Argument - rolling mass negligible or not?

Discussion in 'Cycling Equipment' started by Jacobe Hazzard, Oct 5, 2003.

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  1. I have seen some references in RBT to the rolling mass of a wheel, IE that its moment of inertia
    about the axle contributes to the overall inertia of the bike as well as its mass, or some rule of
    thumb like weight on the rim counts as double compared to weight on the frame. Usually this idea is
    immediately dissmissed as having been 'thouroughly debunked'.

    As far as I can see, the theory is sound - and since the much of the mass of the wheel is
    concentrated near its perimeter, the moment of inertia should not be negligible compared to the
    overall inertia.

    Why is this not important in real situations?

    Adam
     
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  2. Jacobe Hazzard wrote:
    > I have seen some references in RBT to the rolling mass of a wheel, IE that its moment of inertia
    > about the axle contributes to the overall inertia of the bike as well as its mass, or some rule of
    > thumb like weight on the rim counts as double compared to weight on the frame. Usually this idea
    > is immediately dissmissed as having been 'thouroughly debunked'.
    >
    > As far as I can see, the theory is sound - and since the much of the mass of the wheel is
    > concentrated near its perimeter, the moment of inertia should not be negligible compared to the
    > overall inertia.
    >
    > Why is this not important in real situations?
    >
    > Adam

    OK I did some calculations and figured that weight on the rim counts for only about 10% more than
    weight on the frame, but surely this is significant in racing situations?

    Possibly I have misunderstood the arguments of both sides, but I'm still interested in what people
    have to say.

    Adam
     
  3. Jacobe Hazzard wrote:
    > Jacobe Hazzard wrote:
    >> I have seen some references in RBT to the rolling mass of a wheel, IE that its moment of inertia
    >> about the axle contributes to the overall inertia of the bike as well as its mass, or some rule
    >> of thumb like weight on the rim counts as double compared to weight on the frame. Usually this
    >> idea is immediately dissmissed as having been 'thouroughly debunked'.
    >>
    >> As far as I can see, the theory is sound - and since the much of the mass of the wheel is
    >> concentrated near its perimeter, the moment of inertia should not be negligible compared to the
    >> overall inertia.
    >>
    >> Why is this not important in real situations?
    >>
    >> Adam
    >
    > OK I did some calculations and figured that weight on the rim counts for only about 10% more than
    > weight on the frame, but surely this is significant in racing situations?
    >
    > Possibly I have misunderstood the arguments of both sides, but I'm still interested in what people
    > have to say.
    >
    > Adam

    Err, maybe that should be 30%....
     
  4. Mark Hickey

    Mark Hickey Guest

    "Jacobe Hazzard" <[email protected]> wrote:

    >Err, maybe that should be 30%....

    Say a pound on the rims IS equivalent to 1.3 pounds on the frame.

    An extra pound (2.21kg) will slow you down a whopping 1 second in a flat, no turns 25 mile (40km)
    time trial, including the extra time to accelerate.

    So I guess if the extra pound was on the rims (heavy rims!) it would slow you down 1.3 seconds.

    I won't be losing sleep over that kind of loss any time soon (considering the adjusting mechanism on
    my shoes will account for at least 10x that much loss).

    Mark Hickey Habanero Cycles http://www.habcycles.com Home of the $695 ti frame
     
  5. W K

    W K Guest

    "Jacobe Hazzard" <[email protected]> wrote in message news:EePfb.104546

    > OK I did some calculations and figured that weight on the rim counts for only about 10% more than
    > weight on the frame, but surely this is
    significant
    > in racing situations?

    Bike racing is not drag racing. There are hardly any timew when any accelleration happens, and when
    you see lance zooming away from Ullrich, the actual accellerations are tiny.

    Also - accellerating a bike is far easier than accellerating the person on the bike.
     
  6. Bruce

    Bruce Guest

    Most cycling is relatively steady state, and angular acceleration is neglible. In racing there are a
    few situations where acceleration matter - final sprint, sprints out of corners, ... For examply if
    carring extra weight means you are 10cm or 3" behind out of each corner that can add up time to tire
    you out. And final sprints can have differences that small.

