Fork Shudder

Discussion in 'Cycling Equipment' started by Mark Hickey, Mar 2, 2003.

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  1. Mark Hickey

    Mark Hickey Guest

    Hey, whadaya know?

    I'd observed what appeared to be significant amounts of fork shudder when braking with dirty
    rims on my road bike - fork deflections that appeared to be the almost an inch in fore/aft
    movement... but the
    r.b.t. consensus seemed to be that there was no reasonable (non-destructive) way to deflect a fork
    that much.

    Today I figured out a really easy (5 minute experiment), cheap ($0) way to figure out if what I was
    seeing was an optical illusion or reality.

    I'd noticed that by locking the front brake solidly I was able to oscillate the rest of the bike
    back and forth easily with just a couple fingers by applying a little fore/aft pressure on the top
    tube. It sure LOOKED like the fork was flexing, even when sighting down over the handlebars (using
    them as a reference point and watching the hub move back and forth, just like that bumpy, wet
    descent I mentioned). But how was I going to measure the apparent deflection?

    I was thinking of taking a series of high-speed digital images against a background containing a
    reference grid (nerdly idea, huh?). Suddenly it occured to me that If I want to know if the back of
    the bike is moving, all I have to do is measure how much the REAR wheel rotates.

    I rubber-banded a gold paint marker onto the rear triangle of my fixed gear bike, aligned so that
    the marker's tip contacted the rear tire's tread just off-center. Then I locked up the front brake
    and applied gentle force to the top tube, back and forth with two fingers.

    The result was 14mm of movement (or plus / minus 7mm) of the rear wheel (I verified the front wheel
    didn't move at all).

    I held the top tube a bit more firmly and applied the same sort of motion to the top tube, but with
    a little more force. This time I got 19mm of fore/aft movement at the rear tire.

    I repeated both tests several times, and got nearly identical results each time.

    My conclusion is that with only a few pounds of force applied intermittently AT THE RIGHT RATE, a
    significant amount of fork deflection can be produced. It doesn't seem unlikely at all that similar
    deflections can occur under certain braking conditions, lending credibility to those who claim to
    have seen their front hub dancing back and forth in inch (2.54cm) or so.

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


  2. S. Anderson

    S. Anderson Guest

    I watched my brother descend in the Austrian Alps with a Kinesis MTB alu. fork in the rain and I
    swear his fork was oscillating a centimetre or two in the fore-aft direction. I didn't perform any
    on the spot anaylysis, but I swore never to ride a Kinesis fork after witnessing that!! ;-)

    Cheers,

    Scott..
    --
    Scott Anderson

    "Mark Hickey" <[email protected]> wrote in message
    news:eek:[email protected]...
    > Hey, whadaya know?
    >
    > I'd observed what appeared to be significant amounts of fork shudder when braking with dirty
    > rims on my road bike - fork deflections that appeared to be the almost an inch in fore/aft
    > movement... but the
    > r.b.t. consensus seemed to be that there was no reasonable (non-destructive) way to deflect a fork
    > that much.

    <<snip..>>

    > My conclusion is that with only a few pounds of force applied intermittently AT THE RIGHT RATE, a
    > significant amount of fork deflection can be produced. It doesn't seem unlikely at all that
    > similar deflections can occur under certain braking conditions, lending credibility to those who
    > claim to have seen their front hub dancing back and forth in inch (2.54cm) or so.
    >
    > Mark Hickey Habanero Cycles http://www.habcycles.com Home of the $695 ti frame
     
  3. Frank121

    Frank121 Guest

    Maybe the frame is not stiff enough. You know, you can't build a really stiff and stable bike from
    anything other than steel ;-)

