Different FTP on same trainer???

Jan 31, 18 14:49

Post #1 of 24 (1737 views)

Riddle me this:

I take an FTP test on my Tacx, using a (rode) Trek Madone, Garmin 2S PM and hit 280w for the 20 min.

[Week later]

Do another test on my Tacx, using my (Tri) Orbea M20, different PM but Garmin 2S nevertheless, and can only muster 220w for the 20min.

**I have been fitted professionally on both bikes as well.

I tried my best to replicate everything. Also, this wattage discrepancy is the norm even when I do Trainnerroad/Zwift workouts on the different bikes. I even torqued each PM on the pedal to the same newton metre. But no matter what, there is a whopping 60w difference between bikes.

I would expect lower watts outside on my tri bike, due to less drag ... but indoors?

Why is this frustrating to me? Because now my WPK is waaaay lower my tri bike, and I based all of my self-esteem and worth on my watts-per-kilogram.

Is life worth living? Nay, I say !

Last edited by: p3: Jan 31, 18 14:51

Re: Different FTP on same trainer??? [p3] [ In reply to ]

Jan 31, 18 15:07

Post #2 of 24 (1715 views)

p3 wrote:

Riddle me this:

I take an FTP test on my Tacx, using a (rode) Trek Madone, Garmin 2S PM and hit 280w for the 20 min.

[Week later]

Do another test on my Tacx, using my (Tri) Orbea M20, different PM but Garmin 2S nevertheless, and can only muster 220w for the 20min.

**I have been fitted professionally on both bikes as well.

I tried my best to replicate everything. Also, this wattage discrepancy is the norm even when I do Trainnerroad/Zwift workouts on the different bikes. I even torqued each PM on the pedal to the same newton metre. But no matter what, there is a whopping 60w difference between bikes.

I would expect lower watts outside on my tri bike, due to less drag ... but indoors?

Why is this frustrating to me? Because now my WPK is waaaay lower my tri bike, and I based all of my self-esteem and worth on my watts-per-kilogram.

Is life worth living? Nay, I say !

If I am reading this correctly you are using two different sets of Garmin 2S pedals. If this is true, then should answer your question of where the difference is originating. It should be easy to swap the pedal sets between bikes?

If I read this wrong and you are moving one set of Garmin 2S pedals between bikes, I have nothing to offer. This is single sided PM so perhaps an imbalance L/R between bike positions. But 60 watts would be extreme for fit differences.

Re: Different FTP on same trainer??? [SummitAK] [ In reply to ]

Jan 31, 18 15:15

Post #3 of 24 (1703 views)

2 PM's, one on each bike.

Single pod on Trek, and a different pod on the Orbea - both torqued the same. I have 2 PM's so I don't have to switch all the time.

Re: Different FTP on same trainer??? [SummitAK] [ In reply to ]

Jan 31, 18 15:20

Post #4 of 24 (1698 views)

Have you tried swapping the power meter between the bikes to double check?

Re: Different FTP on same trainer??? [p3] [ In reply to ]

Jan 31, 18 16:32

Post #5 of 24 (1670 views)

Your hip angle is a lot more open on a road bike so I would expect to produce more watts. From what I am told in earlier Garmin power meters accuracy is an issue so unless you used the same meter I would question the results based on that. Drag has nothing to do with watts either. You may have more speed for the same watts being more aero but watts is how much power you are producing period nothing to do with drag.

Re: Different FTP on same trainer??? [p3] [ In reply to ]

Jan 31, 18 16:55

Post #6 of 24 (1650 views)

Which model Tacx? If it's a Vortex, look no further. It is wildly whacked unless tyre pressures are identical, pressure knob is identical, and warm-up temperatures are identical. In other words, it is a random number generator, IMHO.

If it's a Neo, then indeed the pedals, bike frame, etc. as suggested above are all possible sources of the discrepancy.

Re: Different FTP on same trainer??? [p3] [ In reply to ]

Jan 31, 18 19:43

Post #7 of 24 (1583 views)

+1 for swap the pedals next time and see.

If you have access to a "smart" trainer, maybe you can compare your PM readings with the trainer's readings? They won't be identical, but you can shouldn't see different offsets between the power the trainer records and the power your pedals give you as you go between bikes. If they are, it points to a calibration difference.

I also agree with the comments about the different bikes using different muscles. Some of the smart people around here can probably give some interesting details about this, but I know I'm about 15-20 watts stronger on my TT bike.

If all else fails, assume your highest power when calculating your worth as a rider. I mean, uh... w/kg.