    To do some computations go to http://www.analyticcycling.com/WheelsInertia_Page.html

    -Bruce

    "Jacobe Hazzard" <[email protected]> wrote in message
    news:[email protected]...
    > I have seen some references in RBT to the rolling mass of a wheel, IE that its moment of inertia
    > about the axle contributes to the overall inertia of the bike as well as its mass, or some rule of
    > thumb like weight on the rim counts as double compared to weight on the frame. Usually this idea
    > is immediately dissmissed as having been 'thouroughly debunked'.
    >
    > As far as I can see, the theory is sound - and since the much of the mass
    of
    > the wheel is concentrated near its perimeter, the moment of inertia should not be negligible
    > compared to the overall inertia.
    >
    > Why is this not important in real situations?
    >
    > Adam
     
  7. Timh

    Timh Guest

    My experience as a 140lb rider on a 19lb bike is that there is a noticeable difference when I stomp
    on the pedals between a 1300g wheelset and an 1800g wheelset. The lighter wheels spin up and the
    bike accelerates to the desired speed more quickly. Not subtle, but obvious. Not my theory, but my
    experience. The lighter wheels provide a similar advantage in my situation in steep climbs.

    "Jacobe Hazzard" <[email protected]> wrote in message
    news:[email protected]...
    > I have seen some references in RBT to the rolling mass of a wheel, IE that its moment of inertia
    > about the axle contributes to the overall inertia of the bike as well as its mass, or some rule of
    > thumb like weight on the rim counts as double compared to weight on the frame. Usually this idea
    > is immediately dissmissed as having been 'thouroughly debunked'.
    >
    > As far as I can see, the theory is sound - and since the much of the mass
    of
    > the wheel is concentrated near its perimeter, the moment of inertia should not be negligible
    > compared to the overall inertia.
    >
    > Why is this not important in real situations?
    >
    > Adam
     
  8. Harris

    Harris Guest

    "Jacobe Hazzard" wrote:

    > >> I have seen some references in RBT to the rolling mass of a wheel, IE that its moment of
    > >> inertia about the axle contributes to the overall inertia of the bike as well as its mass, or
    > >> some rule of thumb like weight on the rim counts as double compared to weight on the frame.
    > >> Usually this idea is immediately dissmissed as having been 'thouroughly debunked'.
    > >>
    > >> As far as I can see, the theory is sound - and since the much of the mass of the wheel is
    > >> concentrated near its perimeter, the moment of inertia should not be negligible compared to the
    > >> overall inertia.
    > >>
    > >> Why is this not important in real situations?
    > >>
    > >> Adam
    > >
    > > OK I did some calculations and figured that weight on the rim counts for only about 10% more
    > > than weight on the frame, but surely this is significant in racing situations?
    > >
    > > Possibly I have misunderstood the arguments of both sides, but I'm still interested in what
    > > people have to say.
    > >
    > > Adam
    >
    > Err, maybe that should be 30%....

    I didn't see the word "acceleration" in your posts. When riding at a constant speed, the weight of
    the rim counts the same as the weight anywhere else. The rotational inertia effects only come into
    play when accelerating or decelerating. You're not carrying "extra weight" throughout the entire
    ride/race.

    And since you have to have rims, tires, and tubes on the bike, we only need to be concerned with the
    *differences* in weight between light and heavy items.

    If you study a wheel by itself, you might find that a heavy wheel has 20% more rotational inertia
    than a light wheel. That sounds impressive, but when you consider the total energy required to
    accelerate the bike and rider, the effect of those differences are down in the noise. Rims weigh 400
    grams; riders weigh 160 pounds!

    Here's an experiment. Mark off a distance of about 200 meters on a flat road. Have someone time
    you riding that distance from a standing start. Do five runs with a light set of wheels, and five
    runs with heavy wheels. This is a worst case scenario. There should be a difference, but it will
    be very small.

    In road races, the differences will be *much* less than in the above case, because riders typically
    accelerate from 25 to 30 mph rather than from 0 to 30 mph. And most of the time they're not
    accelerating at all.

    Aerodymanic effects also come into play in racing. A heavy "aero" rim will more than offset inertial
    effects at high speeds.

