
RoadBikeReview Member
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Curt Kinetic Trainer Convert Speed to Watts?
Probably will buy this soon:
Kinetic by Kurt Road Machine Indoor Bicycle Trainer
Is there a formula that converts speed (forerunner cadence/speed sensor) to watts given the configuration of the trainer?
Any other way to get at this data (maybe a different trainer)? Or do most people have a PM on their bike when riding on a trainer?
best,
Jeff

A good habit is as hard to break as a bad one..

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I got angry at first but in the end admired the ingenuity.

Using the same wheel and near identical pressure, you can establish a relationship showing gains in fitness, but there are no real substitutes for a power meter. A few friends of mine purchased KK's speedometer that attempts to convert speeds to watts, but they weren't happy with it.

The computer they offer that reads power is only done through an algorithm utilizing your speed and cadence. It's not a direct measurement.
In the world of training, it may have some value if the numbers are consistently repeatable, even if they're not entirely accurate.
Originally Posted by bigrider
Teh Lounge "Its not just for weirdos anymore. It is for those trying to escape the noobsauce questions."
Originally Posted by QuiQuaeQuod
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The Daily Grind Cycling Journal & Tailwind Coaching

Originally Posted by robdamanii
The computer they offer that reads power is only done through an algorithm utilizing your speed and cadence. It's not a direct measurement.
In the world of training, it may have some value if the numbers are consistently repeatable, even if they're not entirely accurate.
Supposedly, the numbers are consistently repeatable on the trainer.
Also, the measurement with the calculation is only good for average power. You can't tell instantaneous power  as in, you stomp on the pedals to sprint, you won't be able to tell what your power is at the moment because the wheel hasn't gotten up to speed. So you can't tell spikes. But for doing a 20 minute lactate threshold test, you can get decent idea of your average power using the formula.
Oh, the algorithm only uses speed. Cadence will depend on your gearing choice, but won't effect the calculation, as it's only based on speed.

Originally Posted by Jwiffle
Supposedly, the numbers are consistently repeatable on the trainer.
Also, the measurement with the calculation is only good for average power. You can't tell instantaneous power  as in, you stomp on the pedals to sprint, you won't be able to tell what your power is at the moment because the wheel hasn't gotten up to speed. So you can't tell spikes. But for doing a 20 minute lactate threshold test, you can get decent idea of your average power using the formula.
Oh, the algorithm only uses speed. Cadence will depend on your gearing choice, but won't effect the calculation, as it's only based on speed.
Interesting. I thought there was a KK cadence sensor that was used on the computer as well.
I think the other problem with the KK computer is that it guarantees nothing if you don't use a KK trainer (it's calibrated to the resistance curve of the KK unit.)
Originally Posted by bigrider
Teh Lounge "Its not just for weirdos anymore. It is for those trying to escape the noobsauce questions."
Originally Posted by QuiQuaeQuod
Trolling the lounge is like noodling for piranha.
The Daily Grind Cycling Journal & Tailwind Coaching

Originally Posted by robdamanii
Interesting. I thought there was a KK cadence sensor that was used on the computer as well.
I think the other problem with the KK computer is that it guarantees nothing if you don't use a KK trainer (it's calibrated to the resistance curve of the KK unit.)
Oh, there may be a cadence unit, but it's not for calculating power. I have a Kurt, and I sometimes take the calculator out afterward and use the equation to determine my average power (equation saved in my calculator).
And, yes, the KK computer only gives you accurate power data for use on a KK trainer. Not accurate on the road or other trainer, not even a KK wind or magnetic trainer. KK does provide equations you can use on other brands' fluid trainers (Well, used to. I just glanced through their site and couldn't find it).

Originally Posted by Jwiffle
Supposedly, the numbers are consistently repeatable on the trainer.
Also, the measurement with the calculation is only good for average power. You can't tell instantaneous power  as in, you stomp on the pedals to sprint, you won't be able to tell what your power is at the moment because the wheel hasn't gotten up to speed. So you can't tell spikes. But for doing a 20 minute lactate threshold test, you can get decent idea of your average power using the formula.
Oh, the algorithm only uses speed. Cadence will depend on your gearing choice, but won't effect the calculation, as it's only based on speed.
Well yeah, but so what? A stomp is measuring weight more than anything else. Doesn't mean a thing to what you can do on the bike. But you could take the speed data downloaded to training peaks, etc. and gather "average" data for short enough periods to be honestly meaningful as a 'peak.' (Agreed that's not applicable to the KK head unit, but would work with any datalogging device, which is a basic need for any meaningful power analysis anyway.)
And cadence is irrelevant to power, so that's not an issue either way.
A good habit is as hard to break as a bad one..

In lieu of spending the $ on the KK "power meter" I simply took the power vs speed equation an put it in a spreadsheet to create a table showing power vs speed in .2 mph increments.
If you want a copy, pm me.

