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Help me understand my power numbers
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I normally ride indoors on a Cycleops Hammer trainer, which won't turn on since last Friday, so I took out my travel trainer, a Feedback Omnium (without progress resistance), and have been riding it for the last a few days. Now the power is coming from the Quarq crankset on the bike.

This is what I found strange, for the same perceived effort, the power number on the Omnium is noticeably higher than the Hammer (both have been calibrated correctly and regularly). For example, for the same Zwift ride in London, the average power difference is over 20 watts, but the average speed only varies by less than 0.5 mph. My understanding is that the Quarq measures the power by direct force, hence, it should be accurate. If that's the case, does that mean the Hammer has been wrong, or is my feeling of perceived effort being way off?
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Re: Help me understand my power numbers [dalava] [ In reply to ]
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2 things off top of my head, one is that quarq measures before drive train losses, hammeris after and 2nd, your omnium is going to have way less inertia so you may well have a higher perceived effort
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Re: Help me understand my power numbers [jeffp] [ In reply to ]
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jeffp wrote:
2 things off top of my head, one is that quarq measures before drive train losses, hammeris after and 2nd, your omnium is going to have way less inertia so you may well have a higher perceived effort

Inertia - likely. Also, was the hammer in sim mode, simulating the hills? That may have made it feel more road-like, maybe easier.

Speed/power at the same RPE for two rides is way fraught with error. I'd ignore that. There are many reasons to explain either way.

Your best bet is if you have any rides where you recorded both quarq and hammer data. Then you could check the offset and compare it to your roller/quarq offset.
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Re: Help me understand my power numbers [dfroelich] [ In reply to ]
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dfroelich wrote:
jeffp wrote:
2 things off top of my head, one is that quarq measures before drive train losses, hammeris after and 2nd, your omnium is going to have way less inertia so you may well have a higher perceived effort


Inertia - likely. Also, was the hammer in sim mode, simulating the hills? That may have made it feel more road-like, maybe easier.

Speed/power at the same RPE for two rides is way fraught with error. I'd ignore that. There are many reasons to explain either way.

Your best bet is if you have any rides where you recorded both quarq and hammer data. Then you could check the offset and compare it to your roller/quarq offset.

Now that I noticed this difference, I will definitely do a comparison when my Hammer is back online. I think the inertia is the key and my perceived effort is probably off - my legs seem to be more sore when I get off the Omnium comparing to Hammer.
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Re: Help me understand my power numbers [dalava] [ In reply to ]
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It might be okay.

Going 0.4 mph faster on say 22 more watts is not crazy. If you were to assume a speed of 22 mph then to go 22.4 mph would require about 11 more watts (assuming flat ground, no wind, .31 cda, 85 kg).

As already mentioned, Quarq is reading before the drive train losses. That could be a few watts.

Then of course power meters are inherently built with 1-2% error. At 220 watts, that could be a few watts as well. So all added up, you may have 16-20 watts accounted for.
Last edited by: Jason N: Feb 12, 19 9:54
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Re: Help me understand my power numbers [dalava] [ In reply to ]
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Your smart trainer is reading lower. Many do. Mine is a consistent 10% lower than my power meter.
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Re: Help me understand my power numbers [rubik] [ In reply to ]
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I have the opposit going on, did a step test with lactate testing at a sports lab and tried to verify over multiple rides on my Tacx Flow Smart how far off my power numbers were for a given heart rate to see if I would have to adjust for power during intervals etc. I noticed my Tacx was reading 15-20W higher (let's say about 10%) for all heart rates during the step test. I did the test without a fan blowing and on the trainer I always have a fan on but still, I thought that was quite a big difference.
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Re: Help me understand my power numbers [Tri_Joeri] [ In reply to ]
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Tri_Joeri wrote:
I have the opposit going on, did a step test with lactate testing at a sports lab and tried to verify over multiple rides on my Tacx Flow Smart how far off my power numbers were for a given heart rate to see if I would have to adjust for power during intervals etc. I noticed my Tacx was reading 15-20W higher (let's say about 10%) for all heart rates during the step test. I did the test without a fan blowing and on the trainer I always have a fan on but still, I thought that was quite a big difference.
HR against power is not a consistent correlation
This might be telling you the Tacx is reading high compared to the lab but it might just be telling you your HR was higher while in the lab. I don't think that wouldn't be unusual. HR is often higher for races or other special events. Similar to whitecoat syndrome for BP testing!
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