    And of course, to win a race, you have to finish it. If you puncture a tire or collapse a wheel
    because you were using stupid light tires/rims, you've gained nothing.

    Art Harris
     
  9. Harris

    Harris Guest

    "Mark Hickey" wrote:

    > Say a pound on the rims IS equivalent to 1.3 pounds on the frame.
    >
    > An extra pound (2.21kg) will slow you down a whopping 1 second in a flat, no turns 25 mile (40km)
    > time trial, including the extra time to accelerate.

    Ah, but the "extra weight" effect is only there during the initial acceleration. Once you're at
    constant speed, 100 grams on the rim is equivalent to 100 grams on the frame. And in a time trial,
    the small "flywheel effect" is actually a benefit once you're at cruising speed.

    Art Harris
     
  10. G Huang

    G Huang Guest

    Jacobe Hazzard wrote:
    > I have seen some references in RBT to the rolling mass of a wheel, IE that its moment of inertia
    > about the axle contributes to the overall inertia of the bike as well as its mass, or some rule of
    > thumb like weight on the rim counts as double compared to weight on the frame. Usually this idea
    > is immediately dissmissed as having been 'thouroughly debunked'.
    >
    > As far as I can see, the theory is sound - and since the much of the mass of the wheel is
    > concentrated near its perimeter, the moment of inertia should not be negligible compared to the
    > overall inertia.
    >
    > Why is this not important in real situations?
    >
    Depends on what you are trying to do with the inertia. If you want to use the inertia to store the
    kinetic energy in a descent and let it help you through the next ascent, the advantage is probably
    negligible. On the other hand, there will be perceptible difference if you do a 0-30 mph in x
    seconds type of comparison.
     
  11. Harris

    Harris Guest

    "TimH" wrote:
    > My experience as a 140lb rider on a 19lb bike is that there is a
    noticeable
    > difference when I stomp on the pedals between a 1300g wheelset and an
    1800g
    > wheelset. The lighter wheels spin up and the bike accelerates to the desired speed more quickly.
    > Not subtle, but obvious. Not my theory, but
    my
    > experience. The lighter wheels provide a similar advantage in my
    situation
    > in steep climbs.

    Jacobe Hazzard is arguing the "1 gram on the wheel is equivalent to 'x' grams on the frame" theory
    based on inertia. Certainly, any perceived advantage on steep climbs is due solely to weight, not
    inertial effects.

    Also, I don't see how you can differentiate between the bike accelerating faster, and the wheels
    "spinning up" more quickly. You can't have one without the other.

    Art Harris
     
  12. Mark Hickey

    Mark Hickey Guest

    "TimH" <[email protected]> wrote:

    >My experience as a 140lb rider on a 19lb bike is that there is a noticeable difference when I stomp
    >on the pedals between a 1300g wheelset and an 1800g wheelset. The lighter wheels spin up and the
    >bike accelerates to the desired speed more quickly. Not subtle, but obvious. Not my theory, but my
    >experience. The lighter wheels provide a similar advantage in my situation in steep climbs.

    I too can feel a difference between my heavy aero rims and my very light tubular wheels... but I
    know that it makes very, very little difference in overall speed.

    To overstate what I'm feeling, when I'm out of the saddle or pushing very hard, the bike
    "accelerates under me" more with the light wheels
    - whether my butt is holding it back or not. However, that doesn't really mean I'm accelerating
    faster, just that the lighter bike is "storing" less energy on every pedal stroke. With the
    heavier wheels on, the bike surges a little less every pedal stroke, but the extra weight means is
    DEcelerates less before the next pedal stroke. Net/net, it's basically a wash.

    And FWIW, I like the "light wheel feel"...

    Mark Hickey Habanero Cycles http://www.habcycles.com Home of the $695 ti frame
     
  13. Tim McNamara

    Tim McNamara Guest

    In article <[email protected]>, "Jacobe Hazzard"
    <[email protected]> wrote:

    > I have seen some references in RBT to the rolling mass of a wheel, IE that its moment of inertia
    > about the axle contributes to the overall inertia of the bike as well as its mass, or some rule of
    > thumb like weight on the rim counts as double compared to weight on the frame. Usually this idea
    > is immediately dissmissed as having been 'thouroughly debunked'.
    >
    > As far as I can see, the theory is sound - and since the much of the mass of the wheel is
    > concentrated near its perimeter, the moment of inertia should not be negligible compared to the
    > overall inertia.
    >
    > Why is this not important in real situations?

