Power Meter Differences

markwb79

Never know where to put questions like this...

Me and a friend both have SRM's. We recently calibrated them, with only subtle changes to both. We test regularly and our FTP's are quite similar.

But when we ride side by side, I am always reading about 25watts higher. This is really strange as he is slightly bigger built. 6kg heavier. So I would have thought he should be riding slightly higher.

This also correlates to our heart rates. When we test, my HR is about 6-7 bpm less for a 20minute effort.

We have looked at loads of data and I still cant get my head around the reasons for the difference.

I cant imagine loosing that much through drive train efficiency, I have recently replaced my BB for example. And everything 'feels' smooth.

Thoughts?

thegibdog

He is more aero than you?

meanredspider

Could you try swapping bikes? Just to see if you're still putting out 25W more for the same speed. Maybe you're an un-aero shape?

My contention has always been that the real number doesn't matter (Angry Asian also wrote something similar) and the whole question of accuracy of PM's is a bit of a red herring (with the rather niche exception of aero testing) provided they're consistent.

markwb79

thegibdog wrote:

He is more aero than you?

No, both side by side, on the hoods. Compared loads of times too.

markwb79

meanredspider wrote:

Could you try swapping bikes? Just to see if you're still putting out 25W more for the same speed. Maybe you're an un-aero shape?

My contention has always been that the real number doesn't matter (Angry Asian also wrote something similar) and the whole question of accuracy of PM's is a bit of a red herring (with the rather niche exception of aero testing) provided they're consistent.

Thats our next plan, swapping bikes. By Dutch standards I am the most aero person in this country.

Thats the thing I cant get my head round. Both have the same FTP, eg 280 watts. High HR is average about 175bpm, mine is 167bpm.
Put us side by side yesterday for example, HR's both 140, I am at 220 watts he is at 190watts.

Obviously better training for me!

cougie

Take a photo of him head on and then one of yourself head on and see if there are any differences ?

He may be heavier than you - but that maybe his position on the bike is more aero than you are - I think thats what thegibdog was getting at ?

markwb79

cougie wrote:

Take a photo of him head on and then one of yourself head on and see if there are any differences ?

He may be heavier than you - but that maybe his position on the bike is more aero than you are - I think thats what thegibdog was getting at ?

Yeah I understand what he was getting at. We have also thought about that, and its not because of that. If anything its the other way.

BeaconRuth

Markwb79 wrote:

This is really strange as he is slightly bigger built. 6kg heavier. So I would have thought he should be riding slightly higher.

Just goes to show that power/weight is a very long way from being the most important thing affecting speed on the road (on the flat).

You may be a bit smaller but there may be lots of things which make your aero drag greater. Your helmet could have a big effect, as could the cut, fabric and wrinkles in your clothes, the width of your shoulders, the design of your shoes... Also have you confirmed that the rolling resistances of your tyres are identical?

And regarding your riding positions - it's very unlikely they are identical, however much they seem to be similar. It only takes a difference of mm in riding positions to change aero drag by the equivalent of 10W or more.

Ruth

meanredspider

Markwb79 wrote:

meanredspider wrote:

Could you try swapping bikes? Just to see if you're still putting out 25W more for the same speed. Maybe you're an un-aero shape?

My contention has always been that the real number doesn't matter (Angry Asian also wrote something similar) and the whole question of accuracy of PM's is a bit of a red herring (with the rather niche exception of aero testing) provided they're consistent.

Thats our next plan, swapping bikes. By Dutch standards I am the most aero person in this country.

Thats the thing I cant get my head round. Both have the same FTP, eg 280 watts. High HR is average about 175bpm, mine is 167bpm.
Put us side by side yesterday for example, HR's both 140, I am at 220 watts he is at 190watts.

Obviously better training for me!

Maybe next time I should join you (if only to prove that I am the most aero person in this country )

JamesFree

I personally think it may just before 1 powermeter is reading high or low or a combination of both. I have ridden with people who's PM's have obviously over read compared to mine.

Try riding next to each other up a climb and see if the difference exists

Perhaps do a static torque test on both powermeters with a know (same) weight

RChung

Markwb79 wrote:

Me and a friend both have SRM's. We recently calibrated them, with only subtle changes to both.

