Accuracy of Watts vs other PM

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cheoherrera
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Joined: Sun Dec 06, 2015 9:42 am

Accuracy of Watts vs other PM

Post by cheoherrera »

Hi, i know this is an old topic, but i have been talking with a couple of training buddies about the newton accuracy... while i find the Newton very consistent, when i compare my readings to other people using different brands of PM their readings are usually lower... according to my Newton my FTP right now is 277 or 4.42 W/Kg (my weight is 146 lbs). they keep telling me that those numbers are too high for me.

How can i truly know if my newton is well calibrated and my readings are as accurate as they can be? i consider myself a decent amateur triathlete, having completed more than 10 halfs and one full ironman, with a PR for Half-Iron of 4:57. on a recent Half-Iron i finished the bike portion in 2:27 (sadly, probably due to the fact that i had to travel and assembled the bike the day before, my power meter did not worked that day.. ).
Velocomp
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Re: Accuracy of Watts vs other PM

Post by Velocomp »

Please post a ride file, preferably one where you did your FTP test.
John Hamann
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Re: Accuracy of Watts vs other PM

Post by Pete »

cheoherrera wrote:Hi, i know this is an old topic, but i have been talking with a couple of training buddies about the newton accuracy... while i find the Newton very consistent, when i compare my readings to other people using different brands of PM their readings are usually lower... according to my Newton my FTP right now is 277 or 4.42 W/Kg (my weight is 146 lbs). they keep telling me that those numbers are too high for me.

How can i truly know if my newton is well calibrated and my readings are as accurate as they can be? i consider myself a decent amateur triathlete, having completed more than 10 halfs and one full ironman, with a PR for Half-Iron of 4:57. on a recent Half-Iron i finished the bike portion in 2:27 (sadly, probably due to the fact that i had to travel and assembled the bike the day before, my power meter did not worked that day.. ).
The best way is to borrow one of your buddies' meters and do a few rides to compare readings.
I borrowed a mate's PT wheel for a few weeks (mainly to derived a curve for rollers) and used it on a few rides. I was happy with the closeness.
brookside
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Re: Accuracy of Watts vs other PM

Post by brookside »

My readings seem too high at times as well, some things can be corrected, some I don't think can be.

Correctable - don't use the IBike suggestions for Crr and CDA, they're probably too high causing larger power numbers than you're really pulling. With good tires (e.g., almost any 320 tpi tire, Conti GP4000s, Conti TT, Conti Supersonic, Bontrager R4, Specialized Turbo Cotton) and latex tubes your Crr should be set @ .0028 to .0035, not .004. See https://docs.google.com/spreadsheets/d/ ... gid=624765 for some test results, and there are other sources on the web if you search a bit. For CDA it all depends on your exact position, clothing, helmet, water bottle placement and type, wheels and tires, but in general I think the Ibike recommendations are too high. After experimentation I use .3 for rides I do primarily in the drops or 90 deg bent armed on the hoods (which is slightly faster than in the drops).

Correctable post ride - on many rides that are essentially out and back I see wind readings that show an average 1 to 2 mph headwind for the entire ride. I use Isaac to set average wind at 0 in those cases. Even if I do regular wind calibrations in my garage before going out with hand cupped over front of Newton, this is too frequent an occurrence to believe the wind changes direction that often when various weather sources report it as not changing. The iBike also usually thinks I end at a higher elevation than I started, fix this in Isaac as well.

The Newton's readings just won't be the same as a DFPM because of all the estimation going on. A couple days ago I was tired and on a recovery ride but still the power numbers the Newton was showing seemed way too low, in this case, for the effort involved. About 14 miles in another cyclist comes by me who has a Powertap wheel, and I accelerate a bit to stay with him. Going up a 2 to 3% grade we're riding at a steady pace I ask what his power number is, he says 315, the Newton is showing 230. Two miles later, again up a shallow incline at steady pace I ask again and he says 300 and the Newton is showing 305. Go figure.

