Explanation of drag factor

[ilan]

Hello,

I have failed to understand the explanation regarding drag factor. All explanations compare this to gearing on a bicycle, but that does appear to be formally correct. Gearing would be using different sprockets to make the flywheel turn, as shown in Figure 3 of the original Concept 2 rower US Patent 4396188 https://patents.google.com/patent/US4396188A/en

Using different gears would mean modifying the leverage that the user inputs to make the flywheel turn, or in the case of the bicycle to make the rear wheel turn, where only the mechanical transmission has changed. This is comparable to changing the length of the oar on a boat in water. Since transmission losses are apparently only a few percent, there is little difference in energy loss when using different gears, it is a question of user efficiency at different cadences.

On the other hand, drag factor involves modification of frictional forces, so user effort no longer (more or less) directly translates into energy being transferred to the flywheel, but instead to added friction which does not contribute to flywheel energy.

There is therefore a fundamental difference between adding drag and modifying leverage.

Can you please address this issue.

Thanks,

-ilan

[johnlvs2run]

The entrance to the rowing forum is here. <---- Click the link and post your rowing questions there.

[Allan Olesen]

You have asked your question in the forum named "Concept2 BikeErg". That is the place where people write about the BikeErg. Your post is about the rower, not the BikeErg.

[Citroen]

Thread moved to the right place (and cleaned of cruft).

https://www.concept2.com/indoor-rowers/ ... etting-101

[ilan]

Thread moved to the right place (and cleaned of cruft).

https://www.concept2.com/indoor-rowers/ ... etting-101

Ok, the link does not answer my question, in particular, it does not explain how distance is computed.

-ilan

[NavigationHazard]

See http://eodg.atm.ox.ac.uk/user/dudhia/ro ... meter.html

[ilan]

See http://eodg.atm.ox.ac.uk/user/dudhia/ro ... meter.html

Thanks, this answers all my questions. I still find it interesting that this it not easy to find this information.

-ilan

[jamesg]

Drag.
The C2 erg fan pumps air which slows the flywheel between strokes. The Drag Factor quantifies the deceleration torque at any fan speed. What drag gives us is control of flywheel speed when we take the catch. This is handy since we each have an optimum speed/force characteristic that allows maximum power transfer: neither too fast nor too slow.

Distance.
The erg measures the work we do on the handle in each single stroke and calculates average power. Speed is a function of power as to W=kV³, where k=2.8 and V is in m/s. Distance follows immediately.

[ilan]

See http://eodg.atm.ox.ac.uk/user/dudhia/ro ... meter.html

If I understood this correctly, the basic principle can be stated without any explicit formulas as follows:

Constant user input power (e.g., at the chain) corresponds to a constant displayed speed, independent of damper setting or drag factor.

This means that the Watts to speed calculator on the Concept 2 website https://www.concept2.com/indoor-rowers/ ... calculator is actually a definition of how the machine computes speed and therefore distance.

In effect, for any given constant friction (damper setting, drag factor) constant user power input (e.g., to the chain) is in one to one correspondence with constant flywheel rotational speed. The Concept 2 software chooses a nominal friction (damper setting, drag factor) and for any other friction (other damper setting or drag factor) computes a flywheel speed corresponding to the nominal friction.

-ilan

[Ombrax]

The Concept 2 software chooses a nominal friction (damper setting, drag factor) and for any other friction (other damper setting or drag factor) computes a flywheel speed corresponding to the nominal friction.

You have it backwards.

The PM calculates the flywheel speed (and more importantly, the flywheel acceleration and deceleration) based on what the flywheel sensor tells it. Then, based on the deceleration, calculates what the drag must be, and based on the acceleration calculates what the input from the user must have been for a given stroke.

[ilan]

I don't have it completely backward, because the virtual speed and distance computation is done once the user input is estimated.

Assuming that you reserve response only for perceived error, it appears that the rest of my previous post was perceived as correct.

-ilan

The Concept 2 software chooses a nominal friction (damper setting, drag factor) and for any other friction (other damper setting or drag factor) computes a flywheel speed corresponding to the nominal friction.

You have it backwards.

The PM calculates the flywheel speed (and more importantly, the flywheel acceleration and deceleration) based on what the flywheel sensor tells it. Then, based on the deceleration, calculates what the drag must be, and based on the acceleration calculates what the input from the user must have been for a given stroke.

[johnlvs2run]

The C2 erg fan pumps air which slows the flywheel between strokes. The Drag Factor quantifies the deceleration torque at any fan speed. What drag gives us is control of flywheel speed when we take the catch. This is handy since we each have an optimum speed/force characteristic that allows maximum power transfer: neither too fast nor too slow.

Is there a way to calculate the difference in fan speed / RPM at different drag factors?
For example, is a drag factor of 80 approximately double (or triple, etc) the fan speed / RPM of a drag factor of 160?

[jamesg]

Ergdata shows drive speed.

On drag 85 I see drive speed 1.85 m/s; on drag 200, speed 1.45 m/s, for roughly the same force.

The sprocket develops 89 mm, so flywheel speed is around 1000 rpm.

[Allan Olesen]

I don't have it completely backward, because the virtual speed and distance computation is done once the user input is estimated.

You have it half backwards. You think that the calculations goes like this:

flywheel speed, acceleration and drag -> power -> virtual flywheel speed.

It doesn't. It goes like this:

flywheel speed, acceleration and drag -> power -> virtual boat speed.

The flywheel speed is of no interest in itself as an end result. The (rotational) speed and accelerations of the flywheel are just variables used to calculate the power input. None of us know how fast the flywheel spun during our row, and we don't care.

[ilan]

None of us know how fast the flywheel spun during our row, and we don't care.

Have you done a poll of every single person in order to come to this conclusion?

-ilan