Thoughts?

How did you do the calibration?

supermurph09

What about cadence? Could the difference be your pedalling efficiency?

markwb79

BeaconRuth wrote:

Markwb79 wrote:

This is really strange as he is slightly bigger built. 6kg heavier. So I would have thought he should be riding slightly higher.

Just goes to show that power/weight is a very long way from being the most important thing affecting speed on the road (on the flat).

You may be a bit smaller but there may be lots of things which make your aero drag greater. Your helmet could have a big effect, as could the cut, fabric and wrinkles in your clothes, the width of your shoulders, the design of your shoes... Also have you confirmed that the rolling resistances of your tyres are identical?

And regarding your riding positions - it's very unlikely they are identical, however much they seem to be similar. It only takes a difference of mm in riding positions to change aero drag by the equivalent of 10W or more.

Ruth

Thanks!

I know we don't have identical riding positions. I would say I definitely have the lesser frontal area.

We are both not first year amateurs, we both have tight fitting clothes. We have done the checks on several rides.

Rolling resistance is a thought. But if that was the case then everyone would be riding 27mm Roubaix tires. Both on Nemesis wheels actually.

As for the calibration, we went through this calibration process on both bikes last Friday.

http://weightweenies.starbike.com/forum ... =8&t=77383

The difference was there before and after unfortunately.

It all just seems weird to me, can't get my head around it all.

xavierdisley

If the power meters are calibrated correctly, on the flat it's rolling resistance and aerodynamics. Don't worry about it (or get a more aero position/clothing/helmet/better tyres to close the gap).

meanredspider

xavierdisley wrote:

If the power meters are calibrated correctly, on the flat it's rolling resistance and aerodynamics. Don't worry about it (or get a more aero position/clothing/helmet/better tyres to close the gap).

25W is not to be sniffed at so if the OP is using that much more to go the same speed (assuming the SRMs are accurate - a big IF) then it's worth finding out where it's going. My guess is that it's a combination of measurement inaccuracy and some real differences (hardly Einstein). Worth finding the real differences though.

frisbee

I would say a total 10% difference between 2 different people, 2 different bikes and 2 different power meters is actually pretty good.

Most power meters claim 2% accuracy, that's pretty good even for a static force measurement, let alone dynamic. A few watts here and there aero drag and rolling resistance and you get your 25 watts.

meanredspider

frisbee wrote:

Most power meters claim 2% accuracy, that's pretty good even for a static force measurement, let alone dynamic. A few watts here and there aero drag and rolling resistance and you get your 25 watts.

I tend to agree - but that's why accuracy claims are worth less than they first might appear. But, I have to say, 10% was more than I was expecting between similar riders (especially if it's opposite to what you might expect from frontal area)

Interested to see what happens with a bike swap.

imposter2.0

Clearly, one of the riders has a more expensive pair of wheels...**

** that was a joke, btw..

meanredspider

Imposter wrote:

Clearly, one of the riders has a more expensive pair of wheels...**

** that was a joke, btw..

It would have been funny had that been the final answer

JackPozzi

meanredspider wrote:

frisbee wrote:

Most power meters claim 2% accuracy, that's pretty good even for a static force measurement, let alone dynamic. A few watts here and there aero drag and rolling resistance and you get your 25 watts.

I tend to agree - but that's why accuracy claims are worth less than they first might appear. But, I have to say, 10% was more than I was expecting between similar riders (especially if it's opposite to what you might expect from frontal area)

Interested to see what happens with a bike swap.

To be fair, aero is about a hell of a lot more than just frontal area, and from what I've heard from people that have had tunnel time, even a tiny tweak position wise can make a sizeable difference to drag.

meanredspider

JackPozzi wrote:

meanredspider wrote:

frisbee wrote:

Most power meters claim 2% accuracy, that's pretty good even for a static force measurement, let alone dynamic. A few watts here and there aero drag and rolling resistance and you get your 25 watts.

I tend to agree - but that's why accuracy claims are worth less than they first might appear. But, I have to say, 10% was more than I was expecting between similar riders (especially if it's opposite to what you might expect from frontal area)

Interested to see what happens with a bike swap.

