converting spm and watts to time

[BrianC]

Just purchased the model D after trying my neighbors for the first time - great machine. I'm sure this has been discussed but couldn't find exactly what I was looking for.

Obviously there are many methods to achieve the same time for 500m by decreasing your SPM and increasing your Watts (force). While there has been discussion on the optimum SPM I am interested in how is the time for 500m calculated by the PM3 using SPM and watts. Is anyone aware of a formula relating SPM, watts and rate (sec/m)

Too new to say this is for training, just interested in the math and to try different rates.
Thanks group!

[johnlvs2run]

Pace and Watts have a directly proportional relationship.

The stroke rate doesn't have any affect on the P:W relationship.

[BrianC]

John
I understand about pace and watts relationship (watts = 2.80/pace^3). But if you have a slow SPM say 18 with constant pull versus a fast SPM say 28 this the same pull wouldn't this show a different pace?

The PM3 must calculate some average speed(ie pace) with a given SPM and flywheel rotation to show the pace for 500m.

I admit I VERY new and don't know much. It's just the old math teacher in me trying to figure this out.
Thanks

[johnlvs2run]

Brian,

You might have less watts at 18 spm, then the pace would be slower.

[Alissa]

There have been a number of threads discussing accuracy of measurments/methods of measurement of force applied to the erg. I believe PaulS ](who created/sells ErgMonitor which also measures various parameters of the C2 stroke) has been involved in most of them. Here's an example of such a post from Paul discussing how such calculations are made.

If you want to see more, you might try searches for "ErgMonitor" as a keyword and user (alternatively) either "PaulS" or "[old] PaulS". That should find most of them for you, I'd think.

Enjoy!

Alissa

[Jim Barry]

Here's how I understand it.

The erg measures three things every stroke:

The drag on the flywheel (how fast it slows down)

The number of revolutions for each stroke.

The time it took since the end of the last stroke.

The drag is relatively constant. It can be changed by using the lever that allows more or less air to be moved (given energy) per revolution.

When the flywheel stops accelerating (no positive speed change is detected) the PM calls it a stroke and records the revolutions and the time it took. With the drag factor and the revolution count in time the PM has everything it needs to come up with watts, the measure for work done in time, or power. Boat speed on the PM is formula based on watts that fits a real world boat speed and estimated single rower's power plot ( I think for a lightweight 4). It's just an approximation, but with that we get erg racing, distance challenges, and a English Professor that calls himself "Ranger". Similarly, Calories is also a formula based on the watts calculated plus a metabolic rate "base" for sliding up and down an erg for someone of average size at a normal stroke rate.

[michaelb]

You may want to ignore John Rupp like many others here on this forum. I am not at all an expert on this. I don't believe the PM3 uses the SPM in the math calculation, so if you produce 300 watts at 18 SPM or 30 SPM, the PM3 will show you going the same "speed" and your time for the piece will be the same. I don't understand the math, but that may be because the PM3 uses average watts per stroke. Most people will find producing 300 watts at 18 SPM to be "harder" though.

As Alissa says, there are lots of discussions about these issues, mostly here on the old forum. More recently there have been highly technical discussions on the forum in the UK, like this thread/post by NavHaz:

http://www.concept2.co.uk/forum/viewtop ... 501#243501

[Citroen]

I am not at all an expert on this.

Read all you need to become an expert at: http://www.atm.ox.ac.uk/rowing/physics/ergometer.html

[haboustak]

While the PM3 uses Power to calculate Pace, the root unit is Energy (in Joules). Stroke Power (in Watts or J/s) is: Energy (Joules) over Stroke Time (seconds). So SPM does factor into the pace calculation because it's the inverse of stroke time (divided by 60 to change from seconds to minutes) and is indirectly included in the unit of Power.

If Stroke Time is: (1/SPM)*60 or 60/SPM.
Then Power is: Energy / (60/SPM) or (Energy * SPM) / 60.

So holding the Energy input to the system constant:
As SPM goes up, Power goes up.
As SPM goes down, Power goes down.

"Holding Energy constant" is an easy phrase to type, but it's the very thing that makes rowing difficult. So the question becomes, how hard is it for you, the inefficient power source, to produce the same amount of Energy with less time per stroke to work with.

Different strokes have different capabilities of generating the necessary energy. The higher the stroke rate the less time you have to generate energy.

Mike

[becz]

Then Power is: Energy / (60/SPM) or (Energy * SPM) / 60.

What units are "strokes" in? Sorry, just being glib.

To make the math work, it's really energy/stroke * stroke/min / 60. So in essence "strokes" cancels out, and you're again just left with energy produced per unit time, regardless of the stroke rate.

[haboustak]

To make the math work, it's really energy/stroke * stroke/min / 60.

That's absolutely right. There's a tacit "per stroke" on Energy and Power everywhere I used them in my earlier post.