That difference is not uncommon going from an uncalibrated fluid trainer (which was never made to have an exactly replicable power curve, has issues with tire compression variances, etc.) to a direct drive trainer designed for accuracy.

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Fluid trainers will slowly lose oil over time even if there is no visual leak. This results in a gradual decrease in the resistance of the units over the course of years. This decrease is most noticeable when a trainer is fully up to temperature and working hard to dump heat as that is when the reduced volume really effects the resistance level. This means you don't necessarily feel it on a day to day basis but an FTP test creates a perfect storm for the trainer to struggle. The differences you are seeing are not a-typical for a unit that has seen usage. Other factors can come into play but they are consistent between units.

When you say kinetic power and outside power are you referring to use of a separate power meter?

I have a road machine and used to use TR virtual power. When I got favero assioma duo PM I found the virtual power about 25w higher than than the PM, which is what I expected from reading up on it. Still use the road machine as my turbo.

Of course if you're using a power meter other than on the kicker then that is a significant difference.

Not necessarily. Mine never lost fluid, but as the unit aged (nearly 20 years) the oil became more viscous to the point that I could only ride it in the small chainring on the easiest gears. Spinning at low power was impossible. I drained and replaced the fluid with some hydraulic oil, and it’s useable again.

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I have a stages on the tt and a giant dual sided power meter on my roadie. They both read and track very well with each other.
The road machine ftp being 265, my best races on the stages tt around 230ish. Ftp test on kickr core was 236. I tend to bike fairly well generally despite the low numbers to many here. I’m 165 pounds too so no weight weenie. Just a bigger difference than I was expecting.

Sounds like the core is out then? Would say the others are more representative of your ability. Just get the power readings from the PMs rather than the core, and let the core control the resistance.

I used a Road Machine and virtual power for a couple of years before getting a power meter, and later a Tacx Neo 2T.
The Road Machine resistance unit (shared wit hthe Rock'n'Roller I think) is probably the one "dumb" trainer which was aimed at having a pretty replicable power curve. There is some variation with temperature but it's minimal compared with other fluid trainers I've seen which change characteristics massively over the course of a session. I found the RM very consistent in terms of the perceived effort, HR and my ability to hold a given estimate wattage for a given time. So I was reasonably confident it was repeatable, but didn't know how accurate. When i eventually got a PM (Power2Max NGeco - so importantly it's total power not single sided) I compared the PM readings to the virtual power figures for the RM. They were within just a couple of % and stable once warmed up (I think about 10-15% off for first few minutes of warm-up, reaching equilibrium after 10-15mins). I got lucky with my setup as I know that will effect things significantly. But I always used the same tyres and tubes (Felt TTRs with Butyl tubes), tyre pressure (100psi), and clamping compression (exactly 3 turns of the knob from the point of first roller contact).
Friends trying to use virtual power on Elite, Tacx and Cycle-Ops Fluid and Magnetic trainers were unable to get any sort of valid numbers.

Again the Road machine may be a lot better than most in this respect. It uses a magnetic coupler to drive the rotor in the fluid chamber, so there's no shaft seal likely to be very slowly leaking throughout the trainer's life. Unless there's leakage across the gasket sealing the chamber, or breakdown of the resistance fluid, it should be pretty unchanging.

How do you like your Neo? Some reviews complain of technical defects and had to return. Looks like a great trainer.

I've had no problems and I've found it great.

Awesome, I'll probably get one. Thanks!

My 1:1 is that every dumb trainer person from Zwift using virtual power in our bike race team or a-group ride has had very sad realizations when they went to a real smart trainer with good published accuracy.

It sucks, just learn to work with what you have and stay consistent. That eliminates disappointment in the long run, especially if you instead just focus on gains in power and gains in CdA as they relate to improved results in races. Does no good to have "cool" looking wattage numbers or a "low" looking CdA only to always be slower than you think you are.

Agree with the virtual power aspect being out for most, but what I think the OP is saying is that they weren't using virtual power for the test, but a power meter that just happened to be on a road machine (could have been any dumb turbo brand), and the power meter used tracks very closely to another power meter on another bike. Yet the kickr has now given a lower result, although the result tracks better with outside wattages. So either both independent power meters overestimate by the same amount, or the core has underestimated. Or the test done on the road machine was an anomaly. Something doesn't add up. I'd suggest the OP compare the kickr numbers to one of the pm numbers over an interval session recorded separately. Then maybe try and use someone else's kickr to see how they compare. Could well be an issue with the kickr.

I've not seen any leakage from my well-used RM either. Out of curiosity, I think I'll go look for a fill/drain plug (but I don't expect to find one.) Thanks for mentioning,
-Eric

there should be an allen key somewhere on the unit, the manufacturer needs a way to fill them after assembly. on the Fluid 2 it's on the outside of the resistance unit near the bottom.

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If so, a Kickr is more like a Powertap in that it is after any crank or spider measuring device.

So, subtract 2% minimum. 2% of 300w is 6w. Not a trifle if at threshold. 6w can break you.

If you analyze your outdoor rides for how fast you routinely go at what power.........you can get a good guess on which is likely more accurate.

I used to think I had a world-class CdA with some stellar speeds on stupid low watts. It was that my left only Stages grossly under read my power.

If he's a tad slow outside for the power being made, that would lend itself to the Kickr being better. If he's a tad faster outside or just right for the power being made, the Kickr is off.

30w makes a big difference outdoors in average speed. That's gotta be a whole mph.

stages single sided? the giant is a shimano crank-based PM, correct.

GPLama and DCRainmaker have both reported issues with shimano-crank based dual sided PM's. It could be a combo of the PM's reading high (depending on L/R balance and right side inaccuracies on the road bike) and the kickr being off.

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Huh...my quick look revealed no Allen plug/etc. Now I'm really curious; gonna' take the fluid unit off it's mount before my next training session and check it out.

Fairly close

I have used the road machine for years, same psi same dial turns etc. very consistant power I had always found.
My stages is on my tri bike- I use that power figure for outside and always reads lower than the road machine. When training I didn’t use stages power numbers unless outside. Sounds silly and I should use an actual power meter indoors too, but I really did find the kk virtual power curve pretty good.
My tcr has shimano crank based dual powermeter and despite being a road bike, the numbers pretty much fall in line with the stages on the tri bike. I have only had this for a few weeks though admittedly.

The kickr core also falls in line with the stages and giant power meter.
What was suprising is that the road machine was so far above the rest. I always just figured the stages (and giant) read quite low because an ftp of 236 is crap and I can often be at the pointy end of the field for a bike split. The road machine of 265 ftp still isn’t good around here, but that figure from the road machine seemed to correlate better with my outside performance.
In any event maybe I should just do an ftp test and use powermatch on the kick with the stages from my tri bike. And let the stages control the kickr as another posted mentioned

I think the best practice is if you have a power meter available, use the power meter, and if you're going to use multiple different power meters, do some testing with something like dc rainmaker's analyzer to compare them before you use any data from them.