feel softer and smoother.

When air flow into the flywheel is restricted, that is just what happens. The Concept2 has a centrifugal impeller; air enters at the right side of the flywheel and is thrown out toward the periphery. The damper on the right side of the fan cage restricts air flow into the fan.

Another major contributor to how the stroke feels is the diameter (or tooth count) of the drive gear. If a smaller drive gear is used the flywheel makes more revolutions per foot of chain pulled. The pull will feel harder. With a larger drive gear, fewer flywheel revolutions per foot of chain, and a softer feeling pull.

The Concept2 Model B has two drive gears: 13 tooth and 15 tooth. The flywheel cage is made of made of wire mesh, like a bird cage. For the Model C and subsequent, Concept2 used to a 14t drive gear. The cage is much more restictive to air flow. The damper ring also appears to be larger. If true, this would give the newer models a wider range of adjustment to the air flow.

Compared to the Model B, the Models C, D, and E have a lighter air drag component to the pull. The flywheel inertia component of the pull is somewhere between the two drive gears of the Model B.

(My opinion, the pull with the Model B 13t gear is just too heavy. Suitable maybe for strength training, building muscle bulk. Probably require more frequent replacement of drive chain. Not suitable for endurance or weight loss training.)

Energy storage in the flywheel and energy loss to air drag are the two most important places energy goes when the handle and chain are pulled.

Some energy is lost to friction in the flywheel and seat bearings, etc. Small by comparison.

DRAG FACTOR. SOMETHING THE PERFORMANCE MONITOR NEEDS TO CALCULATE WORK AND POWER.

As the handle and chain are pulled on the power stroke the flywheel turns. The flywheel passes some of that energy to the air and keeps some (kinetic energy) as momentum. On the recovery stroke the flywheel slows as it continues to pass energy to the air.

The kinetic energy of the flywheel depends on the mass distribution (moment of inertia) and the speed (angular velocity) of the wheel. Concept2, which designed the wheel, certainly knows the mass distribution.

There are three small magnets imbedded in the left side of the Model B flywheel (presumedly other models also) and a magnetic sensor on the left side of the flywheel cage. The sensor is triggered each time a magnet passes. With the PM clock marking time, angular velocity of the flywheel is known.

Thus the flywheel inertia component of the power stroke is known. But what of the energy passed through the flywheel and lost to drag?

Mostly it is loss to air drag and depends on air density (temperature, pressure, humidity) and flow rate - factors too numerous and difficult to account for.

But in a steady cruise each power stroke applies just enough energy to to replace energy lost during the preceeding recovery stroke. Since no new energy is applied during the recovery any change in flywheel speed can be counted as drag loss. By noting velocity loss during recovery the PM calculates the energy loss to drag (not just air drag but flywheel bearing drag also) and has a good estimate for the drag loss it could not sense directly during the preceeding power stroke.

Although the rower adds no energy during the recovery, the PM needs information it can gather only during that time to properly calculate the total energy of the power stroke.

In the calculations the information appears as a single number which accounts for air mass flow and other drag components. The PM displays this as 'drag factor.'

Drag factor does not depend on the force of the power stroke, nor on flywheel gearing. It cannot; it is measured during recovery when no power is applied.

To test this, go to your machine and read the drag factor after a few very light pulls of the handle. Reset the machine and read again after a few of your most forceful pulls. There will be very little difference.

Now adjust the damper and read drag factor for soft and hard pulls.

TO CONCLUDE

Drag factor and damper setting are intimately related, but the relationship cannot be easily described. Indeed, drag factor is arrived at by mechanical engineering formula to represent the unknowables (mostly the mass flow rate determined by air density and damper setting) when everything else in the formula is known.

A few words about the SPEEDRING. With its 13t and 15t drive gears the Model B can never be made to feel exactly like a 14t gear C, D, or E Model. A better correspondence however can be had by lowering the drag factor of the Model B. This is easily done by blocking off the right side of the flywheel cage outboard of the damper. A ring can be cut from plastic or other material; inside diameter 14 inches, outside diameter 18 inches, and fixed in place with wire ties. Such may be available from Concept2 as part #372.

FOLLOW ON READING

http://www.atm.ox.ac.uk/rowing/physics/ergometer.html