Discrepancy between power readings: Tacx Vortex v Powertap P1

Mar 9, 17 6:23

Post #1 of 24 (4289 views)

Hi Slowtwitch,

I've been reading the forum for over 3 years now, but never posted until this point - hope this is ok!

I'm hoping someone here might be able to advise with an issue I'm having. I'm aware that there can be a small variance between a smart trainer reading and the one given by a power meter. However, I'm seeing quite a large one.

For example, when I'm riding at what my Tacx Vortex tells me is 306w, my powertap P1s are giving me a reading of 260w - this seems like a massive gap to me. What is also odd is that riding at the same Tacx Vortex power but at a higher cadence (say 100rpm instead of 90rpm), gives me an even lower reading from the powertaps for what feels like exactly the same effort.

Is there something I've missed here? I calibrate the trainer on trainerroad before every workout and also calibrate the pedals as per powertaps instructions.

Any help would be massively appreciated - I've attached the two power files to highlight the difference I'm talking about!

Thanks.

Re: Discrepancy between power readings: Tacx Vortex v Powertap P1 [spraggz] [ In reply to ]

Mar 9, 17 6:29

Post #2 of 24 (4279 views)

Are you sure you have the crank length set properly for your P1's in whatever computer you are using to grab the data?

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Last edited by: rsmoylan: Mar 9, 17 6:29

Re: Discrepancy between power readings: Tacx Vortex v Powertap P1 [rsmoylan] [ In reply to ]

Mar 9, 17 6:31

Post #3 of 24 (4271 views)

Crank length is definitely correct - have set this on my garmin 920xt (the device I'm using to record the P1 data) - vortex data is coming from the trainerroad app on my mac.

Re: Discrepancy between power readings: Tacx Vortex v Powertap P1 [spraggz] [ In reply to ]

Mar 9, 17 6:51

Post #4 of 24 (4249 views)

Is this just a recent delta between the two?

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Re: Discrepancy between power readings: Tacx Vortex v Powertap P1 [rsmoylan] [ In reply to ]

Mar 9, 17 7:00

Post #5 of 24 (4239 views)

So I've only just acquired the pedals - had them for a week or so now - brand new. As I said, I calibrate the tacx before every workout and I know there's sometimes a small difference but to me this seems pretty large? Have tried 4/5 times and get roughly the same difference each time.

Re: Discrepancy between power readings: Tacx Vortex v Powertap P1 [spraggz] [ In reply to ]

Mar 9, 17 7:13

Post #6 of 24 (4223 views)

I have the same setup, but I also have the advantage of a stages on one of my bikes as well.

The stages and the P1 agree, and the vortex overreads to a similar level to you. And, like you say, a bit inconsistently with regards to cadence.

So I just trust my P1's. They're right.

Re: Discrepancy between power readings: Tacx Vortex v Powertap P1 [spraggz] [ In reply to ]

Mar 9, 17 7:17

Post #7 of 24 (4223 views)

Do you also calibrate the pedals? Do you always pump the rear tire to the same psi? Do you always tighten the wheel down the same amount?

I don't know the exact details of how the Vortex works, but I know the CycleOps Magnus uses a power curve to approximate the power and a clutch-style knob to make sure the wheel always has the same pressure on the resistance unit. If the Vortex is reading too high, it might be a question of tightening the wheel down further. When I first used the Magnus, I was surprised at how firm the pressure was once the knob 'clicked' into place.

Re: Discrepancy between power readings: Tacx Vortex v Powertap P1 [spraggz] [ In reply to ]

Mar 9, 17 7:21

Post #8 of 24 (4215 views)

Do you spindown the vortex after 10-15 minutes? Once it warms up, the calibration curve shifts.

Re: Discrepancy between power readings: Tacx Vortex v Powertap P1 [dfroelich] [ In reply to ]

Mar 9, 17 7:30

Post #9 of 24 (4207 views)

Hi OP -

I have Tacx Vortex Smart and have been using it for many months.

I recently got the Power Tap GS Hub.

The Vortex reads roughly 11% higher in watts from the two rides I have done so far. None of the rides involved any coasting which is supposed to 0 calibrate the hub. I want to do a test with that yet.

dfroelich wrote:

Do you spindown the vortex after 10-15 minutes? Once it warms up, the calibration curve shifts.

