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gym bike fudge factor, whats to watts

springtide9springtide9 Posts: 1,731
I'm trying to estimate my power using a standard StarTrac gym bike.

I know it gives you an average watt value during and at the end of the session, but something tells me it over estimated.
Anyone compared or can give a rough idea what to use as a fudge factor?

I understand that this is really not ideal, but just trying to get a rough idea. Up until now, I have just been calling them "whats" rather than "watts" :)

Cheers
Simon

Posts

  • P_TuckerP_Tucker Posts: 1,878
    Fudge factor? Between 0.00001 and 100000. But it's probably not a constant. Hope this helps.
  • springtide9springtide9 Posts: 1,731
    P_Tucker wrote:
    Fudge factor? Between 0.00001 and 100000. But it's probably not a constant. Hope this helps.

    Funny enough, not really.

    You would have thought it was pretty simple measuring real watts on a gym bike, since I thought all they do is pretty much drive a dynamo - and measuring the electrical power is pretty simple (as well as knowing roughly dynamo efficiency)

    It's not like it needs to do anything clever, so I not convinced. The electronics involved is the same as a turbo - and they are cheap! Powertaps are obviously a lot more complex.

    So are the power output readings from turbos also useless?
    Simon
  • AnonymousAnonymous Posts: 28,799
    P_Tucker wrote:
    Fudge factor? Between 0.00001 and 100000. But it's probably not a constant. Hope this helps.

    Funny enough, not really.

    You would have thought it was pretty simple measuring real watts on a gym bike, since I thought all they do is pretty much drive a dynamo - and measuring the electrical power is pretty simple (as well as knowing roughly dynamo efficiency)

    It's not like it needs to do anything clever, so I not convinced. The electronics involved is the same as a turbo - and they are cheap! Powertaps are obviously a lot more complex.

    So are the power output readings from turbos also useless?

    All the Gym bikes I've used have, based on effort/HR are miles out. Most turbos are too...

    Inaccuracy is one thing, inconsistency is another, and worse!
  • The best you can hope for really is to use the same machine each time and hope that it is consistent from day to day. In that way at least the inaccuracy will be consistent and you are measuring apples with apples.

    But as NapoleonD points out, even these machines can be inconsistent from day to day and/or drift in their calibration during a session.
  • bompingtonbompington Posts: 7,363
    or drift in their calibration during a session.
    Absolutely, the one we've got at the school clearly increases resistance over the course of a few minutes - I find at first I'm pushing 400W just fine but after a while what claims to be the same power output is way harder :wink:
  • P_TuckerP_Tucker Posts: 1,878
    P_Tucker wrote:
    Fudge factor? Between 0.00001 and 100000. But it's probably not a constant. Hope this helps.

    Funny enough, not really.

    You would have thought it was pretty simple measuring real watts on a gym bike, since I thought all they do is pretty much drive a dynamo - and measuring the electrical power is pretty simple (as well as knowing roughly dynamo efficiency)

    It's not like it needs to do anything clever, so I not convinced. The electronics involved is the same as a turbo - and they are cheap! Powertaps are obviously a lot more complex.

    So are the power output readings from turbos also useless?

    Yes. Unless it has strain gauges, it's basically a magic 8-ball.
  • PigtailPigtail Posts: 424
    I use a gym bike quite a lot. They're Technogym ones and I always try to get the same one.

    I usually go for a specific heart rate or else an interval session. I find the power listed drifts downwards a little over a longer session whilst keeping my heart rate the same, however it appears to me to be pretty consistent in its readings - I've shown a gradual improvement over the last 5 months or so, which is what I would expect given the work I have put in!

    I've no idea how it would compare with a power meter though - I see the figures as a guide for my progress, rather than an accurate measure of power output.
  • doyler78doyler78 Posts: 1,951
    Pigtail wrote:
    I use a gym bike quite a lot. They're Technogym ones and I always try to get the same one.

    I usually go for a specific heart rate or else an interval session. I find the power listed drifts downwards a little over a longer session whilst keeping my heart rate the same, however it appears to me to be pretty consistent in its readings - I've shown a gradual improvement over the last 5 months or so, which is what I would expect given the work I have put in!

    I've no idea how it would compare with a power meter though - I see the figures as a guide for my progress, rather than an accurate measure of power output.

    I use the same type of gym bike and what you are experiencing is cardiac drift and is exactly what should be happening. By now you should have a good grasp on your watts that you should be able to sustain. Go for that and forget the heart rate.

