Strange results from power meter on rollers
 

It's too damned cold outside now so this week I start riding indoors. I track my best efforts on the Sufferfest videos to give me something to shoot for. I started with the Hunted and I can't understand the power / IF numbers coming from my Powertap. I've always found that power #s on rollers are about 10% lower than outdoors and have found links on the net to support this, but I'm asking about power #s indoors this year compared to last year. Here's last year's best effort:

http://connect.garmin.com/activity/239160975

Time: 55:00
Distance: 37.13 km
Avg/Max Speed: 40.5 / 51.9 kph
Avg/Max HR: 155 / 177 bpm
Avg/Max Power: 253 / 456 watts
IF: 0.893

(Ignore the speed as it is on rollers, but as a comparative number to the next ride it's relevant)

Here's today's ride:

http://connect.garmin.com/activity/391339342

Time: 55:19
Distance: 43.63 km
Avg/Max Speed: 47.3 / 61.4 kph
Avg/Max HR: 161 / 186 bpm
Avg/Max Power: 267 / 606 watts
IF: 0.901

I don't get it. By every metric I totally kicked my last year's ass yet the IF is barely higher. Even the avg power doesn't seem to be correct. I have the newest version of the firmware and this one now asks me to calibrate the power meter before every ride whereas the old firmware didn't do that, it was just auto calibrated.

But my issue is that I use "speed" as a metric to determine what % of max effort I'm putting out within the video and speed should be comparable ride over ride given that I'm using the same bike on the same roller. So when the video says to go 80% I typically go about 48 kph but today I was upping those numbers throughout the entire video. And my HR numbers show evidence of that. My previous max HR on any video was 182 and I was riding at near puking levels today. So I don't get why the power #'s seem so low.

I did tons of rides on my rollers with a power meter last winter. I did not have a trainer. My cycling coach at the time said that the setup of the two was not optimal. I was having to spin a crazy cadence to hit some of the power numbers for my drills. I could not put as much "force" into the pedals with the lack of resistance on the rollers and just using gearing.

They are in Strava if you want to see them. It was all sorts of intervals, power tests, etc...I have cadence, HR, wheel speed, power, etc...

I've got TruTrainer rollers and there is sufficient resistance to make me work as hard as I'm able at a cadence of 90 without being in my top gears.

I bought my rollers 2nd hand and the belt was missing for the resistance unit. I need to hit the auto parts store to see if I can find a suitable replacement. That would help for sure.

Someone on the forums suggested placing a folded towel under the front roller to add a little resistance.

Ok, but this thread is really not about resistance on rollers.

IF calculates off of FTP. If you ride the entire ride at your FTP (no warmup etc), then the IF is 1.0. Was FTP the same in both years?

This.

As for your power numbers seeming incorrect, that's hard to say. Powertaps are pretty darn fool proof. What are you using that asks you to calibrate before every ride? Normally you can just stop pedaling for a few seconds and it'll auto-recalibrate without you even knowing. If you never stopped pedaling the entire time, maybe you did have a dud calibration.

Ah, good point. I see that my FTP did go up from 297 to 305. So that explains why it will be harder to increase the IF on the same ride at a similar speed. I'm surprised that on all of the same equipment an increase in avg speed of 17.5 % only represents an increase in NP of 3.8%.

It's a Garmin 500. Ever since the last firmware update it asks me if I want to calibrate before each ride. I didn't stop pedaling during either ride.

I had similar results on Local Hero today and I think I've figured it out. I assume I'm a bit stronger than last year, as evidenced by the increased power, but the one change I forgot is that I'm using clipless pedals now. Last year I was less confident on the rollers so I swapped my pedals for platforms. Apparently the clipless allows the same power to translate into greater speed. Otherwise it doesn't make sense to me that an increase in power of 4% yields an increase in speed of 17.5% given that all other equipment is the same. On Local Hero my power increased by 2% and speed by 8.5%. I thought that a X% increase in power would yield about X/4% increase in speed. Given that it's indoors and there's no wind resistance maybe it's closer to a 1 to 1 relationship but certainly not 1 to 4.

If you have a power meter why are you using speed as a measure of intensity?

other factors will play into speed - different tires, inflation pressure, ambient temperature etc, etc.

I'm not using it as a measure of intensity. I'm just surprised that the marked difference in speed exists and was trying to think what other factors could have come into play. I'm using the same size/brand of tires, I get the inflation the same before rides and it's indoors so environment is the same. The pedals are different though.

Edit: I see what you mean. I use speed as my number when the video says to go 8/10. I could pick a power level and try and hit that but power bounces around all over the place, or if you go to 30sec power it lags brutally. So I use 60kph as 100% intensity and scale from that... when it says to hit 8/10 I stay above 48kph and so on. Typically that speed yields about 80% of my FTP. What I'm finding this year is that I am using 70kph as my 100% power as I have energy to spare. So I assumed that a roughly 10kph increase in speed would yield a similar increase in power.

Try using 3second power for your pacing. It smooths out the readings and is only slightly lagged. I find it fine for me, altho not nearly as exact as using an ergometer.

This is extremely easy to explain. Last year, you had, for whatever reason, a little more resistance on your rollers (tires, even at the same pressure and same brand, can vary in roller resistance depending on fairly subtle variables such as wear and air temperature). This year, because you had a little less resistance on your rollers, you chose a higher gear to ride in to keep you at 90rpm, hence the higher speed.