Kurt Kinetic users: question about power
 

Is Kurt's formula accurate? Anyone compare with what your power meter says? Here's the formula: Watts=S*5.24482+(S*S*S*0.01968) (where S is speed in mph). I was apparantly able to hold about 350 Watts for a 20 minute effort, which seems too high.

If you were to go do some constant wattage with your power meter on your trainer, what is your speed? If you use Kurt's computer, all it is doing is using this formula, so I'd really like to hear from anyone who has an independent way of measuring power.

Thanks!

I've found the KK to be within about 10% of my powermeter. It is important to make sure the tire is pumped to the same pressure each time and that there is no wheel slippage or you may end up with an overly high estimate from the KK formula.

350W for 20 min seems very good for this time of year. What level are you racing at?

Thanks for the information gregf83. I don't have tire slip as long as I don't accelerate quickly, but for the sake of being consistent, I'll be sure to check my tire pressure before future tests.

Something isnt right here..... With an FTP that high you (in theory) would be tearing up the female pro ranks.

Yeah, I think something is not right... so if any KK users also have a power meter, I'd love to hear what kind of wattages you're getting at various speeds. My bike computer is accurate, so it must be something about the trainer. Maybe my resistance unit doesn't resist as well as it should???

At least it's good enough to give me a means to track my progress... that's really the important thing.

It sounds like your roller is not tight enough if you are concerned about not accelerating quickly. I tighten mine at least 3 full turns after the roller first touches the tire. It helps if you put some grease on the threads attached to the roller. If you want an independent check of power, time yourself up a steep hill (preferably with a constant grade) and use one of the online power calculators to estimate you power.

I'd say call KK and ask them what they think. I use one of their bike computers and ran 280 watts average for a 20 minute FTP test a couple weeks ago. That was after about 6 weeks off of anything but occasional recreational rides. I'm not Spartacus but I'm not a bike pathathlete either.

Well, I use a KK, and on a 20min threshold test I can hold 360-370W according to the watt computer. I race Elite MTB and Cat1/2 Road so I guess it could be about right. 68kg when in shape.

And that's why I was not and will never be a cat 1/2. Well that plus I'm old enought to post in the complaining old man forum now.

If anyone with a way to measure power (not the Kurt computer though; it just uses their formula) feels like 'calibrating' their trainer for me and reporting back, that would be wonderful! I'm pretty sure my stats ability is up to calculating confidence intervals on this formula they provide. I'd need several participants, though... 3 at the very least? Pick a few wattages, and let me know what your speed is while holding each wattage... How about 150, 200, 250, 300, and 350 watts?

I'll report back with the results, which would be helpful to those of us without powermeters. Much like in the second reply to this thread, a confidence interval would allow you to say (with a certain high degree of confidence) that your power estimate is within a certain percentage or number of watts of your true power. Maybe Kurt trainers allow you to estimate your power to within a few watts, or maybe trainer-to-trainer there is enough variation that the estimate could be out by 100 watts, and is therefore basically useless. Right now, I have no idea.

Thanks

Here are some numbers from my last session:
Speed Power (Calc) Power (Actual) Error
27.5 188 199 6%
32.8 274 280 2%
28.6 204 197 -3%
32.0 259 254 -2%
28.3 199 189 -5%
30.3 230 221 -4%

I gots me a Powertap wheel comin and it'll be interesting to see how the numbers compare with the KK computer formula.