    I too have found it be consistent however it way underestimates watts (although the humidity and lack of proper cooling in the gym is going to mean my watts are lower too so probably a combo of both under reporting and my inability to sustain outdoor watts).

    I too improved my power (measured with a powertap) consistently whilst doing the vast majority of my riding on the gym bike at the start of last year when going outside wasn't possible given the ice.

    I always use the same bike, unless I'm doing a recovery ride in which case it's just easy spinning so not worried about watts. If it isn't free I will wait.
  • rolf_frolf_f Posts: 15,962
    NapoleonD wrote:
    Inaccuracy is one thing, inconsistency is another, and worse!

    Lack of precision rather than inconsistency but yes, far worse! Two variables - accuracy and precision. Imagine a target; accurate and precise = shots clustered around the bullseye. Inaccurate but precise = shots clustered somewhere else. Accurate but imprecise = on average, on the bull but widely spaced. Innaccurate and imprecise = random but not dispersed around the bull. (I always think of Edward Fox in Day of the Jackal adjusting his rifle - always precise, initially innaccurate!)

    Basically, accuracy doesn't really matter as long as you are precise unless you are comparing with someone else; even then you can probably correct for the errors in accuracy. Low precision just means too much noise. The target is well worth remembering next time someone on television tells you how accurate their statistics are................
    Faster than a tent.......
  • amaferangaamaferanga Posts: 6,789
    Rolf F wrote:
    ...Two variables - accuracy and precision....

    You've gotta be a physicist. No-one else gets so hung up on the EXACT definition of accuracy and precision :wink:
    More problems but still living....
  • AnonymousAnonymous Posts: 28,799
    Rolf F wrote:
    NapoleonD wrote:
    Inaccuracy is one thing, inconsistency is another, and worse!

    Lack of precision rather than inconsistency but yes, far worse! Two variables - accuracy and precision. Imagine a target; accurate and precise = shots clustered around the bullseye. Inaccurate but precise = shots clustered somewhere else. Accurate but imprecise = on average, on the bull but widely spaced. Innaccurate and imprecise = random but not dispersed around the bull. (I always think of Edward Fox in Day of the Jackal adjusting his rifle - always precise, initially innaccurate!)

    Basically, accuracy doesn't really matter as long as you are precise unless you are comparing with someone else; even then you can probably correct for the errors in accuracy. Low precision just means too much noise. The target is well worth remembering next time someone on television tells you how accurate their statistics are................

    Yeah!
  • rolf_frolf_f Posts: 15,962
    amaferanga wrote:
    Rolf F wrote:
    ...Two variables - accuracy and precision....

    You've gotta be a physicist. No-one else gets so hung up on the EXACT definition of accuracy and precision :wink:

    Lol - not quite but there are a few numbers in my job! It's more that I get hung up about how statistics are quoted in the press. If you don't interpret what people are saying exactly you might actually believe they aren't trying to deceive you!
    Faster than a tent.......
  • springtide9springtide9 Posts: 1,731
    Thanks for the replies. Pretty mixed opinions, but with some commons.

    Think the StarTrac gym bikes are the better ones... seem to have a reasonable flywheel (and electronics)... not like the really cheap ones out there.

    I've done a few 'constant power' sessions, even across bikes, and they seem pretty consistent... as in, also using a Garmin to track my HR, monitoring the 'level' as well as the cadence. i.e. the last three weeks, did a 1 hr constant 'power session' (think I used two bikes), and with the same level... a 3% increase in cadence (at the same level) seemed to result in roughly a 3% increase in power - as well as around 4-5% increase in HR.

    What I do not believe to be accurate is the' watt power. I know this is inaccurate... but maybe this doesn't really matter to me.... as long as I'm comparing 'apples' with 'apples' (maybe just keep calling them 'whats' !)
    Ideally I'd like a expensive turbo... but just don't have the room as well as the inclination to use a home based training unit. Memories of owning a C2 rower... was that there was never really enough space.... and it was always too hot and not enough airflow. I also didn't exactly like dripping a litre of sweat into the carpet every session :)

    Thanks again.
    Simon
  • ZachariahZachariah Posts: 782
    I use the same Powersport exercise bike every time.

    The wattages are sadly fictional (or I'd be national TT champ) but they are consistent for perceived effort, and the cadence is certainly accurate/precise.

    My home turbo is an Elite Cronomag. It takes 5-10 minutes to reach a consistent level of resistance but is also subject to the vagaries of my bike's tyre pressure, transmission cleanliness and so on. The same combination of intervals can feel very different from day to day.

    However, all exercise is good (unless you're ill)!
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