Powertap SL+on Tacs Flow - does it all work?

[Malcolm P]

I have a Tacx flow turbo trainer and have quite happily been making sense of it over the winter. In early May I had the great pleasure of discovering the PT SL+ and am now a convert to power based training. I never really did take the power on the Tacx all that seriously.

Anyway some time has now past and I am gathering some data and starting to understand things. My problem is that my power readings from my PT seem to be understated when I do sessions on the turbo.

For instance in an attempt to make my testing protocol very sound I have done one FTP test on the turbo. The result produced a 30 min average power of 250W, but on the road I have several data files demonstrating that I have managed 275W+ during 40-45 min TT like sessions. This puzzled me but given my recent arrival to power I ignored this and carried on gathering data and learning. Equally on the road my zones feel more correct when assuming an FTP of c275W using the road data.

Today I did an interval session on the turbo which is in Hunter and Coggans book including 2min, 1min and 30sec intervals. This seemed to have the same problem, the power readings were much lower than expected and lower than the rest of my file history is showing for these interval durations. I simply could not hit the ranges that were targetted based upon my road FTP.

To be clear, I am ignoring the computer on the Flow. I have done sessions on the road using my PT and Edge 500 and got data, when I do similar sessions on the Tacx flow using my PT and Edge 500 the results show me putting out much less power.

There two options:
1 - I put out much less power when on the turbo, why would that be, position, its the same bike? It implies that there is additional resistance between me and the PT torque meter.
2- The power readings from my PT and Edge 500 are much lower when operating with the Tacx Flow, ie there is some sort of interference.

Does anyone have any ideas as it is making the controlled environment of the turbo hopeless as it is on a totally different baseline to data from the road.

Thanks for you help.

 

[jollyrogers]

1. Are you zeroing the CPU every ride?

2. Alex's Cycle Blog: Turbocharged Training

 

[Malcolm P]

1. Are you zeroing the CPU every ride?

2. Alex's Cycle Blog: Turbocharged Training

yeah I am zeroing on every ride. In fact I am quite certain that all the technical bits are working fine. I have no problem gathering all manner of data from my Edge 500 and PT when training outdoors or indoors. I am using, laps, workouts and downloading to Golden Cheetah and everything is pretty clear.

That is the strange part, I see no technical reason for this difference.

Quite simply the data is all working fine, it is just showing up considerably underperformance at every level on the turbo. I would discount motivation and probably even heat. Yesterdays intervals sessions was a relatively short one and I could not hit the target power on the first one and this was certainly before any major overheating due to being indoors.

I have read your link which is very interesting. It make sense to me that it could be something to do with the inertial load. I love the turbo because of its convenience in winter and also because the relentless lack of freewheeling does seem to compress a session. But I was also hoping to use it as a very controlled environment for my power testing, but that is not looking like a good idea.

Does anyone successfully use the turbo trainer as their testing environment?

If I did I would immediately have a 10% boost the moment I went back outdoors.

 

[KeldaS]

Hi

I have exactly that problem!

I ride with a Powertap Pro Plus and use the Tacx Flow for indoor training.

I rode a 10 in June with the PT hub and recorded 27:10 for 188 watts and around 170 bpm hr.

Yesterday I sat on the turbo and did a 6 x 6 min interval session and worked according to RPE, speed and heart rate at 10 pace but the watts recorded for each interval were in the low 150s?

I can't work out why this should be either other than some function of the braking mechanism perhaps in the turbo? I played around with that this morning during my warm up to work out if I could set it differently so the PT readings for indoors were nearer those outside but couldn't see any pattern to be honest.

I even took the wheel out to check the hub was running freely.

I'm coming back to training after some down time so would antcipate my power to be down but then so would the speed surely? I could understand covering less distance with lower power for the an effort that would normally be faster, what is odd that all the readings are correct for a 10 effort except the watts!

Kelda

 

[JibberJim]

Originally Posted by KeldaS . I'm coming back to training after some down time so would antcipate my power to be down but then so would the speed surely? I could understand covering less distance with lower power for the an effort that would normally be faster, what is odd that all the readings are correct for a 10 effort except the watts!

Lots of people find the power lower on the turbo for identical percieved effort. There are many opinions about to why, lots of people think it's heavily to do with heat, but this is often removed by people who can put out the numbers on rollers. The braking effect of the turbo is often very different to the forces on the road, the inertia of the system is completely different and this can heavily impact your ability to produce power as your pedalling mechanics have to change.

Personally I would just accept the lower watts, consider a new turbo with more road like inertias if you're looking for another in the future. The training benefit on Percieved Effort is likely to be driving adaptations that you want. The actual number is less important.

