Not related to this topic, but I’m a milk drinker and proud of it! Now the question!
I did my first FTP test on a computrainer. The tester had us do 2 x 30 second, 1 x 5 min, and 20 min max effort intervals. I have rode with a PT for the last year, but didn’t have my PT on my bike as it’s broken and I’ve ordered the part. What really confused me was the significant difference in power at the RPE and HR I was cycling at. My 20 min test was 83% of the power I sustained in OD races in July and August for 40k and my HR was 5 - 10 beats higher than I’ve ever carried for an OD race. I know that fitness at this time of year will be a factor, but the numbers are significantly different. My question is, do the computrainer and PT give similar wattage readings or are they different? If they’re different, as a general rule, which one reads higher?
There are potentially alot of things that could cause that kind of a difference, but my first question would be, did you do the test indoors or outdoors? If indoors did you have a fan on you for cooling?
The test was indoors but I wasn’t hot. We had a door and a window open on either side of the room, and I live in Alberta so there was lots of cooling going on! I understand the point about indoors and outdoors, but the PT on my old mag trainer read higher indoors last year too. Very good point… I hadn’t thought about that though. I just looked and my average wattage for the 20 minute test on the CT was almost the same as IMCDA wattage. (within 4 watts.)
I will put the PT on when I get the head back… that’s the piece that’s broken. I am just so disgruntled with my results from this test. It was really hard, I could not have gone one ounce harder, and I’m shocked at the difference in watts from what I’ve been able to produce with the PT.
Out of the box, my Computrainer registered power that was about 15 watts lower than the Powertap. Others have had similar experiences, though a few have had it read higher than the Powertap.
It was probably the low inertia of the CT which forces you to pedal in an unnatural fashion (more circular). That greatly increases PE and power output usually takes a big hit.
The small flywheel of the CT (the cause of the low inertia problem) and its accuracy problems are the 2 deal killers which make the Velodyne worth the $1000 more.
All power measurement tools will eventally become uncalibrated. Therefore, it should come as no surprise that two powermeters do not necessarily measure the same numbers. In the best conditions, a computrainer should be within 5% of real wattage. 5% results in 15 watts at 300watts.
If you want to ensure accuracy, the power unit of the computrainer should be sent back to the factory for recalibration after every X hundred hours. I have seen at least one place where they do testing for athletes and they had no idea they were supposed to do that. The standard computrainer unit is calibrated at one point (one cadence and one power). For an extra 100$ you can get a 6-point calibrated unit (calibration at 200 and 300 watts and cadences of 80, 90 and 100 rpms, if I recall correctly). That is money well spent.
I have a power tap and 2 computrainers, and they all read within the 5% margin.
To conclude, well maintained (calibrated) powermeters should have similar wattage readings. I have seen innacurate readings from several computrainers, and as well from a couple of power tap units.
If you undergo repetitive testing, you should consider always using the same unit, or at least comparing the testing unit to your own power meter. If 2 powermeters don’t read within 5%, one or both is incorrect.
All power measurement tools will eventally become uncalibrated.
I’ve had/had access to the same Velodyne for 17 y, the same PowerTap for 5 y, and the same SRM for 3 y, all of which get used extensively. None has shown any change in calibration over that time.
I often have my PT on when riding the Computrainer, and I can make the power values comparable or not comparable depending on the calibration setting. For my CT, if I calibrate the unit (after a good warmup) at 1.9-2.0, the values will be quite close. This is highly repeatable. As my bike never lives in the CT, calibration is essential each time I ride. As an aside, I think indoor efforts feel harder too.
after quite a lot of wrangling on the old yahoo CT list, a procedure was developed that allowed one to ‘align’ a CT with your PM of choice.
“Wrangling”…I like that.
I would not simply accept a 5% difference. For me, that’s been an entire year’s worth of FTP gains. Also it’s ~ 20W delta for hard L4/L5 work – at that level 20W is a BIG deal (at least to me!).
rmur
The 5% stated difference is the difference between the computrainer and ‘real’ power. If you are very carefull with your computraine setup, and that means, alway using the same tire, same inflation pressure and same calibration number - 3.0, you can get repeatability within 2% (that’s from folks at computrainer). Best case scenario would be about 6 watts difference at 300 watts.
Practically speaking if you are trying to measure any difference smaller than 10 watts, I don’t think the computrainer is the right tool. Computrainer is a great training tool, but its design (measuring power at the wheel) makes it a less precise tool than SRM and Power Tap.
I’ve had/had access to the same Velodyne for 17 y, the same PowerTap for 5 y, and the same SRM for 3 y, all of which get used extensively. None has shown any change in calibration over that time.
**
My power tap has been fine for the 3 years that I’ve owned it (static test). I’ve reshipped one of my computrainer once for recalibration. A gym nearby home runs power classes and has 10 computrainers that run about 4 hours a day for 8 months of the year. They need recalibration pretty much every summer.
As to all power tools all becoming eventually uncalibrated, I stand by what I’ve said. The important thing is that whatever system one has, accuracy should be verified once in a while.
“As to all power tools all becoming eventually uncalibrated, I stand by what I’ve said.”
And why not, it’s impossible to disprove that statement. For any example that remains in calibration, you can always claim wait longer and it eventually will go out of calibration. The thing is, I don’t really care that my PT will eventually go out of calibration as long as it stays good for say 80 more years. So what is the time frame over which you claim all units become uncalibrated, 1 year? 10? 100? 1000? without some scale, your statement is meaningless.
The best people to answer that question would be Saris.
My experience with torque cells and strain guaging is hit and miss, when something works well, it normally continues to work well and the only thing that knocks it out of cal is more stress than it was origionally designed to take. You could define stress as the load we put on it or abuse the hub may take from temp, humidity or trauma.
Of course I deal with companies like Lebow, Himmelstein and Key and not your inexpensive bicycle torque cells for the general public.
It would be nice to find Saris’s opinion on this, does anyone have a link to their policy on this?
sticking to the CT versus PT part of the equation (leaving aside fitness, test conditions, etc):
…
2. Subsequent to that and after quite a lot of wrangling on the old yahoo CT list, a procedure was developed that allowed one to ‘align’ a CT with your PM of choice. That could be an SRM, PT, Ergomo or other power meter. I use the word align versus calibration as there’s no inherent guarantee that your given PM is properly calibrated, zero-offet etc. Of course you can confirm your SRM/CT calibration with a static weight check.
…
Can you give us an explanation of this procedure ?
