damper/cadence/watts

[scottyucsb]

I've been using the erg for a few months and recently joined zwift. I previously had a smart trainer and used trainerroad. I use to love the mindlessness of having the smart trainer set resistance based on cadence and whatever workout I was doing. I am finding some joy in using damper and p5 to "hit" the watts on a ride or workout. I do have a preferred cadence, 90, which might go to 100+ when riding outside with a group or racing. I just worked through the damper from 2-5 with half damper steps and noted watts at 90 rpm, here it is:

Damper_#, Cadence, Watts
2, 90, 150
2.5, 90, 175
3, 90, 203
3.5, 90, 240
4, 90, 284
4.5, 90, 319

I haven't calibrated, but am close to one million metres, so will check again after that. What are other people's experience with these settings. In other forums I read that people felt like damper was too hard, which I totally get. I am lucky that at damper 1 and my ftp, I can spin that with little effort (damper 1, cadence 80 or less), so the bottom end is fine with me, but maybe NOT low enough. At the top end I have no idea what damper 10 and 90 cadence (RPM) would be, but there are non-sprinters, but avid cyclists might want to explore 1000+ watts for short bursts.

[CaseyClarke]

Quoting damper setting alone isn’t enough info. Need to refer to drag factor at these settings. The older models have a range of about 50 drag at 1 to 215 drag at 10. The newer models go down a good bit lower at the bottom end and higher at the top end. Around 30 on 1 to 240 on 10.

I think it depends on relative fitness and preferred RPM as to whether the bottom end is too stiff for some people. If you’ve got a relatively low FTP (say 150w) and you like high cadence riding (say 95rpm+) then the bottom end damper settings will be too high for anything other than hard sessions. Ie there’s not a damper low enough to do say 100w at 95rpm, etc etc, so you’d be forced to ride at a heavier gearing than you’d prefer at a lower cadence to stick to that 100w.

[scottyucsb]

Thanks. I was wondering if the million metre recalibration is adequate, should I recalibrate each ride? I did quote cadence with my #s. I've added 4 more cadences and will record watts at each (85, 90, 95, and 100).

[johnlvs2run]

Calibrating every million meters should be fine. Be sure to check the drag factor settings too.
I've used different colors of tape to mark drag factors from 100 to 150 plus a few other readings on the cage.

[ArmandoChavezUNC]

Drag factor at any point of your erg's damper will change due to atmospheric conditions as well as dust build-up inside the fan. I check mine before every row, but you probably don't have to be so anal about it. 1,000,000 meters might be too long, though. It's a simple enough process - just do it once a week or so.

[wilsonc10e]

I'm trying to work this out myself because fiddling with the damper mid-ride is annoying. I have enough workouts to have a plot of RPM, damper, and Watts and was going to just do a line of best fit, but I wasn't sure what the trend line equation would be.

Anyone who is better at math than me have the answer to that question?

[Nomath]

The general formula for the relation between power input and angular speed of an air-braked flywheel is : P = C * ω³ + ω * J * dω/dt
P is input power [W]
C is drag coefficient [kg m²]
ω is angular speed [rad/s]
J is moment of inertia [kg m²]
For a BikeErg at constant cadence you may assume that the variation in angular speed during a pedal turn is very small.
The equation then simplifies to : P = C * ω³
The drag coefficient C [in kg m²] multiplied by 10^6 equals the drag factor displayed by the PM5. E.g. for a drag factor 120, C equals 120 * 10^-6.
Hence at constant cadence the input power is proportional to the drag factor. Input power increases with cadence^3 !

However, the relation between the damper setting and the drag factor is non-linear and depends on environmental conditions such as altitude (=air density) and barometric pressure. It also depends on the cleanness of the flywheel and the air vents.

After cleaning the air vents and the flywheel blades, I recently measured the following pairs of damper setting and drag factor :
(1 - 78), (2 - 87), (3 - 97), (4 - 107), (5 - 122), (6 - 133), (7 - 152), (8 - 171), (9 - 189), (10 - 200)
Note the unequal steps in the drag factor!

Now, assuming that the BikeErg flywheel of the topic starter scottyucsb follows roughly the same trend between damper setting and drag factor as my RowErg, you can see that his measurements nicely fit the expected linear relation.