Puzzling power question for the data geeks

matthewbward

So this morning I did 3x6' 90-95% efforts on the trainer.  My power efforts were means of 219, 220, and 213 with medians pretty close to that.  I went to the data to find out why I dipped in the 3rd effort.  Gearing, bike position, and cadence were the same for all 3 efforts.  Torque declined in the third effort (8.1, 8.1, 7.9) by just a little.  But here' the kicker:  Speed was up in the third effort (32.5, 32.4, 32.6).

Is this just an artifact in the data or is there some relationship I'm missing?  I don't understand how a decline in force (torque) leading to a decline in power while holding everythign else constant could lead to a gain in speed?

 

matthewbward

BTW, the standard deviation and quartiles on the speed were basically the same on all 3 samples so I don't think it's an outlier issue. My question is really more about the physics and what I'm not accounting for when it comes to generating speed.

Peter Greagg

Matthew — physics tells me that if torque (and power) went down but speed went up, resistance must have gone down.
But you probably already knew that, so the real question is why did that happen.
Perhaps your rear tire was hotter during the last interval (how much of a warm up did you do?)?
Your trainer resistance may have changed due to higher temperature of the resistance medium (oil? magnetic?)?
What sort of trainer do you have?

tuckerm

I get the physics equations but my short answer is it's a trainer, always ignore speed.

matthewbward

@Peter - It's a Kurt Kinetic fluid trainer. I thought about the resistance changing but it just seemed like such a sudden and significant drop, there might be something else at play. For instance, body position or pedaling mechanics or foot angle? But it seems like those changes would be reflected in the torque response which is another reason it's puzzling.

In all likelihood, a hot tire is probably a part of it. As the set progressed, resistance dropped yet cadence stayed constant, force decreased a little, and that created a drop in power. But the tire was hot to decrease resistance such that a 3% drop in power yielded a little less than a 1% increase in speed.

It would be interesting to know the rolling resistance the trainer offers.

kkiefer

Well, maybe you were more aerodynamic ....



1.) you look at differences are for most PMs "insignifcant", SRM tolarance band is at +/- 1.5% on Watt and that is considered as Gold.

2.) temperature effects on the tire, trainer and PM is a factor.... 2-5% on Watts are common here.



Train on Power, forget speed on the trainer, look at speed/watt if you want to improve your aerodymamics

BTW: rolling resistance ... many trainers power meters do a roll-down test ... look at newtons laws of motion 1 and 2, get the the time to roll-down from e.g. 40-10km/h and the mass of your trainer drive wheel you will be able calculate the resistance force imposed by the trainer.

mgraffeo

almost certain your calibration changed. Did you fully stop pedalling before the start of the last interval? I've found with my KK trainer that if I let the rear tire roll down to a stop, it can throw off calibration and show low power numbers (thus I always hit the brakes to stop the tire if I'm going to stop pedaling).

Coach Patrick

xWHATEVER on the calibration. That's close enough for any standard deviation...keep pushing hard to hold the "same" watts and you know you'll be working uber-hard on the last rep!