Rankings

[idearle60]

I find keeping my logbook up to date very motivational, and comparing myself to others in the rankings even more so, but I have a query about how accurate the rankings can be. Thinking specifically about the bike erg, in order to rank any workout time and distance have to be entered, but NOT the level of resistance used during the workout. There will be a lot of difference in time or distance achieved using light resistance as against heavy resistance (you can't ride up Alpe d'Huez as fast as along a canal bank). So, without knowing and specifying the level of resistance used, how much credibility can the rankings have?

[jamesg]

you can't ride up Alpe d'Huez as fast as along a canal bank

The C2 machines all measure power delivered, not speed. This is estimated without reference to gravity.

C2 machines consider only hydro- or aerodynamic drag loads, using a Power-Speed cube law. But when climbing hills, clearly gravity dominates.

A 70kg all up cyclist at 10km/h on the flat may not need to pedal at all. But on a 15% slope if at 10km/h he or she needs to deliver 10000x70x0.15x 9.81 / 3600W = 286W.

If you want to estimate your climb rates for any given Power delivered, you could use:

(Speed in m/s) x (Slope as a fraction of 1) x 9.81 x (Weight in kg) = Watts.

[MPx]

but NOT the level of resistance used during the workout.

There is no "resistence" on any of the C2 ergs. What you are probably referring to is the damper lever position which alters the drag factor (between about 90 and 220 on a clean new machine). The DF simply changes how heavy it feels. The erg measures the watts you put in and callibrates itself for any DF so all scores can be directly compared. Some prefer a light feel with very high cadence others something tougher but lower cadence. Doesn't matter which you choose, it records the watts you put in.

[Cyclist2]

There will be a lot of difference in time or distance achieved using light resistance as against heavy resistance (you can't ride up Alpe d'Huez as fast as along a canal bank).

To say what MPx says in another way; Think about what you refer to as the "resistance", which is actually the drag factor, as gears on that bike. You can ride up Alpe d'Huez in a low gear (high cadence) or high gear (low cadence) but it takes the same energy in each case. It's just a matter of what cadence you feel best at. How FAST you ride up Alpe d'Huez is purely up to you (the power you put into it), not the machine settings.

[gvcormac]

C2 devotees always quibble about the distinction between "drag factor" and "resistance." I think it is a distinction without a difference.

What you need to know is that the PM5 (and all previous PMs) measure drag factor by how quickly the flywheel spins down when you aren't applying force to it. Based on the drag factor and velocity, it calculates power. "Speed" is just a formula based on power. It is not the speed of the flywheel -- it is an estimate of how fast you'd be going on water/snow/pavement for that power output.

With the rower or the skiErg, it is easy to measure drag factor between power strokes. On the bikeErg, that isn't possible, because you pedal continuously for long periods of time. For this reason, the PM5 incorporates a calibration phase that you need to do before any verified workout. It involves pedaling to speed and coasting down while the PM5 figures out what drag factor results from each damper position. So while you are cycling, it just uses damper position.

The net effect is the same. The Erg measures power in watts, and then estimates speed/distance/calories based on power and time.

So changing the damper setting makes no more difference than using different gearing on your bike.

[jamesg]

Adjusting drag let us adjust the impedance of the machine, to set it at a level that best suits ours, in terms of pull speed and force.

The rankings are ordered as to average power delivered.

According to Newton, force applied and resistance are equal and opposite.

[mjhatten]

I may have been living in a fool's paradise but I thought that the C2 measured torque or resistance over time to calculate distance. I thought that whatever the settings were, the calculator converted its to common reference so that for the same stroke rate/pedal RPM/ski cadence you earned more distance with high resistance and less distance with low resistance. The rankings (weight corrected?) are on the common reference.

[JaapvanE]

I may have been living in a fool's paradise but I thought that the C2 measured torque or resistance over time to calculate distance. I thought that whatever the settings were, the calculator converted its to common reference so that for the same stroke rate/pedal RPM/ski cadence you earned more distance with high resistance and less distance with low resistance.

The PM5 (and everything back to the PM2) measures the rotational position, and based on that it calculates drag (what you call resistance, which isn't an entirely accurate name), angular velocity and angular acceleration. And indeed, the higher the drag, the more distance per flywheel rotation.

[jamesg]

The PM sees a string of impulses from the fan-flywheel.

The basic physics, to calculate Power, requires knowledge of the speeds of the flywheel: in rowing (which is discontinuous), at release, catch and next release and the times between these events.

Ergspeed is then calculated from Power using W=2.8V³, V in m/s.

[CaseyClarke]

I find keeping my logbook up to date very motivational, and comparing myself to others in the rankings even more so, but I have a query about how accurate the rankings can be. Thinking specifically about the bike erg, in order to rank any workout time and distance have to be entered, but NOT the level of resistance used during the workout. There will be a lot of difference in time or distance achieved using light resistance as against heavy resistance (you can't ride up Alpe d'Huez as fast as along a canal bank). So, without knowing and specifying the level of resistance used, how much credibility can the rankings have?

Strange and bizarre post?!

What on earth does the resistance level have to do with the ranking?

Some might like a lower resistance and to use higher RPM (spinning) ,some might like a higher resistance and to use a lower RPM. The bike measures the pace/power correctly if it’s calibrated properly. Damper / resistance is totally irrelevant. Same on row/ski.

[JaapvanE]

The basic physics, to calculate Power, requires knowledge of the speeds of the flywheel: in rowing (which is discontinuous), at release, catch and next release and the times between these events.

Ergspeed is then calculated from Power using W=2.8V³, V in m/s.

Actually, one only needs the average angular speeds across drive and recovery. Speeds at the catch or release are useless as the forces governing acceleration and deceleration are not constant.

[jamesg]

How can you calculate energy loss from a single average speed?

[gvcormac]

How can you calculate energy loss from a single average speed?

The PM5 measures flywheel position -- and hence rotational velocity -- continuously. It also measures the damper position, and based on a previous calibration, calculates the resistance as a function of rotational velocity and damper position. From this it calculates power. A simple fixed formula translates power to road speed and calories.

Unlike the BikeErg, the RowErg and SkiErg do not use damper position or prior calibration at all. They recalculate drag on every recovery, from the rate that the flywheel slows down between strokes.

[JaapvanE]

How can you calculate energy loss from a single average speed?

You don't. A C2 Model D gives 6 impulses per rotation, older machines give 3 impulses per rotation. You can calculate drag in the recovery easily and extremely robust by doing linear regression over the increase in the time between impulses over time. On OpenRowingMonitor we do that structurally, and that has an R^2 that is structurally over 0.98, which suggests there is a pretty good fit (please note that C2 has some weird pattern in their new magnets actually causing this 0.02 difference)

Unless you want things like peak force, average force, force curves, handle speed curves, etc., you only need drag and the average speed of the flywheel through the stroke (which is easily determined by measuring the angular displacement divided by the stroke time).

[jamesg]

Unless you want things like...

I certainly do, which is why I use a C2 with a PM5 and Ergdata. Otherwise we can't know what we are actually doing nor how.

Clearly if we count the impulses and know the time, release to release, we can calculate the average flywheel speed. We must also find and use an "average" drag torque; taking account that drag torque has a cubic relationship with speed. Does that lead to complications?

Basic physics would seem to be much simpler: if we know the flywheel speeds and times at release and catch, knowing its inertia we know the amounts of energy lost to air by the flywheel and don't even have to measure drag.