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Question on Power display for tacx flow trainer

Old 01-27-08, 09:30 PM
  #1  
jazzy_cyclist
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Question on Power display for tacx flow trainer

I'm in a group training program that uses Tacx Flow trainers, which have power meters, a great thing. My question is the watts that are displayed when using a higher gear versus using the "brake" (control that lets you change gradient).

I would have thought that "watts are watts" and that you could either shift to a higher gear or use the brak (increase the gradient) to arrive at the same resistance. But when I use the brake and increase to, for example, a +9% grade, I can put out a much smaller wattage than when I increase the resistance by shifting to a higher gear.

As a further data point, I would assume that watts = speed (a given wattage should result in a certain speed). Since my speedo is also hooked up, I've tried the following test:

1. Use the brake to increase to 4% grade. Check the wattage (say, 200). Speed is 17 mph.
2. Don't use brake, but increase gears to output 200 watts; speed is 19 mph.

Can anyone explain this?

Thanks,
Jim
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Old 01-29-08, 01:47 PM
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If you have grade of 4%, of course you will go slower at 200W, than on a flat road at 200W. Resistance goes up with speed (non-linear via wind resistance) or with gravity (going uphill). Putting the resistance setting to a +4 is like going up a hill, while a setting of 0 and gearing up is like going fast on a flat road. With the +4 setting, a significant part of your power is going into the equivalent of climbing the hill with less power remaining for speed (wind resistance).
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Old 01-29-08, 01:53 PM
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Kind of simplified but 300 watts going up a hill could have me going 7 mph. 300 watts on flats could have me going 23 mph. Kind of the same thing you are doing with the brake I believe.
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Old 01-29-08, 03:27 PM
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I used the i-Magic which is the same as the Flow but with the VR.

There is no break - the computer changes the brake and if it sets a hill if I maintain gear & cadence the watts goes up which is what I'd expect.

Is the watts given by the trainer accurate?

A couple of data points from the graph:

Slope 1.8 - Speed 19.3mph - Power 259w
Slope -1 - Speed 26.3mps - Power 252w

Sort of makes sense...

Does the break simulate wind resistance?

For example if I can average 22mph on the Tacx will I manage that on the road for the same period if it's not windy?

Last edited by musto_skiff; 01-29-08 at 03:39 PM.
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Old 01-30-08, 03:05 PM
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The power measurement on the flow is derived from the speed and grade. In other words, a given speed at a given grade will always show the same power output. Unfortunately, things like tire pressure, tension knob adjustment and choice of tires have an effect on the actual power output.

I used my Powertap with the flow last winter, and found it to be highly variable (readings of up to 60 w higher with the flow). I did manage to get it in line by using the same tire, the exact same tire pressure, and 2.5 turns of the tension dial on every ride. Sadly, when I wore out the tire and tried a new one, the readings were way off again even though I was using the same set-up method.

I ended up using the flow's resistance setting primarily for dialing in my preferred cadence, while relying on my Powertap for the actual readings.
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Old 01-30-08, 03:27 PM
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AH ... I guess that makes sense ...

Regardless of the accuracy I guess if the power keeps going up then things are moving in the right direcction :-)

Last edited by musto_skiff; 01-30-08 at 04:01 PM.
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Old 01-30-08, 04:50 PM
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Originally Posted by musto_skiff View Post
I used the i-Magic which is the same as the Flow but with the VR.

There is no break - the computer changes the brake and if it sets a hill if I maintain gear & cadence the watts goes up which is what I'd expect.

Is the watts given by the trainer accurate?

A couple of data points from the graph:

Slope 1.8 - Speed 19.3mph - Power 259w
Slope -1 - Speed 26.3mps - Power 252w

Sort of makes sense...

Does the break simulate wind resistance?

For example if I can average 22mph on the Tacx will I manage that on the road for the same period if it's not windy?

The wattage numbers aren't very accurate. If you can do 26.3mph up a 1% grade with only 252 watts, you are both very light and very aero.

According to Kruezotter, if you were riding a TT bike, and weigh 160 lbs, 26.3 mph on a 1% grade should take 401 watts.

On a road bike in the drops 450 watts.

If you weigh 130lbs, have a very aero position, skin suit, no gloves, TT helmet, ful disc, and an 808 front, you might get the number closer to 300-350 watts.

