what makes a light bike really light?

Discussion in 'Cycling Equipment' started by [email protected], Feb 27, 2006.

  1. Hello

    being that I'm into steel road frames only and don't wish to get
    suckered into the buying frenzy of putting carbon and titanium and
    other exotic materials on my bike just to make it lighter(in order to
    make me go faster :) ), I was told by my LBS that if you wanted to
    lighten up your bike, the most important part to upgrade would be the
    wheels (rotating weight). Would cranks and road cassettes also count as
    "rotating weight"? Possibly anything that "rotates" while pedaling,
    like pedals as well?
     
    Tags:


  2. Mike Reed

    Mike Reed Guest

    Weight is overrated, but if you want to reduce it, go for the highest
    stuff first, as that will lower your center of gravity and give a
    lighter feel when standing. Saddle, bars, stem, seatpost, brifters.

    -Mike
     
  3. Leo Lichtman

    Leo Lichtman Guest

    <[email protected]> wrote: (clip) Would cranks and road cassettes also
    count as "rotating weight"? Possibly anything that "rotates" while pedaling,
    like pedals as well?
    ^^^^^^^^^^^^^^^
    As you accelerate, the parts of a bike that rotate have an extra inertial
    component called "moment of inertia." This uses up a little of your
    pedaling energy while you accelerate. But moment of inertia depends on the
    diameter as well as the weight. The wheels have most of their weight (mass)
    at the outside, so the effect is larger (though not large.) Things like the
    crank and chainrings are small in diameter by comparison. I'm not sure how
    to think about the pedals--they don't spin on their axis, since they are
    pressed against your foot, but they do go around in circles. I think I
    would include their weight with the cranks.

    One of the best places to remove weight, for many of us, though it doesn't
    rotate. is the waistline. :)
     
  4. Paul Hobson

    Paul Hobson Guest

    Mike Reed wrote:
    > Weight is overrated, but if you want to reduce it, go for the highest
    > stuff first, as that will lower your center of gravity and give a
    > lighter feel when standing. Saddle, bars, stem, seatpost, brifters.
    >
    > -Mike
    >


    I can't quantify this, but my beater (Reynolds 531) has horribly heavy
    rims. My nicer bike (Columbus tubing from '87) has Velocity Aeroheads
    that are incredibly light (to me). The difference is amazing in both
    the heft of the whole bike and how easily it accelerates uphill. Also,
    I notice quite in the difference in stopping as well (when not using
    brakes as they are fixies).
    \\paul
    --
    Paul M. Hobson
    Georgia Institute of Technology
    ..:change the f to ph to reply:.
     
  5. Tires are a great and fairly cheap way to shave off a few hundred
    grams. Then, if you can find a scale, pull your stem, your saddle, and
    your seat post and weigh them. Often you can replace such parts for
    even rather inexpensive new stuff, and knock off a pound.

    I'm not obsessive about weight myself, but dropping nearly 750g by
    going from Brooks to Flite saddle (and it's more comfy for me) made my
    bike feel like a new much faster version, especially in the hills, as
    it's "swung" weight.

    Saving weight via wheels can get expensive and unreliable, start with
    the basics as I said. Sometimes you can drop 200g with a new $20
    seatpin you found at Nashbar on sale, a lot more friendly to the pocket
    than spending $500 to drop that same 200g via wheels.
     
  6. Dan Connelly

    Dan Connelly Guest

    Leo Lichtman wrote:
    > <[email protected]> wrote: (clip) Would cranks and road cassettes also
    > count as "rotating weight"? Possibly anything that "rotates" while pedaling,
    > like pedals as well?
    > ^^^^^^^^^^^^^^^
    > As you accelerate, the parts of a bike that rotate have an extra inertial
    > component called "moment of inertia." This uses up a little of your
    > pedaling energy while you accelerate. But moment of inertia depends on the
    > diameter as well as the weight. The wheels have most of their weight (mass)
    > at the outside, so the effect is larger (though not large.) Things like the
    > crank and chainrings are small in diameter by comparison. I'm not sure how
    > to think about the pedals--they don't spin on their axis, since they are
    > pressed against your foot, but they do go around in circles. I think I
    > would include their weight with the cranks.
    >
    > One of the best places to remove weight, for many of us, though it doesn't
    > rotate. is the waistline. :)
    >
    >


