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The Powermeter Thread

Moderator: King Boonen

05 Mar 2013 01:38

CoachFergie wrote:That there is scientific proof that a power meter (an SRM at least) does what it claims it does. Validly and reliably measures power.


First shown (in the scientific literature) in 1998:

http://www.academia.edu/239368/Martin_JC_Milliken_DL_Cobb_JE_McFadden_KL_Coggan_AR._Validation_of_a_mathematical_model_for_road_cycling_power._J_Appl_Biomech_1998_14_276-291
acoggan
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05 Mar 2013 01:44

CoachFergie wrote:Swart and Robinson papers


...are crap, and should have never been published.
acoggan
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05 Mar 2013 03:11

Fergie, any thoughts on the Stages Cycling Crank based Power Meter? Quite cheap but measures one leg only.
[color="DarkGreen"][SIZE="2"]"I thought of that while riding my bicycle." ~ Albert Einstein on the Theory of Relativity[/SIZE][/color]
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05 Mar 2013 03:45

Polyarmour wrote:Fergie, any thoughts on the Stages Cycling Crank based Power Meter? Quite cheap but measures one leg only.


Again nothing in the literature (nor Quarq, Pioneer, Brim Bros's, Polar Pedal Based system) but the one leg only blows it right out of the water for me. I wouldn't suggest a power meter where you can't change the slope if the calibration is out. Why spend any money to measure power if you can't trust the data?

http://www.dcrainmaker.com/2013/01/stages-power-meter-in-depth-review.html

I picked up a 2nd hand wired SRM from Bermuda of all places (far away from NZ) for US$600 and $40 freight. Plus kinda handy the Oceania SRM service centre is in Christchurch.
Hamish Ferguson
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User avatar CoachFergie
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05 Mar 2013 04:08

CoachFergie wrote:Why spend any money to measure power if you can't trust the data?
A better question: Why spend any money (let alone a lot of money) if there is no demonstrated benefit to the user?

Come on Fergie, you were the one who started this thread, where is the benefit to the user?
Life is short, both reading my posts and training with PowerCranks will make it seem longer
FrankDay
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05 Mar 2013 04:13

CoachFergie wrote:That there is scientific proof that a power meter (an SRM at least) does what it claims it does. Validly and reliably measures power. Also as a discussion point for any issues or questions people have with their power meter. For me mounting a wired SRM and for another person calibration when using Q-Rings.

Considering some people do believe that a power meter will actually make them perform better (as opposed to training, diet, recovery etc) there is clearly a need to educate people about their use.
Well, a HRM validly and reliably measures HR. Where is the evidence that any of this information (knowledge of actual power or HR) actually helps the athlete?
Life is short, both reading my posts and training with PowerCranks will make it seem longer
FrankDay
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05 Mar 2013 04:14

FrankDay wrote:A better question: Why spend any money (let alone a lot of money) if there is no demonstrated benefit to the user?

Come on Fergie, you were the one who started this thread, where is the benefit to the user?


Yup, no benefit whatsoever. My $600 2nd hand SRM or the NZ $6000 one of my riders spent on a Rotor model SRM with PC7 is never going to make us or anyone faster on the bike. But we know that's not why you measure something.
Hamish Ferguson
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User avatar CoachFergie
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05 Mar 2013 04:21

CoachFergie wrote:Yup, no benefit whatsoever. My $600 2nd hand SRM or the NZ $6000 one of my riders spent on a Rotor model SRM with PC7 is never going to make us or anyone faster on the bike. But we know that's not why you measure something.
Perhaps you could explain to me why you are measuring this then. $1,500-$5,000 is a lot of dough for no benefit.
Life is short, both reading my posts and training with PowerCranks will make it seem longer
FrankDay
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05 Mar 2013 04:22

FrankDay wrote:Well, a HRM validly and reliably measures HR. Where is the evidence that any of this information (knowledge of actual power or HR) actually helps the athlete?


More trolling. Neither helps the athlete perform better.

Evidence Based Training helps the rider perform better.

Evidence Based Nutrition helps the rider perform better.

Well planned recovery helps the rider perform better.

Improved Aerodynamics helps the rider go faster.

Pacing helps the rider perform better.

The power meter and to a far lesser extent the HR monitor helps us to measure this.
Hamish Ferguson
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User avatar CoachFergie
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05 Mar 2013 04:24

FrankDay wrote:Perhaps you could explain to me why you are measuring this then. $1,500-$5,000 is a lot of dough for no benefit.


More trolling.
Hamish Ferguson
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User avatar CoachFergie
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05 Mar 2013 04:27

CoachFergie wrote:More trolling. Neither helps the athlete perform better.

Evidence Based Training helps the rider perform better.

Evidence Based Nutrition helps the rider perform better.

Well planned recovery helps the rider perform better.

Improved Aerodynamics helps the rider go faster.

Pacing helps the rider perform better.

