Another indexed steering thread

Discussion in 'Cycling Equipment' started by ZeeExSixAre, Jun 10, 2004.

  1. ZeeExSixAre

    ZeeExSixAre Guest

    I've been making observations lately between two of my bikes: my BMX bike
    and my road bike.

    Whenever I rode my road bike (100psi), I would look down and see the wild
    fore-and-aft deflections of the fork dropouts in relation to me over bumpy
    pavement. The deflections were probably up to a 1/2 inch. I naturally
    attributed this to the fork's lighter steel construction and relatively thin
    blades.

    After I got my BMX bike, I was riding it home (60psi) when I looked down and
    saw the fork doing the exact same thing! This is on a bike that has legs
    that are about 4 inches shorter, has fork blades that are 1 1/4" in diameter
    and a wall thickness of 6mm!

    The fork fore-aft motion can't possibly be attributed to much else other
    than the headset, unless the fork is bending that much at the crown.

    In relation to the indexed steering thread, does this mean that the bearings
    are being pushed into the races enough to cause the deflection, and since it
    does not hit the yielding stress of the materials, the bearings bounce back
    with no deformation?

    --
    Phil, Squid-in-Training
     
    Tags:


  2. Mark South

    Mark South Guest

    "ZeeExSixAre" <[email protected]> wrote in message
    news:F%[email protected]
    > I've been making observations lately between two of my bikes: my BMX bike
    > and my road bike.
    >
    > Whenever I rode my road bike (100psi), I would look down and see the wild
    > fore-and-aft deflections of the fork dropouts in relation to me over bumpy
    > pavement. The deflections were probably up to a 1/2 inch. I naturally
    > attributed this to the fork's lighter steel construction and relatively thin
    > blades.
    >
    > After I got my BMX bike, I was riding it home (60psi) when I looked down and
    > saw the fork doing the exact same thing! This is on a bike that has legs
    > that are about 4 inches shorter, has fork blades that are 1 1/4" in diameter
    > and a wall thickness of 6mm!
    >
    > The fork fore-aft motion can't possibly be attributed to much else other
    > than the headset, unless the fork is bending that much at the crown.
    >
    > In relation to the indexed steering thread, does this mean that the bearings
    > are being pushed into the races enough to cause the deflection, and since it
    > does not hit the yielding stress of the materials, the bearings bounce back
    > with no deformation?


    Did you allow for the fact that your eyeballs are not rigid?
    --
    Mark South: World Citizen, Net Denizen
     
  3. jim beam

    jim beam Guest

    ZeeExSixAre wrote:

    > I've been making observations lately between two of my bikes: my BMX bike
    > and my road bike.
    >
    > Whenever I rode my road bike (100psi), I would look down and see the wild
    > fore-and-aft deflections of the fork dropouts in relation to me over bumpy
    > pavement. The deflections were probably up to a 1/2 inch. I naturally
    > attributed this to the fork's lighter steel construction and relatively thin
    > blades.
    >
    > After I got my BMX bike, I was riding it home (60psi) when I looked down and
    > saw the fork doing the exact same thing! This is on a bike that has legs
    > that are about 4 inches shorter, has fork blades that are 1 1/4" in diameter
    > and a wall thickness of 6mm!
    >
    > The fork fore-aft motion can't possibly be attributed to much else other
    > than the headset, unless the fork is bending that much at the crown.
    >
    > In relation to the indexed steering thread, does this mean that the bearings
    > are being pushed into the races enough to cause the deflection, and since it
    > does not hit the yielding stress of the materials, the bearings bounce back
    > with no deformation?
    >


    almost all the flex you describe /is/ the frame [and fork], not the head
    set.

    all "diamond" frames flex in plane about the head tube, even sturdy ones
    like bmx. even tho it's called a diamond frame, the tip of the diamond
    is usually truncated by the head tube, so you don't get perfect
    triangulation. if you built a straight parallelogram from erector set,
    you'd have free play about the axes. if you built a triangle, it would
    be as rigid as the materials could stand. if you built a truncated
    diamond like a frame - a hybrid between the parallelogram & the
    triangle, you'd get minimal play about the "seat tube", and exaggerated
    play about the "head tube".

    the head tube exaggeration can be minimized by making the tubes and
    their joins at this point as rigid as possible by using big thick tube
    ends, and in the case of giant mtb frames, even flare the end of the
    tube to make the contact with the head tube even larger then normal, but
    the tendency is still there.

    regarding the bearings, yes, they deform elastically under load, but
    their deformation is measured in microns, not halves of inches.
     
  4. ZeeExSixAre

    ZeeExSixAre Guest

    > almost all the flex you describe /is/ the frame [and fork], not the
    > head set.


    Oh.

    > all "diamond" frames flex in plane about the head tube, even sturdy
    > ones like bmx. even tho it's called a diamond frame, the tip of the
    > diamond is usually truncated by the head tube, so you don't get
    > perfect triangulation. if you built a straight parallelogram from
    > erector set, you'd have free play about the axes. if you built a
    > triangle, it would be as rigid as the materials could stand. if you
    > built a truncated diamond like a frame - a hybrid between the
    > parallelogram & the triangle, you'd get minimal play about the "seat
    > tube", and exaggerated play about the "head tube".


    Yes... this is likely, as the frame size for this BMX bike is about 10.5"
    and the top tube is 20.5". The top and down tubes are about 2 inches in
    outer diameter.

    > the head tube exaggeration can be minimized by making the tubes and
    > their joins at this point as rigid as possible by using big thick tube
    > ends, and in the case of giant mtb frames, even flare the end of the
    > tube to make the contact with the head tube even larger then normal,
    > but the tendency is still there.


    My Giant XTC has this Fluidform applied to the downtube and top tube...
    looks cool but dunno if it makes as big of a diference as they claim.

    > regarding the bearings, yes, they deform elastically under load, but
    > their deformation is measured in microns, not halves of inches.



    Okay, so if this is true, then that means the frame, around the headtube, is
    deflecting a degree or two, enough to cause the 1/2" deflections at the
    dropout, and fork flex is minimal in comparison?

    --
    Phil, Squid-in-Training
     
  5. jim beam

    jim beam Guest

    ZeeExSixAre wrote:
    <snip>

    > Okay, so if this is true, then that means the frame, around the headtube, is
    > deflecting a degree or two, enough to cause the 1/2" deflections at the
    > dropout, and fork flex is minimal in comparison?


    no, forks flex also. the fork crown area tends to be quite rigid, but
    the blades have flex as does the steerer tube.
     
Loading...
Loading...