    What really matters is the speed relative to the bike speed. The outside
    of the rim, independent of wheel diameter, rotates at a speed equal to the bike
    speed (approximately). The ratio of pedal speed to bike speed is
    crank length / effective wheel radius * rear cog teeth / front cog teeth.
    Since kinetic energy is proportional to the square of speed, pedal rotational
    kinetic energy is small. A larger factor is shoe weight, since shoes must
    be supported isometrically.

    Dan
     
  7. Matt O'Toole

    Matt O'Toole Guest

    On Mon, 27 Feb 2006 07:32:25 -0800, mxd1007 wrote:

    > being that I'm into steel road frames only and don't wish to get
    > suckered into the buying frenzy of putting carbon and titanium and other
    > exotic materials on my bike just to make it lighter(in order to make me
    > go faster :) ),


    A light steel frame is half a pound heavier than a typical Ti or carbon
    frame, although the lightest carbon frames are half a pound lighter still.
    So all else being equal, that's the difference in having a steel frame.

    If you want to go faster, ride more.

    > I was told by my LBS that if you wanted to lighten up your bike, the
    > most important part to upgrade would be the wheels (rotating weight).
    > Would cranks and road cassettes also count as "rotating weight"?
    > Possibly anything that "rotates" while pedaling, like pedals as well?


    The rotating weight issue is severely overblown. If bike salesmen really
    understood physics, do you think they'd be selling bikes for a living?
    Most of the weight savings in light wheel sets is in the hubs anyway.

    However, it's easy to save some weight with a lightweight wheel set.
    Carbon cranks are several ounces lighter than aluminum ones too.

    There's a website devoted to bike weight freaks:

    www.weightweenies.starbike.com

    Matt O.
     
  8. Mike Reed wrote:
    > Weight is overrated, but if you want to reduce it, go for the highest
    > stuff first, as that will lower your center of gravity and give a
    > lighter feel when standing. Saddle, bars, stem, seatpost, brifters.
    >
    >



    Let's say you lighten the bike by two pounds via the installation of a
    lighter saddle, lighter bar and stem, etc. (not likely, but let's
    pretend)

    Now, take a 160lb rider standing whilst pedaling a 17lb bike. Where is
    the center of gravity, and how much did the loss of those two pounds
    lower it?
     
  9. Peter Cole

    Peter Cole Guest

    [email protected] wrote:
    > Hello
    >
    > being that I'm into steel road frames only and don't wish to get
    > suckered into the buying frenzy of putting carbon and titanium and
    > other exotic materials on my bike just to make it lighter(in order to
    > make me go faster :) ), I was told by my LBS that if you wanted to
    > lighten up your bike, the most important part to upgrade would be the
    > wheels (rotating weight). Would cranks and road cassettes also count as
    > "rotating weight"? Possibly anything that "rotates" while pedaling,
    > like pedals as well?
    >


    It's really pretty incremental, there's no obvious single place to drop
    a lot of weight (or everyone would). Like the man said: "light,
    reliable, reasonably priced -- pick any 2".

    "Rotating weight" arguments are overstated, but it's not unreasonable to
    have a spare set of wheels with light tires & tubes. Some things make a
    bike feel lighter, without actually doing much.

    Outside of climbing hills, aerodynamic drag is the most important thing
    to improve for speed. I'm always surprised by riders with very light and
    expensive bikes who wear flappy jackets and/or have very upright
    positions on the bike.
     