Re: Different FTP on same trainer??? [p3] [ In reply to ]

Jan 31, 18 20:57

Post #8 of 24 (1537 views)

p3 wrote:

Riddle me this:

I take an FTP test on my Tacx, using a (rode) Trek Madone, Garmin 2S PM and hit 280w for the 20 min.

[Week later]

Do another test on my Tacx, using my (Tri) Orbea M20, different PM but Garmin 2S nevertheless, and can only muster 220w for the 20min.

**I have been fitted professionally on both bikes as well.

Assuming that you have a recent-ish Garmin head unit that displays the applied torque on the pedal after you do a zero offset, you can hang a weight (in the 20 pound range) off of the toe of the left pedal, after you do a static zero calibration. Do this on each bike and you'll know whether or not the Vector pedals are part of the discrepancy. Regardless of their absolute accuracy, hanging the same weight off each pedal should give you the same result.

Less is more.

Re: Different FTP on same trainer??? [JRT_Racing] [ In reply to ]

Jan 31, 18 21:12

Post #9 of 24 (1530 views)

Nope. On Neo.

I'm not planning on installing the same Vector on both bikes to test ...... That's why I bought two!

Last edited by: p3: Jan 31, 18 21:15

Re: Different FTP on same trainer??? [woodnat1] [ In reply to ]

Jan 31, 18 21:14

Post #10 of 24 (1529 views)

woodnat1 wrote:

+1 for swap the pedals next time and see.

If you have access to a "smart" trainer, maybe you can compare your PM readings with the trainer's readings? They won't be identical, but you can shouldn't see different offsets between the power the trainer records and the power your pedals give you as you go between bikes. If they are, it points to a calibration difference.

I also agree with the comments about the different bikes using different muscles. Some of the smart people around here can probably give some interesting details about this, but I know I'm about 15-20 watts stronger on my TT bike.

If all else fails, assume your highest power when calculating your worth as a rider. I mean, uh... w/kg.

What???? The comments above said the opposite. Tri bike = fewer watts, because of decreased hip angle.

Re: Different FTP on same trainer??? [Big Endian] [ In reply to ]

Jan 31, 18 21:16

Post #11 of 24 (1527 views)

Big Endian wrote:

p3 wrote:

Riddle me this:

I take an FTP test on my Tacx, using a (rode) Trek Madone, Garmin 2S PM and hit 280w for the 20 min.

[Week later]

Do another test on my Tacx, using my (Tri) Orbea M20, different PM but Garmin 2S nevertheless, and can only muster 220w for the 20min.

**I have been fitted professionally on both bikes as well.

Assuming that you have a recent-ish Garmin head unit that displays the applied torque on the pedal after you do a zero offset, you can hang a weight (in the 20 pound range) off of the toe of the left pedal, after you do a static zero calibration. Do this on each bike and you'll know whether or not the Vector pedals are part of the discrepancy. Regardless of their absolute accuracy, hanging the same weight off each pedal should give you the same result.

Head unit is a Garmin 800.... Don't think it'll work....

Re: Different FTP on same trainer??? [p3] [ In reply to ]

Jan 31, 18 21:19

Post #12 of 24 (1525 views)

p3 wrote:

Head unit is a Garmin 800.... Don't think it'll work....

Yeah, that's unfortunate.

Less is more.

Re: Different FTP on same trainer??? [p3] [ In reply to ]

Feb 1, 18 6:34

Post #13 of 24 (1384 views)

I haven't seen anyone else point this, so I will:

The apparent discrepancy you have observed is probably due in part (if not entirely) to differences in L/R balance between the two bikes (positions).

Instead of two 2S pedals, you would have been better off buying a dual-sided system. Sure, you would have had to swap pedals around periodically, but IMO that beats junk data.

Re: Different FTP on same trainer??? [Andrew Coggan] [ In reply to ]

Feb 1, 18 6:39

Post #14 of 24 (1371 views)

But ... Is it really "junk data" if my 220w sucky FTP is repeatable on the same bike?

Furthermore, I have a history with dual sided as being 48%/52% (tested on Computrainer).

Not sure how an L/R imbalance equates to such a discrepancy in power.

Re: Different FTP on same trainer??? [p3] [ In reply to ]

Feb 1, 18 6:41

Post #15 of 24 (1363 views)

p3 wrote:

I'm not planning on installing the same Vector on both bikes to test ...... That's why I bought two!

You mean you're unwilling to swap pedals as a test? I guess you can just live with two inaccurate power meters but it seems kind of pointless having power meters if they're not remotely accurate.

