Wednesday, February 24, 2016

Power tamed

Recently I have been spending a lot of time on my Garmin Vector power meter. Here, and on the bike. More like experimenting on the bike and reporting it here. After some weird issues with total drop-outs which cleared up after a battery change the thing that was driving me crazy was getting reliable, balanced readings at the start of a ride. I am happy to report that I have solved the problem.

A word of caution. My advanced age provides me with a depth of understanding typically missing from younger folks. When I say "problem solved" it is with my fingers crossed behind my back. With me?

Bottom line up front: When you move the pedals between bikes, take the receiving bike out for a calibration spin immediately after installation. Do not install the pedals in the evening, put the bike in the car the next morning and drive to the ride start, then attempt the calibration ride. The results will be awful. The fix in that situation is to remove power, wait a bit, then power up as if you just installed the pedals and then do the calibration ride.

The reason this is important to me is that it answers the question of what to do before a race. Get the bike fully calibrated, rack it, and it will be fine coming out of T1.

In order to do a good calibration the head unit should be configured to display left-right power. I do not have this on my main page as it takes up too much space, but I do have it on a separate page.

In the photo the balance data is at the top of the display and reads 87%-13%. I took this during a calibration spin up on a trainer and it shows how whacky the system gets when the pedals are moved and not immediately calibrated. In this case cycling power fixed the problem.

The calibration ride goes like this:

1. Select the correct bike.  The most significant impact of this setting is crank length. On the Edge 800 the sequence is

Menu > wrench > Bike Settings > Bike Profiles > (name of bike)

2. Ride for 1-2 minutes with a cadence of 70 or higher. Best done on a trainer, or on a long, flat road where you can ride without slowing down, turning, etc. The important thing is to maintain a continuous spin with smooth, even pressure on the pedals. It takes a few seconds with cadence above 70 for the display to kick in, and at first the balance numbers may jump around a bit. This can be done on a slight downhill grade as long as a brake drag is added -- spinning with too light a force will result in an incorrect calibration.

3. Stop, unclip from both pedals, rotate the cranks until they are parallel to the ground. Do not move the bike or touch the pedals or cranks. Go to the head unit Power menu and select Calibrate. On the Edge 800 the sequence is

Menu > wrench > Bike Settings > Bike Profiles > (name of bike)  > ANT+ Power > Calibrate

Wait until the head unit says the calibration is complete.

That's it.

The next time you go for a ride on the same bike the calibration should be good enough for anything short of a detailed pedaling drill, providing power was not interrupted. By that I mean the pedal pod connectors were not unplugged and the battery doors were not opened. If you want really precise balance data, repeat steps 2 and 3 before the ride. The good news is that these steps do not need to be performed coming out of T1 as long as they are done just before the bike is racked.

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