Power output calculator

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  • Power output calculator
  • mudshark
    Member

    I’m unsure how accurate the power measurements on my turbo are and want to calibrate it. I figured that if you know a rider’s weight, speed and ascent then a reasonable estimate of power could be estimate. I’ve had a look and found this calculator so wonder what anyone who knows about these things thinks:

    http://bikecalculator.com/wattsMetric.html

    Seem sensible? Interesting to see how position and tyres affect things.

    cynic-al
    Member

    Whoever has calculated that will know more than anyone here about its accuracy I’d have thought – they’ve got input for various positions, tubeless tyres vs clinchers…impossible to know whether he’s got that right or not…and does it account for aerodynamics of bigger rider?

    One would hope it’s going to be more accurate at low speeds on hills where wind resistance has less effect and the calculation is simpler.

    What are your figures on the turbo?

    mudshark
    Member

    Initially the figures seemed very wrong so have changed the parameters to get closer to what I think seems correct. There are 3 values I can play with – 1 for flat and the other 2 for different ascent levels. I think I have around 160 watts for 32kph on the flat which fits the calculator well so seems maybe I’ve fiddled quite well.

    BTW, I mostly posted this as thought would be of interest to others.

    cynic-al
    Member

    Fair enough, there are a few on here with power monitors etc so someone may be able to help.

    160W for 20mph…I’d have thought more (in fact this was discussed on a wheel thread of mine IIRC, I’ll have a look)

    LabMonkey
    Member

    160W for 20mph…I’d have thought more

    On a ‘flat’ ‘smooth’ road – 20 mph costs me ~250W and I am ~78 kg.

    edit: that is on a Cervelo aero road bike, Zipp 303 aero rims, and with Conti’s race tubular tyres.

    cynic-al
    Member

    Sorry I can’t find it, it’s 1+yrs old and I CBA!

    GregMay
    Member

    *twitch twitch*

    My science sense was twitching, I knew something was amiss.

    Weight is not a factor. Lean muscle mass is a factor. By assuming weight matters we assume that a 78kg athlete with 4% body fat produces the same as a 78kg athlete with 66% body fat. This is very much not the case.

    Power ‘calculators’ are better left aside, they mean nothing.

    NOW, once you bring speed into it we go into a whole different world of aerodynamics and trinomial increases in power needed to increase speed. The whole co-efficient of drag thing is great, but a pain in the ass when it comes to estimating by speed.

    In short. Go buy a power meter….a real one….not one on a turbo.

    Now back to my PhD.

    *cries in the corner of his lab*

    LabMonkey
    Member

    To correlate speed with power (outside, not on a turbo), WEIGHT, rolling resistance, and aerodynamic drag DO matter – as do the physiological mechanisms that provide the power output. (and a host of other bits and pieces that have a small effect also).

    OP – get someone with a powermeter to ride on your turbo to check the calculations.

    hazeii
    Member

    From experience, Bikecalculator seems pretty accurate,certainly where uphills are involved (the nice thing about hill climbs is pretty much everything except weight and power can be ignored).

    A reasonable, low-tech approach to calibration is to do a hill climb wearing a HRM, and calculate the power required from the ascent and the distance. All else being equal, riding the turbo to the same heart rate for the same duration means you are putting out the same power (but do note the “all else being equal”).

    mudshark
    Member

    STW is blocked at work due to sexual content….

    Unfortunately I don’t know anyone with a power meter but a chap I know online has the same turbo (Elite RealPower) and has borrowed a bike with a power meter so I’m trying to get him to compare his calibration with bikecalculator and then look at sorting mine out. I’m not too bothered about anything more than ball park accuracy though these turbos can be used for racing online against other users so helpful for everyone to have their turbos calibrated correctly.

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