Computrainer and Powermeters

tridoc3

Does anyone have expirience riding a computrainer combined with your regular powermeter? What would be considered a reasonable disparity in readings?

Porkyboy

Hi

I use a CompuTrainer for my indoor training and a PowerTap 2.4 SL for outdoors, I callibrate both as per the instructions and according to the blurb they both have accuracy in the range +/- 1.5% so I take that at face value. 275W-300W feels about the same on both so that's good enough for me.

I guess I could start mucking about callibrating them against each other riding the PowerTap wheel on the CompuTrainer but frankly I can't be bothered! The small difference there may well be simply wouldn't make any difference to what I do so there is just no point in having it and I'd then of course have to keep checking the difference, how often, daily? weekly? monthly? I honestly wouldn't know what to do with the information. Any testing I do is done on the CompuTrainer (Lab version) and that seems very stable though I would prefer higher numbers

I reckon physiology and everything else varies so much from day to day that small differences in the readings of meters are no big deal and anyway people who know more than me have already done all the comparisons and that does me.

I suppose if I thought something was way out of whack I might think about it but whilst it all "feels" Ok that's Ok with me.

Cheers.

PBUK

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rmur17

err ... yes I collected a bunch of data on my CT (circa 2002) vs. my PT PRo and later SRM in 2004.

Based on several months worth of tests, I concluded the CT was highly repeatable -given careful attention to warmup protocol - and highly linear wrt. to the road PM's.

However, the slope of my CT was substantially off - around 4.5% - which some might consider acceptable but I did not.

Sometime in the fall of 2004, with considerable pressure placed upon Racermate (online at least!, thanks AC ), a method was developed to adjust the CT to match one's power meter of choice. It's isn't a pure calibration but if you have a 'known' PM, it's relatively easy to adjust your CT to match it.

if you want the detailed procedure, pm me. i used it to adjust two other local PT's which were off from 4-8% and there's been no issue since.

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Bill Black

I regularly ride my CT with my SRM Pro on my bike and the SRM log shows slightly higher power average and a bit spikier surges, but the difference is consistent. The difference is explained by the SRM measuring watts further "upstream" (i.e. at the cranks vs. at the rear tire/resistance unit interface).

I regularly (once every 4 mos. or so) calibrate the slope of my SRM.

Best,
Bill Black

anneslam

I have an older model computrainer (circa 1998) and it has always been off in watts by 25-30 watts. Obviously, when I was using it in the early days I had no idea since I had no powermeter to check its accuracy, nor did it matter as it only had to be reliable form session to session, and that it was. I based my training off a MAP test I did on the CT so it was all relevant. I knew my CT was reading low in watts when I completed a MAP test on another CT and went 30 watts higher.

Now that I use my PT on the CT I can tell that indeed it is reading low in watts, and thus I use the PT as the accurate measure while using the CT. It is still consistent as long as I calibrate it the same every time. This means proper warm-up of the wheel, setting the load at 2.00lbs every time, and keeping it in the same gear every workout. If I control these variables it seems to be very reliable.

By the way after 10 years of use my CT is still going strong! I had to repair the CPU as the buttons finally failed, and when I sent it back to Racermate they upgraded my CPU to the newer version,and fixed everything so it is as good as new all for $60!! I have nothing but good things to say about that machine!

If I were you I would test your PT on the CT to check for accuracy. Once you know the difference (if any) you can adjust the CT in the future.

JIM WV

Don't want to cause work for rmur, but take him up on his offer if consistency between your devices is important to you. I used his calibration protocol on my older computrainer (circa 1996 -- upgraded to 3D a few years ago) and it's within 1% of my SRM.

Once I followed the protocol, it has stayed quite accurate, I believe. As a matter of fact, the SRM slope drifts more often than the CT if my experience is correct. I hadn't checked my SRM slope for quite a while and happened to toss the bike with SRM on the trainer -- SRM was a bit off and was reading a bit low compared to CT. Checked the SRM slope and it was off by a bit. Plugged in the new slope to the SRM and voila, it was back in tune with the CT.

allenpg

My CT and PT 2.4 are usually within 1-2% each other in 3D mode, but the CT is about 7% lower than the PT when I use Erg mode. I simply adjust the CT wattage in a spreadsheet when using Erg mode. I've checked several times and the relationships between the CT and PT in each mode are similar. Weird, but okay for me.

rmur17

that is weird. Mine was off around 4.5% across 3D and ergo modes and very consistent in both before adjustment.

just for future reference, how old is your CT and are you using Ergo manually or using CS?

thanks

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