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