Joule Calculations
Debated about putting this in the Gear forum but thought more Joule users would see it here.
New PT and Joule user. Very happy with my joule but had a couple of questions I wanted to confirm with the the team.
1. CADENCE- According to DC Rainmaker, " In the case of the PowerTap you’ll pair it as a Power Meter, but it’ll actually provide you with Power, Speed and Cadence. It knows Power and Speed based on measurements via reed switches in the hub, but with cadence it actually determines that mathematically."
Is the mathmatically calculated Cadence Calculation accurate? It seems a little low to me. My only other reference is a spin bike at the gym so maybe that bike is just a little high. Wondered what others thought.
2. Is the SPEED calculation accurate? just as with the cadence calculation I don't have a speed sensor but just the PT and Joule providing me witht he calculation. The correct wheel size is entered into the Joule.
3. CALIBRATION. I've read some of the other forum discussions on calibration but here's another question. in my Power Agent under Joule setup I agreed to have it auto calibrating any time the numbers aren't the same...Is this truley a set it and forget it thing or do I still need to do a manual calibration?
4. Variability Index (VI). Does the Joule provide me with this number. Am I just not seeing it or do I have to do the math equation myself after my workout to find out what my VI was for the session.
Thanks for the help...
Comments
1) I think CoachR has said before that the PT "estimates" the cadence, it's not the true 'mechanical' cadence that you might expect. You can buy secondary sensors to pair with the Joule to get the true 'mechanical' cadence. I have a garmin speed/cadence sensor for each of my bikes (when you switch the crank from one to the other you have to manually select the other sensor in the Joule menu)
2) I can't imagine the PT providing an accurate speed, so you'd have to have a speed sensor. The Joule is only recording on Speed or HR (menu setting), so if you don't have speed at all, it wouldn't record the whole ride - but again, I don't think the PT speed is accurate. Again, I have separate speed/cadence sensors.
3) cannot comment
4) VI is nothing more than NP/AP, so you can do the math yourself, or the software (WKO+, Golden Cheetah, etc) will/should do it for you.
1. I just use the calculated cadence and its close enough for me. I think I'll check it out on my next trainer session though. I'll find a nice easy gear, spin up until the Joule says I'm at 80, then count strokes for a minute and see how far off it is.
2. I have never had an issue with speed being off on my PT/Joule as long as my wheel size was set correctly. When I've had the battery cover off, seems like I saw a magnet inside there that works just like the old tried and true sensor and magnet. Just know there is a setting to include time at zero or not, May throw off your thinking some.
3. don't know really, just have mine set to auto zero and have never given it a second thought.
4. Haven't found VI in the display options either. I just do it in my head like Scott suggests. Like if my NP =220, AP =200, then I know my VI is 1.1
The Joule gives you a lot of data to look at, but I usually focus on current watts, cadence and NP when racing. And only cadence to make sure I keep it above 85 wherever possible.
@Scott, thanks for the info and I will check out the IF readings to see if i notice any difference
@Dave, you are obviously smarter than I because I can't do math like that on the fly
from what I am hearing (reading) I think I can live with the CAD given from the PT to the Joule, I might look into a speed sensor once I get ouside and i feel like its way off. For no other reason than I think its Cool to know how fast I'm going...
1. Close enough for me but others have found otherwise
2. Yes. It works just like a cateye magnet/sensor system. I've always found it very accurate, distance spot on, etc, assuming accurate wheel measurement.
3. Yep
4. Nope. You'll need to do that math in your head. I display average power and normalized power on the same screen so I can stare at them with my Math or Marines math skills and eventually figure out my VI….takes a while though :-)