bike physics and spensive wheels (reading involved)

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  • bike physics and spensive wheels (reading involved)
  • Tracker1972

    Unless the speed advantage is due to aerodynamics. Which with equal power it must be down to, surely?
    You aren’t mixing up velocity and acceleration are you?


    The way I understood it was that the amount of power needed to accelerate a rider and bike from inertia depends a lot more on other factors than it does on lightweight wheels.

    The speed advantage, taking initial acceleration into account is minimal for having light wheels.

    b r

    Not going to argue with your formula, but you need to factor a different set of numbers.

    Ride = 80kg
    Kit = 5kg
    Bike = 11kg
    Wheels/Tyres/Rotors etc = 4kg

    But lighter wheels/tyres etc always feel easier to pedal up to speed than heavier ones. And any weight impacts the ability to climb – simple physics.


    But in the grand scheme of things the 200g’s you may save by slinging on a set of lighter rims (not taking into account spokes or hubs) is absolutely bugger all compared to you’re average (and i think 80kg’s is being a littler generous) rider on here


    Then why do you notice the difference of 200g between different tyres?


    You notice because you just spent 40/50/60 whatever on a pair of tyres, probably did some maintenance whilst you were at it and are really up for the ride so expect them to be better/grippier/faster…


    Theres an interesting idea.
    If you were to get on your bike without seeing the wheels and someone changed them to a diff. set would you actually notice one way or another?

    I´m pretty sure that I wouldn´t, tires on the other hand, even blindfolded I could, but that would be more down to the vibrations from the tread.


    Found a biker physicist blog and came across this little gem, basically it says for the amount of energy needed to spin up and maintain speed with lighter vs heavier wheels it makes little difference overall (except to your pocket).

    The total mass of a light rider (60 kg) plus bike (8kg) plus associated gear like helmets and water bottles (2 kg) is roughly 70 kg. Of that mass, the wheels (including tires and tubes) account for roughly 2 kg of that number. The total kinetic energy of a cyclist is therefore:

    KEtotal = KEwheels + KE bike =

    Where v is the linear velocity, I is the moment of inertia and r is the radius of the wheels. Now let’s say that we’re moving at 10 m/s (36 km/hr).

    KE wheels = (100 J + 20 J) = 120 J.

    KE bike = 3400 J.

    So you can see that the wheels account for about 3% of the total kinetic energy of a cyclist. The rest, 97%, is dedicated to overcoming the inertia of you and the rest of your gear. Therefore, a large change in your wheels is going to have a relatively small effect. However, a moderate change in body mass is going to have a huge effect. That said, small changes can add up to big ones.

    My point is that changing your wheels will not dramatically change how fast you go. It will make a small, but significant change. You should also beware anyone who tells you that brand X wheels give you a 3 km/hr advantage. They probably don’t. As shown above, a 10% change in wheels gives less than 1% change in total mass and kinetic energy.

    The full text is on here:


    Basically, all that stuff about lighter wheels being something that transforms your bike, makes it climb like a mountain goat on crack, fly like a scalded cat, insert unlikely metaphor of your choice, is rubbish.

    The whole ‘a pound of rotating weight is worth 2 pounds off the frame’ stuff is nonsense too.

    It goes to show that the placebo effect is powerful, and that cyclists, just like everyone else, are suckers for a bit of pseudo scientific babble, especially when it costs loads of money.

    In summary, wheels account for almost 10% of the total power required to race your bike and the dominant factor in wheel performance is aerodynamics. Wheel mass is a second order effect (nearly 10 times less significant) and wheel inertia is a third order effect (nearly 100 times less significant).

    …baggies in downhill racing, don’t get me started..

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