Wattage drop when switch from road tire to trainer tire

Have a Tacx Vortex Smart, running Zwift on Apple TV.

My first season with Zwift, used a plain road tire for the rear, my FTP per Zwift was at approx 3.4 w/kg.

My second season, after a few weeks on a worn down road tire, I switched it out for a Vittoria trainer tire. Suddenly, at the same effort level (am basing this on my heart rate), my FTP has durably dropped from 3.4 to 2.8. (Am in my third season with Zwift now, FTP has never recovered to 3.4)

Tire pressure is the same, used Tacx Utility to calibrate regularly, both on old tire and new.

Any ideas on what is happening here?

my kickr snap has a target “spin down time” of 12-16 seconds. when i do a spindown with the kickr utility, i also check that my spin down time is correct.

it’s possible that your new tire has a longer spin down time than your older one, which i think would make you lose power.

it’s also possible that the reverse was true; your first tire may have had too short of a spin down time.

I suppose it’s possible… visually comparing the two tires:

  • due to prolonged trainer wear, the old road tire was quite ‘squared off’, so to speak
  • the newer trainer tire is more rounded (due to less wear)

I say that the former would have had much more surface contact with the trainer roller than the latter… which may go to support your theory that the trainer tire (with less surface contact) has a longer spindown time → less power.

That said, last night I took some time and tested my trainer tire at higher/lower tire pressures and also tighter/looser roller tension. After each tweak (and recalibrating in Tacx Utility each time), I rode a quick 2 km stretch on flat road in Watopia to see if the wattage would change.

Remarkably, the wattage stayed very consistent in all scenarios (at most 3-5% variance). Couldn’t at all get to a setting that I could explain the 20% drop in wattage I had experienced with the tire switch.

I still have the old road tire, I may very well try swapping it back on and seeing what it does.

Wanted to share on the forum for input as reading through the posts, I was not coming across much of anything regarding a power drop due to a switch from road to trainer tire.

I have a similar experience but with a dumb trainer setup.

I just started using Zwift to see if I like it and started out with my roadbike on the trainer, which has a 25mm road tire on it.
First thing I did in Zwift after getting everything setup was an FTP test.
I scored an FTP of 423watts@83kg = 5.1W/kg. Well, I knew a dumb trainer setup wasn’t going to be very accurate but this seemed a little extreme.
I don’t want to be a cheater and also I want my speed and distance numbers to be somewhat realistic. I checked the setup again but couldn’t find anything wrong with it except for the roadtire that wasn’t a trainer tire.

So I put my old MTB on the trainer. This has a 29x1.35 (622-35mm) trainer tire on it and when I started a ride in Zwift I instantly noticed that it was significantly harder to get up to speed.
I tried another FTP test, but spun out because of the gearing on this bike. I did manually adjust the FTP to 300W and try a workout. I didn’t even make it through the warm-up.

So yeah, a trainer tire makes a big difference. I guess I should try to get a 700c23 trainer tire for the roadbike.

Same problem - just swapped out my old trainer tyre (schwalbe insider) for a new Continental home trainer tyre and I’m having a shocker! I have a very average 215 FTP, but struggling to get above 180 at the moment - spindown on my Kickr Snap (2017) takes 18 to 20 secs with 2.25 turns so will keep on tightening to get back to sub 12 secs like before and see what happens, but I suspect this will be way beyond the recommended 2 turns…

Hate to drudge up an old thread but there didn’t seem to be any concrete resolution here and thought I would share my story in case it helps someone else.

I have been on Zwift for the better part of a year and got my first flat tire on the trainer. Seems a few little pebbles had made their way in back when I was still riding outdoors and the tightening on the Snap got it in the wrong spot. In any case, I had the LBS replace it with a Continental Home Trainer.

The next day I had a WTRL race and did my normal routine - pump tire, screw to 2.5 turns past contact, warm up the trainer, and spin down. Hop on the race and total disaster. Average heart rate near normal while average watts down almost 100 from the previous week. I was killing myself just to ride a flat course and my TTT team had to leave me behind in the first 2 minutes.

I was convinced that I was going to have to quit Zwift for 60 days so that ZPower and CE would drop me down to a D rider as apparently I couldn’t make more than 2 wkg any more (not that I’m overdramatic or anything…)

So that night I did a couple things:

  1. Filled tire to specifications on the side wall rather than LBS recommendation (90 PSI if I remember correctly.)
  2. Wahoo Spindown, Tighten, Spindown tighten until I got to the 12 second range (note, this was MUCH tighter than I was going with my road tire.)
  3. Factory spindown in Wahoo app
  4. Spindown in Zwift
  5. Final Spindown in Wahoo app

Not sure which of these ultimately solved the issue, but I ultimately got the Home Trainer to report similar power to what I was getting before the change. I even set a new FTP the following week in ZRL. My hypothesis is that the tire wasn’t getting the same grip as my old hybrid on/offroad tire and thus needed to be tightened a LOT more than I was used to. The factory spindown is just good hygiene for the Snap, and then making sure the spindown is in that sweet spot of 12 seconds is crucially important.

In any case, don’t despair - a wheel on trainer is always going to have more parts of the drivetrain in the equation than direct drive which means any change can have dramatic results on the output wattage. Just treat it like you are starting from scratch with a whole new bike - don’t assume that your existing routine and tightness level are going to carry over. Run a factory spindown, keep working until you hit the 12 seconds, and you’ll be right back to your old self in no time.