Power, Speed & Polar Power Output

Anything about Power (watts)

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KevinOB
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Power, Speed & Polar Power Output

Post by KevinOB » Wed Jul 09, 2008 10:33 pm

I new to all this I Have A Polar Power Output Sensor fitted How Come The Readings Differ the Cadence is Correct as you would Expect.To Match My Speed with Polar I Put the Slope up on the Catalyst Free ride 1.4 I E-Mailed Tacx and asked the Question My Reply was don't use Them Together as you will get a Double Heart rate Reading I am New to Cycling but am putting A lot of effort into my Training I just want to do It Right I calibrated the other day it gave me 0.6 am i doing this Right any Other Setting I could do would be Appreciated

JohnWilky
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Polar v TACX

Post by JohnWilky » Sun Jul 13, 2008 1:30 pm

Which polar power unit are you using? The CS600 or the earlier one? Some people note that the Polar units are not too good on the turbo, the unit works on sensing chain vibration to calculate the power output and I have read that some users can't get there Polar units to work indoors but are fine on the road. I presume the TACX unit uses some algorithm to determine what the power is being produced rather than a direct measurement, there for the values are likely not to correspond but as long as they are constant that is the key.

All I would say is that make sure your TACX is calibrated before every ride to ensure you are getting the same/similar readings. You can then base your training levels on the readings you see whilst undertaking a typical FTP test. You may have to do a similar test out on the road the the Polar unit to see what those levels are for when training outdoors.

I have the CS600 but have yet to try it with the TACX as it's on a different bike. If I get chance I will have a play around this week and see what the difference is between my 2.

Cheers,

John.

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ms6073
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Re: Power, Speed & Polar Power Output

Post by ms6073 » Sun Jul 13, 2008 9:45 pm

KevinOB wrote:I E-Mailed Tacx and asked the Question My Reply was don't use Them Together as you will get a Double Heart rate.
Again, if you are using the Polar CS600 WIND w/Power then the HR and power data are transmitted at 2.4Ghz and will have no effect on the TACX. Unfortunatley the method in which the Polar system measures power output (similar to pickup used on electric guitar), you will not get reliable power measurements from the Polar while the bike is being used on the Fortius. If you are trying to verify the calibration of the Polar, then the TACX is not going to be of much help with the Polar.
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Post by mcorn » Tue Jul 29, 2008 5:29 pm

We have two Polar CS600 computers with power that we use on two tandems. Each of us have our own computer that we switch between the tandems. Both tandems have their own set of permanently-mounted sensors. We've been using the CS600's for about four months and in general I am pretty happy with the units. However, both of the original power/cadence sensors failed (one was DOA when new and the other failed recently due to moisture leakage from a bad seal in the case). Polar promptly replaced them.

I don't think the CS600 wattage measurement is particularly accurate, but the rest of the system is pretty good. The CS600 basically calculates wattages by measuring both the strain and speed of the chain. However, if the chain is too far away from the sensor (which happens with certain gear combinations), accuracy is affected.

I have not tried the CS600 with either the Fortius or Imagic, which is also set up with an older tandem dedicated to the trainer. On the tandem trainer, we have a PowerTap Pro hub, which is pretty accurate. Using the current Version 2.02 software and the latest motorbrake, I find that I have to set the Scalefactor in the Fortius software to 1.11 in order for the Fortius wattage reading to approximate the PowerTap wattage. Otherwise, the Fortius overstates the wattage by about 10%.

For calibration of the Fortius or Imagic, I use a value close to zero as I use very high tire pressure (150 psi) with a minimum number of turns on the tension screw to prevent slippage (about 1.5 turns). Through a lot of trial and error and experience, I find that this makes for the smoothest ride. We do not ride a lot of step hills, but this amount of tension is usually adequate to prevent slippage with the amount of power we generate up to about a 10% grade.

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PowerTap tricks if any?

Post by VT Hillclimber » Tue Sep 16, 2008 12:05 am

Hi, I am just now setting up a PowerTap Pro (wired version) that I bought. Interestingly, once I got it going, I found that the PowerTap was fairly regularly running at about 50% of the reading of the Tacx. I don't think the Tacx was that overstated. When I was running at about 210 Watts on the Tacx, I'd be showing around 100 Watts on the PowerTap. And I was pushing at a pretty good pace (22-ish mph), so I'm pretty sure the PowerTap was seriously understating the Watts. Anyone have any issues with integrating PowerTap into the Tacx Fortius? Thanks, Tom

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Post by mcorn » Tue Sep 16, 2008 1:05 am

As indicated above, I found a 10-15% difference between my wired PowerTap Pro and the Fortius, which I corrected with the scale factor.

