caloso

Armstrong had those shoes.


Neil Armstrong.

globecanvas

For Catskills, yes in the past, I'd be surprised if that changed for this year. More on this specific race should probably go in the Northeast Racing thread.

Matt2.8NJ

@tetonrider

Question for you - I have two powermeters - a wired SRM (a la @carpediemracing and a Powertap. The SRM reads considerably lower power (20% lower)than the Powertap, but I've got 6 years of data from the PT and trust it. The SRM was calibrated about a year and 2kmiles ago.

I've done a bunch of rides with both powermeters running. If I were to adjust the slope of the SRM until the rides matched the PT output, is there any downside to this approach? I would leave the slope permanently changed, making the data more useful.

shovelhd

Trust the SRM.

revchuck 

I've got a wireless SRM and three Powertaps, and they agree pretty well. Since you've had your SRM calibrated more recently and the PT doesn't agree, I'd tend to disbelieve the PT.

Matt2.8NJ

Agreed re: trusting the SRM - however, the 20% difference in power kinda throws the last 6 years of power training out.

As another data point the PT's output matches well with files from friends from riding together.

Not scientifically conclusive... but I'm way more concerned with consistency than accuracy.

shovelhd

No it doesn't. Just make note of it. History only matters in context. What matters now is now.

spectastic

how is the new srm more accurate than what's available now? and why is accuracy that big of a deal? don't the errors cancel out over a long ride? your average power over 30 seconds, 5 minutes, an hour will be nearly identical despite the accuracy of the power meter.

echappist

static calibration test to see if the slope is indeed correct. anything else that anyone else says is just a guess.

Ygduf

and slope error would mean that the 20% offset is at a point, not at all points. Is it 20% AP over 4 hours or Is it 20% at 1 minute, e.g.

tetonrider

since you asked...

what makes you trust the PT? is it jus that you got the PT first and ran it for a while, and your idea of what 100, 200, 700w is is based on that? or, have you been periodically checking the calibration of the PT?

it's a sincere question.

i'm going to assume it is something more like the former than the latter. no worries--this type of thing happened to me years ago.

you've got a situation where one meter is right -- or neither is. the only way to tell for sure is to hang a weight off of each pedal and do a static calibration test. it's super easy; i've written about it before.

if i had to place a bet in the absence of any other info, i'd wager the PT is probably reading high.

my experience is that SRMs tend to be more accurate in terms of slope as received from the factory and also tend to have a more stable slope over time. this is the #1 reason i ride them. doesn't mean that other units are not bomber nor does it mean that there is never a dud SRM--it's just that some meters have more issues than others.

the funny/sad thing is it is almost ALWAYS the case that a meter other than SRM was reading HIGHER. it's a funny thing i've noticed. this makes it especially painful during your adjustment period. it's kind of demoralizing to now do your threshold intervals at 290w instead of 330, or whatever. Or to work all season and never come close to that 450w 5' effort from 2 years ago on your PT.

in fact, it REALLY sucks. sure, @shovelhd is right....but historical data is really important as a tool to measure progress AND the demoralizing aspect is real. we work SO hard for small gains, and even 2, 3 years from now when you are working to improve you'll still remember those peaks from the meter that was reading high as if they were accurate.

(also, it is possible that your PT was wrong from the factory, or that it drifted uniformly over time.)

i would NOT adjust the slope of one meter to match the other, particularly when you do not know which one is correct. that is throwing good money after bad, IMO. the PT cannot be adjusted by the end user (which is one reason why i, personally, don't use them; this is important to me), but you can check it and if it is wrong you CAN send it back to them.

hope this helps. feel free to ask any other questions. i'm really sorry this happened. i lost a few years of data.

i've often wondered if some manufacturers tweak their products to read a bit high--i've seen it so often when a few brands. kind of makes sense--human nature would make us predisposed to like that meter just a bit better.

tetonrider

this is an important point. if a meter's slope is seriously off (20%), then it will not be off by a uniform amount from a properly-functioning meter.

however, it's a function of torqueXcadence not time (though the 2 tend to be related--higher power at shorter durations--it is misleading to say a meter is right at 1' but wrong at 4h).

in some cases a bad meter may be accurate at one value, while it is lower at some and HIGHER at others, depending on where the lines cross.

tetonrider

don't really have time to address this but...

few things are accurate yet precise. the two often go hand-in-hand, as far as meters are concerned. so....if one isn't accurate, it probably has precision issues, too.

accuracy is quite important because most users will own multiple meters in their lifetime. for example....Quarq's great customer service sends you a new unit when yours gets wet; Stages sends a new arm when the battery cover fails....or whatever warranty issue arrises for ANY meter.

Or...you come into some money and decide you don't want to swap meters between bikes any more (e.g. during a stage race), so you now have 2 or more.

