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Watts per Kg Vs Watts - the value over a triathlon bike leg

  • 05-03-2014 03:26PM
    #1
    Fazz
    Registered Users, Registered Users 2 Posts: 1,121 ✭✭✭


    Following on from a recent few q's in the Ask Tunney thread, this post is to try and learn more and better understand the differential and value in real world racing terms over a typical triathlon bike leg of watts/kg vs watts.

    A basic understanding says that a better watts/kg figure will get you up the hills quicker.

    However if on the flat, and a 75kg rider is pushing 4 w/kg so 300 watts whereas a 65kg rider is pushing 4 w/kg so 260 watts then the heavier rider should be increasing a lead and biking faster due to more power on the flats where weight is not as important.

    Now, I see it that logic dictates that weight will always count towards some value, and that the lighter rider will not need to push the same wattage so 300watts to match pace of the heavier rider because they are powering a lighter load and effectively should get more bang for buck if on same wattage. So perhaps 280 or 290 watts would match the heavier rider’s 300 watts and so on.

    Tunney disagree's to this theory and suggests weight once rolling is negligible which is a fair point.

    So let's try find examples to see what the real world suggests.

    Unfortunately, there will be a number of variables surrounding equipment,aero position and actual performance regarding staying in aero/getting heldup/drafting etc.

    So let’s ignore these for now and see what can be discovered on the basic levels first.

    Perhaps we can try find out an approx formula, such as if a lighter rider is running 5% less wattage, then they must have 10% greater w/kg to overcome this etc...


    A recent few posts highlighted the below

    Kilkee’13 Bike leg
    44km, 320m elevation so bar the initial climb, it can be considered a relatively normal slightly rolling course.

    Not a hilly course by any description imo.

    Fazz – 1:08 bike split

    [http://www.trainingpeaks.com/av/5ZRK4HM7OUIHUCJTRZ43SJFQI4IV636VM4R65BY

    4.01 w/kg, AP 268w, NP 276w

    MD1983 –1:15 bike split

    3.41w/kg, AP 280w, NP 290 (est)

    So a 7minute loss despite biking a relatively flat course at higher wattage.

    Ratio –4.5% greater watts for MD1983, but 17.5% less w/kg.

    Variable –equipment, aero position, time in aero position.


    Conclusion– W/kg trumps higher watts in above ratio case over average olympic bike leg.



    That suggests the w/kg is a factor on the flat’s over a bike leg as in the above example, I biked at 5% less watts over a non hilly course but still gained 7 mins.

    So we need more data points to try find out more...

    Anyone else got any race files they can share or match up to courses?

    I have –Lost Sheep, Tri an Mhi, Humbert, for last year if anyone can match those?


    I reckon a fair ratio could be heading towards the direction of 10% greater watts needed to overcome a 10% lower w/kg figure or thereabouts on an average bike course...

    Of course this is all course dependant, but the point is w/kg still counts on a flat course to a degree it appears.



Comments

  • #2
    Fazz
    Registered Users, Registered Users 2 Posts: 1,121 ✭✭✭Fazz


    I should add open to discussion on this and amending incorrect assumptions I've made above.

    There are a number of variables, some perhaps real, others perhaps not.

    There's a couple of pro files I'll dig out later and try compare those also.

    If anyone has any other opinions or thoughts let's hear them.


  • #3
    MD1983
    Registered Users, Registered Users 2 Posts: 645 ✭✭✭MD1983


    I have files for 3 OD races last year:

    Kilkee http://connect.garmin.com/activity/335421158 (the file you referred to above)

    Athy http://connect.garmin.com/activity/321545580

    Kilkenny http://connect.garmin.com/activity/355111893


  • #4
    MD1983
    Registered Users, Registered Users 2 Posts: 645 ✭✭✭MD1983


    its an interesting one for sure, if you put a 7 mins delta between me and you over a flat 40km course it would have you around a flat course like Athy in 56 minutes last year, yet there are very few who actually break an hour down there if memory serves me correctly and the only person i remember crushing it was BMC with a 56 min 40km i think. i think the winner there last year did 1 hour 1 min?

    Kilkee didnt feel that flat to me that day!


