I just spoke with Jim from quarq (www.quarq.com) about how their Cinqo bike powermeter picks up and sends power signals to head units. This isn’t covered in their FAQ’s in depth, so I thought I’d post what I understood here:
- The Cinqo reads torque at 60HZ (60 samples a second)
- At each pedal revolution completion the torque values for the rotation are computed to come up with a power figure for the full rotation, using an averaging method rather than a “latest data” method.
- 4 times a second the data from the latest rotation is checked and if there’s a new value (i.e. you just finished a pedal stroke), transmitted via ANT+ to the head unit
You can use the above to go through some scenarios, but the take-home is that when you’re spinning at 90RPM, which is 90/60s = 1.5 rotations a second or a rotation every .67s, you’re generating a new value a little better than every second, and within a 1/4 second of that value being generated, it’s transmitted to the head unit. That means that at worst, the lag between the new value (which is generated every .67 seconds) and the transmission (every .25 seconds) would be limited mostly by the .25 seconds between sends.
The head unit, (mine a Garmin edge 500) on the other hand, is a different matter. Some say that the head unit takes the power measures and just shows, and records, the latest value, which is a bad idea – it should average multiple values in a second, if it receives them, at least for storage purposes, which is widely reported to be 1 measurement stored per second.
On a test ride last night, I saw changes in the head unit that correlate roughly with the above. Tonight I will try to spin up to 200RPM, but am guessing that I’m going to have a hard time reading the display at that RPM to determine if I’m getting faster updates.
There are rumors that head units like an upcoming one from cinqo will show multiple values per second, and that they will also allow for much better data resolution, which may be important for sprinters, or those interested in peak power in general.