Also, the above statement is huge and I need to try this test.

Separately, consistency is key. Always run your FTP tests off of the same device and same variables. Even if the numbers are inflated or deflated, as long as the power meter is consistent you can use it for your training and track your gains.

After some more testing with the Vortext if I can't get the numbers between devices to get close, I will switch to the PowerTap as my main power output. I will just have to make note of it in my personal log for the dip in FTP.

"If it costs you 30 minutes at Maryland so what" -dwreal

Re: Discrepancy between power readings: Tacx Vortex v Powertap P1 [spraggz] [ In reply to ]

Mar 9, 17 7:40

Post #10 of 24 (4190 views)

spraggz wrote:

Hi Slowtwitch,

I've been reading the forum for over 3 years now, but never posted until this point - hope this is ok!

I'm hoping someone here might be able to advise with an issue I'm having. I'm aware that there can be a small variance between a smart trainer reading and the one given by a power meter. However, I'm seeing quite a large one.

For example, when I'm riding at what my Tacx Vortex tells me is 306w, my powertap P1s are giving me a reading of 260w - this seems like a massive gap to me. What is also odd is that riding at the same Tacx Vortex power but at a higher cadence (say 100rpm instead of 90rpm), gives me an even lower reading from the powertaps for what feels like exactly the same effort.

Is there something I've missed here? I calibrate the trainer on trainerroad before every workout and also calibrate the pedals as per powertaps instructions.

Any help would be massively appreciated - I've attached the two power files to highlight the difference I'm talking about!

Thanks.

Are you transmitting over BT or Ant+? P1 pedals can't transmit data from both pedals over BT, they only transmit (I believe) the right pedal and essentially double that number to give you your full reading. They can only transmit both left and right through Ant+

Re: Discrepancy between power readings: Tacx Vortex v Powertap P1 [teichs42] [ In reply to ]

Mar 9, 17 8:10

Post #11 of 24 (4172 views)

teichs42 wrote:

Are you transmitting over BT or Ant+? P1 pedals can't transmit data from both pedals over BT, they only transmit (I believe) the right pedal and essentially double that number to give you your full reading. They can only transmit both left and right through Ant+

Almost there...
From: https://www.powertap.com/support/faqs/p1-pedals

WHAT'S THE DIFFERENCE BETWEEN HOW ANT+ AND BLUETOOTH READ DATA FROM THE P1 PEDALS?
Basically there is a fundamental difference in the way the two communication protocols operate. With ANT+ the pedals are connected to each other in a master/slave relationship. The pedal that serves as the master, the left in our case, connects to the display device as well as the slave pedal and does the lion's share of the work.
For BLE (Bluetooth Low Energy or Bluetooth Smart) each pedal is paired individually to the display device and it is up to the display to combine the two power values. If a display device does not have the ability to pair to two individual power sensors (most do not) a user will see ½ power. Unless that display device has some built in smarts to double the value when only one pedal based power meter is connected.

Re: Discrepancy between power readings: Tacx Vortex v Powertap P1 [spraggz] [ In reply to ]

Mar 9, 17 8:14

Post #12 of 24 (4167 views)

spraggz wrote:

Is there something I've missed here? I calibrate the trainer on trainerroad before every workout and also calibrate the pedals as per powertaps instructions.

Any help would be massively appreciated - I've attached the two power files to highlight the difference I'm talking about!

Thanks.

Welcome.

Yes, you're missing a big thing! The Vortex does NOT have a power meter. It's a factory calibrated resistance unit. Effectively, it's simply using the correlation between resistance and speed to estimate power. The estimate can be way off. The calibration app can be a useful aid, but really only gets you in the ball park of what your actual power may be.

Your P1 is a much more accurate representation of power.

If you can, use your P1 output to control resistance of your Vortex (trainer road and Zwift both have these functionalities).