  • #5
    peter kern
    Registered Users, Registered Users 2 Posts: 3,359 ✭✭✭peter kern


    good stuff Fazz

    other points to consider


    not being rude fazz as i know you rode legal but you being in a pack is also an huge factor for time differnce ( especially in kilkee on the way home ) that could be some 3 min difference

    on the other hand one female last year had and ftp of 4.9 watts per kg and was pretty much the same pace then as MD 1983 in Athy.
    ( that female athlete rode the fastest male bike split in a duathlon lately)
    so that would bring your thinking a bit into trouble


    the athelte in comparasion (MD ) has v wide shoulders your are v narrow.
    which is as important as watts per kilo ( and on track workouts many guys but lead on the wheels so the flater the course the less important weight gets)

    so I would say there is a good chance 3 min you you gained riding in a group ,
    75 sec on gear
    some 60 sec being a very very narrow frame vs big shoulders.
    kilkee is not an easy course ( not difficult either so more middle of the road )

    I will look later at the files but I know how md rides and saw you passing me in kilkee.

    what i would say is the truth is kind of in the middle its watts per kg and wind resistance per cm2


  • #6
    couerdelion
    Registered Users, Registered Users 2 Posts: 2,910 ✭✭✭couerdelion


    You're also assuming that both power meters are accurate in relation to each other.

    Edit: and the scales you both used to weigh yourselves.


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  • #7
    peter kern
    Registered Users, Registered Users 2 Posts: 3,359 ✭✭✭peter kern


    btw if it makes you happy at least in 2008 cycling ireland for their track program selected atheltes purely on watt per kg ratio ?????????
    (scratches my head )


  • #8
    sconhome
    Registered Users, Registered Users 2 Posts: 5,468 ✭✭✭sconhome


    Been scratching my head a little on this. Forgive me for I am an innocent when it comes to power meters.

    Surely 4w/kg in a heavy cyclist is the same as 4w/kg in a light cyclist if all other things wind, aero, attitude are equal?

    The heavy guy is pushing 300w to overcome the friction caused by his weight. The inertia caused by his weight might help keep the bike moving at speed but friction (ground and wind) would eventually bring him to stop the same as the lighter guy.


  • #9
    lopesc
    Registered Users, Registered Users 2 Posts: 107 ✭✭lopesc


    Why don't you do an initial test in to this? For example: find a nice flat stretch of road with no junctions, say 2Km long. Hold constant pace over the 2Km with and without full water bottles on the bike. Then compare power needed for both efforts?

    All the variables in terms of kit would be the same, and if the weather is good then wind resistance should be approximately equal too.


  • #10
    d4r3n
    Registered Users, Registered Users 2 Posts: 707 ✭✭✭d4r3n


    There's plenty of numbers posted here you can use to sample.


  • #11
    tunney
    Registered Users, Registered Users 2 Posts: 12,585 ✭✭✭✭tunney


    A heavier rider will be larger.
    A larger rider will have a larger frontal area.

    On hills its watts per unit mass (i.e. W/kg), on the flat it's watts per unit frontal area (W/m^2)

    So there will be a penalty for the larger rider but an indirect result of their weight and it can be optimised away, or at least to insignifcance or tending towards

    No course is completely flat either so there will segments thats W/kg is more relevant.

    However if we had a flat course, completely, and the course was completely straight ( to remove bike handling factors) and two pairs of calibrated SRMs (sorry the rest don't cut it here) then its all down to the frontal area and CdA. There is NO impact of weight.

    "but but but Dave my N=2 numbers don't lie...."

    On an inclined plane the force exerted by gravity on a body can be resolved into two components. F(perpendicular) and F(parallel) in a frictionless vaccum F(perpendicular) can be calculated as m*g*cos(ang) where m= mass of object, g=gravity (9.8 ms-1) and cos(ang) is the cosine of the angle of inclination. F(perpendicular) acts to counteract the normal force. F(parallel) can be calculated as m*g*sin(ang) where m= mass of object, g=gravity (9.8 ms-1) and sin(ang) is the sine of the angle of inclination. F(parallel) is not balanced by any other force. F(parallel) is the force that must be overcome to move up the inclination.

    So lets take a 65kg rider and put him on a zero degree inclination. Lets call the force(parallel) here F1
    Lets also put a 85kg rider and put him on a zero degree inclination. Lets call the force(parallel) here F2

    Now we can calculate the force due to mass(weight if you must) and gravity that must be overcome to move forward.