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Originally Posted by Blue CheeseHead
In lieu of spending the $ on the KK "power meter" I simply took the power vs speed equation an put it in a spreadsheet to create a table showing power vs speed in .2 mph increments.
If you want a copy, pm me.
That's what I did. Quick excel spreadsheet taped to the wall next to my trainer  quick glance of my speed on the chart tells me my power.

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Although conversion to Watts would be nice, I have an old Blackburn trainer (not fluid) which I roughly converted to road speed for relative workouts. My TT speed is about 21 MPH in a 20K, so my trainer is set accordingly & approx = my FTP. I use the same tire pressure, & resistance on the roller for each session. Resistance on the roller must be at the high point on the rear wheel, which is not prefectly round.

Formulas...
I have a Kurt Road Machine trainer and demoed their computer along with my Powertap.
The calibration of the computer is actually pretty good. I'd say it was +/ 5% of the PT when at a consistent wattage  but therein lies the rub.
The Powertap is constantly calculating power based upon TORQUE. I have my display update every 15 seconds.
The KK computer averages speed every 15 seconds to give a computation of power. This is all right to come up with a general target zone for 1520 minutes, but lets the numbers roam far too much for a short 35 minute interval.
For those who say they just have the equation done on their own  how did you determine the resistance of the Kurt fluid chamber? While modelled on the road, clearly the resistance of, and thus power required, to spin the trainer at a given speed is not equal to that required to pedal a bicycle on the road at the same wheel speed.

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Originally Posted by Argentius
For those who say they just have the equation done on their own  how did you determine the resistance of the Kurt fluid chamber?
Why would you have to? Kurt provides the equation for converting between speed and power.
While modelled on the road, clearly the resistance of, and thus power required, to spin the trainer at a given speed is not equal to that required to pedal a bicycle on the road at the same wheel speed.
True, but I ask: so what? It doesn't have to be. Power is power, the wheel speed on the trainer is secondary (in that it doesn't matter at all).
Asad

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Riding a trainer is a closed system. So it is very easy to calculate wattage based on speed and gear ratio and have a number that is repeatable over time. You have no wind or hill slope to worry about to mess with a calculation like you do outside, so you don't really need to use a regular powermeter on an indoor trainer bike.

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From Kurt's website:
Stationary Indoor Bike Trainer  Kinetic by Kurt
Kurt claim the calculation to be within +/ 3 %
"If we let S stand for “speed” in miles per hour, and P stand for “power” in watts, the formulas are as follows:
Kinetic Cyclone:
P = (6.481090) * S + (0.020106) * S3
Kinetic Road Machine:
P = (5.244820) * S + (0.01968) * S3
For example, to calculate how much power is produced at a speed of 16.1mph while riding the Kinetic Road Machine, plug 16.1 in for “S.”
P = (5.244820) * (16.1) + (0.01968) * (16.1)3
P = (5.244820) * (16.1) + (0.01968) * (16.1) * (16.1) * (16.1)
P = 84.4416 + 82.13017
Power = 166.571 watts
The Kinetic Road Machine power formula is accurate for all riders because the variables experience on an outdoor ride are controlled indoors. We are so confident in the accuracy of the Kinetic Trainer; we encourage you to try this test. Complete a 20 minute time trial on the trainer using a rear mount bike computer or Kinetic PC (Power Computer) Complete a similar ride on a flat course with a neutral wind. The distance covered will be within a range of + / 3%."
In the FAQ's: "Can I use my Kinetic Power Computer for outdoor riding?
Yes and no. You can use it for the basic bicycle computer functions such as speed, distance, and ride time etc. but the “Power in Watts” feature will not be accurate. The Kinetic PC is calibrated specifically to work with the resistance curve of the Kinetic Trainer and there are too many variables when riding outdoors such as wind and terrain which the Kinetic PC cannot account for and so the “Power in Watts” reading will not be accurate."

Originally Posted by asad137
Why would you have to? Kurt provides the equation for converting between speed and power.
That's my point. Kurt has done the fancy math for their resistance units. If you're not using their trainer, or, their equation, I'm curious how one would estimate watts.
Originally Posted by asad137
True, but I ask: so what? It doesn't have to be. Power is power, the wheel speed on the trainer is secondary (in that it doesn't matter at all).
Those using these math equations don't have a power meter, so, are estimating it from wheel speed on the trainer.
People in general have gone and looked at those "average power" equations  You know, it takes 300 watts to go 25mph, or whatever  and then tried to project that to a trainer, and figure out their watts from there.
Or, folks use a Kinetic / Tacx power estimator / calculator to determine that, when wheelspeed on the trainer is 25mph, they are using ~300watts, and then presume that this is accurate on the road.
I have not seen a power calculating (as opposed to metering) device which provides accurate enough information to use in training.

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Originally Posted by Argentius
I have not seen a power calculating (as opposed to metering) device which provides accurate enough information to use in training.
If you are training indoors, you don't have a lot of variables and don't need a powermeter. It's wasted money. And, this thread is about training indoors.
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