Power Tap

[The doc]

Hi guys,
I am new. I come from Italy.

I'd like to build a roller and installing the Power Tap on it... so that, I can know if there is a disparity between the two legs.


Having the power tap on my ergometer / roller, removing the rear wheel I can hook the chain to my roller and pedaling i can measure the instantaneous power, so that I could see any asymmetry.

But, I do not know actually how it can measure the cadence.

[SIZE=-1]Do you think it is possible?

Do you have any suggestion in how to build it?

Thanks
Marco
[/SIZE]

 

[gman0482]

How would powertap measure each leg separately ?

2 powertaps ? Don't get it.

 

[The doc]

I see what you mean, but I forgot to say that I would use power tab in a motion analysis laboratory, so that, with my system I could know when each crank is at 0 deg, 90 deg, 270 deg and 360 deg.
If I know the current power (from the power-tab), putting these two information together I would be able to know which is the power for each leg... Do you understand, now?

Marco

 

[alienator]

Why not just use the rear wheel as intended by Power Tap? You can still determine which part of the power output belongs to which leg (although the power tap won't be able to completely isolate the power from each leg) by setting a time, t=0, with a known crank position. This can easily be done by having the rider start with the cranks at a known position. In fact, this can easily be done with Matlab, Labview, or some other instrument control software. If you use Power Tap's cadence sensor, you can monitor cadence as a function of time to determine crank phase. Hell, output the PowerTap information to a Garmin device (500 or 705), and you'll have a file with all of the information you can get, without the need for continuous external position detection. You of course can do this with output from a PowerTap head unit, too. You'd have power data, cadence data, and the sampling interval. Therefore with a known starting position, you'd be able to find the crank position at any time by using the average cadence to that point in time.

You'll be able to determine which peaks belongs mostly to which leg, but the Power Tap won't discriminate at some off-peak positions. All the Power Tap see's is the sum output of both legs. Still, knowing the original crank position and having a plot of power output over time will at least allow you to identify which leg is producing more power.

To get actual power from each leg, you'd have to have force data for at least one leg, something no bike power meter currently gives. There are couple in the works, but they haven't been released yet.

 

[The doc]

Yes, you are right.
I have never seen a power tab, so that, I do not know If there are some wires which I could connect to a computer or some sort of interface...
If I can do that, I would be allowed to see on-line the powertap's signal.... Got it?

Marco

 

[Yojimbo_]

I thought the powertap takes many measurements per revolution, but it then does some sort of averaging and outputs results every 1 second at best. In which case, there isn't enough resolution to distinguish between each leg.

 

[The doc]

I thought the powertap takes many measurements per revolution, but it then does some sort of averaging and outputs results every 1 second at best. In which case, there isn't enough resolution to distinguish between each leg.

If this is true... I need another way to solve the problem...
Are you sure about that? I need to be sure.... Does anyone know how the power tab works?

Thanks

 

[alienator]

If this is true... I need another way to solve the problem...
Are you sure about that? I need to be sure.... Does anyone know how the power tab works?

Thanks

You're problem can be easily solved, especially if you're using a wired PowerTap. If you're using a wired PowerTap, you can connect the output wire from the PowerTap to a data acquisition device which will give you access to the full analog signal. Likewise, if the cadence sensor is wired, you can take the output wire and run it to a DAQ. With the cadence raw signal, you won't need to do anything special to locate the crankarm in phase because each signal from the cadence sensor will come, obviously, when the crankarm magnet passes the sensor, i.e. at a known position, known crank phase angle.

With respect to the signal from the PowerTap hub, you can use your DAQ software to calibrate the signal, i.e. to remove DC offset and null any slope in the signal.

If you're using a wireless PowerTap, you can still get access to the cadence and PT signal, but that will require an ANT receiver used in conjunction with your DAQ.

You won't know absolute power numbers unless you do a procedure to establish absolute power, but since you're apparently wanting to compare power output between legs, using normalized power should be just fine.