Could be slipping of the tire, not enough contact, or maybe not enough inertia. The Vortex is a good trainer but if you have progressed and are doing quite well you may have outridden it.
Your gearing seems fine,
I know when I switched from a vortex to a kickr core I dropped from 292 to 252. I worked back up to 272.
Now that my kickr core is dieing I am moving on to a H2. Im interested to see where I test at on it.
From memory the absolute power ceiling of the Vortex is about 900W at 40kph, which is a cadence of 90 in a 50-14 gear ratio.
If you’re in the 50-25 gear combination (third sproket on an 11-32 cassette) at a lower cadence then your speed could be as low as 25kph, which, depending on how linear the power curve of the Vortex is, and how much power you’re putting out in the last couple of steps, could quite reasonably be close to the ceiling of the trainer.
Tacx did have a power band calculator at one point but I can’t find it now.
My advice would be to start in a higher gear (smaller sprocket) so long as you can get the power low enough at the start of the test.
Mike
Thanks for the advice on the Vortex. So previously I think I was using 52-23, cadence of about 100. Last night I used 52-21, again at about 100rpm. It went much better, but I still think the Vortex short changed me on the final step of the test - I was in ERG mode, actually increasing my cadence, but power was much lower than it should have been. Screenshots show the previous test and the one from last night. TR gave me an FTP of 239, but I feel like it should have been at least more like 242.
You could just change gear. It won’t invalidate the test.
This is what I need. How do you work it out?
19m30 to match your current FTP. Everything beyond that is improvement.
Like @brenph just said, after 19m30 you will get an improved ftp. If you have a goal in mind you need to multiply it by 0.75 and complete 1 m at that watt number.
So if you fail with an average watt of 400 for the last minute your ftp should be around 300 watts.
if you hover above the watt line of the ramp test in yout browser or app you can look up the time which matches your set goal.
Personal top tip, don’t try and do a ramp test 2 days after a huge race. What a rookie mistake, compounded by doing a Baxter the day after the race. Sometimes the “High of racing” lulls us into a false sense of power!
I also have similar experience related dropping ftp. With Kurt Kinetic road machine my ftp was 296W. Then I bought Kickr (or clikr) 2018 and performed a ramp test again. Result was 267W. Now I have worked my ftp to 277W and today I should perform a new ramp test.
For me the high cadence (95-100) is a key when performing a ramp test. When cadence start to drop I will be taxing out pretty soon. Still wondering should I use big gear or not (= does inertia “help” there)? I have used small ring on the front…
I’m getting a little frustrated with the ramp test. Done it a few times, and i consistently seem to get a result around 295, which i’m fairly certain is too low. I’ve been basing my zones off an outdoor ftp test which was 20min x 330W *.95 = 313. I regularly get 60-90 min peaks in races of around 300. During the ramp test, i my HR wasn’t even maxed out, but my legs simply couldn’t turn over such a hard gear. I’m definitely highly aerobic rather than anarobic, so not sure if that makes a difference?
All my workouts have been going ok with my current FTP setting, and when i move it down everything seems way too easy.
Thoughts/suggestions? Maybe i just don’t have that mental ability to truly go to failure.
cardiac drift is always less than a few % with my current zones too. Hard to know where i should set my ftp for the best results from TR workouts.
Sounds good and I’d just continue as you have been. The ramp test is not spot on for everyone and if you feel you don’t perform well on it and/or it gives you an inaccurate result, use the 20min or 2x8 version. Then going forward always use the same FTP test. There is no requirement to use the ramp test, it’s simply one method to determine FTP.
Any tips for doing a test on a wattbike?
I seem to top out at the same point as I run out of air gears and need to move to mag. That switch massively upsets my rhythm than breathing goes and then I fall apart. The simple answer is up my cadence so I don’t need the extra gear, but I find I’ve just done 10+min at that cadence and kind of locked in mentally with it.
It’s normal for your outdoor FTP to be significantly higher than your indoor FTP. Proper cooling helps minimize the difference, but it’s pretty tough to match the cooling you get outdoors.
I think max HR can vary slightly from session to session (e.g. based on temperature?)… If your HR was significantly below max, try lifting your cadence a little next time to shift more of the burden onto your cardiovascular system and help your legs last a little longer.
This is my playlist for music to ride by:
Uh…’shut up legs’?
I find two tunes from Metallica’s most recent album especially helpful: Now That We’re Dead and Moth into Flame. Give those a try 
Yeah Ive wondered if those who do LV/MV base plans perform better than those doing HV?? Seeing as the LV/MV workouts target above FTP, so the rider has a better grasp of those higher efforts… Or HV riders can handle the duration, focus better, mentally? Maybe all been asked before but i haven’t seen anything yet.
I’ve come off HV bases and done a handful of ramps and always feel like i come just short. New plans get easier quickly. Ramps are the only times i’ve seen my ‘max’ HR in months haha! and certainly not a fan of VO2 work… only a week into HVBuild and already missing some SS work 
Thanks! I’ll check them out!
I did my latest ramp test today and got a small bump in FTP, which I’m OK with. The thing I’ve noticed about the last few ramp tests (basically since I got a smart trainer) is that my legs aren’t my limiter, my breathing (or by the end difficult in breathing enough) is what causes me to shut it down. My legs don’t hurt like they do in a TT for example, I just can’t process the volume of air required. It also seems to co-incide with a higher than “natural” cadence - my last minute today was >110 rpm.
Should I consider changing up (to a harder gear) when I get above 100rpm? When I was doing ramp tests on the dumb trainer and having to shift, it felt very much more like how it feels at the end of a TT (or when you’re just falling of the back of a paceline and you just don’t have the legs to get back on).
https://www.trainerroad.com/career/duncanm23/rides/56564070-ramp-test
Do the Ramp Test in ERG mode and pedal at whatever cadence you want. Typical failure at the end of a Ramp Test is sudden, not gradual (i.e, falling off the back of a pace line). YMMV
Higher cadence does stress the cardio system more, but at the same time makes that higher power at the end of a test more doable/sustainable. That said, typically you will be unable to keep the pedals turning and suddenly go into the ERG spiral of death. Fun times! 
