Will a direct drive mess up my FTP estimate?

I’m looking at getting a direct drive trainer. My power meter is a powertap hub so if I get a direct drive like the kicker core, I can’t use my power meter any longer and Trainerroad will have to use the kickers internal power meter to do so. My concern here is that the kickers measurement will be way different than my power meter thus the Trainerroad workouts will not be at the same intensity any longer (currently using dumb rollers). In your opinion would the kickers power measurement be close enough to my powertap or would they be way different causing an issue? Thanks for any insights.

Your most accurate numbers are certainly going to come from re-testing, but to take that even further I would say there would be value in doing ramp tests with a day of rest in between or so with each set up. This way you can give yourself a correction variable for riding outside with your powertap. I think every company claims to be within 1-3%, but actual accuracy (even among the same brand) seems to vary more than that.

Thanks, makes sense Alex, my other option would be to get a wheel on trainer but I would hate to compromise. I guess once I establish an accurate offset between the powertap and the trainer I can adjust FTP/power targets when I jump back and forth between outdoor/indoor rides. Kind of a pain but unless I get a different power meter this might be the only way to use a direct drive.

Get the direct drive trainer and retest. Wheel on trainers are a substantial drop in experience over direct drive

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I like using a power meter with my smart trainer because I find that the trainer power drifts over time. I am using a wheel-on trainer, so I may experience more drift than a direct drive trainer, but I like being able to use powermatch to keep things accurate. When the power is high by even a few percent it can be very hard to complete the most demanding intervals.

I wouldn’t worry too much about it. Over time, you will probably be able to spot the difference in power readings between the two and then learn to adjust accordingly (if that’s even necessary). If you have the willpower and drive, then doing two ramp tests with a day of rest as others have mentioned would be good.

Thanks for all the input guys, I ended up getting a kickr core and so far the power accuracy has been satisfactory.

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Wahoo Kickr is a really nice direct-drive family of trainers. Like any trainer, we can nit-pick details (and I have) but overall it is really solid and I’m very happy with my 2017 model.