Kurt Kinetic power curve too high?

I've been using a Kurt Kinetic Road Machine indoors this winter. Kurt has graphs and charts that you can use to calculate power from speed. I remember reading in these forums some time ago that if you calibrate the KK so that it takes 13.5 seconds to coast to a stop from 20 mph that the power numbers from Kurt w/b pretty accurate. This morning a friend of mine came over with his Power Tap. We put it on my bike. I set my bike up on the trainer, warmed up for ~15 minutes and adjusted the roller pressure so I got my 13.5 rolldown etc. Then I did a 20 minute interval and compared the numbers. The KK says I did 285 watts and the PT says I did 249W.

Anybody out there have experience w/this? Anybody using a KK w/a power meter? I don't understand why there is such a disparity between what Kurt claims to be accurate power estimates, and what the PT says.

thanks

 

From what I've been told, the readings can depend on a number of things--tire pressure and tightness of the roller, for example. But if you keep the tire pressure constant and the number of rotations constant, you can at least get repeatable results. I did it that way using speed and heart rate before getting a PT.

 

Similar experience here, except my numbers were a little bit lower.

 

One of my teammates advised his power tap reading is usually between 10-15% lower than his Kinnetic computer reading. Burst my Kinnetic bubble....I purchased a Kinnetic this year along with their computer and thought my watts/kg were OK for an old guy in MI.......

 

I was dissappointed too when I saw how low my numbers actally were. They were lower than what I originally posted. For that 20 minute interval the KK said 300W and the PT said 249. That's a huge difference. I went to the analytic cycling website and plugged in some numbers and it's likely that the PT is right. Oh well. As someone said in an earlier post, if the numbers are consistant then at least you can chart your progress and choose intensitities and durations appropriate for your training. I'm faster than I was in the fall of last year it's just that the numbers aren't what I thought they were. I'm over it.

 

Wow. A 20% differential makes the KK numbers useless for comparing to others.

 

I also have a KK Road Machine and PT. There is one other thing I've noticed about the KK that supposedly isn't supposed to happen. In order to keep a constant power in my intervals, lets say a 2x20 or 3x20, I need to maintain a higher average mph in each subsequent interval on the KK. I can see the gradual rise in mph within each interval as well. My KK is set up in an unheated garage; wintertime temperatures in there are usually 35-45F. This is after a 20 minute warm-up that includes 5 minutes at LT, so the unit should be fairly warmed up by the start of the first interval. Just one other thing to beware of.

 

First of all, you need to be aware that there is a longtime typo on the KK website in the road machine power equation, which now seems to have made its way into the computer instruction manual. The Road Machine curve fits much better to P=S^3*0.019168+S*5.24482 than it does to P=S^3*0.01968+S*5.24482 as the website used to say. There are some old threads on here to that effect if you need some backup on that.

Second, I always ride a PowerTap on my KK Road Machine trainer rides, and a couple years back went through the process of plotting calculated power based upon average speed for long, steady intervals vs my PT avg power. The chart is attached below. The reason that I did this is that I began to notice inconsistent speed v. power data in 2008 and began to wonder whether something was going wrong with my KK. I created a full curve in 2008 by taking readings at various speeds and did confirm that my readings were amiss. Upon performing a static torque test of my PT, however, I discovered that my PT torque was reading approx. 7% low rather than anything to do with my trainer. Long story short..... the trainer was so consistent that it actually told me when my PT had gone out of whack. (Note: I do not do coastdown tests before riding since I have a PT. As a result there's going to be more scatter and variance from the curve then one might observe if they were more careful about consistent starting conditions)



Thirdly, I'd suggest torque-testing your PT or checking your wheel rollout. /img/vbsmilies/smilies/wink.gif

 

There is some resistance shift as the temperature heats up, but I'd expect that at least part of what you're seeing has to do with the length and power during your 'warmup' phase. That's a pretty low starting temp and you'd need to get the unit up around 120F or so before it's really consistent (see the warmup part of the post linked below).

For a lot of in-depth discussion of KK Road Machine temperature performance, check out this post and the associated thread.

 

Good stuff there Frenchy.
Thanks