PowerTap efficiency

Apr 25, 09 17:44

Post #1 of 25 (4813 views)

A question for those who engage in the endless pursuit of getting that last bit of speed out of their bike:

A power meter, by definition, taps a small percentage of power from either your cranks or hubs - I am no expert, but I assume it would have a miniature dynometer in order to determine the amount of power being turned into rotational energy. This means that the amount of power that is reaching the back wheel is going to be fractionally less than if there were no power meter in the mechanical linkages from legs to wheel (based upon the law of Conservation of Energy).

Does anyone know what percentage of total power is being lost to the meter? (it would be very small, but for the obsessive amongst us, it would conceivably add up to a noticeable time difference over 180km).

A couple of other factors I would be interested in hearing comments on:
1. The extra weight (50-100g) of the hub over a standard hub.
2. The use of a wireless power meter means that even more power must be extracted to power the RF transmitter in place of a simple electrical signal along wires.

Your thoughts and comments...

Re: PowerTap efficiency [SwBiRu] [ In reply to ]

Apr 25, 09 17:56

Post #2 of 25 (4804 views)

Is this triathlon's version of the Heisenberg Uncertainty Principle?

Re: PowerTap efficiency [SwBiRu] [ In reply to ]

Apr 25, 09 18:04

Post #3 of 25 (4792 views)

In Reply To:

A question for those who engage in the endless pursuit of getting that last bit of speed out of their bike:

A power meter, by definition, taps a small percentage of power from either your cranks or hubs

you've jumped the tracks right at the start...

In Reply To:

I am no expert, but I assume it would have a miniature dynometer in order to determine the amount of power being turned into rotational energy.[/quote] That's not how the most common PM's calculate power. For an overview of how they actually measure power, give this a read.

In Reply To:

This means that the amount of power that is reaching the back wheel is going to be fractionally less than if there were no power meter in the mechanical linkages from legs to wheel (based upon the law of Conservation of Energy).[/quote]
not necessarily--it will only be less if an SRM spider somehow 'absorbs more power' or a PT hub 'absorbs more power' than a conventional crank or conventional hub.

In Reply To:

Does anyone know what percentage of total power is being lost to the meter? (it would be very small, but for the obsessive amongst us, it would conceivably add up to a noticeable time difference over 180km).[/quote]
If a PT hub 'lost' a significant amount of power versus a regular hub, you'd be able to measure it by comparing the power differential among different rear wheels while measuring power at the crank. I've certainly never seen any evidence that the hub is costing any power. While I wouldn't say it's not costing any energy at all, my guess is it's infinitesimally small, as in tiny fractions of a single watt.

In Reply To:

A couple of other factors I would be interested in hearing comments on:
1. The extra weight (50-100g) of the hub over a standard hub.[/quote] yep, it's heavier. You can go to analyticcycling.com to see the impact of 50 grams on your performance...

In Reply To:

The use of a wireless power meter means that even more power must be extracted to power the RF transmitter in place of a simple electrical signal along wires.[/quote] umm...that's what those little batteries are for...

Re: PowerTap efficiency [sjPaul] [ In reply to ]

Apr 25, 09 18:05

Post #4 of 25 (4789 views)

It could be, but at least this would prove to be quantifiable. My guess is that the power meter manufacturers have tested how much power is lost by the addition of meter, but obviously publishing it would be bad for business.

Personally I think the gains from efficient pacing will always far outweigh the losses due to the meter, but for those who obsess over carbon brake levers or shifters it is worth thinking about.

Re: PowerTap efficiency [SwBiRu] [ In reply to ]

Apr 25, 09 18:08

Post #5 of 25 (4784 views)

We're talking about very tiny fractions of a watt here. Strain guages soak up effectively no energy at all, and according the http://www.thisisant.com/products, the ANT+ protocol used for wireless takes up milliwatts.

If you list the top 50 things which eat up your energy output, the energy consumed by a PowerTap (even wireless) wouldn't make the list.

Weight is a different issue. The added weight is a handicap, but I can't quantify how much of a handicap it is.

-

-Mark Rebuck, http://www.markrebuck.com/

Re: PowerTap efficiency [roady] [ In reply to ]

Apr 25, 09 18:13

Post #6 of 25 (4779 views)

Roady,

Just the sort of article I was looking for - strain gauges as a means of measuring torque rather than a dyno setup mean that the power lost would indeed be infinitesimal, and certainly not measurable. 50 to 100g at the hubs is not really going to be worth worrying about (as opposed to at the rims), and if the wireless ones have little teeny weeny batteries, then that is no power removed from the system.

So I guess it is just the extra weight you have to worry about, and the fact that you are going to be staring at the display rather than the road ahead...

Last edited by: SwBiRu: Apr 25, 09 18:14

Re: PowerTap efficiency [SwBiRu] [ In reply to ]

Apr 25, 09 18:27

Post #7 of 25 (4761 views)

Powermeters don't drain any energy to power themselves (they run on batteries), and rather than having any sort of dyno, they just use a strain gauge to see how much torque is being produced (and along with cadence can compute power).

On a theoretical level, conservation of energy means that there are small drains for any cyclecomputer (ex. the energy required by the leaf switch in a speed sensor when the wheel magnet passes is drawn from the wheel). We are talking thousandths of a watt (if that). None of these are at the level to have a measurable performance impact.

ECMGN Therapy Silicon Valley:
Depression, Neurocognitive problems, Dementias (Testing and Evaluation), Trauma and PTSD, Traumatic Brain Injury (TBI)

Re: PowerTap efficiency [Titanflexr] [ In reply to ]

Apr 26, 09 7:21

Post #8 of 25 (4637 views)

what does cadence have to do with using torque to measure power?

king of the road says you move too slow
KING OF THE ROAD SAYS YOU MOVE TOO SLOW

Re: PowerTap efficiency [ttocsmi] [ In reply to ]

Apr 26, 09 7:31

Post #9 of 25 (4627 views)

In Reply To:

what does cadence have to do with using torque to measure power?