    F1= 65*9.8*sin(0) = 65*9.8*0 = 0
    F2= 85*9.8*sin(0) = 85*9.8*0 = 0

    Do I need to go on?

    As much as I have enjoyed my trip down memory lane and remembering how much I loved physics in school and college I'm tired and going to bed.

    Just in case a picture might help

    5Pfp0jY.jpg


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  • #12
    Fazz
    Registered Users, Registered Users 2 Posts: 1,121 ✭✭✭Fazz


    peter kern wrote: »
    good stuff Fazz

    other points to consider


    not being rude fazz as i know you rode legal but you being in a pack is also an huge factor for time differnce ( especially in kilkee on the way home ) that could be some 3 min difference

    on the other hand one female last year had and ftp of 4.9 watts per kg and was pretty much the same pace then as MD 1983 in Athy.
    ( that female athlete rode the fastest male bike split in a duathlon lately)
    so that would bring your thinking a bit into trouble


    the athelte in comparasion (MD ) has v wide shoulders your are v narrow.
    which is as important as watts per kilo ( and on track workouts many guys but lead on the wheels so the flater the course the less important weight gets)

    so I would say there is a good chance 3 min you you gained riding in a group ,
    75 sec on gear
    some 60 sec being a very very narrow frame vs big shoulders.
    kilkee is not an easy course ( not difficult either so more middle of the road )

    I will look later at the files but I know how md rides and saw you passing me in kilkee.

    what i would say is the truth is kind of in the middle its watts per kg and wind resistance per cm2


    Don't worry I'm not taking this the wrong way but 3 mins for being in a group?

    If you mean catching and riding with the top 10 guys then perhaps, but I never had that luxury in Kilkee.
    As you know due to my crappy swim, I was playing catch up the whole bike and didn't have any legal group riding options.
    Came off the bike around 10th I think.

    If you mean benefit of catching and passing riders so doing a lot of overtaking, and getting minor draft benefits during the 5-10 seconds it takes to pass then the 3 min would be a gross overestimation, if applicable at all.

    I'm not sure of MD's gear but perhaps the 75sec on gear may be accurate alright, and the frontal aero as you know the 2 athlete builds in question so maybe 2-3 mins explained through those alright.

    Truth as you say is defo somewhere in the middle, largely due to not many bike courses being pan flat so w/kg will always have a factor in perhaps 50% or so of bike courses?

    Athlone may be an interesting one. I didn't race that one with power but know BMC pushed 412 watts for a 53 odd bike split (slightly short course ~2km). so maybe 4.65 or so w/kg but massive power figures obviously.

    Another Athlone bike may be useful.




    V interested to hear of the 4.9 w/kg female athlete?
    Guessing this can only be Siobhan Horgan the Duathlon Champ and ex biker?
    Or a smaller female athlete with great w/kg but even so, 4.9 is quite massive and pro levels! :eek:


  • #13
    Fazz
    Registered Users, Registered Users 2 Posts: 1,121 ✭✭✭Fazz


    tunney wrote: »
    Just in case a picture might help

    5Pfp0jY.jpg

    Very interesting thanks.

    A picture paints a thousand words in this case of lots of technical jargon!

    So the above states that on a 0% incline then gravity has no effect once rolling - so watts trump w/kg.
    Slight possible advantage to the rider with less rolling resistance - and this could be the lighter rider assuming tyres not inflated to 150 psi etc :D;):pac:

    With even 1% gradient, then gravity will come into play, and w/kg will count for 24% of the energy demand, but frontal area still the primary factor.

    And so on...

    I wonder if we could formulae a race profile.

    So in the example of Kilkee - 44km of which 323m has elevation.
    That's not even 1% in my books so w/kg should have been very negligible in the above?

    Granted you descend from this also where a heavier rider has an advantage but it seems less so compared to a lighter rider on the ascent.

    Perhaps not possible to formulae or take a given ratio etc.

    Frontal Area seems paramount and unfortunately the hard to measure for the age grouper so too much of a variable it appears.


  • #14
    sconhome
    Registered Users, Registered Users 2 Posts: 5,468 ✭✭✭sconhome


    We could all take your bike for a week each which would give us a constant and the variables then being weight, course and frontal area per cyclist we could work out a linear value be great craic ! :D


  • #15
    EC1000
    Registered Users, Registered Users 2 Posts: 1,336 ✭✭✭EC1000


    Fazz wrote: »
    So in the example of Kilkee - 44km of which 323m has elevation.
    That's not even 1% in my books so w/kg should have been very negligible in the above?

