Computrainer power Vs Joule power
Help!!
For all of you in da haus who have experience with a computrainer and a powertap....Why do I have a 20-30 watt gap between measuring devices? (joule reads lower)
I Cal my CT each time I ride and I have the joule set to auto cal...so any ideas/solutions on the reason they vary so much?
Thanks
0
Comments
The way that the power is calculated on your CT is highly dependent on the Crr (rolling resistance) of the tire/CT contact point. Crr is affected by a bunch of things, tire pressure, contact patch, etc, but temperature is one of the biggest. As you ride on a CT and the tire heats up, the power reading is noticeably affected, thus it's very important to calibrate after you have sufficiently warmed the tire up, and not a bad idea to do subsequent rolldown tests another time or two during your ride, such as during rest intervals.
It's also pretty reasonable to expect that there will be differences between your CT and PT power, and I usually assume that CT is the less accurate of the two. Generally this is not really an issue, since for the purposes of training at least, while we'd love for it to be both accurate and consistent, it matters more that it is consistent than accurate.
All that said, 30W is a pretty big difference, especially between a PT and the CT versus a crank based PM where you can use the "drive-train inefficiencies" explanation. I've found with proper rolldowns, my CT is reasonably accurate compared to my Quarq, so I guess I dunno, hopefully someone else has a better idea.
I do the roll down each time I ride. I clean the tire. I do everything you tought me to do and have this Huge gap. I want to do the OS with Trainer Road and the CT, but am worried about pushing 225 on the CT and the joule reading 202??? Maybe thats just the nature of the beast? I am going to have someone come over and test ride it for fun in a couple of weeks, I will have them bring their PT and see what kind of readings they get. I have tried both of my PT wheels and have about the same gap...so at least the error is consistant...
Barring anything more obvious to try, that is what I would suggest, have someone with either another PT or a Quarq hop on your CT and see how different their readings are. If they are consistently off as well, you could contact Racermate and see if they have any insight on the matter.
In terms of training over the OS, this goes back to my comment about consistency earlier. For example, my RPE on the trainer seems to be quite a bit higher than it is on the road for the same number of watts pushed. I can average 300W+ outside for 10-15min+ on climbs and it feels pretty doable, however indoors I can only hold 300W+ for 2 or 3 minutes before I start to pop. I know that I'm not hotdogging it inside, there is something physiologically going on, or there is some error in what the CT is reporting, but my heart is working hard and I know I am working hard regardless what the number in TR says.
There is an element of disconnect and not worrying about the raw numbers versus your outdoor or even PT power readings, and instead just accepting the baseline relative to what feels very hard on the trainer and working from there throughout the outseason. In your case the CT is actually reading higher, where in my case my watts are much lower than expected based on RPE, but regardless as long as it is consistent it still will be an effective basis for you to conduct your OS training.
In terms of pacing your efforts outdoors for both training and racing, if there is a large discrepancy between your indoor and outdoor watts (or your CT and PT watts for that matter), it's simply a matter of re-testing outdoors on your PT to establish your FTP under those conditions and using that number for your outdoor training and racing.
Should it turn out for example that your PT is reading consistently low and always has; well there is really no harm in that other than sacrificing bragging rights with others. Again, what is important is consistency, if you set your goal watts in a race based off of your measured FTP as say 190W, it almost doesn't matter whether that might come out to 210W on someone else’s setup. Your results will still be the same regardless of what the # on the little screen says.