Re: Different FTP on same trainer??? [p3] [ In reply to ]

Feb 1, 18 7:00

Post #16 of 24 (1347 views)

Yes, it is "junk data", as it clearly isn't trustworthy.

In particular, the fact that a CT's pseudobalance indicates only a 4% error is no guarantee that the error isn't even larger or - more importantly - variable from day to day, from one bike (position) to another, etc.

Especially for a triathlete interested primarily in power over longer durations only, the benefits of a power meter are going to be far smaller than the discrepancy you have observed. I for one would never be willing to "just live with it", as it largely defeats the purpose of having spent the money on a power meter (actually, two) in the first place.

Last edited by: Andrew Coggan: Feb 1, 18 7:01

Re: Different FTP on same trainer??? [gregf83] [ In reply to ]

Feb 1, 18 7:01

Post #17 of 24 (1343 views)

gregf83 wrote:

p3 wrote:

I'm not planning on installing the same Vector on both bikes to test ...... That's why I bought two!

You mean you're unwilling to swap pedals as a test? I guess you can just live with two inaccurate power meters but it seems kind of pointless having power meters if they're not remotely accurate.

Yes, I am unwilling. Because I spent money to avoid switching. It's now out of principal.

Just like I said to Andrew, if the 220w pathetic FTP is repeatable, then how can I say it's 'wrong'?

(we may be at a impasse here....)

Re: Different FTP on same trainer??? [Andrew Coggan] [ In reply to ]

Feb 1, 18 7:04

Post #18 of 24 (1337 views)

Andrew Coggan wrote:

Yes, it is "junk data", as it clearly isn't trustworthy.

In particular, the fact that a CT's pseudobalance indicates only a 4% error is no guarantee that the error isn't even larger or - more importantly - variable from day to day, from one bike (position) to another, etc.

Especially for a triathlete interested primarily in power over longer durations only, the benefits of a power meter are going to be far smaller than the discrepancy you have observed. I for one would never be willing to "just live with it", as it largely defeats the purpose of having spent the money on a power meter (actually, two) in the first place.

Thank you. Now I am convinced. I will be calling Garmin customer service today. I will look into this deeper, and if I have to switch pedals to test, I will. I just needed to hear other opinions, weigh them, and then make a decision. Thank you.

Re: Different FTP on same trainer??? [p3] [ In reply to ]

Feb 1, 18 7:04

Post #19 of 24 (1337 views)

p3 wrote:

gregf83 wrote:

p3 wrote:

I'm not planning on installing the same Vector on both bikes to test ...... That's why I bought two!

You mean you're unwilling to swap pedals as a test? I guess you can just live with two inaccurate power meters but it seems kind of pointless having power meters if they're not remotely accurate.

Yes, I am unwilling. Because I spent money to avoid switching. It's now out of principal.

Just like I said to Andrew, if the 220w pathetic FTP is repeatable, then how can I say it's 'wrong'?

(we may be at a impasse here....)

Repeatability has nothing to do with accuracy.

Re: Different FTP on same trainer??? [p3] [ In reply to ]

Feb 1, 18 7:04

Post #20 of 24 (1336 views)

Do not own a Tacx, but when jumping back and forth with PM's on smart trainer make sure the watts you are getting are coming from the meter on both and not the Tacx on one.

Re: Different FTP on same trainer??? [ktm520] [ In reply to ]

Feb 1, 18 7:06

Post #21 of 24 (1334 views)

Good point. On Neo, I pair with my Garmin pedals, not the trainer.

Re: Different FTP on same trainer??? [p3] [ In reply to ]

Feb 1, 18 7:11

Post #22 of 24 (1326 views)

You said you're using a Neo? What does that report for both bikes. I'd trust the data from the Neo way more than either of the Vector 2s.

Just because a result is repeatable does not mean it is accurate. You would have been better off buying a single double sided Vector and swapping. Or two of them if you wanted to avoid swapping. Fortunately I believe they are upgradeable so you can rectify your mistake.

I'll leave this image here as well, right now it seems your Vectors fall in the first case. Ideally you would be in the fourth case.

Re: Different FTP on same trainer??? [p3] [ In reply to ]

Feb 1, 18 7:13

Post #23 of 24 (1322 views)

I have had to "forget this device" to get configuration cleared on various trainers to connect to new/different PM to drive the workout, otherwise it will just pull data right from the trainer unit which is often substantially different than an external PM, even a one sided like Stages and Garmin.

Re: Different FTP on same trainer??? [p3] [ In reply to ]

Feb 1, 18 7:16

Post #24 of 24 (1317 views)

FTP will be higher on the road bike. Typically.

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