To be fair, aero is about a hell of a lot more than just frontal area, and from what I've heard from people that have had tunnel time, even a tiny tweak position wise can make a sizeable difference to drag.

For sure - it's frontal area x drag coefficient. But, if small differences are making (more than) 10% differences, we should all be getting tunnel time.

markwb79

meanredspider wrote:

Imposter wrote:

Clearly, one of the riders has a more expensive pair of wheels...**

** that was a joke, btw..

It would have been funny had that been the final answer

Damn....was just checking for wallet, but then remembered we was on the same wheels.

Different tyres I will add though.

markwb79

meanredspider wrote:

JackPozzi wrote:

meanredspider wrote:

frisbee wrote:

Most power meters claim 2% accuracy, that's pretty good even for a static force measurement, let alone dynamic. A few watts here and there aero drag and rolling resistance and you get your 25 watts.

I tend to agree - but that's why accuracy claims are worth less than they first might appear. But, I have to say, 10% was more than I was expecting between similar riders (especially if it's opposite to what you might expect from frontal area)

Interested to see what happens with a bike swap.

To be fair, aero is about a hell of a lot more than just frontal area, and from what I've heard from people that have had tunnel time, even a tiny tweak position wise can make a sizeable difference to drag.

For sure - it's frontal area x drag coefficient. But, if small differences are making (more than) 10% differences, we should all be getting tunnel time.

And quit giving money to coaches!

Alex_Simmons/RST

meanredspider wrote:

Could you try swapping bikes? Just to see if you're still putting out 25W more for the same speed. Maybe you're an un-aero shape?

My contention has always been that the real number doesn't matter (Angry Asian also wrote something similar) and the whole question of accuracy of PM's is a bit of a red herring (with the rather niche exception of aero testing) provided they're consistent.

That was a crap article.

If your meter is consistent, then accuracy is just a matter of calibration.

markwb79

Alex_Simmons/RST wrote:

meanredspider wrote:

Could you try swapping bikes? Just to see if you're still putting out 25W more for the same speed. Maybe you're an un-aero shape?

My contention has always been that the real number doesn't matter (Angry Asian also wrote something similar) and the whole question of accuracy of PM's is a bit of a red herring (with the rather niche exception of aero testing) provided they're consistent.

That was a crap article.

If your meter is consistent, then accuracy is just a matter of calibration.

Totally agree, thats why I am at a loss to understand it. Any ideas would be great?

Alex_Simmons/RST

Markwb79 wrote:

Never know where to put questions like this...

Me and a friend both have SRM's. We recently calibrated them, with only subtle changes to both. We test regularly and our FTP's are quite similar.

But when we ride side by side, I am always reading about 25watts higher. This is really strange as he is slightly bigger built. 6kg heavier. So I would have thought he should be riding slightly higher.

This also correlates to our heart rates. When we test, my HR is about 6-7 bpm less for a 20minute effort.

We have looked at loads of data and I still cant get my head around the reasons for the difference.

I cant imagine loosing that much through drive train efficiency, I have recently replaced my BB for example. And everything 'feels' smooth.

Thoughts?

The HR comparison is irrelevant. Everyone's HR response is different.

As for the power difference, providing the data is accurate, and it sounds like it probably is (make sure zero offset is correct as well) then on flat roads it's likely down to aerodynamic differences.

I have a mate who (when we are both fit) is same height and mass as me and we have same threshold power. However his coefficient of drag area (CdA) is 25% less than mine, meaning his power output at same speed is significantly less than mine (and he can sustain a higher speed than me at threshold).

His shape and frontal area is different - he is aerodynamically gifted.

Apart from frontal area, shape matters a lot in aerodynamics, as does the nature of the surfaces.

markwb79

Alex_Simmons/RST wrote:

Markwb79 wrote:

Never know where to put questions like this...

Me and a friend both have SRM's. We recently calibrated them, with only subtle changes to both. We test regularly and our FTP's are quite similar.

But when we ride side by side, I am always reading about 25watts higher. This is really strange as he is slightly bigger built. 6kg heavier. So I would have thought he should be riding slightly higher.

