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  • How many joules….
  • paddlemyowncanoe
    Free Member

    …does it take to accelerate a 100kg bike+human from 0 to 50km/h?

    Reason I ask is I’m interested in acceleration losses in roadie uber light wheels vs boggo ones.

    aracer
    Free Member

    KE = 1/2mv^2 = 1/2 * 100 * (50000 / 3600)^2 = 9.7kJ

    Though if you’re interested in speeds that high, aero is far more important than weight.

    Wally
    Full Member

    He is right 9645J.
    But this takes time out of the equation
    It is the power (energy/time) to deliver this energy that is important.
    So acceleration losses (change in velocity/time) will need a time element.
    (P.S. I haven’t quite left school yet 8O)

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    paddlemyowncanoe
    Free Member

    Thanks – this is really really useful. I’m glad they are teaching useful stuff at school these days.

    Roues Artisanales quotes 70J to get a super light Lew VT-1 wheel rotating up to 50km/h and approx 130J for a boggo Mavic with a 600gm rim. I was agonising about blowing another wedge on lower inertia wheels. So 60 difference sounds like an extra 80-90% on top of the Lew but is peanuts compared to the ten thousand J I’m wasting getting my bulk up to speed. Even two wheels is 120J difference, so barely 1%. About the difference a full waterbottle makes.

    I may need to find another way to justify those Lews then…..

    aracer
    Free Member

    As I said before, you really should look at the aero. Once you’re at 50km/h a VT-1 absorbs 23.4W in aero drag, whilst a Zipp 808 (which takes 30J more to get there) only absorbs 16.7W. Therefore it will take less than 5s at that speed for the Zipp to save you more energy. Even looking at a Mavic Cosmic at 60J more, that saves 1.5W in aero drag, so will only take 40s to be on the positive side.

    Of course you might have been looking at a Ksyrium which has rubbish aero as well (only really beaten by an R-Sys!)

    molgrips
    Free Member

    You’re kind of asking the wrong question. Most of the energy going to that speed goes into aero and road resistance. And time is important – theoretically you could put in a small amount of power and reach that speed – but the limiting factor is how long your legs can manage the power levels involved.

    I calculated once that saving a lb off your wheels resulted in an effect equivalent to gaining 0.1W on the climbs (so negligible) but gaining 10W when accelerating your bike in typical singletrack – significant. (Note this was a while ago and I may have forgotten the numbers – it might’ve been 0.5lbs not 1lb)

    Aerodynamic and friction considerations won’t change (asuming you’re not getting aero wheels) so you only need to consider the weight. Calculate the energy needed to impart appropriate amounts of angular momentum to the wheels in an appropriate time, and that’s your answer.

    aracer
    Free Member

    Aerodynamic and friction considerations won’t change (asuming you’re not getting aero wheels)

    What’s an “aero” wheel? There’s a good 5W difference between a couple of normal Mavic wheels there (the cheaper Aksium is that much better than a Ksyrium!)

    njee20
    Free Member

    Interesting stuff! I’d also like to add that a 600g rim is a proper beast on the road, that’s not a ‘bog standard’ wheel!

    paddlemyowncanoe
    Free Member

    Both of you have got it. I’m going for the Mavic Carbone SL – pants weight and inertia but 20W drag AND real world braking surface. Also decent lateral rigidity which is good for the crits/sprints I do. And they look good.

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