Froome's SRM data on Ventoux

[Escarabajo]

Re: Re:

Thanks for the answer.

To the bolded. Is that possible with power meters today? do some power meter have this capability?
The integration of the Torque times the angular velocity is what gives me power which equivalent to the summation. In that case we would not need a correction factor if well calibrated, right?

To the best of my knowledge, no power meter on the market samples angular velocity more than once or twice per revolution - so no integration and therefore some inaccuracy. To add an encoder would definitely add cost, weight, complexity as well as demand more computing resources which is probably why nobody does it. As it is right now, it can be a bit of an effort to stuff a device full of strain gauges and get meaningful data. Then you have to deal with things like vibration, temperature drift, calibration, etc, etc. Oh, and try to get the cost down to a couple hundred bucks so you can make a living at it.

Of course, this doesn't mean current power meters are useless even if they are inaccurate. Taking large time-based samples allows you to average out the data and get a pretty good picture of performance - which is why Froome *really* hates that the Ventoux data got out into the wild. Talk inaccuracies and 6% errors all you want, there's more than enough data to get a good picture of how well he's riding. And when you back that up with different methods (a la Vayer, Ferrari et al) you can have a lot of confidence in your conclusions.

Which is why Sky's "release of data" is just so much bullsh!t. Instead of feeding us the conclusions, hand over the raw data. It'll pretty much confirm that he did 6.1+ W/kg so we'll never see it.

John Swanson

Thanks again for the answer.

So there has to be a factor. It looks like they picked the highest factor and the highest weight as well.

 

[Escarabajo]

BTW, I estimated Quintana's power at around 6 watts/kg for PSM. His VO2 max was measured around 91.

 

[Dear Wiggo]

Re: Re:

Thanks for the answer.

To the bolded. Is that possible with power meters today? do some power meter have this capability?
The integration of the Torque times the angular velocity is what gives me power which equivalent to the summation. In that case we would not need a correction factor if well calibrated, right?

To the best of my knowledge, no power meter on the market samples angular velocity more than once or twice per revolution - so no integration and therefore some inaccuracy. To add an encoder would definitely add cost, weight, complexity as well as demand more computing resources which is probably why nobody does it. As it is right now, it can be a bit of an effort to stuff a device full of strain gauges and get meaningful data. Then you have to deal with things like vibration, temperature drift, calibration, etc, etc. Oh, and try to get the cost down to a couple hundred bucks so you can make a living at it.

Of course, this doesn't mean current power meters are useless even if they are inaccurate. Taking large time-based samples allows you to average out the data and get a pretty good picture of performance - which is why Froome *really* hates that the Ventoux data got out into the wild. Talk inaccuracies and 6% errors all you want, there's more than enough data to get a good picture of how well he's riding. And when you back that up with different methods (a la Vayer, Ferrari et al) you can have a lot of confidence in your conclusions.

Which is why Sky's "release of data" is just so much bullsh!t. Instead of feeding us the conclusions, hand over the raw data. It'll pretty much confirm that he did 6.1+ W/kg so we'll never see it.

John Swanson

There is one. It uses an inclinometer rather than a reed switch(s). Don't remember which one. Ah a quick google says this: http://www.gizmag.com/limits-pedal-cycling-power-meter/37287/

I asked on twitter and apparently they are potentially going to do continuous measure.

 

[Escarabajo]

It is not pretty!

It will probably get better over time.

 

[.Froomestrong.]

Its been mentioned upthread, but some clarity is in order regarding power meters.

Direct force type power meters are currently being used at World Tour level, in two types- spider based (Quark, SRM, Power2Max), and crank arm based (Pioneer, Stages, etc)

In 2013, Sky used SRM, which are spider based. This type of PM "may" suffer from drift, or other data anomolies when paired with oval rings. This is debatable, and despite what has been claimed in this thread by the "experts", there is little actual evidence to suggest that this generalizes.

In 2014 and 2015, Sky uses Stages, which is arm based. On most of the Sky bikes at the tour, Stages has supplied a dual sided PM, with each arm having strain gauges. This version of Stages is due to be released to the public later this year, to augment their existing left side only PM.

Arm based PM's measure force at the crank arm- before the force goes to the spider and rings- and are therefore not subject to inaccuracies incurred by using oval rings. While Stages uses velocity as part of their algorithm, the force which causes arm deflection is the important event.

The accuracy of the Stages systems vs SRM is another discussion entirely, but there is little doubt that these things work very well indeed.

 

[franic]

Re:

Its been mentioned upthread, but some clarity is in order regarding power meters.

