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Curious as to how a power meter works on a road bike. Seems to me that if you know what gear you are in, how much you and the bike weigh, velocity and grade, you should be able to get a rough watt measurement. If my previous statement is correct, wouldn't a simple formula produce the metric in a normal bike computer that has an altitude feature (assuming you are able to input your weight, the bike weight and what gear you are in).

Thoughts??
 

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I don't know much about it, but wouldn't things like wind, whether your tires are underinflated, and any other quantifiables unique to you and your situation cause calculated results to differ from actual measured results? I'm asking, I don't know.
 

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There is no way to measure wind resistance though. I have heard that while riding in the flats that wind resistance creates 80% of the drag, not completely sure if its true, but its sounds believable.
 

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Discussion Starter · #4 ·
studiddy said:
I don't know much about it, but wouldn't things like wind, whether your tires are underinflated, and any other quantifiables unique to you and your situation cause calculated results to differ from actual measured results? I'm asking, I don't know.
I would think it would, but how does one of those expensive power meters account for those variables?? I suspect they don't, which brings me back to the original question. Why can't a regular cyclometer be programmed to tell you your watts as accurately as a power meter?
 

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Power meters like an SRM or a Powertap measure power by the tenison or strain on the crank(SRM) or hub(Powertap). If you are going 18 in a headwind then the tension will be higher than going 18 in a tailwind. They measure the actual torque on the drivetrain.
 

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LyncStar said:
I would think it would, but how does one of those expensive power meters account for those variables?? I suspect they don't, which brings me back to the original question. Why can't a regular cyclometer be programmed to tell you your watts as accurately as a power meter?
They wouldn't directly measure wind or rollling resistance, but a meter would measure what effect they had on your ride, since they are part of what you are working so hard to overcome. Like the 18mph headwind requiring more power than the tail.
 

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Depends on your training goals.

For many, they are necessary and provide instant training feedback while racing, doing intervals, and recovering. Formulas do not. HR is a stress response, Power is the stress.
 

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LyncStar said:
I would think it would, but how does one of those expensive power meters account for those variables?? I suspect they don't, which brings me back to the original question. Why can't a regular cyclometer be programmed to tell you your watts as accurately as a power meter?
Those expensive power meters don't need to account for those variables because they directly measure the two variables (force and velocity) that allow one to calculate power. Force (really torque) is measured by tiny strain gauges that deflect ever so slightly when your muscles exert force on the cranks combined with cadence (velocity) you can accurately calculate power. That is what is so attactive about power meters they remove all the variables that confound any other measure of how hard you're working.
 

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There is just such device --

All reports say it works terribly.

Wind + rolling resistance + pedalling efficiency + drag coefficient + other frictional losses are just too hard to estimate in the constantly changing cycling conditions. Maybe in an indoor velodrome you might do a decent estimate...

Cyclists have trained, raced, and succeeded for years without a powermeter. They're absolutley not "necessary" to ride a bike, but, pros all say they appreciate the specific feedback, since the GT's are won and lost on 1-2% differences.
 

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Argentius said:
There is just such device --

All reports say it works terribly.

Wind + rolling resistance + pedalling efficiency + drag coefficient + other frictional losses are just too hard to estimate in the constantly changing cycling conditions. Maybe in an indoor velodrome you might do a decent estimate...

Cyclists have trained, raced, and succeeded for years without a powermeter. They're absolutley not "necessary" to ride a bike, but, pros all say they appreciate the specific feedback, since the GT's are won and lost on 1-2% differences.

The HAC4 has this imputed power reading. I had one. It was a joke. I have been running Powertaps for a while now. World's of difference.

May try an SRM next.

Loves me the electronic doo-dads. . .