    Becaue that mass is only about 550-700 grams (assuming a racing wheel).
     
  14. Timh

    Timh Guest

  15. Mark Hickey <[email protected]> wrote:

    > I too can feel a difference between my heavy aero rims and my very light tubular wheels... but I
    > know that it makes very, very little difference in overall speed.

    I think it's very possible that an experienced cyclist might feel the difference. However, feeling
    the aerodynamic advantage of your heavier more aero wheels is almost impossible without a stop
    watch. I believe on everything but extremely hilly terrain the heavy aero wheels should be faster.

    -as
     
  16. > equivalent to 100 grams on the frame. And in a time trial, the small "flywheel effect" is actually
    > a benefit once you're at cruising speed.

    Is it? If so, why?

    --
    Phil, Squid-in-Training
     
  17. Bruce

    Bruce Guest

    > Mark Hickey <[email protected]> wrote:
    >
    > > I too can feel a difference between my heavy aero rims and my very light tubular wheels... but I
    > > know that it makes very, very little difference in overall speed.

    Perhaps what you percieve is when you are out of the saddle and you are throwing the bike around -
    sideways. Then you might feel the difference between a 20 and 19 pound bike. But you certainly won't
    notice it make the bike and rider, as a whole, accelerate.

    http://www.analyticcycling.com/WheelsClimb_Page.html

    Try this with a wheel that has the same aero values and weighs front wheel mass, inertia, drag
    1. 1.25 .08 0.1 .05 .05 rear wheel, same
    2.5 1.75 .1 0.12 .05 .05 (all other parameters are the standard on that page)

    Set the time and distance to be 20 seconds and 200 meters After 200 meters the light wheel is ahead
    by 92 centimeters, or less than one percent.

    -Bruce
     
  18. Harris

    Harris Guest

    "Phil, Squid-in-Training" wrote: Harris wrote:
    > > And in a time trial, the small "flywheel effect" is actually a benefit once you're at cruising
    > > speed.
    >
    > Is it? If so, why?

    Yes, but the effect is small.

    When you're accelerating, the rotational inertia has to be overcome. But when you're cruising, it
    tends to keep you're speed constant despite slight variations in pedaling force. It's the old
    "objects in motion tend to stay in motion" thing.

    Art Harris
     
  19. Steve Knight

    Steve Knight Guest

    On Sun, 5 Oct 2003 08:52:40 +0000 (UTC), "W K" <[email protected]> wrote:

    >Bike racing is not drag racing. There are hardly any timew when any accelleration happens, and when
    >you see lance zooming away from Ullrich, the actual accellerations are tiny.
    >
    >Also - accellerating a bike is far easier than accellerating the person on the bike.

    I also wonder since the wheel is in contact with the ground how it would effect the measurements?

    --
    Knight-Toolworks & Custom Planes Custom made wooden planes at reasonable prices See
    http://www.knight-toolworks.com For prices and ordering instructions.
     
  20. W K

    W K Guest

    "TimH" <[email protected]> wrote in message news:[email protected]...
    > My experience as a 140lb rider on a 19lb bike is that there is a
    noticeable
    > difference when I stomp on the pedals between a 1300g wheelset and an
    1800g
    > wheelset. The lighter wheels spin up and the bike accelerates to the desired speed more quickly.
    > Not subtle, but obvious. Not my theory, but
    my
    > experience. The lighter wheels provide a similar advantage in my
    situation
    > in steep climbs.

    Which is why an effect that has no possible basis still gets the dollars out of the pockets.

    The bike may accelerate better for short periods in your pedal strokes, but your body doesn't and
    the lightness of the bike means it slows down quicker in other parts of the stoke. ie you feel the
    bike surge forward when standing up, and it feels much better - the total opposite of the feeling
    of a tandem.

    I wonder whether this effect is just an illusion or whether the rythmn it sets up helps you to
    pedal better.
     
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