Kurt Kinetic power discrepency with different wheels?

[JulesT]

Hello People

I've had my KK for about 4 weeks and on it has been my Look road bike with a zonda rear wheel (2006) and a trainer tyre. My FTP is around 330 and I do my 3 X 20 at around 320 watts.
Boxing day I decided to put my TT bike on the KK(to do some 20's on this bike).
I can only manage 300 watts on this bike, either on or off the TT bars.........why?
The rear wheel on my TT bike is a R500 with **** bearings + a training tyre (my TT warm up wheel). Could I be losing 10% through pants bearings? or could it be that I haven't ridden the bike for 3 months?
Anyone had a similar experence?
TIA

 

[jetnjeff]

10% seems a lot for bearings. It may be that you are not as powerful in that position. TT bikes are a compromise between the lowest frontal area and producing the most power.

And yes it maybe that you have not trained in this position in a while as well, but maybe not.

Hello People

I've had my KK for about 4 weeks and on it has been my Look road bike with a zonda rear wheel (2006) and a trainer tyre. My FTP is around 330 and I do my 3 X 20 at around 320 watts.
Boxing day I decided to put my TT bike on the KK(to do some 20's on this bike).
I can only manage 300 watts on this bike, either on or off the TT bars.........why?
The rear wheel on my TT bike is a R500 with **** bearings + a training tyre (my TT warm up wheel). Could I be losing 10% through pants bearings? or could it be that I haven't ridden the bike for 3 months?
Anyone had a similar experence?
TIA

 

[frenchyge]

Boxing day I decided to put my TT bike on the KK(to do some 20's on this bike).
I can only manage 300 watts on this bike, either on or off the TT bars.........why?

An SRM or Ergomo will measure the same regardless of the bearings or tires being used, and it doesn't sound like you're using a Powertap since the bearings are apparently different on your TT bike. How are you measuring power?

 

[TiMan]

Hello People

I've had my KK for about 4 weeks and on it has been my Look road bike with a zonda rear wheel (2006) and a trainer tyre. My FTP is around 330 and I do my 3 X 20 at around 320 watts.
Boxing day I decided to put my TT bike on the KK(to do some 20's on this bike).
I can only manage 300 watts on this bike, either on or off the TT bars.........why?
The rear wheel on my TT bike is a R500 with **** bearings + a training tyre (my TT warm up wheel). Could I be losing 10% through pants bearings? or could it be that I haven't ridden the bike for 3 months?
Anyone had a similar experence?
TIA

It sounds to me that you are simply using the Kurt power meter. If that's the case you must recalibrate the power meter for EVERY tire/wheel you use. A different wheel and tire can make a big difference.

Also, you need to always do a roll down test. From 20 mph stop pedaling and time how long it take for the wheel to come to a dead stop. Adjust the tension on the rear wheel until it takes about 13.3 seconds for the wheel to completely stop......With the tire I am using it takes about 2.25 turns of the tensioning knob to achieve this roll down.

......if you re-calibrate as I suggested and also do the "roll down" your Kurt will be "spot on" with as power tap at wattages above 100.

Look into the Kurt power meter manual to find out how to re-calibrate ...it's not hard to do.

Kurt is most accurate with a slick(tire).... a cheap one is just fine.

 

[JulesT]

Thanks TiMan

Time to read the manual.
I will recalibrate and do a run down test with the TT bike in place.

 

[frenchyge]

......if you re-calibrate as I suggested and also do the "roll down" your Kurt will be "spot on" with as power tap at wattages above 100.

Look into the Kurt power meter manual to find out how to re-calibrate ...it's not hard to do.

Do you actually change the equation constants as a result of this 'calibration', or just adjust the tension knob for a consistent roll down?

 

[JulesT]

I measured the wheel circumference of my R500 and it was 2175. The computer had 2090 in it from my Zonda rear wheel. I also did the run down test and setup the rear tension accordingly. This difference made up the missing watts and I came out with a FTP of 335 this morning . The eqaution is correct for the KK road machine, which I have.

 

[TiMan]

Do you actually change the equation constants as a result of this 'calibration', or just adjust the tension knob for a consistent roll down?

Just the tensioning knob for the roll down to about 13.3 seconds.

......equation constants for the wheel/tire combo after the "roll out" test.

Some have thought that different wheel weights would make a difference in the roll down, and that makes sense, BUT the Kurt fly wheel is so heavy that it pretty much eliminates this "issue". That's why Kurt says that tightening the knob 2 to 5 turns, using a slick, and pumping the tire up to 100-110 psi is just fine.

.....but I am a "stickler" for accuracy and that is why I do the roll down test.

 

[cuttr]

Just the tensioning knob for the roll down to about 13.3 seconds.

......equation constants for the wheel/tire combo after the "roll out" test.

Some have thought that different wheel weights would make a difference in the roll down, and that makes sense, BUT the Kurt fly wheel is so heavy that it pretty much eliminates this "issue". That's why Kurt says that tightening the knob 2 to 5 turns, using a slick, and pumping the tire up to 100-110 psi is just fine.

.....but I am a "stickler" for accuracy and that is why I do the roll down test.

Do you do the roll down test with the trainer warmed up or cold?

 

[TiMan]

Do you do the roll down test with the trainer warmed up or cold?

It really doesn't matter since the Kurts fluid does not change in viscosity much when warm. It's the only fluid trainer that has thremodynamically stable fluid.

ie.....after the trainer was warmed up the calibration resulted in a time of 13.5 seconds and the another at 13.4 for me......then when done with the trainer cold the roll down took 13.2 and then another at 13.3 seconds.
So I "ball parked" the roll down at 13.3 or so.
It's hard to be spot on with your stop watch, and pedaling at exacting 20mph before starting the roll down anyway.

 

[cuttr]

thanks