Saturday, March 28, 2009

New SRM maiden voyage

Managed to get the new SRM on the bike and head out today for a ride this arvo.
Nice, fine calm day, but a bit of traffic around (Grand prix weekend). SRM and Garmin behaved nicely, but think I set the offset incorrectly in the Calibrate power section, was 449, and should have been set to 719 - maybe. The offset process works like this:

On the Garmin 705 to set the offset:
Menu button: select Settings: Ant+Sport:Accessories
Spin the SRM crank backwards 5 or 6 revolutions (or maybe a few more) to activate the power meter.
Finish with the crank horizontally, and put no pressure on the crank.
The power meter should be detected, and the Calibrate Power screen should activate.
The offset number should stabilise.
At this point, press Enter (by clicking the joystick down in the central position) to save the offset value.
Press Enter again to leave the config screen.
For best results, you should leave the bike at the same ambient temperature for 30 mins or so before doing the offset. The offset shouldn't vary more than a few points generally.

Note: This is with firmware 2.60 - at least Garmin has an easy way to update the firmware on the Edge, as with all their devices. Check Web updater on the garmin website.

The other thing to have calibrated is the slope of the power meter. Each SRM has a slope value in Hz, which varies as a result of the manufacturing process... over time the resins used for the strain gauges can settle in a little more, leading to changes in slope. SRM recommend checking slope every month or so when new - problem is that you can set the slope in the Edge 705, but not read the calibration values - you need a Powercontrol SRM head unit for that.

To set the slope in the Garmin Edge 705:

Have look on the back plate of the SRM. The slope value will be listed in Hz (mine is 21.3).
Menu button: select Settings: Ant+Sport:Accessories
Now use the joystick and press left-right-left (intuitive eh?)
A new screen appears with the SRM serial number (mine is #14999, missed by 1)
and the slope field.
Use the joystick to enter your slope value.
Select mode to exit these screens.

The other interesting issue is the refresh rate - there must be a 3 sec or so averaging of power, so when you eyeball the current power output it isn't actually showing what your effort is (unless you've been constant power for 3 sec). In reality not really an issue for steady state riding, but might make some difference if you want to see what you are actually produding right now. Doesn't appear to be a setting for this either in the Garmin 705.

No comments:

Post a Comment