  10. well I was poking fun by saying I want to make my bike lighter so I can
    go faster :) it seems thats the reason why most people switch to
    lighter parts...I'm a firm believer, 80% of riding has to do with the
    rider him/herself, how fit they are, how often they ride, how they
    ride, how the bike fits rather than spending a lot of $$$ on parts. But
    as a quick and easy way to shave some "dead weight" is to start with
    the wheels. Then "work your way up" the bike if you have some left over
    $$$ and get a lighter stem or seat or seat post. Most people say to
    start with the wheels first....kind of interesting to see how others
    would focus on other components

    hopefully this strict weight training regimen I have been doing over
    the winter will pay off once the snow is gone and I can ride again :)
    I envy you people in Southern Cali and Arizona
     
  11. p.k.

    p.k. Guest

    Ozark Bicycle wrote:
    > Mike Reed wrote:
    >> Weight is overrated, but if you want to reduce it, go for the highest
    >> stuff first, as that will lower your center of gravity and give a
    >> lighter feel when standing. Saddle, bars, stem, seatpost, brifters.
    >>
    >>

    >
    >
    > Let's say you lighten the bike by two pounds via the installation of a
    > lighter saddle, lighter bar and stem, etc. (not likely, but let's
    > pretend)
    >
    > Now, take a 160lb rider standing whilst pedaling a 17lb bike. Where is
    > the center of gravity, and how much did the loss of those two pounds
    > lower it?


    Errm, I'm 230lb, i know the best way to lighten my bike - make it as heavy
    and inefficient as possible and work the weight off my waistline!

    pk
     
  12. Art Harris

    Art Harris Guest

    [email protected] wrote:

    > I was told by my LBS that if you wanted to lighten up your bike, the most important part to upgrade would be the wheels (rotating weight).


    Drilling holes in your water bottle has been known to save a pound or
    more of weight.

    Seriously, unless you can save a couple of pounds, it's not going to
    make much difference. Calculate the combined weight of you and the
    bike.Then calculate the amount of bike weight you want to save. The
    percentage of saved weight to "total package" weight will probably be
    about 1 percent. And that will only make a difference when climbing.

    In many (if not most) cases, cutting weight will reduce reliability.
    Unless you're into competition, don't worry about an extra pound or
    two.

    Art Harris
     
  13. Leo Lichtman

    Leo Lichtman Guest

    "Dan Connelly" wrote: (clip) A larger factor is shoe weight, since shoes
    must be supported isometrically.
    ^^^^^^^^^^^^^^^^^
    I don't know what you mean by "isometrically."

    I see the shoe weight being lumped in with the pedal weight. But when you
    get your thinking to that point, where do you stop. Rotating weight would
    include the foot, and part of the lower leg.
     
  14. Werehatrack

    Werehatrack Guest

    On 27 Feb 2006 07:32:25 -0800, [email protected] wrote:

    >Hello
    >
    >being that I'm into steel road frames only and don't wish to get
    >suckered into the buying frenzy of putting carbon and titanium and
    >other exotic materials on my bike just to make it lighter(in order to
    >make me go faster :) ), I was told by my LBS that if you wanted to
    >lighten up your bike, the most important part to upgrade would be the
    >wheels (rotating weight). Would cranks and road cassettes also count as
    >"rotating weight"? Possibly anything that "rotates" while pedaling,
    >like pedals as well?


    If you want to make your bike lighter, take it apart, weigh the
    components, and then shop for alternate components that have less
    mass. You may discover that there's a specific area in which your
    current bike is already at the limits of practical lightness, and/or
    you may find that there's no one place where a change is going to make
    a big difference. You may also discover that there's a hidden boat
    anchor that you can change out for something more svelte which will
    bring the total weight down a lot. Until you know what's heavy and
    what's light among the bits you have, however, it's pointless to
    speculate.
    --
    Typoes are a feature, not a bug.
    Some gardening required to reply via email.
    Words processed in a facility that contains nuts.
     
  15. IMHO the bottom, farthest from the center of gravity, is more
    important. The role of the bike is basically to move back and forth to
    support the rider and maintain balance. Light bikes are similar to
    light pogo sticks -- easier to get them where they need to be under the
    rider.
     
  16. Mike Reed wrote:
    > Weight is overrated, but if you want to reduce it, go for the highest
    > stuff first, as that will lower your center of gravity and give a
    > lighter feel when standing. Saddle, bars, stem, seatpost, brifters.


