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Stages Power Meter Issues

Old 01-18-18, 05:56 PM
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Stages Power Meter Issues

Hey everyone,

I acquired a stages Ultegra single crank arm power meter about a month ago. It has treated me pretty well when I use it on the rollers and usually outdoors as well. I had an issue with it this morning, though. The numbers it was reading were simply not accurate.

About a month ago, I did some 8 minute power intervals on the rollers in 53x11. My cadence at 380 watts was around 97 rpm. This was on Gatorskins. This morning I was going even harder on 8 minute intervals in 53x11, holding 106 rpm at what my power meter claimed was 290 watts. This was on gp4000s. Since my last power test I have done an unbelievable amount of training, including a 3-day training camp with my team. I calibrate my power meter before most workouts with the stages app. What could be wrong? Is it a massive decrease in rolling resistance from the tire change? Did my bombproof kreitler 4.5" rollers magically change resistance? Is it the power meter?

By the way, I'm 6'5", 200 lbs. Based on my results in races and such, I think the higher reading is probably more accurate.
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Old 01-19-18, 07:34 PM
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Originally Posted by Kevin44
Hey everyone,

I acquired a stages Ultegra single crank arm power meter about a month ago. It has treated me pretty well when I use it on the rollers and usually outdoors as well. I had an issue with it this morning, though. The numbers it was reading were simply not accurate.

About a month ago, I did some 8 minute power intervals on the rollers in 53x11. My cadence at 380 watts was around 97 rpm. This was on Gatorskins. This morning I was going even harder on 8 minute intervals in 53x11, holding 106 rpm at what my power meter claimed was 290 watts. This was on gp4000s. Since my last power test I have done an unbelievable amount of training, including a 3-day training camp with my team. I calibrate my power meter before most workouts with the stages app. What could be wrong? Is it a massive decrease in rolling resistance from the tire change? Did my bombproof kreitler 4.5" rollers magically change resistance? Is it the power meter?

By the way, I'm 6'5", 200 lbs. Based on my results in races and such, I think the higher reading is probably more accurate.
Hi Kevin44,

I have a Stages single-side power meter also and I've never experienced a problem. I'm also very familiar with both the Conti tires you mentioned and have ridden many thousands of miles with both models. The rolling resistance of the Gatorskins is not very good. For example, the Gatorskins require 19.3 watts versus 12.2 watts for the Grand Prix 4000S II according to the Bicycle Rolling Resistance website. However, these numbers are only for relative comparisons between the tires -- they do not equate to power you use on rollers. I would expect the rolling resistance of the tires to have a bigger effect on your measured power at the pedals for roller use compared for road use because rollers typically offer less resistance than pushing a bike through air on a road.

I think the power levels you measured are plausible for two reasons: First, the Gatorskins have 58.2% greater rolling resistance than the Grand Prix 4000S IIs and this difference is probably amplified on rollers. Second, the higher cadence probably results in greater tire slippage against the rollers, further lowering the resistance when you used the Grand Prix 4000S IIs.

Other things to consider:

1 - tire pressure
2 - battery level in the power meter
3 - ambient temperature

Kind regards, RoadLight
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Old 01-22-18, 11:51 AM
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Originally Posted by Kevin44
Hey everyone,

Since my last power test I have done an unbelievable amount of training, including a 3-day training camp with my team.

Assuming that you put out the same level of intensity / effort both times, neither the tires nor the roller should have had anything to do with your power output measurement. They are "downstream" of the measurement. It is sometimes easier to post a higher number against resistance (outdoors, a positive grade or a headwind; indoors, more resistance on a trainer) because they make it easier to discipline yourself to make a higher number for the entire duration of the test. But if you put out the same level of effort (without any coasting or backing off), the equipment shouldn't have made the difference.

But your quote above raises a question. You did an unbelievable amount of training. How much did you ramp up your training compared to the period before the first "test?" Did you do TOO much, without adequate recovery? Once upon a time, a few years back, my training was going GREAT. My FTP was climbing very nicely. Even I was impressed. So I decided to increase the intensity of my workouts by another 10% and cut my recovery days by 33%. I figured I could handle it. But after one training cycle, my FTP dropped by 10%. (10% is nothing. I figured I just had a bad test day.) After two training cycles, my FTP had dropped a total of nearly 30%. I had trained too hard without taking adequate recovery days.

Another possibility: What were the data sampling settings on your bike computer for the two "tests?" If you had it set NOT to accept zeroes during your first test, but you had it set to record zeroes during your second test, the difference could easily be explained -- and the power meter could be dead-on accurate both times. The second test averaged in all the time you weren't applying power (as zero values) and that would have resulted in a lower average.

The first variable I would look at would be your data sampling setting. The second would be your training volume, intensity, and recovery schedule. You may have been tired or even overtrained for the second "test." And don't discount the possibility that you just had an inconsistent test.

Last edited by FlashBazbo; 01-22-18 at 04:35 PM.
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