SRM/Powertap output contrast - Page 2

Let me re-phrase without using the word "cramming"

SRM's method will distort the true power signal.

I used the term "cramming" to point out that under normal operating conditions, the power signal is distorted or "forced" to fit into an arbitrary time scale of the user's selection. Under some circumstances this may work while in others it may not.

As I have a CT, PT and SRM and use the PT and CT 90% of the time (SRM on the TT bike), I chose to slightly de-rate my SRM power to match my PT and CT. It was around 2% higher IIRC - in a non cross-over gear at 45 kph.

rmur

Which comes from winning races presumably....

This sounds like a good idea to me. Currently the SRM's are showing a 10% higher output than the PT. Is it just a matter of changing a number using the SRM s/ware?

Short answer: YES - the SRM slope (only) is readily adjustable in s/w or from the PC setup itself. Increasing the slope will reduce the reported power.

Long answer: I'd recommend doing at least one 'stepped' power test ideally on an ergometer and checking your SRM versus PT power slope and zero-offset (simple linear regression in Excel) before adjusting anything.

Uhm - that assumes you're being quite careful about zeroing both meters of course! Aside from that, the SRM should definitely not read 10% above the PT.

There's a stomp test available to check your PT should there be reason to doubt the meter ... and a static weight check available for the SRM. It's best to be sure which one is off before making any adjustments.

Mine read pretty much as expected - some 2% high across my tempo to V02max training range so I increased my slope 2% to match the PT. Haven't needed to touch it in some time now.

rmur

Thanks - that's a great help. I moved it up 0.4(guess), and tonight it was down to ~2% over, so I've re-adjusted for tomorrows session. I plan to test at 80%, 100%, and 120% FTP, as I plan to use it mainly for 10, 25 and 50 mile TT's.

sounds good. BTW I see you're in Guildford - I worked in Leatherhead in 98/99 and recall that's not too far ... southwest of there?

rmur

Leatherhead is 5 miles(east) down the road from me, on the way to Box Hill.
Small world .

ah I remember Box hill - nice climb with those switchbacks ... well-treed if i remember correctly....

Hi Andy, When I was deciding on a power meter I read the following link on biketechreview.com:

http://www.biketechreview.com/archive/pm_review.htm

this seems to indicate that contrary your observations that the PT consistently recorded higher values than the SRM.... though I do agree with your comments based on the science, and I'd like to add 2.5% to my readings, I'm not sure...

Any comments on this data and its validity?

Since the SRM is 'upstream' of the drivetrain and the PowerTap is 'downstream' of it, the only way for a PowerTap to provide a higher power reading than an SRM is if one or both are not properly calibrated. In that regard, it is well known that you can't always trust the factory calibration of an SRM crank, and should always check it yourself to be sure it's correct (especially when attempting to compare it to another powermeter).