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One Smart Trainer, Two bikes, Different wattages

I have a Tacx Smart Trainer which gives different wattage readings by around 30 watts difference for my two bikes. Have worked this figure out over the last 30 or so rides on Zwift.

I calibrate the bikes using the trainer app prior to riding. The only real difference in the bikes are one is 50/34 with 25mm Grand Prix 4000's on and the other is 52/36 with 23mm Gatorskins fitted.

For instance, just completed the Zwift Figure 8 route on the compact with an average wattage of 256 but yesterday did the same route on the semi-compact with an average of 232. This is a common occurrence.

Anyone know what's going on?

Thanks for any informative replies.


  • StillGoingStillGoing Posts: 5,209
    Is it a Bushido? If it is, there's your problem.
    I ride a bike. Doesn't make me green or a tree hugger. I drive a car too.
  • Websta24Websta24 Posts: 162
    Its impossible to compare in this instance, maybe you just worked slightly harder in the second instance

    Is your heart rate, speed, cadence etc all identical?

    In the grand scheme of things, 26 watts is absolutely nothing i would say its a completely normal fluctuation.
  • slowbikeslowbike Posts: 8,498
    Whilst it's impossible to compare the two rides exactly - if it's a general trend that the power output is lower on one bike rather than the other then you have to consider that it's most likely down to something mechanical rather than just the rider.

    Power is being measured by the Tacx device - so entirely separate to the bike. Any change in power efficiency on the bike will result in differences being measured on the Tacx device - unlike say a crank PM where as long as the wheel is turning, it doesn't matter what losses are being taken up in the drive chain.

    An obvious question is body position. Perhaps the OP is in the wrong position and cannot put the power out in the position on the semi-compact compared to the compact.

    Another obvious one is tyre pressure & contact pressure - if the 23mm tyre is being pumped up to the same pressure as the 25mm then it will deform more than the 25mm which will take more power than the 25mm - although 26 watts is a lot to lose in tyre pressure - Tacx uses the screwin the lever type method to apply pressure to the back wheel that has to be adjusted for each different outer circumference - so a combination of lower tyre pressure and higher contact pressure could easily account for the loss (although calibration should sort that?)

    I doubt the OP is losing 26watts in the drive chain - but a few watts may be lost in an inferior or dirty groupset - heck even the pedal spindles dragging would consume a fraction of a watt...
  • fat daddyfat daddy Posts: 2,605
    calibration works on braking resistance ..... so would drivetrain losses / tyre losses make a diference to wattage, as the braking spin down of the trainer should figure this out shouldn't it ? .... or doesn't it work like that

    position on the bike would be my guess if the above is not applicable
  • slowbikeslowbike Posts: 8,498
    I don't use one - so don't know - do you have to pedal during the braking resistance check? If not then drivetrain loses are a consideration - although I'd think it be very minor
    Far more likely to be position...

    Some TTers tend to have a more upright TTing position because they're not as efficient when most aero - it's a trade off ...
  • fenixfenix Posts: 5,437
    Keep the same wheels on and see if that helps ?

    Other than that - I'd not think that two different bikes on a turbo would give you the same readings.
  • Figured out what it was. The tyre requires warming up otherwise wattage can be out by as much as 80 watts on the Tacx Vortex. Bought a Quarq that tells the truth every time!
  • MiddleRingerMiddleRinger Posts: 678
    Good to know. Yep with those wheel-on smart trainers the tyre pressure, tyre size, rolling resistance and temperature all have an effect on the (lack of) accuracy of the power numbers given.

    Enjoy your new powermeter! :D
  • It didn't seem so enjoyable when my FTP test was 50 watts lower on the Quarq than on the Vortex!

    Not as fit as I thought! :-)
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