Wattage drop when switch from road tire to trainer tire

(J M Falconer) #1

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?

(Dan) #2

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.

(J M Falconer) #3

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.

(Lion Bok) #4

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.