DQOTH (Dumb question of the day) on the PM5

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pagomichaelh
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DQOTH (Dumb question of the day) on the PM5

Post by pagomichaelh » June 17th, 2025, 1:35 pm

What is the scale of the time (horizontal 'x') axis on the force curve?
5'7" 152# b. 1954

JaapvanE
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Re: DQOTH (Dumb question of the day) on the PM5

Post by JaapvanE » June 17th, 2025, 2:15 pm

As far as I know: distance, where the total length is the drive length

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Ombrax
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Re: DQOTH (Dumb question of the day) on the PM5

Post by Ombrax » June 18th, 2025, 12:15 am

The post on the C2 blog linked below says that the X axis units are time.

https://www.concept2.com/blog/improving ... kJE3k6J1tg

But it also says that the Y axis is "power application" which is clearly incorrect, since power and force are two totally different things.

So, take it all with a grain of salt.

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Carl Watts
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Re: DQOTH (Dumb question of the day) on the PM5

Post by Carl Watts » June 18th, 2025, 1:15 am

It has no clearly defined units, what you are really looking for is the right overall shape.

Things like the ideal seamless transition from the legs to the arms etc.
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Re: DQOTH (Dumb question of the day) on the PM5

Post by JaapvanE » June 18th, 2025, 4:08 am

Ombrax wrote:
June 18th, 2025, 12:15 am
The post on the C2 blog linked below says that the X axis units are time.

But it also says that the Y axis is "power application" which is clearly incorrect, since power and force are two totally different things.
With OpenRowingMonitor we generate both, and they are indeed slightly different curves. Bad transitions will be visible on both, but slightly different as handle velocity becomes more present. According to the BLE specifications, the PM3 and PM5 generate force curves in forces-lbs (which can be transformed in Newtons).
Ombrax wrote:
June 18th, 2025, 12:15 am
So, take it all with a grain of salt.
Looking back at the PM3 setup, which introduced the force curve, it would be highly impractical to take a time based approach: the sensor reporting flywheel position is triggering based on flywheel position (i.e. a magnet passing the coil), not time based. As speed and even acceleration are far from constant (hence the need for a force curve), there is no easy way to provide nice consistent curve. In theory one could do a x-y plot where both x and y vary, but that doesn't translate well to the CSAFE interface that gets the same (???) force curve data without x-axis data.

During the drive, the flywheel position and handle position are directly linked (via the chain). And thus each position report by a sensor always represents x cm handle travel. This makes the measurement points extremely easy to plot and providing only Y-data is then extremely acceptable. Additional benefit is that these points can easily be projected onto physical handle position in the stroke for easier diagnosis of issues. OpenRowingMonitor and RP3 thus explicitly report using the length of handle travel on the X-axis of the curve. As force x distance = work, it also is a more meaningful measure, as "the area under the curve" is in fact a meaningful thing in physics.

The new sensor setup with a PM5 could allow a time-based measurement setup. But a PM5 also can be retrofit to the old sensor, and BLE and CSAFE still get the data without x-axis data. So to keep things consistent across versions and backwards compatible, it would make sense to keep the same approach. But then again, nothing is impossible.

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