Or....you decide that pedal-based meters aren't cutting it and you want a crank-based meter...

Or...the new new thing comes out in 2 years and you want to try it.

In all of these scenarios precision without accuracy creates problems IF you care about your data. Some folks I've sold meters to just want them as an expensive toy; one guy doesn't even download his data. I tried to talk him out of it, but whatever. For him, neither accuracy nor precision matters. For everyone else it does. Problem is we often don't realize how important it is until an issue like the one described above arises.

tetonrider

i suspect those guys are repeating the (poor) translation of an article that appeared in a german magazine. this was also based on a pre-prod device.

not many people really have these in hand.

i've been watching real-time GPS accuracy and am seeing 1.6m, with occasional blips to 2.1m. it is inline with other GPS devices i've used (i've used them on the bike and for mountaineering for >15 years). much has been made of Garmin's use of GLONASS & such; i don't see it making any difference IRL.

if one wants/needs high precision for speed/distance (e.g. field aero testing), any GPS is insufficient. otherwise what i'm seeing from any hand-held device is good enough.

spectastic

yea, I meant precise, not accurate.

from what I see in the powermeter comparisons, the difference between the different brands are well within 10W. To me, that's a tolerable margin of error.

as for precision, I maintain my previous point that the errors cancel out.

the main complaint with my stages is that there's a 2-3 second time delay between the pedal and the reading.

tetonrider

hey--whatever works for you. if you want to assume that the errors are random, then sure they might cancel out. my experience is that errors tend to be systemic.

every brand just repeated the marketing of "2% accurate" from SRM. SRM, in fact, produced more accurate units and for the longest time never bothered to update the marketing. some models are still more accurate within their line-up than others. other companies use that tagline so they can say "we're the same...but lower price." definitely results in a bunch of sales; who would pay more for something that is the same, right?

not sure where you pull the 10w from, but we all have our own values and tolerances. there's a guy in this thread who might lose years of data...from a device that is "2% accurate", so keep that in mind.

as an engineer i'm surprised you wouldn't want to check the calibration of your device. IME with a variety of brands, i would not blindly trust a factory calibration, though some are better than others.

some folks work all year to achieve <10w gain in power. if their meter happens to start reading low, they might work all year for NO apparent gain. in my world, that's not a great thing (and is demoralizing), so i do all i can to improve the chances of accurate data. that takes me all of about 5-10' per year for a meter.

if your slope is wrong, 1% or 2% accuracy is pretty meaningless.

as always, ymmv, but i think you're simplifying the situation and placing trust in a place where it may not be earned.

mattm

lololol

spectastic

there's a guy who posted a pretty extensive review comparing stages against all the other types. He pretty much did all the work for everyone, and then some

Stages Power Meter In-Depth Review Update | DC Rainmaker

it's pretty clear to me that for a long ride, the average wattage line up pretty closely, regardless of which one you use.

his earlier review brings up something else, which is how the stages power readings tends to jump around a lot, which makes modulation difficult for a tempo. I might try to mitigate that by going off of the 3s avg, or even 10s avg.

tetonrider

with all due respect to DCR, this is a bike racing forum. his tests are overwhelming and give off the impression of being thorough (they are...to a point), but he tests HIS use cases. he is a triathlete.

over the years if you read his stuff you can see this play out time and again.

if all one cares about is AP for a ride, a power meter isn't that useful. RPE works really well. when we look at crit files, or MTB files (take a look at one of those sometime!), or short intervals, that's where differences can be exaggerated and really brought to light.

this may not matter for you, and that's fine--but just don't go thinking that your stages is something it is not.

for commuting or something like TT use (steady-state), the stages tends to more closely match other meters. the more variable the effort (in terms of torque and cadence, with abrupt changes), the more it deviates.

again, none of this may matter to you or to a prospective buyer, but the differences exist.

TMonk

I thought that was good too! haha.

Nice one @caloso.

shovelhd

The reason I said trust the SRM is not because I'm an SRM dealer it's because it was recently serviced and calibrated. Unless something broke, like the spider, allowing water infiltration, these units do not tend to drift. If you have the means to do a static test then you should do it.

Wylde06

Or in my case they shrug off the -3000 offset and make you wait until it gets wet a second time, leaving you wondering if you have an accurate power meter for a month.

grolby

Did an FTP test. Went to upload file to Cycling Analytics. "Could not parse file." ARGH. Is there any hope of rescuing this file?

Edit: nevermind. Obviously it's gone - file size is 4 KB for some reason (!?), normally they're at least 10 times bigger. WHY TODAY OF ALL DAYS.

canuckbelle

I know your feels. The first time I broke 1400w in a sprint was a corrupted file.

caloso

Yeah, I had a monster ride the day after the daylight savings time change, and the Garmin ate the file. At least I was able guess at my 20' power, but I sure would have liked to have the whole data.