Just bought a used SRM that I think is putting out too many watts. It was freshly calibrated (spinning backwards with the crank until stable read-out) so it probably comes down to the slope factor which needs to be recalibrated. Unless of course I'm better fit than I expected

My question, how much do your slope factors change from calibration to calibration? How often do you recalibrate?

The Wattage forum is the place to ask this:

Check these search results

https://groups.google.com/forum/?fromgr ... alibration

0.1 - 0.2 at the most. Most often it is just a verification that they have not shifted. Really, it should not change unless you've taken it apart completely (unbolted the crank arm from the spider), even changing chainrings shouldn't have much, if any, effect. I check it every couple months, it only takes 15 minutes or so once you get the hang of it.

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My Ridley Helium
My Scott Plasma 2

First off, you are not calibrating the SRM, when doing the procedure where you spin the crank backwards etc. You are setting the zero offset, which can change due to tempature etc.

The calibration procedure is a little more comprehensive and includes some weights, but as it was said, it can be done in 15 minutes or less and then you will know.

Calibration should not change to much, but can be effected by the chairings etc. As an example O'symmetric rings will ad something like 10 watt in the power reading (and maybe some more due to the highly praised effect on the pedal stroke).

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Bring the mountains to Denmark!!

+1. The SRM slope process is more detailed than the zero-offset, but can be done at home with a known weight. You should recalibrate any time you open the SRM, replace the cranks, etc. You don't need to re-calibrate if you change rings for new rings of the same manufacture. You should do it if you switch ring brands, as they will have different deflection. It's probably been done to death, but PM me and I'll walk you through it.

Really no need to calibrate the Srm if changing rings?
Just asking as I need to swap my chainrings as soon as I find some spare time.

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ciclismo-espresso.com

Well I found a little difference between to sets of chainrings so I have calibrated everytime I have changed.

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Technical editor at Cykelmagasinet Denmark

My Simplon Pavo II

I tested this out by measuring before (7800 rings) and after (7900 rings). Slope didn't change at all.

GiantDK, Maybe just slight measurement tolerance differences? +-2% difference and it could just be down to the accuracy of the SRM itself.

Also, Backward pedalling obtains a correct zero offset? I would have thought that any movement of the cranks will cause some strain on the gauges and so adversly affect the zero offset value. Normally I just freewheel with the cranks horizontal and as still as possible until the zero offset value settles down.

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Di2x2

The error factor of even the most-accurate SRM is within the range of the difference in deflection of most typical ring sets. Make sure you torque them to the same Nm and you should be completely fine. If you make material changes (e.g., going from a DA 7900 ring set to a O-Symetric ring set) you sould re-calibrate to ensure accuracy. Having said that, the real issue is repeatability between your training sessions. From that perspective, it doesn't matter if you are reading high or low by 20 watts, as long as your periodic testing and your workouts are scheduled based upon the same data.

According to SRM, so long as both chainrings are round there is no need to recalibrate your SRM after a change of rings. I asked this specifically regarding switching from compact to standard chainrings on the THM M3 SRM but the answer applies the same here.