Re: Discrepancy between power readings: Tacx Vortex v Powertap P1 [dfroelich] [ In reply to ]

Mar 9, 17 8:27

Post #13 of 24 (4152 views)

dfroelich wrote:

teichs42 wrote:

Are you transmitting over BT or Ant+? P1 pedals can't transmit data from both pedals over BT, they only transmit (I believe) the right pedal and essentially double that number to give you your full reading. They can only transmit both left and right through Ant+

Almost there...
From: https://www.powertap.com/support/faqs/p1-pedals

WHAT'S THE DIFFERENCE BETWEEN HOW ANT+ AND BLUETOOTH READ DATA FROM THE P1 PEDALS?
Basically there is a fundamental difference in the way the two communication protocols operate. With ANT+ the pedals are connected to each other in a master/slave relationship. The pedal that serves as the master, the left in our case, connects to the display device as well as the slave pedal and does the lion's share of the work.
For BLE (Bluetooth Low Energy or Bluetooth Smart) each pedal is paired individually to the display device and it is up to the display to combine the two power values. If a display device does not have the ability to pair to two individual power sensors (most do not) a user will see ½ power. Unless that display device has some built in smarts to double the value when only one pedal based power meter is connected.

I was close! All I know is, when I've connected to Zwift over BT I get a full power reading but it seems a bit off from when I'm on Ant+. And when I'm on the sensors screen in Zwift, under the power section, only one pedal shows up.

Re: Discrepancy between power readings: Tacx Vortex v Powertap P1 [beston] [ In reply to ]

Mar 9, 17 8:30

Post #14 of 24 (4148 views)

beston wrote:

spraggz wrote:

Is there something I've missed here? I calibrate the trainer on trainerroad before every workout and also calibrate the pedals as per powertaps instructions.

Any help would be massively appreciated - I've attached the two power files to highlight the difference I'm talking about!

Thanks.

Welcome.

Yes, you're missing a big thing! The Vortex does NOT have a power meter. It's a factory calibrated resistance unit. Effectively, it's simply using the correlation between resistance and speed to estimate power. The estimate can be way off. The calibration app can be a useful aid, but really only gets you in the ball park of what your actual power may be.

Your P1 is a much more accurate representation of power.

If you can, use your P1 output to control resistance of your Vortex (trainer road and Zwift both have these functionalities).

If I'm not mistaken (which I was proven to be above) I don't think that will work because the P1's aren't Ant+/BT FEC. I think he could have set up Zwift/TR to get a power reading from the P1's but the controlling aspect has to be done by the Vortex.

Am I right? Because I would like to do that myself. Have my Cycleops Hammer control the resistance but use my P1's as the power source.

Re: Discrepancy between power readings: Tacx Vortex v Powertap P1 [beston] [ In reply to ]

Mar 9, 17 8:30

Post #15 of 24 (4143 views)

Thanks for the responses all - really appreciate it.

I don't warm up the tire before calibrating so I'll do that, and also try some of the other above suggestions. The powertaps are connected to my garmin 920xt so I believe this is done via BT?

Think I'm just going to have to accept that I've been training off an inflated FTP, which for a training effect is fine as I'm using the same measurement each time and not switching between power sources, just a bit annoying to think you're at a level that you're not quite at yet! Ha.

Think it's safe to say I'll use the powertaps for all power readings moving forward as these are by far the more accurate - there isn't anything specific I need to do to calibrate these for riding the trainer as opposed to riding outside is there? Sorry - sounds like such a rookie question.

Re: Discrepancy between power readings: Tacx Vortex v Powertap P1 [teichs42] [ In reply to ]

Mar 9, 17 9:07

Post #16 of 24 (4114 views)

The powermeter doesn't need to be FE-C (Fitness Equipment Control). It simply transmits power information to any compatible receiver.

The trainer DOES have to be FE-C to allow apps like TR or Zwift to control the resistance.

In my case, I have a Stages and a Pioneer PM. I can use either to control my Vortex.

For more info...
https://www.dcrainmaker.com/...d-bike-trainers.html

Re: Discrepancy between power readings: Tacx Vortex v Powertap P1 [spraggz] [ In reply to ]

Mar 9, 17 9:15

Post #17 of 24 (4108 views)

spraggz wrote:

Think I'm just going to have to accept that I've been training off an inflated FTP, which for a training effect is fine as I'm using the same measurement each time and not switching between power sources, just a bit annoying to think you're at a level that you're not quite at yet! Ha.

You can adjust the roller contact pressure If you simply want to get the power readouts to match more closely.