Direct force type power meters are currently being used at World Tour level, in two types- spider based (Quark, SRM, Power2Max), and crank arm based (Pioneer, Stages, etc)

In 2013, Sky used SRM, which are spider based. This type of PM "may" suffer from drift, or other data anomolies when paired with oval rings. This is debatable, and despite what has been claimed in this thread by the "experts", there is little actual evidence to suggest that this generalizes.

In 2014 and 2015, Sky uses Stages, which is arm based. On most of the Sky bikes at the tour, Stages has supplied a dual sided PM, with each arm having strain gauges. This version of Stages is due to be released to the public later this year, to augment their existing left side only PM.

Arm based PM's measure force at the crank arm- before the force goes to the spider and rings- and are therefore not subject to inaccuracies incurred by using oval rings. While Stages uses velocity as part of their algorithm, the force which causes arm deflection is the important event.

The accuracy of the Stages systems vs SRM is another discussion entirely, but there is little doubt that these things work very well indeed.

Thanks. That should mean that post 2013 data shouldn’t be affected by oval rings, should they?

 

[franic]

Re: Re:

Hinault is the greatest of all time. Glad I could clear that up for everyone

Merckx's will headbutt-Hinault the greatest of all time!

You are naming the riders who won the most in cycling. The greatest was born in Lakewood, CA.

 

[ray j willings]

All this talk of POWER ,,,
Do you think its power that spins those granny rings at 100RPM ?

 

[M Sport]

Re:

Its been mentioned upthread, but some clarity is in order regarding power meters.

Direct force type power meters are currently being used at World Tour level, in two types- spider based (Quark, SRM, Power2Max), and crank arm based (Pioneer, Stages, etc)

In 2013, Sky used SRM, which are spider based. This type of PM "may" suffer from drift, or other data anomolies when paired with oval rings. This is debatable, and despite what has been claimed in this thread by the "experts", there is little actual evidence to suggest that this generalizes.

In 2014 and 2015, Sky uses Stages, which is arm based. On most of the Sky bikes at the tour, Stages has supplied a dual sided PM, with each arm having strain gauges. This version of Stages is due to be released to the public later this year, to augment their existing left side only PM.

Arm based PM's measure force at the crank arm- before the force goes to the spider and rings- and are therefore not subject to inaccuracies incurred by using oval rings. While Stages uses velocity as part of their algorithm, the force which causes arm deflection is the important event.

The accuracy of the Stages systems vs SRM is another discussion entirely, but there is little doubt that these things work very well indeed.

Not correct on the bold bit, while you are correct the force is measure through the crank arm the speed is only measured once per revolution. Therefore the irregular chain ring velocity will cause power to be overstated.

From the stages site http://support.stagescycling.com/support/solutions/articles/1000158537-can-i-use-the-stages-power-meter-with-oval-or-osymetric-chain-rings-

Our power measurement through a Stages Power meter is event based, where as an event is one complete pedal revolution. Due to the changes in velocity non-round chain rings produce through the course of an 'event' you will see that your power will be skewed higher than with a round ring, which has a constant velocity throughout each event. Through our own testing, and using a hub-based meter as a control, we conclude that there will be a 4-5% increase on the readings from a Stages Power meter when used with a non-round chain rings.

What I think Sky should have reported yesterday was how the Left Right balance effected the power. They overstated the effect of the rings but neglected the balance issue. That in my opinion could swing it back closer to zero.

Sky will know that variance.

 

[ray j willings]

Power is weight transference is it not? so how can the shape of the ring influence the actual Force? Speed maybe a wee bit depending on the actual point measured on the rotation ,it would be a tiny amount at most.

I think it's Sky bull$$$$

 

[Alex Simmons/RST]

Re:

Its been mentioned upthread, but some clarity is in order regarding power meters.

Direct force type power meters are currently being used at World Tour level, in two types- spider based (Quark, SRM, Power2Max), and crank arm based (Pioneer, Stages, etc)

In 2013, Sky used SRM, which are spider based. This type of PM "may" suffer from drift, or other data anomolies when paired with oval rings. This is debatable, and despite what has been claimed in this thread by the "experts", there is little actual evidence to suggest that this generalizes.

In 2014 and 2015, Sky uses Stages, which is arm based. On most of the Sky bikes at the tour, Stages has supplied a dual sided PM, with each arm having strain gauges. This version of Stages is due to be released to the public later this year, to augment their existing left side only PM.

