Power to Cadence Ratio

I was hoping we could have the option of graphing the Power to Cadence ration. Why? I use the site for cycling and rowing. In rowing, it is very useful to look as the stroke power index (SPI) = Power/Strokes per Minute (where stroke per minute = cadence).

The SPI (ratio of Power to Cadence) provides insight into the power of each stroke. Since rowing speed/rhythm changes over the course of a structured row, your watts will naturally move with changes in speed. However, you want to make sure that your SPI is staying constant. You can infer if and when you are fatiguing by graphing the Power to Cadence ratio over the duration of a row.

Power = torque x cadence. So power / cadence = torque. You can get this information as an option on the graphs of the main activity page, as well as by intervals.

That’s why a quadrant analysis graph would be better; it plots the four types of power/cadence ratios:

Quadrant I (upper right): High cadence, high force. Sprint type work.

Quadrant II (upper left): Low cadence, high force. Hill climbing work, Standing Starts.

Quadrant III (lower left): Low cadence, low force. Easy pedalling, (in the pack or while resting between intervals).
Quadrant IV (lower right): High cadence, low force. Fast pedalling with little resistance, (sitting in the middle of a pack or pedalling downhill.).

The percentages in each corner show the proportion of ride time spent in that quadrant

1 Like

Does @Michael_Webber 's suggestion of using the existing torque chart work out for you? One of the things I have on the near term todo list is to add support for custom plots on the activity timeline page (like on /fitness).

/**
 * Crank torque.
 * https://www.bikehub.co.za/features/_/articles/training-nutrition/performing-and-analysing-torque-intervals-r7368
 */
public static double torque(int watts, int cadence) {
    return watts / (cadence * 2 * Math.PI / 60.0);
}

Thanks for your response. However, I would appreciate it if you could kindly clarify. The posters above are arguing that torque is identical to power/cadence.

However, my understanding is P=τω. So, as your code above suggests, you are not simply dividing Power by cadence. You are dividing the watts by the angular velocity ω, which is 2x Pi x Cadence/60s.

So, the torque calculation is not actually a substitute for the stroke power index which is simply power divided by cadence. I would love if you could add it to the fields and charts. However, I guess I could simply multiply the torque by 0.104716667= 2 * 3.1415 / 60.

I have just added this chart. It’s called “W / rpm” because “Power / Cadence” is too long for the label.

I had forgotten how easy it is to add more of these charts. So long as it’s something that can be computed from other streams and info available on the activity, it’s easy.

1 Like

Thanks. This is extraordinarily useful. Hoping to bring rowers onto the site.

1 Like

Is it also easy to add to the “Fields”? That would be very useful too.

Not quite so easy :slight_smile: Those are pre-computed when the intervals are detected so require server side changes. What are you looking for?

I ideally would be looking for the W/rpm ratio as a Field for the intervals and for the entire workout activity/row. Ideally, the average, max, and mix per interval would be great. For now, I am pulling torque and then multiplying by 0.104717

To motivate my request: As I noted above, rowers refer to W/rpm or W/SPM as stroke power index (SPI). It is indicative of the force of each drive in the stroke. It is a very useful metric because while Power varies with the rhythm (i.e., cadence) of your row, the force of the leg drive should stay constant. By tracking W/rpm (SPI) across intervals, you can see if you have leg drive fatigue. Also, when you really pick up the pace of the row, you have much less recovery time and time at the catch to maximally position yourself for a powerful leg drive. So, it is useful to assess if the SPI is declining during high cadence/power intervals.

I just added min, max and average SPI for intervals. Already had this for torque so I just used your factor, no server side changes needed (duh!). I also renamed the chart to “SPI”. Cycling uses torque instead so might as well use the rowing term for the w/rpm chart.

I don’t have average torque on an activity yet but I will add that (and hence SPI). Probably best to exclude zero power from that to get rid of coasting, stops etc…

1 Like

Wow. Thanks. We rowers will really appreciate it. I already posted this to the rowing Facebook boards encouraging people to join the site so that they can follow their SPI… Yes, it also should have occurred to me that you could calculate SPI by multiplying torque by a constant…Gosh, I hope I gave you the correct coefficient.

If SPI is expressed as SPI=ktorque. The coefficient k should be 2Pi/60.

2 Likes