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A

Post by Tacx Video Production » Tue Sep 16, 2008 9:35 am

VT Hillclimber wrote:Hi, I am just now setting up a PowerTap Pro (wired version) that I bought. Interestingly, once I got it going, I found that the PowerTap was fairly regularly running at about 50% of the reading of the Tacx. I don't think the Tacx was that overstated. When I was running at about 210 Watts on the Tacx, I'd be showing around 100 Watts on the PowerTap. And I was pushing at a pretty good pace (22-ish mph), so I'm pretty sure the PowerTap was seriously understating the Watts. Anyone have any issues with integrating PowerTap into the Tacx Fortius? Thanks, Tom
Sometimes the powertap receiver can have trouble getting a signal with interference from the Fortius (as out on the road also). The solution some have found is to move the receiver so the seat stay is between the line of sight to the motorbreak.

As for accuracy more or less all the powermeters differ in the displayed/recorded watts. SRM for example records the data at the crank before drive chain losses. While the powertap records the data after drive chain losses with the Polar in the middle. The Fortius records the wattage after wheel drag & drive chain loss & then tries to figure out that loss. If there is an error it will be caused by this calculation of drag. Its really vital to warm the trainer up first before doing the spin down test.

The VR worlds watts are calculated in a different way to the more accurate Catalyst/RLV/Ergovideo...Don't ask me why this is but all I know is its hard to change this.

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Re: A

Post by mcorn » Tue Sep 16, 2008 5:48 pm

Phil wrote:Sometimes the powertap receiver can have trouble getting a signal with interference from the Fortius (as out on the road also). The solution some have found is to move the receiver so the seat stay is between the line of sight to the motorbreak.
Phil-

He is using the wired PowerTap Pro, which is the same one I have and use on my Fortius trainer. The only wireless link in the wired PowerTap model is a small "shark-fin" pickup that goes on the chain stay next to the hub. A wire goes from the pickup to the computer. With the wired model, I have not found interference to be an issue. I think it is likely he has a problem with either his PowerTap hub or computer.

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Re: A

Post by Tacx Video Production » Tue Sep 16, 2008 8:01 pm

mcorn wrote:
Phil wrote:Sometimes the powertap receiver can have trouble getting a signal with interference from the Fortius (as out on the road also). The solution some have found is to move the receiver so the seat stay is between the line of sight to the motorbreak.
Phil-

He is using the wired PowerTap Pro, which is the same one I have and use on my Fortius trainer. The only wireless link in the wired PowerTap model is a small "shark-fin" pickup that goes on the chain stay next to the hub. A wire goes from the pickup to the computer. With the wired model, I have not found interference to be an issue. I think it is likely he has a problem with either his PowerTap hub or computer.
I think its the older block shaped pickup that has the problems, Ive got this one & its really tough to get a good signal if its on the top of the seat stay.

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Post by VT Hillclimber » Wed Sep 17, 2008 12:44 am

Guys, This unit seems to be the 'legacy' PowerTap. It has the yellow hub. But, as mcorn indicates, it does have the sharkfin pickup about 2 inches above the hub. I will take it outside assuming the weather holds up here in Vermont tomorrow, and will let you know what results I get. I'm pretty sure that indoors, it's registering about 50%. BTW, I did see another posting from last year from an RJJASICK that I responded to. He seemed to have exactly the same issue. Tom

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Post by mcorn » Wed Sep 17, 2008 6:59 pm

VT Hillclimber wrote:Guys, This unit seems to be the 'legacy' PowerTap. It has the yellow hub. But, as mcorn indicates, it does have the sharkfin pickup about 2 inches above the hub. I will take it outside assuming the weather holds up here in Vermont tomorrow, and will let you know what results I get. I'm pretty sure that indoors, it's registering about 50%. BTW, I did see another posting from last year from an RJJASICK that I responded to. He seemed to have exactly the same issue. Tom
Yes, if you have the yellow hub, that is an older model. My PowerTap Pro has a black carbon and aluminum hub with the shark-fin pickup. It's certainly possible that the older hub, which does have a short wireless link to the shark-fin pickup, is subject to interference. You could try moving the shark-fin pickup to other locations. It is also possible that the computer you have is subject to interference from electronic noise.

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Post by VT Hillclimber » Thu Sep 18, 2008 10:36 pm

Thanks mcorn. I got the bike set up to ride on the road and the PowerTap works fine. It also feels like the claims are true. On flat sections, I was finding 200W a lot more difficult than it is on the Tacx. I'm guessing around 180W is more like the 200W feel on the trainer. I don't need the PowerTap for the trainer, so I probably won't pursue this anymore. But now that I have power histogram capability for my favorite short rides, I would like to focus on mapping the rides into Catalyst as a means to power train thru the winter. I know it's different subject, so I won't even ask the questions here :-)

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