Power Meter Differences

markwb79

Alex_Simmons/RST wrote:

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.

Thanks, I wasn't referring to a possible higher CdA when I said 10% extra weight.

Weight is completely measurable and surely carrying an extra 10% in kg's would make a difference.

10% extra weight means you need to ride at a higher power assuming all things are equal.

So then aerodynamics is making as much as 20% difference between us?

Just doesn't sound right to me I am afraid.

cougie

cyd190468 wrote:

Regarding aero, it often comes as a great surprise to people that at low speeds like up to 40kmph the perfect aero shape is actually short and fat. Think teardrop with about a 2:1 length to width. As people go that's pretty chunky. Tall skinny guys are only aero if you going really fast.

I like this idea ! Anyone know where I can buy 'cake' ?


I think this thread needs photos.

If you put your positions up on something like slowtwitch you'll soon get about 5 pages of comments. Usually conflicting mind you.

Alex_Simmons/RST

meanredspider 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.

I think he was right.

I don't but ce la vie.

meanredspider wrote:

For most people, what does it matter what units you measure power in?
<snip>
And, yes, you can make your meter more accurate by spending more money. But, really, what's the point?

I think you are making rather large assumptions about what other people's requirements might be.

If your usage of power meters is confined to the very narrow paradigm you paint, then I agree with you. Indeed I agree so much that a power meter isn't even required for such uses so save your money and don't buy models that are not consistent or accurate.

But if you use a power meter for a wide range of versatile performance development applications, then the performance of the power meter, in particular accuracy, matters.

Alex_Simmons/RST

Markwb79 wrote:

Alex_Simmons/RST wrote:

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.

Thanks, I wasn't referring to a possible higher CdA when I said 10% extra weight.

Weight is completely measurable and surely carrying an extra 10% in kg's would make a difference.

10% extra weight means you need to ride at a higher power assuming all things are equal.

So then aerodynamics is making as much as 20% difference between us?

Just doesn't sound right to me I am afraid.

Well when riding side by side, do you mean going up a solid gradient, or along flatter terrain?

On steeper positive gradients then speed is directly proportional to W/kg. So if your heavier mate is reading lower power at same climbing speed, then yes, something is up.

But on the flat it's mostly about W/m^2 and unless you know the relative CdA of yourself and your mate, you won't know much about the expected power demand at a given speed for each of you. Simple things like clothing, position, helmets, wheels may easily be a factor in creating a power demand differential. Another may be quality tyres versus poor rolling rubber.

Mass isn't a big factor on a flat road.

e.g. let's assume two riders with same road CdA of 0.35m^2 but a mass difference of 80kg total and 90kg total. On flat road the power difference for both to ride at 30km/h is only ~3W.

markwb79

Found a picture. Yellow over shoes and furthest right with the black shoes.

Really can't imagine 30watts difference between the 2 of us.

BeaconRuth

Then you haven't got the message yet that aerodynamic drag is massively affected by all sorts of details of your position and what you're wearing, and that aero drag is the biggest factor affecting your speed for a given power output.

I can easily imagine 30W difference between two individuals of nominally similar shape and size and I work day in, day out with time triallists who realise that reducing their drag is hyper-critical and very very sensitive to even whether or not they shave their legs.

Ruth

cougie

What was shaved legs like ? 15 watts ? Who would have thought that eh ?

norvernrob

Surely you should just go out together, ride together for a mile or two noting the wattages, turn around and ride back to where you started and swap bikes, then ride the same couple of miles again. Obviously it won't be 100% perfect as you're on each other's bikes, but if you're very similar anyway then it should be easy enough to spot any obvious discrepancies with the PM.

If one bike is showing more power with both riders then it's obviously the PM differences, if the same riders are showing pretty much the same power on both bikes then it's down to the riders.

RChung

Which are you? Is this how you two were dressed on the day you made this observation? The eye is a poor wind tunnel but my eye wouldn't be shocked if the guy on the right had lower aero drag than the guy with yellow over shoes.