This also correlates to our heart rates. When we test, my HR is about 6-7 bpm less for a 20minute effort.

We have looked at loads of data and I still cant get my head around the reasons for the difference.

I cant imagine loosing that much through drive train efficiency, I have recently replaced my BB for example. And everything 'feels' smooth.

Thoughts?

The HR comparison is irrelevant. Everyone's HR response is different.

As for the power difference, providing the data is accurate, and it sounds like it probably is (make sure zero offset is correct as well) then on flat roads it's likely down to aerodynamic differences.

I have a mate who (when we are both fit) is same height and mass as me and we have same threshold power. However his coefficient of drag area (CdA) is 25% less than mine, meaning his power output at same speed is significantly less than mine (and he can sustain a higher speed than me at threshold).

His shape and frontal area is different - he is aerodynamically gifted.

Apart from frontal area, shape matters a lot in aerodynamics, as does the nature of the surfaces.

Cheers Alex. Although I still find it unlikely. Its something that I will certainly check. Especially if i can save 15% of my power at the same speed.

but....It was (and always is) completely flat. Wouldnt his 10% extra weight negate some/any aero gift he happens to have?

Alex_Simmons/RST

While in general additional mass means a larger rider and an expected increase in CdA (although allometric scaling and impact on CdA is not linear with mass), it still does not automatically imply any individual will have a higher CdA just because they are heavier. I know riders lighter than me with higher CdA and riders larger than me with lower CdA.

Aerodynamics is complex.

frisbee

Alex_Simmons/RST wrote:

meanredspider wrote:

Could you try swapping bikes? Just to see if you're still putting out 25W more for the same speed. Maybe you're an un-aero shape?

My contention has always been that the real number doesn't matter (Angry Asian also wrote something similar) and the whole question of accuracy of PM's is a bit of a red herring (with the rather niche exception of aero testing) provided they're consistent.

That was a crap article.

If your meter is consistent, then accuracy is just a matter of calibration.

How many points is it measuring torque around the pedal stroke? How is it filtering and averaging these measurements? A different pedalling style could change the measured power.

Alex_Simmons/RST

frisbee wrote:

Alex_Simmons/RST wrote:

meanredspider wrote:

Could you try swapping bikes? Just to see if you're still putting out 25W more for the same speed. Maybe you're an un-aero shape?

My contention has always been that the real number doesn't matter (Angry Asian also wrote something similar) and the whole question of accuracy of PM's is a bit of a red herring (with the rather niche exception of aero testing) provided they're consistent.

That was a crap article.

If your meter is consistent, then accuracy is just a matter of calibration.

How many points is it measuring torque around the pedal stroke? How is it filtering and averaging these measurements? A different pedalling style could change the measured power.

Torque is sampled at variable rates depending on the brand of power meter. An SRM samples torque at 200Hz.

It would require large variations in crank rotational velocity during a pedal stroke for such a thing to impact power meter accuracy.

At typical inertial loads, this just doesn't happen.

meanredspider

Alex_Simmons/RST wrote:

meanredspider wrote:

Could you try swapping bikes? Just to see if you're still putting out 25W more for the same speed. Maybe you're an un-aero shape?

My contention has always been that the real number doesn't matter (Angry Asian also wrote something similar) and the whole question of accuracy of PM's is a bit of a red herring (with the rather niche exception of aero testing) provided they're consistent.

That was a crap article.

If your meter is consistent, then accuracy is just a matter of calibration.

I think he was right. For most people, what does it matter what units you measure power in? The number that you get is relevant only to you in the same way that HR is the same. The only direct output of a power meter that I'm aware of is your energy expenditure which, in any case, is also a factor of your metabolism and efficiency which is also unique to you. Tell me that, for the same precision, a power meter with an accuracy of 1% would be worth spending any more money than one that had an accuracy of 2% or even 5%. There are just too many variables for accuracy to matter. You've just added CdA difference of 25% - it makes direct power comparisons between you and your buddy as meaningless in exactly the same way as if you compared HRs. I find it interesting and amusing that accuracy is discussed and advertised by PM companies yet precision isn't discussed.

And, yes, you can make your meter more accurate by spending more money. But, really, what's the point?