Ummm...because Power = Torque X Rotation Rate?

http://bikeblather.blogspot.com/

Re: PowerTap efficiency [Tom A.] [ In reply to ]

Apr 26, 09 7:59

Post #10 of 25 (4609 views)

That's nonsense. I've read that power is power and cadence is not important!

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The Triathlon Squad

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Re: PowerTap efficiency [Paulo Sousa] [ In reply to ]

Apr 26, 09 8:13

Post #11 of 25 (4591 views)

In Reply To:

That's nonsense. I've read that power is power and cadence is not important!

Why are you confusing power calculation with power generation?

http://bikeblather.blogspot.com/

Re: PowerTap efficiency [Tom A.] [ In reply to ]

Apr 26, 09 8:17

Post #12 of 25 (4581 views)

my PT does not measure cadence and it calc's power, now it will give a wild guestimate of cadence, but that gives a lot of false cadence readings in the data output

:)

Re: PowerTap efficiency [jeffp] [ In reply to ]

Apr 26, 09 8:20

Post #13 of 25 (4576 views)

In Reply To:

my PT does not measure cadence and it calc's power, now it will give a wild guestimate of cadence, but that gives a lot of false cadence readings in the data output

:)

Ummm...it does measure the hub rotation rate, no?

Crank based torque measurement - need crank rotation rate.
Hub based torque measurement - need hub rotation rate.

It's just math...

http://bikeblather.blogspot.com/

Re: PowerTap efficiency [Tom A.] [ In reply to ]

Apr 26, 09 8:22

Post #14 of 25 (4573 views)

yes, it does, but it does not use "cadence" for anything. I am done, before I get sucked into something I cannot hope to win

Re: PowerTap efficiency [jeffp] [ In reply to ]

Apr 26, 09 8:23

Post #15 of 25 (4568 views)

In Reply To:

I am done, before I get sucked into something I cannot hope to win

lol

Re: PowerTap efficiency [jeffp] [ In reply to ]

Apr 26, 09 8:26

Post #16 of 25 (4562 views)

In Reply To:

yes, it does, but it does not use "cadence" for anything. I am done, before I get sucked into something I cannot hope to win

Right...because your PT doesn't need to...a crank-based power meter (to which I assume Titanflexer was referring), on the other hand, does need crank rotation rate (i.e. cadence) to calculate power.

http://bikeblather.blogspot.com/

Re: PowerTap efficiency [jeffp] [ In reply to ]

Apr 26, 09 8:29

Post #17 of 25 (4558 views)

Game over

Re: PowerTap efficiency [cheyou] [ In reply to ]

Apr 26, 09 8:37

Post #18 of 25 (4550 views)

I need to remember when I am actually typing, that pink font needs to be used a lot around here...........

that helps, as my poor attempts at sarcasm never seem to make it across

Re: PowerTap efficiency [SwBiRu] [ In reply to ]

Apr 26, 09 10:37

Post #19 of 25 (4508 views)

So, how many beats does wearing a HRM cost you?

Re: PowerTap efficiency [Tom A.] [ In reply to ]

....

Apr 26, 09 10:50

Post #20 of 25 (4499 views)

If the 2.4 calculates cadence directly, and it's always reliable, I must ask...why does power tap also sell a cadence sensor?

Re: PowerTap efficiency [Francois] [ In reply to ]

Apr 26, 09 11:01

Post #21 of 25 (4490 views)

In Reply To:

If the 2.4 calculates cadence directly, and it's always reliable, I must ask...why does power tap also sell a cadence sensor?

Where did you read that it "calculates cadence directly"?

http://bikeblather.blogspot.com/

Re: PowerTap efficiency [Francois] [ In reply to ]

Apr 26, 09 11:29

Post #22 of 25 (4471 views)

PowerTap looks at the changes in torque (no one pedals evenly, we all apply far more force on the downstroke than the upstroke or the "dead spots" at 12 and 6) and derives a cadence from this. They sell the additional sensor for those who want the accuracy of measuring cadence directly.

ECMGN Therapy Silicon Valley:
Depression, Neurocognitive problems, Dementias (Testing and Evaluation), Trauma and PTSD, Traumatic Brain Injury (TBI)

Re: PowerTap efficiency [Francois] [ In reply to ]

Apr 26, 09 11:50

Post #23 of 25 (4447 views)

Because it is not reliable, and that is not in pink text. At least not for me. I get spikes up and down between 20-40 rpm from both steady state efforts, and especially at starts for 1-3 seconds.

I've got plenty of data where I am riding at 300+ watts during an interval and the cadence will suddenly go up and down and then back to where it was with no change in speed, gearing or power output that would make that large a change(30 rpm range). At starts, it likes to spike to 140 or so for a brief period, even though speed is lower and gearing the same as a few seconds later where it drops to a closer, to reality, value.

I gues my pedal stroke must be both smoother and choppier than expected at different times. which I can see being the case, but it makes the cadence about as useful as the HR on my 2.4PT. I won't bother going into the wild flucuations on the HR.

Re: PowerTap efficiency [jeffp] [ In reply to ]

....

Apr 26, 09 11:52

Post #24 of 25 (4445 views)

I know why they sell a sensor... ;-)

Re: PowerTap efficiency [Francois] [ In reply to ]

Apr 26, 09 11:55

Post #25 of 25 (4444 views)

If the 2.4 calculates cadence directly, and it's always reliable, I must ask...why does power tap also sell a cadence sensor?

To use your computer with another wheel without the power tap hub.

Francois in Montreal

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