The primary time the Newton is obviously too high is when coming off the power and just letting my feet follow the pedals, essentially 0 power. The Newton will show 150 to 180 watts for 10 to 12 seconds while soft pedaling in that case. Conversely, if I'm going down a roughly 6% or higher grade but still putting a lot of effort in the Newton will show 0 to 60 watts no matter how hard I'm going, then as the terrain levels if I come off the effort a bit the Newton's power readings will climb even though I'm clearly putting in less effort.

It's just the nature of the beast, unless my particular unit is screwed up.
Velocomp
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Re: Accuracy of Watts vs other PM

Post by Velocomp »

You should be getting much closer results than you're reporting. Please post a ride file.
John Hamann
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Re: Accuracy of Watts vs other PM

Post by EHB »

@ Cheoherrera

277W seems fine to me. If I was to guess, based on your 70.3 time, I'd have said 4.2-4.4 W/Kg, which is very near.

I have a Crank based PM and my FTP is 305 or exactly 4W/Kg (I had to get 4 obv) and would be very happy with less than 5 hrs. Unfortunately, I've only done two myself, once for a laugh and in the second a bolt came off my chainring, so I crashed twice and ended up doing about 25m of a very hilly course in the big ring, but I'd still be happy with anything under 5hrs.
EHB
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Re: Accuracy of Watts vs other PM

Post by EHB »

Actually my guess is based on indoors and most people find they get a bigger value outside, putting my guess even nearer.
cheoherrera
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Re: Accuracy of Watts vs other PM

Post by cheoherrera »

@Velocomp

sorry for the late reply Velocomp!

attached my last FTP ride (july 2015). i also attached a more recent long ride (march 26). any thoughts? i understand that OFPM usually have a greater error margin than DFPM. but for example i completed a half-ironman last sunday, my bike leg time was 2:35:43, with a NP of 253W and Avg Power of 240W. a close friend of mine biked in 2:34:59 with a NP of 210W. we are both very similar in height and weight, and usually have very similar times in races (i ended up beating him in the run leg by 4 minutes 8-)


thanks for the replies!
Attachments
iBike_03_26_2016_0702_75_Miles_HiDef.ibr
(1.77 MiB) Downloaded 311 times
iBike_07_25_2015_0802_25_Miles_HiDef (FTP).ibr
(511.33 KiB) Downloaded 296 times
cheoherrera
Posts: 10
Joined: Sun Dec 06, 2015 9:42 am

Re: Accuracy of Watts vs other PM

Post by cheoherrera »

brookside wrote:My readings seem too high at times as well, some things can be corrected, some I don't think can be.

Correctable - don't use the IBike suggestions for Crr and CDA, they're probably too high causing larger power numbers than you're really pulling. With good tires (e.g., almost any 320 tpi tire, Conti GP4000s, Conti TT, Conti Supersonic, Bontrager R4, Specialized Turbo Cotton) and latex tubes your Crr should be set @ .0028 to .0035, not .004. See https://docs.google.com/spreadsheets/d/ ... gid=624765 for some test results, and there are other sources on the web if you search a bit. For CDA it all depends on your exact position, clothing, helmet, water bottle placement and type, wheels and tires, but in general I think the Ibike recommendations are too high. After experimentation I use .3 for rides I do primarily in the drops or 90 deg bent armed on the hoods (which is slightly faster than in the drops).

Correctable post ride - on many rides that are essentially out and back I see wind readings that show an average 1 to 2 mph headwind for the entire ride. I use Isaac to set average wind at 0 in those cases. Even if I do regular wind calibrations in my garage before going out with hand cupped over front of Newton, this is too frequent an occurrence to believe the wind changes direction that often when various weather sources report it as not changing. The iBike also usually thinks I end at a higher elevation than I started, fix this in Isaac as well.

