First-time Pt user - Realization stinks

Long time lurker, first time poster.

I've been using a 1-year-old Fluid2 trainer with a rear-wheel speed sensor to extrapolate power with the following power number from a spreadsheet I've been using...
I know this isn't exact, but it seemed to be consistent. I determined my FTP to be at roughly 300W with this trainer, a very hard effort that lasted 1 hour with nothing left. Pretty rock solid at 20mph with a couple of dips in the last 10 minutes.

Enter a Powertap Pro. Same wheel circumference and with the PT zero-torqued. At 20 mph I'm showing about 214W on this trainer. I know everyone says you'll realize how slow you actually are but there seems to be a huge discrepancy. Anyone else get this big of a difference? Can the PT be setup wrong? Am I just facing reality?
Thanks.

I think the fluid 2's changed sometime in the last year or two. Your formula is pretty close to what I am seeing with my 3 year old fluid 2. However, your actual reading with the PT is pretty close to what some of my friends are seeing with their new fluid 2's. I understand that Cycleops made a change in the last year or so, that made the trainers have less resistance. Not sure why, but if I had to buy one now, it would be a Kurt Kinetic. Its got a similar resistance curve to the original fluid 2's.

I assume you've already gone through the standard idiot-check items?

-- wheel rollout and correct diameter entered.
-- no tire slippage on the resistance wheel
-- PT computer set for mph v. kph
-- trainer adequately warmed up (for some reason, people have reported Fluid2 resistance *increasing* after warmup)
-- power/speed averaged over at least several seconds to account for the jumpiness

Is this a new PT Pro, or just new to you?

Thanks, I haven't heard of that. I got mine just before the yellow-lever models were introduced.

Everything checks except for the warm-up, I wasn't as thorough as usual. And it's new to me.
Thanks.

Nothin' like checking for idiots

The truth sucks sometimes but hey, better to know than wonder....

I suppose a calibration check of the PT would be in order.

As for speed, ride with both computers to compare.

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Custom Training Plans -- cyclecoach.com -- My Blog -- Power Meter Hire in Australia

Just as a matter of interest, if there were tyre slippage on the trainer, that would skew the results towards a higher power reading, wouldn't it?

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Well, wheel speed increases *relative to the trainer speed* and torque decreases. I highlight 'relative' because since the resistance is all in the trainer, the resistance unit effectively slows down while my pedalling and wheel speed stay roughly constant. I'm not talking about the wheel breaking completely loose, but just those small chirps at the peak torque of the downstrokes which kill the net torque but don't really show as higher wheel speed.

To the OP, if you want to check the calibration of the hub, you'll find one method here: http://www.cyclingforums.com/t451571.html

sorry, I should ahve said : for KK trainer.
Since power is derived from wheel speed.

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Wait 'till you learn your 5s, 1m and 5m power. It's eye-opening (and you may find you have strengths you didn't realize you had).

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www.ghisalloanimation.com

The OP is using a Powertap Pro (meter) on a Cycleops Fluid2 trainer. Power is inferred from the torque measured by the powermeter.

I don't have an explanation, but it might be worth a look before getting too discouraged.

http://www.cyclingforums.com/showpos...6&postcount=41

Sorry, but those numbers look like what I get. Same setup, brand new powertap pro & newer fluid 2 trainer. After the traineer is warmed up ( can take up to 15 minutes sometimes) 20mph is just over 200 watts.

I think OP meant an increase the power shown on the KK computer, not the PT.

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Custom Training Plans -- cyclecoach.com -- My Blog -- Power Meter Hire in Australia

If it made sense to you then maybe I'm missing something. If we assume some wheel slippage in both cases:

with previous real-wheel-speed-power-extrapolator --> wheel speed is higher than trainer speed --> displayed power is higher than trainer curve would indicate.

with powertap --> measured torque is lower than trainer would provide --> displayed power is lower than trainer curve would indicate for the measured speed.

both cases would seem to support the dramatic drop in displayed power that the OP has experienced in switching to the PT.

Well, what does reality say? Can you time trial like a rider with a 300W FTP? How fast can you ride flatland for an hour on a windless day? Can you crush "fast" recreational riders and Cat 5's? The difference between a 300W rider and a 214W rider is huge enough that real-world experience could tell you which is more likely to be correct.