In your case, since the Vortex is reading HIGHER than the P1, you should INCREASE the roller contact force to the tire. This will bring the estimated power of the vortex down to better match the P1. Do it in incremental steps until you are happy with the consistency between the two units.

Keep in mind that the two sources can only ever be the same at steady state speeds. The vortex can't pick up sudden changes in force (like a sprint).

Re: Discrepancy between power readings: Tacx Vortex v Powertap P1 [spraggz] [ In reply to ]

Mar 9, 17 9:49

Post #18 of 24 (4092 views)

I have that same setup. If you're willing to pay for TrainerRoad they have powermatch that will take care of the discrepancy. It essentially takes the Vortex "power" out of the equation and uses the real power from the P1 to adjust the resistance. You still have to calibrate both Units, but its a good way to use both without them fighting for your attention.

Re: Discrepancy between power readings: Tacx Vortex v Powertap P1 [spraggz] [ In reply to ]

Mar 9, 17 10:28

Post #19 of 24 (4061 views)

spraggz wrote:

when I'm riding at what my Tacx Vortex tells me is 306w, my powertap P1s are giving me a reading of 260w

Never ever trust the power readings from a smart trainer. Readings from an actual power meter will be more accurate 99.9% of the time. Not sure why people buy smart trainer when they have a real power meter.

Re: Discrepancy between power readings: Tacx Vortex v Powertap P1 [spraggz] [ In reply to ]

Mar 9, 17 10:30

Post #20 of 24 (4056 views)

spraggz wrote:

The powertaps are connected to my garmin 920xt so I believe this is done via BT?

The PowerTap pedals connect to your 920XT through Ant+. Garmin uses Ant+ for accessories.

spraggz wrote:

Think it's safe to say I'll use the powertaps for all power readings moving forward as these are by far the more accurate - there isn't anything specific I need to do to calibrate these for riding the trainer as opposed to riding outside is there?

There is no specific calibration needed other than setting the offset before each ride. You can set the offset with the 920XT, or if you are in TrainerRoad, you can set it in the TrainerRoad PowerTap device setup screen.

Re: Discrepancy between power readings: Tacx Vortex v Powertap P1 [beston] [ In reply to ]

Mar 9, 17 10:33

Post #21 of 24 (4051 views)

beston wrote:

The powermeter doesn't need to be FE-C (Fitness Equipment Control). It simply transmits power information to any compatible receiver.

The trainer DOES have to be FE-C to allow apps like TR or Zwift to control the resistance.

In my case, I have a Stages and a Pioneer PM. I can use either to control my Vortex.

For more info...
https://www.dcrainmaker.com/...d-bike-trainers.html

That's what I was saying, the trainer has to control the resistance not the pedals. Sure you can have the pedals (or other PM) be the power source, but they won't actually control the resistance, the trainer will.

Re: Discrepancy between power readings: Tacx Vortex v Powertap P1 [spraggz] [ In reply to ]

Mar 9, 17 11:32

Post #22 of 24 (4027 views)

The variance is not small. I have seen as much as 10% difference depending on how warm the tire is, how well are Vortex and P1 calibrated and definitely there is some sensitivity with regards to the ANT+ stick placement.

Which is why I gave up on Vortex altogether.

Next races on the schedule: none at the moment

Re: Discrepancy between power readings: Tacx Vortex v Powertap P1 [RichardL] [ In reply to ]

Mar 9, 17 11:41

Post #23 of 24 (4018 views)

RichardL wrote:

Not sure why people buy smart trainer when they have a real power meter.

Because ERG mode and PowerMatch from Trainerroad.

My Vortex reads low compared to my Stages and Power2Max.

I just run Powermatch and ignore power from the vortex and enjoy all the benefits of ERG mode.

2014 P3 DI2 - RT6, CXR80, Power2Max S, 820, Fenix 3 HR. (gone)

Re: Discrepancy between power readings: Tacx Vortex v Powertap P1 [alex_korr] [ In reply to ]

Mar 9, 17 15:17

Post #24 of 24 (3952 views)

I started with a vortex and was basing my FTP off of that. Buying a powertap hub was a big wakeup call... about 10 to 15% lower than vortex.

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