Downward power drift in PT SL

WTF? Has anyone seen anything like this before? I did my 3 x 25 min SST session indoors and everything looked fine during the first set. The second set, despite the PE and speed being similar to the first, the power displayed was considerably lower. Even more so during the third set. Strangely, this drift didn't seem to happen during the work sets, but during the rest periods. Now that I think about it I did see this before during a 5 x 5 min session, but attributed the drift to my fatigue.

I attached my workout chart as well as a picture of my "sharkfin" positioning. The downward dips in speed were periodic speed dropouts. Strangely, the speed reading would drop, but power was still reading. I'm not sure if it is related to the power drift, but I'm convinced that the speed reading is more reliable here. Also, based on my experience with the Kurt Kinetic power calculator I know the first 25 min set to be most accurate. Help, I only bought this thing a month ago.

Edit: There was no tire slippage and my 20 - 0 mph was 12 seconds.

Attached Thumbnails

   

__________________

I'd stake a paycheck that says your PT isn't decreasing, but the resistance on the KK is getting less due to warm up in the fluid. I've never seen that dramatic a decrease on my KK, but I also only ride it two or three times a month at most. (today will be one of them probably... mumble mumble)

From the naked eye, it looks like everything is lower - speed, cadence, power - which would make sense with no drifting in the power. It only takes a little bit of speed drop to see a "pretty big" power drop.

__________________
Cycling Blog - Training with Power

Thanks for your reply. Well, the KK's are supposed to be thermally stable, plus it didn't feel like there was any less resistance. I've used this trainer for some 3 years now and have a pretty good feel for it. I did slow down somewhat, but only on a scale of maybe 0.3 mph. Note that the power wouldn't drift during any of the intervals, but would simply start at a lower reading on the next one after a 5 min rest. Further, if the resistance got easier I would have most definitely felt "stronger" and picked up speed as the effort progressed. I did change gearing in the last 5 or 10 min of each interval (the torque curve shows it). Otherwise the efforts felt pretty even.

__________________

Perhaps this will be more illustrative without the clutter:

#1:
Power: 247 watts
Speed: 19.0 mph


#2:
Power: 224 watts
Speed: 18.9 mph


#3:
Power: 217 watts
Speed: 18.5 mph


Difference of 23 Watts and only 0.1 mph between first and second interval. The difference between second and third seems more reasonable.

__________________

When i used to train on my Elite Axiom trainer at a constant speed, my power would decrease significantly, with probably a greater decrease than you have. On my Tacx i still get a significant decrease in power at a constant speed, but the difference isn't as big as my Axiom.

If i ride either of the trainers at a constant (Power Tap or SRM) power my speed drifts up, several km/hr.

ric

__________________
http://www.cyclecoach.com

Real quick side bar...

Understanding, and acepting, what you just said here. How accurate is the Tacx power measurment compared to your PT/SRM?

i've had several and they've all given different results, plus they drift during a ride.

ric

__________________
http://www.cyclecoach.com

Thanks.

I understand the drifting problem. I figure the only accuarte way is through a real PM.

Well, again I have a "real PM" and it is behaving strangely, because I doubt my KK is the culprit. Even assuming that my first interval was off and the others were more accurate, why is it showing vastly different power/torque readings for roughly the same effort.

__________________

I have a KK and PT-SL.
Did you coast at all between the 1st and 2nd intervals? I have noticed that the auto-zeroing on the PT-SL can vary by as much as 5 units of torque when I coast then pedal then coast then pedal etc in rapid succession on the trainer. So I always check the zero point on the PT to ensure it looks normal (516+-1 for me) before starting my interval sessions and never stop pedalling between intervals. This approach coupled with a dedicated fan pointed at the rear hub and KK has given me a much more consistent power/speed relationship both inter and intra interval training sessions.

Thanks, I'll check into it.

__________________

Sorry to jump into your thead. I was only trying to find out about the Tacx trainer. I can maintain constant speed and cadence and my power reading drops, or I can maintain constant power and my cadence/speed go up on the Tacx. I was only trying to find out if at some point in time the Tacx trainer had a 'good' power reading.

This is all very off topic to your orriginal question.