Most of the athletes I train have different FTPs on their different bikes.
For example on a road bike and on a time trail bike.
This may be due to the position and or different powermeters.
It could therefore be nice if, in the same way as there is an Indoor FTP - there was also an: TT-FTP, MTB-FTP, Gravel-FTP, etc.
Out of interest, what would the variances be between each âFTPâ value?
If <10W I wouldnât adjust; 2% of 400W is 8W.
The same with <4% due to a +/- 2% variation on PM.
Then, the head unit and /or training app (Zwift/IndieVelo) would have to have a corresponding value to match the discipline used, if thereâs a vast difference.
Hi Gerald,
It is often quite a bit more than the 10 watts. In a current case, the difference between the personâs road bike (371) and his tt bike (347) is a whopping 24 watts.
When I make programs, I almost always use %FTP or %FTP range. I can of course just use watts, and then make the adjustment here. It would just be cool if intervals could make this adjustment automatically 
There is an option for outdoor ftp vs indoor ftp. Perhaps can use that as the differentiator?
Interesting, that the TT is less than road. Unless of course the speed is higher in an aero position.
Has the person been for a bike fit to find an optimal position? Sometime a position too aggressive can compromise power.
He is super fast on both his road bike and his TT. He has been fitted and aero tested on his TT.
I suspect that the difference lies mostly in the two different power meters. Experience this difference in several of the athletes I train, and therefore it could be nice if this difference could be taken into account, on intervals.icu
On Saturday I rode my track bike with an SRM and Powrlink dual pedals. My semi steady state efforts (2-3 minutes average 250-300W) had an average watts difference between 10 and 14 watts, with the SRM always reading greater.
Similarly my road bike with a left only Stages has a difference on the average power of only 2 watts, with the Stages reading higher.
My kickr in erg mode reports 20W lower than the Powrlinks and 12W higher than the Powrlinks in sim mode⌠This was tested using a workout in MyWhoosh and switching erg mode off manually.
I am surprised that the head units donât give the option to adjust the power by bike profile.
Accuracy for different types and brands of real power meters vary by 1 - 3%, plus or minus. The exact place in the drive train also creates differences. A power meter on the pedals will normally show higher wattages compared to crank, bottom bracket, spider, rear wheel hub⌠because those are all further dawn the drive train.
When it comes to indoor bike trainers, most of them donât have a real power meter. They simply do a speed/power conversion following mostly a linear curve that is offset vertically when resistance changes. The better models have device specific calibration from factory. The cheaper ones just have an âaverageâ model.
If you compare 2 different power meters where both have a +/- 3% margin, an 18W difference when riding around 300W is within spec.
If a head unit would give you the option to adjust, how would you decide which one is the âcorrectâ one?
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Oh completely agree with what youâre saying about accuracy between devices, but thanks for the detailed clarification of the situation.
The previous commenters were speculating whether the different FTPs come from different positions or different devices and I was giving my measured difference between different devices, using the Powerlinks as the common denominator.
@MedTechCD To reply to your question; for real world application does it really matter which one is correct? I would just like consistency between devices, even if they are all then consistently off by say 20W. In my situation I would use the Powrlinks as the âcorrectâ one, because then I can move it between bikes to determine how much to adjust each one by.
A few weeks ago I was doing a session on the Kickr in erg mode, I was supposed to be sitting at 350W, with the Kickr reporting 20W too low, I was actually sitting around 370W, I failed on the session and this knocked me mentally for a bit.
The other problem is that often after a track session my Garmin wants to update my FTP, but this is because the SRM on the track bike is over reading from my FTP set from the StagesâŚ
Having different FTPs for different bikes will alleviate some of these issues, your post ride analysis will be more reflective, if youâre building your workouts on intervals.icu and specifying the bike the prescribed wattages will be relevant to you on that specific bike. So for example my Kickr situation wouldnât occur because the workout would be prescribed based on my 'kickr" ftp not my âStagesâ ftp. This wonât change the situation where my Garmin wantâs to change my FTP after track sessions⌠Having an adjustment on the head unit wouldâŚ
My assiomaâs have the ability to increase or decrease the reading by x percentages. This would help to consolidate among all the various PMs you have.
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I donât think it is helpfull to discuss why you see different FTP values. As long as we agree, there are differences. So the question remains, how to handle in the software.
By the way: look at swiming in a wetsuit VS speedos or running carbon/super shoes. Your critical swim speed or pace will vary wrt equipment.
my 2 cents
Thomas