BeaconRuth

cougie wrote:

What was shaved legs like ? 15 watts ? Who would have thought that eh ?

https://www.youtube.com/watch?v=DZnrE17Jg3I

Ruth

markwb79

RChung wrote:

Which are you? Is this how you two were dressed on the day you made this observation? The eye is a poor wind tunnel but my eye wouldn't be shocked if the guy on the right had lower aero drag than the guy with yellow over shoes.

Totally correct, I am in the yellow overshoes. This was a different day, slightly differently dressed. I had a different jacket, aero over shoes. The other guy was similar, but with overshoes too. But yes, with a aero helmet.

What makes you think he has lower drag? Really curious? I told him about this thread and he laughed when I suggested I had the high drag. But totally possible, especially after your observation.

RChung

Well, as I said, the eye is a poor wind tunnel, and if you really want to know for sure you should measure.

So partly my guess was just a guess, and the photo wasn't ideal because from that angle I couldn't really see differences in body position -- but it looked like his clothing was tighter, he had a helmet with fewer vents, and shoe covers are usually an aero loss. None of those things in isolation is much but it's pretty common to see a large total effect by the accretion of lots of small things. One of the things I've learned through measurement of drag is that the clothes you wear can have a pretty large total effect.

markwb79

RChung wrote:

Well, as I said, the eye is a poor wind tunnel, and if you really want to know for sure you should measure.

So partly my guess was just a guess, and the photo wasn't ideal because from that angle I couldn't really see differences in body position -- but it looked like his clothing was tighter, he had a helmet with fewer vents, and shoe covers are usually an aero loss. None of those things in isolation is much but it's pretty common to see a large total effect by the accretion of lots of small things. One of the things I've learned through measurement of drag is that the clothes you wear can have a pretty large total effect.

Cool, thanks. You really had me worried! haha

markwb79

NorvernRob wrote:

Surely you should just go out together, ride together for a mile or two noting the wattages, turn around and ride back to where you started and swap bikes, then ride the same couple of miles again. Obviously it won't be 100% perfect as you're on each other's bikes, but if you're very similar anyway then it should be easy enough to spot any obvious discrepancies with the PM.

If one bike is showing more power with both riders then it's obviously the PM differences, if the same riders are showing pretty much the same power on both bikes then it's down to the riders.

Thats a really good idea, we can do that near enough I think.

JackPozzi

Markwb79 wrote:

NorvernRob wrote:

Surely you should just go out together, ride together for a mile or two noting the wattages, turn around and ride back to where you started and swap bikes, then ride the same couple of miles again. Obviously it won't be 100% perfect as you're on each other's bikes, but if you're very similar anyway then it should be easy enough to spot any obvious discrepancies with the PM.

If one bike is showing more power with both riders then it's obviously the PM differences, if the same riders are showing pretty much the same power on both bikes then it's down to the riders.

Thats a really good idea, we can do that near enough I think.

If you really want to do a test like that, could you swap the cranks rather than bikes? Swapping bikes would mean you're in different positions so discrepancies wouldn't necessarily be down to the PMs. If you swap just the cranks you can be sure you're testing the SRMs head to head with no other changes.

Alex_Simmons/RST

Markwb79 wrote:

NorvernRob wrote:

Surely you should just go out together, ride together for a mile or two noting the wattages, turn around and ride back to where you started and swap bikes, then ride the same couple of miles again. Obviously it won't be 100% perfect as you're on each other's bikes, but if you're very similar anyway then it should be easy enough to spot any obvious discrepancies with the PM.

If one bike is showing more power with both riders then it's obviously the PM differences, if the same riders are showing pretty much the same power on both bikes then it's down to the riders.

Thats a really good idea, we can do that near enough I think.

It's still not going to answer the question.

Different bike = different position and equipment = different aerodynamics.

Just validate the power meter's slope yourself, make sure you are setting the zero offset correctly and check that the auto zero isn't doing silly things.

cougie

Are you both riding the same width handlebars ?

http://www.aeroweenie.com/assets/img/da ... d-bars.png

markwb79

cougie wrote:

Are you both riding the same width handlebars ?

http://www.aeroweenie.com/assets/img/da ... d-bars.png

Interesting, I actually have the narrower bars.

cougie

Break out the tape measure ! I demand a recount !