;)
 

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Been using a Polar Power sensor since last fall, and it's worked fine for my needs. I already had a s720i HRM (for about a year prior), and have been downloading my ride data to the Polar Software, so adding the Power Sensor was a reasonably priced choice. Now my excercises on my road bike are showing wattage in addition to speed/distance, cadence, altitude. It's been pretty cool to see my power output for a sprint, versus a hill and for my 5 different roller interval programs, it's a great measure of actual performance versus my perception of how hard the effort was. I don't know how accurate the Polar Power data is in comparison to the other systems out there. I'm not really comparing my power data to anyone else, so as long as the output wattage measured is consistent for my use, I'm happy.

Another advantage about using the Polar based system, is that I can switch between roadbike's Bike 1 setting with the Power Sensor, and my cyclocross' Bike 2 setting, with basic wireless speed and cadence sensors. I can download excercise files for either bike, which has been great for tracking in the training diary.

It took about an hour to set-up and tweak the Polar Power sensor, and a few rides to get familiar with using it. Now, I'm doing a lot of my solo training rides watching the power data and learning the relationship between my perceived efforts, actual heart rate, and power output.

Like coolhand, I'm a gadget freak. If you're comfortable with setting up and working with digital devices like syncing a PDA phones to your PC or Outlook, the set-up on the Polar Power sensor should be comparably easy.
 

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You could always go the poor man's route and get a Cycleops Fluid Trainer and use the Power Curve that comes with it, for example 16mph = 200W (or whatever it is). It may not be exact but it's definitely repeatable as long as you have rear wheel cadence and speed.

Lou.
 

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Argentius said:
Is polar power the one that figures it out by ... chain noise?
IIRC, it optically measures chain vibration, don't ask me how you figure out power from that. The others all directly measure torque either at the crank (SRM), rear hub (Power Tap) or botttom bracket (Ergomo).
 

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Thought it worked off of the same principal as a guitar string. Caveat is it is sensitive to set-up and certain gear ratios. Still the price is good if your already have the compatible polar HRM. If you don't you are really better off with a Powertap IMHO.

Sadly I am looking at SRM's now. Stupid gadget lust. . .

:D
 

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If you have a Kurt Kinetic trainer, you can buy a virtual power computer for $50. It gives you the power based on the speed of the resistance unit wheel. Its not your actual power, but it is fairly acurate when checked with an SRM unit.
 

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Dwayne Barry said:
IIRC, it optically measures chain vibration, don't ask me how you figure out power from that. The others all directly measure torque either at the crank (SRM), rear hub (Power Tap) or botttom bracket (Ergomo).
I'm pretty sure chain vibration is measured inductively not optically (ergomo is an optical system). Power can be calculated from chain vibration as follows: knowing the mass of the chain (input by the rider) and vibration frequency (measured at the chainstay) gives tension in the chain (force). A sensor in the jockey wheel measures chain speed then force x speed gives power.
 

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necessary? yes, if you really want to train with power, you need to have accurate measurement. guesses of +or - >100W are not acceptable for effective training.
expensive? you get what you pay for.
recently got a pretty good deal on a ciclosport 434. i love the power tap pro on my road bike but hadn't been able to use it on my mountain bike (discs) or my cyclocross bike (135mm spacing) and was curious to see if there might not be a cheaper route to power measurement. there isn't. mounted the ciclosport on the road bike and did a head to head comparison. after a couple weeks of evaluation in a variety of conditions, i've concluded that the ciclosport is absolutely useless for powertraining. in the 434's defence, i do like the altimeter and temperature functions as well as the ease of mounting.
the midweek cycling club has some good power training articles on their website.
http://midweekclub.ca/articles/
 

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Cross Bike: No problem, space the axle out with some washers.

MTB: I've seen a picture of one, but using non-disc brake in the rear.

Hunter Allen's book even shows a BMX bike w/ PowerTap.
 

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grampy bone said:
If you have a Kurt Kinetic trainer, you can buy a virtual power computer for $50. It gives you the power based on the speed of the resistance unit wheel. Its not your actual power, but it is fairly acurate when checked with an SRM unit.
Sounds interesting, do you have one of these? Not sure how it hooks up the the Kinetic Trainer but wonder if it could be adapted to other Fluid trainers?

Lou.
 
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