 

[daveryanwyoming]

+1 on JibberJim's reply

Many folks struggle to sustain the same power on trainers as they typically do outdoors. I'm typically down more than 10% for long interval power at the beginning of winter but even after months on the trainer I still don't quite get back to the kind of numbers I see outside. The good news is that every spring the numbers jump quickly, usually to new highs once I get back outside so all that indoor trainer time, even at reduced power levels, still produces results.

A big fan for cooling, a relatively high inertia trainer with a big flywheel and perhaps some good music or videos for distraction (I personally don't like videos during hard intervals but listen to music instead) can help but in the end do what you can do and put out the effort and it's still good training if you can't ride outside.

-Dave

 

[KeldaS]

I think our point is not that we find it hard to work hard indoors, I know from my heart rate, rpe and the speed and watts the tacx is showing that I'm hitting the right effort the thing I can't explain is why the Powertap records around 30 watts less for that effort on the turbo compared with comparable effort on the road!

 

[daveryanwyoming]

Yes, it certainly seems like we're working just as hard and indicators like HR rise in response to more than just the effort level, lack of adequate cooling alone can drive your HR up quite a bit for the same power output. Many folks running SRMs, Quargs, and Powertaps observe the same thing, power is lower for the same or even elevated RPE and HR while riding indoors. In general the power meter is correct if it's been properly torque zeroed and the rider is simply putting out less power regardless of what it feels like or how high their HR climbs relative to outdoor efforts. The power displayed by the Tacx is meaningless as that's just an uncalibrated look up table related to wheel speed on the trainer with little absolute accuracy and no direct relationship to speed/power relationships on the road.

Lot's of reasons are given with cooling and lack of intertia being high on the list but when it's been looked at closely the power meter is generally correct barring some cases with things like loose locknuts on the PT axles skewing readings but that doesn't explain why crank based Quarg and SRM power meters show the same behavior.

It would be nice to be able to blame the lower indoor power on PT measurement errors, but that's rarely the case.

-Dave

 

[bing181]

Be interested if any of you PT-owning Flow users could just compare numbers (Power). I only have a Tacx Flow, and feel it reads high, even very high, compared to what's really going on, or what goes on out on the road.

 

[KeldaS]

This data below was recorded by an experienced tester who was doing a lactate test for me, we calibrated the Tacx Flow (-2) at the beginning of the test.

The first figure is PT watts, the second Tacx Flow these were taken at 4 minute intervals in a ramp test starting from 100 watts until I fell off the bike!

80, 116 (warm up)
100, 147
120, 178
140, 210
160, 235
180, 269
200, 298



As for the PT be zeroed, yes it was and so was the Turbo calibrated. I phoned the supplier of the PT today and had quite a long chat. He says, depending on how much braking is being exerted by the turbo will affect how the PT reads.

I'm just going to work to the numbers the PT gives me on the turbo after doing a new FTP test on the turbo. I will have to do the same test out on the road and work with those figures when outside, no great drama, I was just curious being of a scientific bent!

Thanks

 

[daveryanwyoming]

Originally Posted by KeldaS . I phoned the supplier of the PT today and had quite a long chat. He says, depending on how much braking is being exerted by the turbo will affect how the PT reads....

He's blowin' smoke. The PT uses a standard strain gauge to measure hub torque, when multiplied by the wheel's angular velocity you get power. The braking or resistive losses provided by the Tacx or other trainer or actual losses encountered on the road will be reflected in the torque and hence power recorded but those braking forces will not affect the accuracy of either the strain gauge nor the measurement of the wheel's angular velocity.

It's possible your PT is out of calibration but you can check that with a static torque test using known weights. As mentioned above some folks have had problems with loose axle locknuts on PT hubs which can influence the accuracy of torque and hence power readings depending on compressive forces exerted by the trainer wheel mount but again you can rule that out by performing static torque tests using known weights, do a set of tests with the bike out of the trainer and another with the bike mounted in the trainer and see if the reported torque or strain gauge accuracy varies. But the braking forces generated by the trainer cannot directly influence the accuracy of the PT strain gauge, if it could then situations like climbing steep hills at low speeds and low inertia vs. going fast and hard in a downhill sprint would also influence the PT strain gauge accuracy and they don't for an in spec. PT hub.

Of the two devices mentioned I'd definitely question the absolute accuracy of the Tacx trainer long before I questioned the accuracy of the PT hub. The PT is a calibrated measurement instrument with published accuracy specifications that can easily be verified by the user. The Tacx is a very different beast and much harder to objectively validate.

Torque test your PT hub if you have doubts, it takes about five minutes. All you need is a fixed weight in the 30 to 50 pound range and one which you're very certain of its actual weight (gym style weight plates vary quite a bit but you can weigh them at postal centers like UPS of Fedex shipping centers to check). It's certainly possible that your PT hub is out of spec, I've tested about fifteen over the past few years and have seen 3 so far that were outside their claimed +/-1.5% accuracy.

-Dave