Newton meters to watts with RPM
by bobnro on 01/15/02 at 07:47:06

Hello;



I have an ergometer which displays Nm & watts, depending on the pedal cadence (RPM).

Is there a way to convert between Nm and watts for different RPM's?

The ergometer only shows values of NM & watts up to 70 RPM, and I want to pedal faster.  

Thanks, Bob

Re: Newton meters to watts with RPM
by Ray Viv on 01/15/02 at 12:27:07

Bob,
Don't quote me but this sounds like a variation of the question I asked. I believe Newton-Meters is a measure of force whereas Watts measure power, or force applied over a period of time. One way maybe to convert Newton-Meters to pounds, RPM to feet per second, foot -pounds/sec to Watts. Once that you've established a known RPM to NM you could work a new speed into the equation. It's a long way around and there is most likely a much better way, but this should work.
Ray Viv

Re: Newton meters to watts with RPM
by bobnro on 01/16/02 at 13:53:25

Ray;

Thanks for the reply.
If what you say is true, does that mean that the watts reading (which is on the same scale as the Nm) has to have the amount of time I'm pedaling at a given NM and RPM figured into the equation to get the actual amount of watts?

Suppose I pedal at 70 RPM, and 20 Nm for 20 minutes.
Is the wattage any different if I pedal at the same NM & RPM for 30 minutes?

Thanks, Bob

Re: Newton meters to watts with RPM
by Robert Fogt on 01/16/02 at 21:44:27

Watt and Newton meter are different, but somewhat related.

1 watt is equal to 1 newton meter per second.
1 newton meter is equal to 1 watt per second.

I don't know if that will help you any.

Re: Newton meters to watts with RPM
by Ray Viv on 01/17/02 at 06:10:55

Bob,
My earlier statement that Watts are a measure of force over a period of time is confusing because time is used as the rate the force ia applied. 746 watts is equal to a horsepower. which is 550 lb/ft (the force) per second (the rate). These are instantaneous values. If you were to pedal for an hour while generating 1000 watts constantly that would be a kilowatt-hour, a cumulative measure of power that the utility uses to charge us for their power monthly.
One lb/ft is equal ro 1.357 newton-meters, both are measures of force, just an English vs. Metric thing.
I hope I haven't caused more confusion, it's the fact time is used in differing ways in different formulas that is confusing to most of us.
Ray

Re: Newton meters to watts with RPM
by Bob Samberg on 01/17/02 at 15:40:11

Ray;

Yes, I think the time makes it confusing.
If I pedal at, say 70 RPM and the meter (a mechanical one that changes depending upon how I set the resistance of the brake pads on the flywheel) indicates 19Nm and 132 watts, then watt :) does that mean with reference to time?   Does it mean 132 watts per hour, and if I pedal for 30 minutes, then does that mean I've only generated (?) 66 watts?  (Is generated the right word, or is it consumed or something else)?

What would be the most meaningful at-home measurement (if there is one) for tracking my exersize progress, to plot on a graph relative to my heart rate? (Average heart rate during the exersize session).  

Unfortunately I no longer have the instructions from the ergometer, having bought it in the 70's.

Thanks again for your replies...Bob

Re: Newton meters to watts with RPM
by Robert Fogt on 01/18/02 at 00:10:05

Well if it says you're generating 19 Nm, then you're generating 19 watts per second. Not sure where they get the 132 watts. I suppose it is just there for a reference.

I believe you should exercise based on your heart rate. There is an ideal exercise heart rate based on your age. (I dont know what it is, you'll have to check a health site maybe)  At the end of your routine, check your heart rate, once it gets below your ideal rate, then next time pedal faster, or increase resistance.

You could use either the Nm or watts to graph your progress. I doubt they are accurate, but it is probably consistantly inaccurate, so you'll still be able to plot your improvement.

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