But clearly, the 252 watts is way off the mark.

Last edited by merlinextraligh; 01-30-08 at 04:56 PM.
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Old 01-30-08, 07:04 PM
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Originally Posted by merlinextraligh View Post
The wattage numbers aren't very accurate. If you can do 26.3mph up a 1% grade with only 252 watts, you are both very light and very aero.

According to Kruezotter, if you were riding a TT bike, and weigh 160 lbs, 26.3 mph on a 1% grade should take 401 watts.

On a road bike in the drops 450 watts.

If you weigh 130lbs, have a very aero position, skin suit, no gloves, TT helmet, ful disc, and an 808 front, you might get the number closer to 300-350 watts.

But clearly, the 252 watts is way off the mark.
Although the power may not be accurate, it is the speed which is truly inaccurate and irrelevant. Stick with the power readings to track your progress.

The 252 w could be close to what you are putting out, regardless of the speed displayed.
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Old 01-31-08, 05:49 PM
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Originally Posted by merlinextraligh View Post
The wattage numbers aren't very accurate. If you can do 26.3mph up a 1% grade with only 252 watts, you are both very light and very aero.

According to Kruezotter, if you were riding a TT bike, and weigh 160 lbs, 26.3 mph on a 1% grade should take 401 watts.

On a road bike in the drops 450 watts.

If you weigh 130lbs, have a very aero position, skin suit, no gloves, TT helmet, ful disc, and an 808 front, you might get the number closer to 300-350 watts.

But clearly, the 252 watts is way off the mark.
Yo missed the sign, read it again ... MINUS 1 ...

Where can I fin these Kruezotter tables ...??
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Old 01-31-08, 05:57 PM
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I have just googled up this page

http://www.kreuzotter.de/english/espeed.htm

If I put in -1 & 252W I get 26mph which matches up with the Tacx figures if I select "on the drops"

The drops to the top of the bar make a huge difference to the speed achieved with the same power .... is that correct?

If it's +1 I need 440w to achiev the same speed as you point out ...

Excellent website ...
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Old 12-28-08, 05:58 PM
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Can anyone explain why you can adjust the Watts on the Flow in the same way as resistance? What does this figure on the settings actually mean? I dont get why this is something that can be adjusted.
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Old 12-28-08, 10:02 PM
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Originally Posted by fuzzthebee View Post
The power measurement on the flow is derived from the speed and grade. In other words, a given speed at a given grade will always show the same power output. Unfortunately, things like tire pressure, tension knob adjustment and choice of tires have an effect on the actual power output.

I used my Powertap with the flow last winter, and found it to be highly variable (readings of up to 60 w higher with the flow). I did manage to get it in line by using the same tire, the exact same tire pressure, and 2.5 turns of the tension dial on every ride. Sadly, when I wore out the tire and tried a new one, the readings were way off again even though I was using the same set-up method.

I ended up using the flow's resistance setting primarily for dialing in my preferred cadence, while relying on my Powertap for the actual readings.

In order to replicate resistance from bike to bike, tire to tire, etc...you need to ensure the rear wheel coast down speed is identical between the different set ups. For example, get your speed up to 20mph with no resistance, stop pedaling and time how long it takes for your rear wheel to stop. Whatever that number is, make sure to replicate it with differing tires, bikes, etc...It's called calibrating your trainer to itself.
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Old 12-29-08, 03:53 AM
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Originally Posted by caballeros View Post
Can anyone explain why you can adjust the Watts on the Flow in the same way as resistance? What does this figure on the settings actually mean? I dont get why this is something that can be adjusted.
It has an ergo mode that will automtically adjust the resistance so that you keep a constant power regardless of cadence.
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Old 12-29-08, 03:55 AM
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Originally Posted by tonyzackery View Post
In order to replicate resistance from bike to bike, tire to tire, etc...you need to ensure the rear wheel coast down speed is identical between the different set ups. For example, get your speed up to 20mph with no resistance, stop pedaling and time how long it takes for your rear wheel to stop. Whatever that number is, make sure to replicate it with differing tires, bikes, etc...It's called calibrating your trainer to itself.
I see. I was using the PT for my data anyway. Ended up selling the tacx.
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