I was wondering if anyone else has noticed this problem (or possible problem.) I'm using a Cyclops Power Tap pro on a cyclops fluid trainer for training right now (Midwest sucks.) It kind of seems like the trainer throws the power off. It seems like I output a higher wattage while on the trainer than on the road, with less effort (judged by HR.) Is there anything you guys can think of that would create such a discrepancy between road and trainer power?

ehm, could it be less "drag". No wind. one wheel that is spinning etc, etc.

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Presumably the cycelops has a speedometer function. I think the issue is that for a given speed, what are the HR and Power readings? Obviously on the road, you should require a higher level of power to attain a given speed than when on the trainer - given drag, higher friction of road surface etc. But then this should be reflected in HR also.

I can't see that for an equivalent level of effort, why the HR and Power readings should be different. Unless of course that you are more consistent on the trainer in terms of your effort and the two just look different given inconsistencies in the sampling. For example, think of how long it takes HR to come down if you have been cycling into the wind and then turn around. You could be putting out massively less watts and still have a relatively high HR for a few minutes.

I understand that the road brings in more variables (like Frankie mentioned) but I figured that power and HR should correlate. For example, if I put out 300 watts going uphill, or 300 watts on flat ground, shouldn't my HR be generally the same? I mean either way you're putting out 300 watts, so road variables dont seem to matter.

The correlation isn't absolute. It you spin faster, you might well have a higher hr for the same power output relative to grinding a bigger gear. With the higher Hr you will produce more lactate, but with grinding a gear, you will fatigue your muscles in a different way. This is reasonably well established in the literature - and might well be the answer to this particular conundrum. To give an example - if I do power intervals on a local hill, at 53 x 13 my HR will generally average 160, whereas if I spin up it on 39 x 16 my Hr will be in the high 170s for the equivalent speed.

tend to agree with homegrown

after all, that is the selling point and the beauty of training with power: it's not affected by as many variables as HR as a training guide

if your average TT power is 300, then whether on the flat or uphill then that's what your average will be

I suspect that this would change when into a headwind, but essentially there should not be a huge difference!

don't know how this affects things, but when you are riding on the road, you call upon your upper body to do a portion of work that doesn't need to be done on the trainer (pulling against the drops when climbing or sprinting etc), this would result in a lower heart rate for the same power output on the trainer.
but the above is just my theory. donno how valid it is.

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I notice average power tends to stay higher on the trainer as there is less fluctuation in power due to coasting, backing off power slightly that occurs when riding outside. It is like riding into a constant strong headwind of climb where you cant back off pedalling for a second.

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While on the trainer you have to realize that you are required to CONSTANTLY keep pedalling or the wheel slows down MUCH quicker. The trainer forces you to do more of a complete pedal stroke. The speed sensor on the trainer is pretty much useless as the gearing is different so wheel speed will not correlate to actual street speed..

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room temp will come into play also, riding in the heat will produce a higher HR, sometimes 10beats higher without a problem.

When campare power to HR, make sure you are comparing Power with Cad to HR. I think someone else also said this.

Also uses less muscle sitting down, so seating down with the same amount of power compare to someone standing up with be about 10-15beats different.

The resistance on the rear tire is quite variable and there is no way of exactly duplicating the resistance level you will have on the road. That will definitely affect readings such as power and heart rate. All other points have been posted I think. Just monitor power and when you do get back on the road you will have that "hey, this is easy" feeling. And YES the Midwest sucks in the winter...and so does New England.

Also, add cadence into the formula and see if it starts to make more sense.