It's a little bit about the bike...

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  • Chiggy
    Chiggy Posts: 261
    neeb wrote:
    SBezza wrote:
    Do both wheels have different tyres, that could quite easily cause a difference. Sorry if I have missed it, but is the difference in the average watts for the complete session, or feel at a particular HR/effort.
    Same tyre (I mean exactly the same blue tacx tyre). 15W difference is average for a particular section of the session that is all uphill (about 7% simulated slope) for 35 mins or so where I am generally trying to keep at threshold / 85-90% of HRmax.
    Certainly for the same power output on different bikes/wheels your speed will be different but there is no way a well maintained bike is losing 15W thorugh drive train losses or frame flex!
    I'm beginning to think it must be some weird interaction between the trainer and the bike/wheel. As I mentioned, the alex wheel is a little out of round. Or maybe frame flex, while not significantly reducing power through the drive train, causes the tyre to move sideways more when in contact with the roller.

    I realise this all seems a bit pointless and speculative :wink: - all I know is that there is a significant difference to the measured performance I get with different bikes on the same trainer, and it's enough to disrupt the measurements I use to gauge fitness / training benefit.

    OUT OF ROUND?? Radially out of round….??

    The tyre's coefficient of rolling resistance and losses due to deformation will be LUMPY.

    Problem solved.
  • neeb
    neeb Posts: 4,473
    Problem solved.
    Quite possibly. Yes, it is very slightly radially out of round compared to the Campag wheel. It's not much, but if the roller is adjusted so that it is just brushing the tyre, it's fairly obvious,
  • amaferanga
    amaferanga Posts: 6,789
    Also, if the two rims have different widths then the profile of the tyre may be different. The contact pressure between the roller and tyre could be different since you're using the exact same number of turns on the adjuster knob.
    More problems but still living....
  • jonmack
    jonmack Posts: 522
    neeb wrote:
    I realise this all seems a bit pointless and speculative :wink: - all I know is that there is a significant difference to the measured performance I get with different bikes on the same trainer, and it's enough to disrupt the measurements I use to gauge fitness / training benefit.

    But if you know that if you train on X bike, you get readings 15W lower than if you train on Y bike, then surely you know to add 15W to your final power readings which will give you a better indication of the effort you put in? Or am I missing something?
  • neeb
    neeb Posts: 4,473
    But if you know that if you train on X bike, you get readings 15W lower than if you train on Y bike, then surely you know to add 15W to your final power readings which will give you a better indication of the effort you put in? Or am I missing something?
    Well, I know that now, but it took me a while to figure it out. And 15W is a pretty approximate estimate. There are enough variables already affecting repeatability/accuracy, so I'd rather just use the same bike...
  • SteveR_100Milers
    SteveR_100Milers Posts: 5,987
    This post just goes to prove that PE / HR / watts; whatever method you use to *quantify* your effort they are all subject to a number of difficult-to-control influences. I wonder how much more beneficial power measurement as a training tool is over RPE for the average amateur cyclist who does not have access to equipment or facilities that can eliminate most or all of these variables.
    http://www.pontypoolrcc.co.uk/
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  • Chiggy
    Chiggy Posts: 261
    SteveR_100Milers wrote:
    This post just goes to prove that PE / HR / watts; whatever method you use to *quantify* your effort they are all subject to a number of difficult-to-control influences. I wonder how much more beneficial power measurement as a training tool is over RPE for the average amateur cyclist who does not have access to equipment or facilities that can eliminate most or all of these variables.

    Yes. The equipment set-up only serves as a comparator. To compare against the last session using that set-up.
    Change one part of the equipment and it won't compare with past results on the original set-up.

    The man-machine interface is the pedal spindle bearings. Any moving part of the power transmission system all the way to the measurement device must stay the same to get a comparative result.
    It will not be absolute. Consumer electronics aren't good enough for an absolute measurement.
  • 58585
    58585 Posts: 207
    Power train losses are insignificant on a well maintained bike.
    If you run a powertap or SRM on different bikes you will get the same reading for the same power input. Calibration on my powertap is a case of pushing 2 buttons for a couple of seconds then there is nothing else to worry about; I know I'm getting an accurate reading irrespective of what temperature it is/which bike I'm riding/if my wheel is true.
    I would argue that this thread is a good advert for buying a proper powermeter and a cheap trainer/rollers rather than an expensive trainer with power measurement...
  • neeb
    neeb Posts: 4,473
    I would argue that this thread is a good advert for buying a proper powermeter and a cheap trainer/rollers rather than an expensive trainer with power measurement...
    I take the point, although the fortius does a lot more that just give an approximate power measurement. I find the simulated climbs linked to the real-life-videos a massive motivation, the motor brake does do a reasonably good job of simulating gradients. And most of the time I can get repeatable power measurements (accurate or not) if I use the same setup.

    I'm just waiting for the long-rumoured affordable pedal-based powermeters to be released - any news on that yet?
  • amaferanga
    amaferanga Posts: 6,789
    neeb wrote:
    I'm just waiting for the long-rumoured affordable pedal-based powermeters to be released - any news on that yet?

    I very much doubt they'll be cheaper than a PowerTap.
    More problems but still living....
  • jonmack
    jonmack Posts: 522
    I've been holding on for the pedal-based powermeters too because I have two bikes and want to be able to get power readings on both of them and don't really want to spend £1500 on a Quarq and have the hassle of moving it between bikes.

    If/when it's finally released the Garmin one comes in at under a grand, I'll jump on it straight away.
  • Chiggy
    Chiggy Posts: 261
    Depends on what you want to achieve.

    An engine test cell measures the power at the flywheel - SRM Cranks.
    This is for base tuning ( cadence efficiencies ) and RAW power measurements.

    A chassis dynamometer measures the power at the roller - Tacx Fortius.
    This is for fine tuning and simulating road conditions.

    Both systems. Evaluation of mechanical losses between engine and tyre contact with road, and lots of fun... :D

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