    On a bicycle, a low C of G is actually a disadvantage. Balancing a bike
    is a bit like swinging an upside-down pendulum, or (a closer
    approximation) balancing a stick on your hand with a weight fixed to the
    one end. If you slide the weight nearer your hand, balancing it becomes
    more difficult because the period of its natural oscillation has reduced
    and you have to react quicker to catch it before it tilts too far. In
    the same way, a high C of G makes the bike wobble more slowly,
    increasing stability.
     
  17. Harry Kim

    Harry Kim Guest

    [email protected] mused:

    >being that I'm into steel road frames only and don't wish to get
    >suckered into the buying frenzy of putting carbon and titanium and
    >other exotic materials on my bike just to make it lighter(in order to
    >make me go faster :) ), I was told by my LBS that if you wanted to
    >lighten up your bike, the most important part to upgrade would be the
    >wheels (rotating weight).


    Exactly correct. Remove the wheels and install an antigrav
    conversion, but make sure that you get one with an interial damper
    field as well; the conversion kits have substantial mass in the
    quantum black hole that supplies their power, and without the inertial
    damper field, the energy required to move them around can be
    substantial. Beware of cheap units from Beta Aurigae IV; they have
    lousy quality control, and if you're not lucky they can induce a nasty
    phase variance that will leave you stuck a couple of microseconds away
    from the rest of the universe.

    Of course, in theory you could simply install an inertial damping
    field generator and leave the wheels in place, but why bother? The
    damping field's power drain means that you'll need the power
    generation capability of the quantum black hole unit anyway, and once
    you've got that, there's no really good reason to do without the
    antigrav...and you get a better deal on the setup as a bundle than you
    would on the individual pieces. Swapping to antigrav also eliminates
    the issue of having to repressurize your tires for different planetary
    gravitational field strengths and local atmospheric densities.

    >Would cranks and road cassettes also count as
    >"rotating weight"?


    Only if they're spinning at the time. Once you eliminate the wheels,
    this is no longer an issue.
     
  18. Mike Reed

    Mike Reed Guest

    Ozark Bicycle wrote:
    > Let's say you lighten the bike by two pounds via the installation of a
    > lighter saddle, lighter bar and stem, etc. (not likely, but let's
    > pretend)
    >
    > Now, take a 160lb rider standing whilst pedaling a 17lb bike. Where is
    > the center of gravity, and how much did the loss of those two pounds
    > lower it?


    I said it would lower the center of gravity, and it will. Ranting about
    how much difference it makes is pointless when I qualified it by saying
    that weight is overrated. Your argument is obvious and applies to
    weight in general, not just center of mass.

    -Mike
     
  19. Mike Reed

    Mike Reed Guest

    [email protected] wrote:
    > IMHO the bottom, farthest from the center of gravity, is more
    > important. The role of the bike is basically to move back and forth to
    > support the rider and maintain balance. Light bikes are similar to
    > light pogo sticks -- easier to get them where they need to be under the
    > rider.


    I see where you're coming from. It's kind of like balancing a vertical
    broom on your fingertip, it's much easier with weight on the end (like
    the Chinese spinning plates).

    But as another poster pointed out, moving the center of mass of the
    bike lower doesn't have significant effect on the bike/rider system. It
    does, however feel a little nicer when you're swinging the bike while
    standing.

    -Mike
     
  20. Dan Connelly

    Dan Connelly Guest

    Leo Lichtman wrote:
    > "Dan Connelly" wrote: (clip) A larger factor is shoe weight, since shoes
    > must be supported isometrically.
    > ^^^^^^^^^^^^^^^^^
    > I don't know what you mean by "isometrically."


    The weight needs to be in part supported without doing any active work. A static force must
    be supplied to support the shoe, preventing it from rotating to its position with the
    COM vertically aligned with the pedal axis.

    >
    > I see the shoe weight being lumped in with the pedal weight. But when you
    > get your thinking to that point, where do you stop. Rotating weight would
    > include the foot, and part of the lower leg.
    >
    >


    The kinetic energy of the feet is negligible, so treating them as "rotating"
    is misleading.

    Dan
     
Loading...
Loading...