Which power to believe? - Quarq vs Computrainer?

salzmadh

I did my FTP test today on my computrainer and was collecting in parallel from my quarq onto garmin 500.  I calibrated the computrainer as per my usual routine, ride for 5 minutes, get tire warm, get up to 25 mph, allow to coast down check value and then repeat to make sure was steady.  The calibration number for today was 3.17. (which is ballpark normal for me)  Usual quarq calibration routine as well, after the 5 minutes of riding and after the coast downs for computrainer pedal backwards at least 4 revolutions to allow for auto zero.  Looking down at garmin and comparing to screen it seemed as if my power was off around 10 watts the entire ride.

At the end I saved both the 3d file as well as the garmin file and upload to WKO+ to compare:

-Computrainer: 5 min VO2 = 262, 20 min FTP = 240

-Quarq: 5 min VO2 = 254, 20 min FTP = 229

I took HR from garmin for both

So... which number is "correct", ie in declaring my new FTP which one should I use.  I am tempted to go off of the computrainer number, not because bigger is better, but I do almost all of my rides currently in the ERG mode and feel that whatever the number 229, 235, 240 it really doesn't matter, but as long as it is consistent and giving me something to work against it will eventually work in my favor.  I'm not riding outdoors in a while so this probably doesn't really matter for now.  Also probably would be beneficial to do the crank arm down calibration of the quarq as I haven't done that in while just to make sure the calibration hasn't drifted too much.  

kens1964

My gut feeling, David, is to use the CT number until you start riding outside more consistently, and then retest. But, what is your feeling about the result? This is a mid-OS retest and how did your first test go? What number have you been using, and how do the new numbers match up? Afterall, these will be used to set up the next 7 weeks of training. Can you work with the new numbers and most importantly meet the goals of the individual training sessions on the bike if you go with the higher number?

salzmadh

Ken-
good questions - my previous FTP was 210, so 95% of 240 would be 228 which would be an 8% increase which seems to be in line with "usual expected gains". Last outseason around this time I was close to 225, then summer was derailed with a shoulder injury so this feels "about right"

John Withrow

Nice work David! Both numbers are right... If you'll be using your CT for the remainder of your OS work, then definitely use that number... Remember, the actual number doesn't really matter as long it is repeatable and gives you good training zones. Once you get outdoors, simply do an outdoor FTP test with your Quarq and use that number for riding outdoors. Many people have different indoor vs outdoor numbers and you are not the first to see different numbers between a Quarq and a CT or a Quarq and a Powertap for that matter. One tiny thing to not is that your Quarq is unlikely to 'drift' during an indoor trainer workout. If the ~10W difference between the Quarq and CT was constant the whole workout, then great. If it changed by a lot over time (would be more dramatic after a couple of hours) then my guess is that the CT might drift as your tire and or the CT itself heats up over time. might be an interesting experiment since you have both...

tim cronk

David like others have said if your training on your CT in ERG mode ..... Use the CT number!

I have the same scenario except reverse.... CT/TR Erg inside training while collecting data on Garmin 500 from Quarq.... Except my Quarq data is almost always higher than the CT data(occasionally its equal but never less) ... I set my FTP to the lower number for inside training while continuing to collect the Quarq data so I know what I can do outside!

Tom Glynn

Power numbers will always be different for different devices. Quark pulls power data from force on the cranks, CT from the giant brake. In general you should have most power right at the pedals and then the various strain gauges would get different readings as the power transfers from foot to pedal to crank to chain to hub etc.

If calibration is not an issue (appears not to be) use the same device all the time and don't worry about differences IMHO.

tom

drmariner

David, 

  My question is your CT calibration method. I typically calibrate after 15 minutes (I believe that's what the manual suggested when I last read it 4 years ago).  Like you, I also calibrate again after several minutes of > 150 watts minimum and sometimes a third time to check consistency.

Would go with the CT number if that is what you will chiefly be using.

bartstevens

Agree with most of above- if you're training with the CT for an extended duration over the winter, go with that. It's the consistency and the trend that matters, only your ego cares about the actual number (in training.) 

FWIW, I'm one that constantly gets lower CT numbers than my Quarq. I've tried to isolate for lots of variables (bikes, tires, friction, etc.) but cannot seem to get to the bottom of it. It's quite stressful to try to hone in on FTP, CP, etc. when I'm going back and forth indoor vs. outdoor every week. 

I believe I can "handicap" the CT to something closer to the Quarq by adjusting [drag ratio] in the CT settings- but that may just be a perception I've concocted to boost my fragile ego. Have you already done the CT drag ratio test? I also generally trust the Quarq numbers because 1.) it's higher and makes me feel less inadequate, and 2.) it's closer to the actual location of power production. 

It might be worth a Trainer Road trial to do some additional experimentation.. just to compare RC! vs. TR?

Gordon Cherwoniak

Not much to add other that to confirm that you use the device you willl be training on.  If you are like most and have a lower number on the trainer than outdoors.  Your quarq numbers outside would probably be very similar to the computrainer numbers.  I always do a test when I first transition outdoors.  I have a pretty good idea what that number will be based on 3 seasons of riding with power but its a good workout so I just plan for a test and hammer myself. 

Gordon