Arm based PM's measure force at the crank arm- before the force goes to the spider and rings- and are therefore not subject to inaccuracies incurred by using oval rings. While Stages uses velocity as part of their algorithm, the force which causes arm deflection is the important event.

The accuracy of the Stages systems vs SRM is another discussion entirely, but there is little doubt that these things work very well indeed.

It matters not where the force is measured (be it spider, crank arms, chainrings or pedals), if the rotational velocity is not also measured with high frequency sampling then the error will still occur.

I'm afraid that Stages still makes the assumption of non-variable crank rotational speed during each crank revolution, and measures crank rotational speed as a single revolution average, and as such they are subject to the very same error when used with non-circular chainrings as are spider based power meters.

Indeed due to the use of accelerometers for determining cadence, the timing of each crank revolution is less precise than when done with a magnet and reed switch (the data is certainly far noisier*) and as such it's also subject to aliasing error, further compounded by the relatively lower torque sampling rate of 60Hz (meaning torque samples straddle one crank revolution to the next).


* So noisy in fact that it took Stages quite some time to work out how to report cadence (i.e. crank rotational velocity once per revolution) with any reliability, let alone attempting to measure crank rotational velocity multiple times per revolution.

 

[Alex Simmons/RST]

Re: Re:

What I think Sky should have reported yesterday was how the Left Right balance effected the power. They overstated the effect of the rings but neglected the balance issue. That in my opinion could swing it back closer to zero.

Sky will know that variance.

Sky may have an idea of general or typical trends for a rider, but power imbalance is variable and multifactoral. It varies with position on the bike, fatigue, power, gear/cadence, inertial load (e.g. flat v climbing), any acute injury or niggle and so on. It varies during a ride and from ride to ride. So if only measuring left leg (although they may be using a two sided measurement device not yet available to purchase) then the error will be variable and unknown.

Asymmetry is also normal.

 

[Alex Simmons/RST]

Re:

Power is weight transference is it not? so how can the shape of the ring influence the actual Force? Speed maybe a wee bit depending on the actual point measured on the rotation ,it would be a tiny amount at most.

I think it's Sky bull$$$$

The issue of non-circular rings and power meter accuracy has been known about since well before Sky even existed as a team.

 

[Alex Simmons/RST]

All this talk of POWER ,,,
Do you think its power that spins those granny rings at 100RPM ?

Cadence is a dependent variable, and is simply an outcome of the power applied (i.e. effort level), gear chosen and the resistance forces.

 

[Alex Simmons/RST]

Re: Re:

There is one. It uses an inclinometer rather than a reed switch(s). Don't remember which one. Ah a quick google says this: http://www.gizmag.com/limits-pedal-cycling-power-meter/37287/

I asked on twitter and apparently they are potentially going to do continuous measure.

It will be interesting to see if it ever makes the light of day (many have tried and failed). For now it's just promises / vapourware. Not sure the extra q-factor will be particularly popular. They've already written some rubbish about q-factor, so let's hope they pick up their game with more factual information in future.

 

[Alex Simmons/RST]

Re: Re:

To the bolded. Is that possible with power meters today? do some power meter have this capability?
The integration of the Torque times the angular velocity is what gives me power which equivalent to the summation. In that case we would not need a correction factor if well calibrated, right?

In short, no current power meter upstream of the bottom bracket has demonstrated this capability. A couple have claimed it, one of those was telling porkies.

The other (Rotor) has suggested they use an alternative means to enable sampling of rotational velocity more frequently but I'm yet to see any valid testing and solid explanation of how this is being done. Even a simple data stream of rotational velocity data would help.

As for changing calibration of meters to account for the error, the problem as I have pointed out a few times already is that the error is variable during a ride, and not a constant. So for instance your can't simply change your SRM's slope to correct for it.

 

[ScienceIsCool]

Re: Re:

There is one. It uses an inclinometer rather than a reed switch(s). Don't remember which one. Ah a quick google says this: http://www.gizmag.com/limits-pedal-cycling-power-meter/37287/

I asked on twitter and apparently they are potentially going to do continuous measure.

It will be interesting to see if it ever makes the light of day (many have tried and failed). For now it's just promises / vapourware. Not sure the extra q-factor will be particularly popular. They've already written some rubbish about q-factor, so let's hope they pick up their game with more factual information in future.

An inclinometer is a horrible way to do this. They might get it working in the lab, but it'll be prone to many failure modes such as response time and vibration artifacts. The best way of course is an encoder. Optical is best because it's contactless and has precise rising and falling edges.

