ergomo vs powertap comparison data - Page 2

The answer may not be as straightforward as it seems. Both Ergometers showed an increase in power with speed. The PT showed 22 watts while the Ergomo showed 10 watts. Without accurately knowing the power curve of the trainer you're using, you still can't say which is correct. Try riding a constant speed and hope that your mag trainer is stable over time.

Afterthought: The slope of the power curve on most magnetic trainers is constant, which means that if you're getting 316 watts at 36.1 kph, the slope is about 8.75 watts per km. Which means that if the power on the Ergomo was correct at 36.1 kph, then it should read 333.5 at 38.15 kph, so it's 7.5 watts low (2.25%). If you use the PT and do a similar analysis, the slope is 9.27 watts per km, so if the PT is correct at 36.1 kph, at 38.15 it should read 353 watts, so it's 4 watts high (1.13%). So it could be that both meters are close, or it could be that the mag trainer itself is drifting. Or it could be that my analysis is all screwed up.

I'm guessing the answer is all of the above, which is why power meters are so entertaining.

Ditto!

I've just never seen a PowerTap "drift" within a ride. Plus, the Ergomo's slight power increase didn't seem comensurate with the speed increase unless that trainer has a really flat curve.

Good idea. Then after that, do a ramp test based on speed. For example, start at 20 KPH and increase 1 KPH every 30 seconds. Then you can define the curve of your trainer (make note of tire pressure and number of turns on the knob).

Hey GettingFaster - are you using the default K Factor supplied on your ergomo BB? (Or did you fudge down the K Factor?) Are you doing an offset procedure before the ride?

I'm really glad you posted this thread to compare ergomo vs PT. Now I'm really glad I got an ergomo but I think my ergomo reads high. But I've only got 1 training ride with it so far. I'm actually happier with the power reading I get from my Polar equipped bikes!

What you should get is an anemomter to measure the wind speed your going with or against. I use a Kestrel 4000 meter.

What category do you race in?

Hi guys, thanks for all the replies

JTE83 - yep I'm using the supplied K-factor. I do the offset procedure before each session although it's steady as a rock anyway, never changes from one calibration to the next. I'm a 4th cat (bottom category here in the UK) at the moment, not done much road racing though.

otb4evr - I didn't install the ergomo, it was done by the retailer from whom I bought it but I think they did chase the BB first yes.

jbvcoaching - the possibility that I have a rt-leg dominance that increases with fatigue has occurred to me, yes. I suppose without something that has the ability to measure leg balance there's no way of testing the theory really but it does sound plausible.

Uhl/ergoman - yeah it's tricky isn't it I rather thought that the increase in speed over the intervals was small enough to be accounted for by the continuing rise in temperature of the tyre causing increased slippage on the roller, which would suggest that the more constant power reading (ie ergomo) was the more accurate. Also I just can't quite believe that my power increased 23W from first to last interval - I know I have a natural tendency to work harder in the later reps (who doesn't!) but it seems like a lot. But then again my whole perception of 'constant power' has been formed over the last 3 months from using the ergomo, so if it does drift downwards then my gut feeling could be way out.

Anyway, I think I'm going to move one of the powermeters off the bike asap. I have quite a bit of data from about 10 rides with both meters on and frankly it depresses me to have two screens staring me in the face all the time with different numbers on them reminding me how unreliable this highly expensive kit can be so I think the best bet is to go to just the PT on the training bike and put my faith in it. And shift my FTP up by at least 10W when I make the switch!!!