Saturday, March 16, 2013
Testing with a Powertap G3
As I mentioned in a previous blog post I’ve been testing my V3 Powermeter. There is going to be a major overhaul with the next iteration but I can still learn a huge amount from V3. Just like how I learned from V2. I’m mainly concerned with testing the consistency of the readings. I’ll be making a post Sunday or Monday with the actual results but suffice to say the results are very similar though my calibration is off, it’s reporting lower power than expected. What you are looking at in the above picture is what I was looking at during and interval session with my prototype along with a Powertap G3 rented from Cycles London. Above you are seeing a movie on one monitor with two custom pieces of software I wrote. One I mentioned previously as my trainer software that I use for indoor training, and one is a program that I pulled together in 3 days of spare time. It connects to heart rate monitor, speed cadence sensor, and two power meters and records them synchronized at 0.25 second intervals. That’s right, 4 times a second.
Here is the catch on why 4hz recording. Powermeters transmitting on ANT+ can do so in a few ways. It can transmit the basic power profile which is mainly power and balance, crank profile which transmits more accurate cadence and torque but no balance, and wheel which does the same as crank. It transmits at 4.06hz. More after the break.
A crank and a wheel meter HAS to transmit the basic profile every 4 transmissions. This means if your cycle computer is only reading basic that it only updates 0.8hz which doesn’t sync well with 1hz recording causing some error. The software I wrote captures every single field –separately. It captures the basic profile of the same channel separately from crank and wheel profiles. In the end with commercial equipment you’d end up with 4 power readings for every two power meters. My prototype only does basic, so I have 3 power readings for two power meters and I can later convert the Kurt Kinetic speed to power.
I can tell you that the Powertap basic and wheel profile numbers are very close but since the wheel is spinning 180+ rpm it’s updating 3hz, wheel profile is updating 4hz capturing this nicely, but basic is updating 0.8hz making the basic profile miss 3 of the sensor updates!
When I meet good people who are helping out my development, whether they know it or not, I figure it is mentionable. Paul, the owner of Cycles London comes off as a nice old school cyclist. He has opinions that he has formed over many years and has no issue freely expressing them. He reminds me of a small shop when I lived in Newfoundland called Earl Industries. Harold was the owner and single employee really, and honestly I didn’t quite care for his opinions on some things – but he had built them up over the years and I respected some of the things he said. Paul is laid back, and with the few minutes I spent with him I determined that he was quite a generous fellow. Sadly, Cycles London is a 100km drive from Guelph. It’s the closest place that rents a Powertap though and that combined with the hospitality will mean I’ll likely go back. With the testing it’ll probably be repeat Powertap Rentals, but you never know.
If you do rent a Powertap make sure to check to see what is included. This rental includes the wheel + tube + tire but not the cassette. This isn’t an issue for me as I keep meaning to pick up a new PG-1050 for my trainer wheel so I have all the necessary removal and installation tools for swapping my cassette. However, if you rent one and don’t have a chain whip and the correct removal tool then make sure to bring your wheel with cassette along to get them to swap it.
I’d generally recommend using your own cassette with a rental or one that is very similar. I didn’t have to do any derailleur adjustment which is how it should be. The wheel is not as true as mine (I use a machinists dial gauge and true them to 0.001” or 0.025mm) is so I had to let out the brake adjuster a bit – but not like I use my brakes on a trainer.