The Newton's readings just won't be the same as a DFPM because of all the estimation going on. A couple days ago I was tired and on a recovery ride but still the power numbers the Newton was showing seemed way too low, in this case, for the effort involved. About 14 miles in another cyclist comes by me who has a Powertap wheel, and I accelerate a bit to stay with him. Going up a 2 to 3% grade we're riding at a steady pace I ask what his power number is, he says 315, the Newton is showing 230. Two miles later, again up a shallow incline at steady pace I ask again and he says 300 and the Newton is showing 305. Go figure.

The primary time the Newton is obviously too high is when coming off the power and just letting my feet follow the pedals, essentially 0 power. The Newton will show 150 to 180 watts for 10 to 12 seconds while soft pedaling in that case. Conversely, if I'm going down a roughly 6% or higher grade but still putting a lot of effort in the Newton will show 0 to 60 watts no matter how hard I'm going, then as the terrain levels if I come off the effort a bit the Newton's power readings will climb even though I'm clearly putting in less effort.

It's just the nature of the beast, unless my particular unit is screwed up.
thanks for the help. i will check Crr and CDA to see if it gets better. at the end of the day i guess the good thing is that the PM is accurate and constant. which i guess its the most important part!
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Russ
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Re: Accuracy of Watts vs other PM

Post by Russ »

Brookside posted spreadsheet link above from google's collection.
I am not well convinced that these final CRR values are really usable except to identify a somewhat faster tire for purchase.

The 'raw' CRR value, then temperature compensated are derives from power meter measurements on a trainer.
No references seemed available for the spreadsheet and a Google search failed to find another source.

So I downloaded the spreadsheet and took a look. The projected flat road (and perfectly smooth I might add) value comes from this formula:
=L3*((1/(1+($'Setup & Analysis Constants'.$B$14/$'Setup & Analysis Constants'.$C$3)))^0.7)*$'Setup & Analysis Constants'.$B$6
L3 is the temperature compensated raw value.

I wonder if anyone would care to validate this and qualify it as valid for, say, indoor wood track use or what.

I also wonder if the 'raw' value vs the projected 'flat' value might be somewhat (or more) useful to understand the 'cost' of riding on a trainer with a small roller vs on the road.

Edit: Forgot to note, they used a 4.5" diameter drum in the tests so our typical trainers would be a greater difference!

Also remember that the CRR for a perfect road must be increased both for roughness and for any additional weight!

Just my 2 cents worth :-)
Think about this and help me understand!

Thanks,
Russ
bobbyperry
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Re: Accuracy of Watts vs other PM

Post by bobbyperry »

yep i agree. 277w seems norm. well done. i,m 308w for 160 pound
EHB
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Re: Accuracy of Watts vs other PM

Post by EHB »

As regards your last 70.3 I have corrected for the difference in our weights (I weigh 10Kg more), which gives 291W and this doesn't sound unreasonable for what I'd expect for a 2:35. Maybe 10-15W higher, but I have gradually worked my way to a fairly extreme position.

Doing the same to your friend's NP comes up with only 242, which actually sounds too small. Maybe he is particularly aero and they are both right.

Well done BTW. I assume your FTP was higher by race day, but, based on 277, that is an IF of 0.91, which is very impressive to be able to run off. I sure as hell couldn't. The VI is also very nearly spot on target too.
cheoherrera
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Re: Accuracy of Watts vs other PM

Post by cheoherrera »

EHB wrote:As regards your last 70.3 I have corrected for the difference in our weights (I weigh 10Kg more), which gives 291W and this doesn't sound unreasonable for what I'd expect for a 2:35. Maybe 10-15W higher, but I have gradually worked my way to a fairly extreme position.

Doing the same to your friend's NP comes up with only 242, which actually sounds too small. Maybe he is particularly aero and they are both right.

Well done BTW. I assume your FTP was higher by race day, but, based on 277, that is an IF of 0.91, which is very impressive to be able to run off. I sure as hell couldn't. The VI is also very nearly spot on target too.
Thanks!
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