    Don't think your interpretation of 323m of elevation is correct. This means an accumulated gain in altitude of 323m, no? I could be wrong and now just look silly......


  • #16
    tunney
    Registered Users, Registered Users 2 Posts: 12,585 ✭✭✭✭tunney


    Fazz wrote: »
    With even 1% gradient, then gravity will come into play, and w/kg will count for 24% of the energy demand, but frontal area still the primary factor.

    And so on...

    I wonder if we could formulae a race profile.

    So in the example of Kilkee - 44km of which 323m has elevation.
    That's not even 1% in my books so w/kg should have been very negligible in the above?

    Assumptions :
    Frontal area 0.5m2
    Cda 0.5
    Crr 0.005
    Bike 10kg
    Rise/ run of 0.008
    Wattage of 250 watts (AP==NP)

    65kg rider will spin at 36kph
    75kg rider will spin at 35.2kph
    Fazz wrote: »
    Granted you descend from this also where a heavier rider has an advantage but it seems less so compared to a lighter rider on the ascent.

    Perhaps not possible to formulae or take a given ratio etc.

    Frontal Area seems paramount and unfortunately the hard to measure for the age grouper so too much of a variable it appears.

    no no its not (okay it might be to some). your coefficient of drag is measurable. I've explained many times how you can scientifically measure your drag, and scientifically optimise your position and equipment. Wondering if that new helmet is faster - well you can scientifically tell. I would have expected that this sort of thing was done as part of the Big Red Cloud but obviously not.


  • #17
    Fazz
    Registered Users, Registered Users 2 Posts: 1,121 ✭✭✭Fazz


    EC1000 wrote: »
    Don't think your interpretation of 323m of elevation is correct. This means an accumulated gain in altitude of 323m, no? I could be wrong and now just look silly......

    Sorry I worded it wrongly - not what I meant.
    Yep elevation means altitude gain over distance,


  • #18
    peter kern
    Registered Users, Registered Users 2 Posts: 3,359 ✭✭✭peter kern


    http://alex-cycle.blogspot.ie/2013/02/pour-me-draft.html

    here you go 12 meter form front wheel to front wheel .

    there was a french rider that was top 10 in hawaii 2 years ago and they did further testing at shorter distances and the as one would expect the watts saved became bigger aat shorter distances it was roman guiliome ( sp)
    if you dig in the net i am sure you will find it.

    what i am really trying to say here to compare watts in a triathlon is not that serious given that some people could ride alone all day
    some in a small group some in a big pack. and anything in between.

    when srm had the life telemetics on for hawaii it was mind boggeling how much power output would change during the race given on the position of cyclists in the field.

    so proper tour ded france tt files would be better than tri files.






    Fazz wrote: »
    Don't worry I'm not taking this the wrong way but 3 mins for being in a group?

    If you mean catching and riding with the top 10 guys then perhaps, but I never had that luxury in Kilkee.
    As you know due to my crappy swim, I was playing catch up the whole bike and didn't have any legal group riding options.
    Came off the bike around 10th I think.

    If you mean benefit of catching and passing riders so doing a lot of overtaking, and getting minor draft benefits during the 5-10 seconds it takes to pass then the 3 min would be a gross overestimation, if applicable at all.

    I'm not sure of MD's gear but perhaps the 75sec on gear may be accurate alright, and the frontal aero as you know the 2 athlete builds in question so maybe 2-3 mins explained through those alright.

    Truth as you say is defo somewhere in the middle, largely due to not many bike courses being pan flat so w/kg will always have a factor in perhaps 50% or so of bike courses?

    Athlone may be an interesting one. I didn't race that one with power but know BMC pushed 412 watts for a 53 odd bike split (slightly short course ~2km). so maybe 4.65 or so w/kg but massive power figures obviously.

    Another Athlone bike may be useful.




    V interested to hear of the 4.9 w/kg female athlete?
    Guessing this can only be Siobhan Horgan the Duathlon Champ and ex biker?
    Or a smaller female athlete with great w/kg but even so, 4.9 is quite massive and pro levels! :eek:


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