How does Bike Erg measure cadence?

[gvcormac]

The Bike Erg differs from the Row Erg and Ski Erg in that there is no drive/recovery pattern. One can pedal pretty much continuously for an entire workout.

So how does the PM5 detect cadence? I'm not entirely sure. I do know that to measure power, it uses an angle sensor on the damper, which the Row Erg and Ski Erg don't have. This must be calibrated from time to time, and immediately before any verified workout.

So I understand power, but it also measures "RPM." I have verified that there is no pedal rotation sensor, so I suppose it must measure fluctuations in acceleration that naturally occur due to pedal rotation. This sounds like a pretty dicey calculation.

Nevertheless, it does seem to be pretty accurate. Occasionally I see it vary by a few RPM, but mostly it is rock solid. If you download from C2 Log, you see per-stroke data (see below).

Any thoughts on precisely how stroke is determined?

Number Time (seconds) Distance (meters) Pace (seconds) Watts Cal/Hr Stroke Rate Heart Rate
1 2.6 5.9 377.3 7 322 27
2 4.5 12 377.3 7 322 27
3 6.3 18.1 377.3 7 322 27
4 8 24.2 312.2 12 339 34
5 9.7 30.2 273.5 17 358 38
6 11.2 36.4 273.5 17 358 38
7 12.7 42.5 273.5 17 358 38
8 14.2 48.5 250.5 22 376 41
9 15.6 54.6 250.5 22 376 41
10 17 60.7 237.7 26 389 42
11 18.4 66.7 232 28 396 43
12 19.8 72.8 232 28 396 43
13 21.2 78.9 226.2 30 404 44
14 22.5 85 226.2 30 404 44
15 23.8 91.1 226.2 30 404 44
16 25.2 97.1 220.8 33 411 45
17 26.5 103.2 220.8 33 411 45
18 27.8 109.3 217.5 34 417 46
19 29.1 115.4 217.5 34 417 46
20 30.3 121.5 213.3 36 424 47

[c2jonw]

The PM sees the flywheel speed and it knows the gear ratio. from there it's a simple division operation.

[gvcormac]

The PM sees the flywheel speed and it knows the gear ratio. from there it's a simple division operation.

That'd be a simple method. I'll check to see if it counts while coasting.

[gvcormac]

Here's the log from my experiment. It appears that the C2 does in fact calculate cadence and strokes from flywheel speed, even when the actual cadence is 0. I stopped pedaling at about 15 seconds, and it kept counting imaginary strokes until it decided I had stopped and paused the "just ride" session even though the flywheel was going pretty fast. The instant I resumed pedaling, it claimed 108 RPM.

In a second coast down (not shown) I tried pedaling backwards and forwards ... no effect unless you pedal forward enough to engage the clutch. Then the RPM estimate appears instantly. So it seems to detect acceleration or lack thereof as a threshold matter. From the calibration it "knows" how fast the flywheel should coast down; I guess it detects the (fairly small) divergence from that.

20 14.2 92.6 98.2 370 1572 134 <== ride in progress
21 14.6 97.3 98.2 370 1572 134
22 15.1 102.2 96.5 389 1640 136
23 15.5 106.7 96.5 389 1640 136 <== stopped pedaling
24 16 111.3 95.6 401 1678 136 <== coasting, until next entry
25 16.4 116 95.6 401 1678 136
26 16.9 120.6 98.5 366 1560 128
27 17.4 125.2 105.1 301 1337 120
28 17.9 129.7 105.1 301 1337 120
29 18.4 134.5 105.1 301 1337 120
30 19 139 112.1 248 1155 112
31 19.5 143.7 112.1 248 1155 112
32 20.1 148.4 119.5 205 1005 105
33 20.7 152.9 119.5 205 1005 105
34 21.3 157.7 127.5 169 881 99
35 22.1 162.5 127.5 169 881 72
36 22.9 167.2 149.9 104 657 75
37 23.6 171.8 149.9 104 657 75
38 24.3 176.4 162.4 82 581 85
39 25 181.1 162.4 82 581 85
40 25.6 185.7 144.6 116 698 95
41 26.1 190.2 144.6 116 698 95 <== PM5 paused, said 0 RPM
42 26.7 195.1 127.4 169 882 108 <== resumed pedaling
43 27.2 199.5 127.4 169 882 108
44 27.7 204.3 115.5 227 1081 116
45 28.2 208.8 115.5 227 1081 116
46 28.7 213.6 108.3 276 1248 123

[Carl Watts]

Some quite different ways of doing things out there....

The WattBike uses a left and right reed switch as cadence sensors and can derive power from a load cell against the chain. You get a "Peanut" shape force graph because the max power into the flywheel is up past 45 Degrees from the top.

I'm surprised that the WattBike setup works to be honest from a mechanical standpoint, but it does and its also very accurate. The loadcell is factory calibrated and the calibration values are stored on the loadcell signal conditioning board. When the Monitor powers up, the first thing it does is read the cal variables. This allows you to switch monitor or switch loadcell and still have it very close to calibrated.

[gvcormac]

I like the peanut display on the Watt Bike, but I like the inertial feel and drag characteristics of the Bike Erg better.

I am not a bike trainer afficionado, but my understanding that it is pretty standard to use strain gauges on the crank arms to measure force and hence instantaneous power, and integrate to get work. I'm not sure how precisely they do this. Probably they can be calibrated well enough to give reliable results for "normal" pedaling.