John Swanson

 

[ScienceIsCool]

Re:

Power is weight transference is it not? so how can the shape of the ring influence the actual Force? Speed maybe a wee bit depending on the actual point measured on the rotation ,it would be a tiny amount at most.

I think it's Sky bull$$$$

No, in this case power is torque time rotational velocity. It doesn't matter if the force changes because you are measuring that several times per second. The errors happen because the velocity is changing, but you only measure it once per revolution.

John Swanson

 

[Escarabajo]

Re:

Power is weight transference is it not? so how can the shape of the ring influence the actual Force? Speed maybe a wee bit depending on the actual point measured on the rotation ,it would be a tiny amount at most.

I think it's Sky bull$$$$

Power in one revolution= Force x Velocity

For an irregular process:

Power in one revolution = Summation in one revolution of (Force x Velocity)

For this case of oval rings there is no summation because there is only one reading for velocity per revolution when you should have several because of its irregular form so the values are skewed. For a round ring velocity is more homogeneous and therefore little correction is needed so the first formula is applied.

 

[zigmeister]

First, all PMs have a variance no matter what crank/rings/pedals or bike it is on. So right there, you have 1.5-3% typically they can be "off". I run QRings, the regular, not super Qs that Rotor makes now which are close to Osymmetric, which are ridiculous. There is variance as well. 6%? Well, taking the standard variance that a PM has already out of the box, then the oval rings, 6% total could be reasonable.

But, that would means around 24w, if we assume the thing always reads high by 6%...not sure how that is determined. Doesn't take much to just put a set of rings on a PM, do some testing, change the rings, do some more testing to determine this, with no calibrating. Should give an idea.

But alas, all the PMs maker's are full of crap and marketing hype, claiming very low variances...when really, they might be higher out of the box as well. Who is going to prove otherwise? Quarq claims a 1.5% accuracy....mmmmkay....

They also claim 300hrs of riding time for the battery to last..funny, mine doesn't last that long ever....another marketing B.S. claim. Actually had to send a PM back and they replaced it due to it eating batteries one time..side bar I know...their customer service is great...but their PMs, like SRM, and the likes, are not perfect...far from it.

 

[AICA ribonucleotide]

Here is the thing I dont get. Wiggins and Froome are two guys renowned for staring at their power meters as they cycle. They are riding to a wattage.
But they using osymetric chain rings. But the power meter fails to display accurate wattage if you are using osymetric chain rings.
What wattage are they then riding to? What are they staring at?

 

[MikeS369]

Is there no way to calibrate a power meter taking the error margin into account?

 

[Ripper]

Froome's more recent data release is priceless. 414 watts at 67.5 kg is over 6.1. However, I realllllly doubt he is heavier the DL, where he was 66, which puts him at 6.3. For all we know, he shed a bit more between the two races in the last bit of sharpening, leading to 65, which would be 6.4. So the thing is, we know that things are being tweaked in any way possible. "Oh, it was 414 watts, but the margin of error could be up to 6% (although more like 4% by the manufacturer), so that's blah blah blah." Oh, we don't know his weight. Oh wait, yes we do ... he's heavier than most of the year.

And then let's not even start with the fact that he is riding faster than others in the race that had supposedly higher output of watts per kg.

Simply amazing!

 

[AICA ribonucleotide]

Is there no way to calibrate a power meter taking the error margin into account?

There might be but Sky obviously aren't doing it if they are having to factor in a 6% reduction in power wattage in the w/kg calculation they released.

 

[malakassis]

Re: Re:

There is one. It uses an inclinometer rather than a reed switch(s). Don't remember which one. Ah a quick google says this: http://www.gizmag.com/limits-pedal-cycling-power-meter/37287/

I asked on twitter and apparently they are potentially going to do continuous measure.

It will be interesting to see if it ever makes the light of day (many have tried and failed). For now it's just promises / vapourware. Not sure the extra q-factor will be particularly popular. They've already written some rubbish about q-factor, so let's hope they pick up their game with more factual information in future.

An inclinometer is a horrible way to do this. They might get it working in the lab, but it'll be prone to many failure modes such as response time and vibration artifacts. The best way of course is an encoder. Optical is best because it's contactless and has precise rising and falling edges.

John Swanson

hi! on which measuring principle are the powermeters based for the force-measure? piezoelectric ?(good for very rigid like crank and very dynamic)
how many sampling points would you suggest per revolution. sorry im new to this but interested.
yeah optics are best - expect you ride strade bianche in rain. or is there any chance to put optics with sufficient number of sampling points INSIDE the tube?