The power meter and to a far lesser extent the HR monitor helps us to measure this.
Then, it should be able to be demonstrated that such help actually makes a difference. If it doesn't, admit it. If you claim it does, where is the evidence to support that claim.

Edit: on another tact, where is the scientific study to support that "evidence based" anything mentioned above helps the rider to perform better.
Life is short, both reading my posts and training with PowerCranks will make it seem longer
FrankDay
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05 Mar 2013 04:41

FrankDay wrote:Then, it should be able to be demonstrated that such help actually makes a difference. If it doesn't, admit it. If you claim it does, where is the evidence to support that claim.

Edit: on another tact, where is the scientific study to support that "evidence based" anything mentioned above helps the rider to perform better.


More trolling.
Hamish Ferguson
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User avatar CoachFergie
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05 Mar 2013 07:07

veganrob wrote:I have a PowerTap hub and my cranks are Rotor 3D+ with Q rings.
...
My question is calibration, would this be done differently because of the Q rings.


CoachFergie wrote:You can check the calibration of a Powertap but you can't change the calibration of one.

Any engineers care to comment about hanging a known weight off a crank with a Q ring to check the calibration of a Powertap.


FrankDay wrote:Just as the calibration of the powertap doesn't change with the size of the wheel or whether the bike is in the small or large chain ring the fact that one has a non-round chain ring has no effect on the calibration or accuracy of the powertap.


The issue Frank is that the common means for users to validate their power meter's calibration themselves is to use the static torque check method, which involves applying a known torque to the meter and comparing that known torque to the reading provided by the power meter.

Typically this is done by hanging a precisely known weight from the pedal spindle of a horizontal crank arm as that's usually most convenient/not all that difficult to do. Known mass + known length of crank = known torque applied.

However when using this method for checking a Powertap, since the measured torque is applied to the crank and not the hub, then you also need to include the chainring and cog sizes in the calculation of what torque is "felt" at the rear hub.

Hence, what should one do when using a chainring with variable effective ring size?

The way around this problem for a Powertap user with non-round rings is not to use the crank to apply the torque, but apply the torque to the wheel via another method that is not transmitted via the drivetrain.

e.g. you could hang a known mass from the edge of the wheel - all you then need to know is the radius from centre of wheel axle to the point you are hanging the mass (and the mass of course) and to make sure the point at which the mass is being applied forms a horizontal line with the wheel axle. Slowly rotating the wheel when near horizontal will help find this point.

Another means would be to use a hanging weight scale fixed at one end connected via rope to the outer edge of the rear wheel from the other end so that the "rope" forms a tangent to the wheel and then use a rope ratchet to apply some tension. The scale will then give you the equivalent "weight" of the force being applied, multiplied by the radius to the point of attachment to the wheel and that's the known hub torque you can use to compare with what the Powertap reports.
User avatar Alex Simmons/RST
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05 Mar 2013 07:10

veganrob wrote:Coach,
Any opinion on the new Rotor cranks coming out that measure power. Pros, cons, is it more data than is needed etc. Thanks

rob


too early really, early users are reporting teething problems but that's pretty typical for new power meters when they hit the market.
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05 Mar 2013 07:16

Polyarmour wrote:Quite cheap but measures one leg only.


That about sums it up.

Aluminium crank options only if that matters (which it will for some).

It'll have its place in the pantheon of power meter offerings, and as long as people realise that left-right power is neither symmetrical, nor consistently asymmetrical.

As a left leg amputee, I reckon the power numbers would suck :D
User avatar Alex Simmons/RST
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05 Mar 2013 12:14

Alex Simmons/RST wrote:The way around this problem for a Powertap user with non-round rings is not to use the crank to apply the torque, but apply the torque to the wheel via another method that is not transmitted via the drivetrain.


I've used a chainwhip for this purpose before - makes it pretty easy to test differences between cogs/alignment on a single cog.
acoggan
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05 Mar 2013 12:15

FrankDay wrote:Where is the evidence that any of this information (knowledge of actual power or HR) actually helps the athlete?


Ask Bradley Wiggins.
acoggan
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05 Mar 2013 15:53

acoggan wrote:...are crap, and should have never been published.


With all due respect, that's rich coming from you and your defence of another very crap paper.
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05 Mar 2013 16:12

Thank you Alex.
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05 Mar 2013 16:12

Frank - I'm not sure if you don't get it or are deliberately being obtuse, but please let this be a discussion about power meters and please do not let it degenerate into the farce that is the multitude of crank/pedalling threads.

Power meters do not improve performance. They are a measuring device, just the same as a heart rate monitor and anything else that measures data. However, there are many advantages for using a power meter to measure data compared to a heart rate monitor because it is a more accurate reflection of your work at that time regardless of road and weather conditions. In a training situation, it can allow you to quantify improvements over a given course. In a race situation, it can help you pace yourself so that you don't blow up.

Andrew Coggan's books on power meters and their use are excellent resources. I presume you have read them, but if not I would definitely recommend them.
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