    "Mark Hickey" <[email protected]> wrote in message
    news:eek:[email protected]...
    > Hey, whadaya know?
    >
    > I'd observed what appeared to be significant amounts of fork shudder when braking with dirty
    > rims on my road bike - fork deflections that appeared to be the almost an inch in fore/aft
    > movement... but the
    > r.b.t. consensus seemed to be that there was no reasonable (non-destructive) way to deflect a fork
    > that much.
    >
    > Today I figured out a really easy (5 minute experiment), cheap ($0) way to figure out if what I
    > was seeing was an optical illusion or reality.
    >
    > I'd noticed that by locking the front brake solidly I was able to oscillate the rest of the bike
    > back and forth easily with just a couple fingers by applying a little fore/aft pressure on the top
    > tube. It sure LOOKED like the fork was flexing, even when sighting down over the handlebars (using
    > them as a reference point and watching the hub move back and forth, just like that bumpy, wet
    > descent I mentioned). But how was I going to measure the apparent deflection?
    >
    > I was thinking of taking a series of high-speed digital images against a background containing a
    > reference grid (nerdly idea, huh?). Suddenly it occured to me that If I want to know if the back
    > of the bike is moving, all I have to do is measure how much the REAR wheel rotates.
    >
    > I rubber-banded a gold paint marker onto the rear triangle of my fixed gear bike, aligned so that
    > the marker's tip contacted the rear tire's tread just off-center. Then I locked up the front brake
    > and applied gentle force to the top tube, back and forth with two fingers.
    >
    > The result was 14mm of movement (or plus / minus 7mm) of the rear wheel (I verified the front
    > wheel didn't move at all).
    >
    > I held the top tube a bit more firmly and applied the same sort of motion to the top tube, but
    > with a little more force. This time I got 19mm of fore/aft movement at the rear tire.
    >
    > I repeated both tests several times, and got nearly identical results each time.
    >
    > My conclusion is that with only a few pounds of force applied intermittently AT THE RIGHT RATE, a
    > significant amount of fork deflection can be produced. It doesn't seem unlikely at all that
    > similar deflections can occur under certain braking conditions, lending credibility to those who
    > claim to have seen their front hub dancing back and forth in inch (2.54cm) or so.
    >
    > Mark Hickey Habanero Cycles http://www.habcycles.com Home of the $695 ti frame
     
  4. Mark Hickey <[email protected]> wrote:
    : It doesn't seem unlikely at all that similar deflections can occur under certain braking
    : conditions, lending credibility to those who claim to have seen their front hub dancing back and
    : forth in inch (2.54cm) or so.

    well, i'm one of those people. take a paul neo-retro brake and put it on one of many cyclocross
    forks (independent fabrications and the wound-up i've seen) and pull the straddle down low. ride the
    bike at 5mph or so and hit the brakes and you'll see shudder of that magnitude.

    i didn't realize there wasn't credibility to it, tho? independent fab. knows about it, a few of the
    shops more familiar with cyclocross do as well. it's pretty easy to demonstrate (you don't need to
    be going fast).

    never seen it on a caliper brake, tho.

    i rode my bike into the wall (starting at the wall, of course) to see if the deflection was
    possible. yep.
    --
    david reuteler [email protected]
     
  5. On Sun, 02 Mar 2003 19:30:45 -0500, Mark Hickey wrote:

    > Hey, whadaya know?
    >
    > I'd observed what appeared to be significant amounts of fork shudder when braking with dirty rims
    > on my road bike - fork deflections that appeared to be the almost an inch in fore/aft movement...
    > but the r.b.t. consensus seemed to be that there was no reasonable (non-destructive) way to
    > deflect a fork that much.

    I don't think that is accurate. The consensus is that the blades themselves do not flex. The
    assumption of some was that the purpose of a curved fork blade was to provide some "shock
    absorbsion" or spring, which of course it does not.

    Forks do flex. At the crown.

    --

    David L. Johnson

    __o | Some people used to claim that, if enough monkeys sat in front _`\(,_ | of enough
    typewriters and typed long enough, eventually one of (_)/ (_) | them would reproduce the
    collected works of Shakespeare. The internet has proven this not to be the case.
     
  6. "David L. Johnson" <David L. Johnson <[email protected]>> wrote:
    : Forks do flex. At the crown.

    yea, that's right (as in what i remember the consensus being) .. i can tell you, tho, that the
    (straight) fork blades certainly do *APPEAR* to flex. but that doesn't mean they are. it'd be nice
    to see with high-speed photography especially since i'm not willing to subject my fork to enuf force
    to test otherwise.
    --
    david reuteler [email protected]
     
  7. Jobst Brandt

    Jobst Brandt Guest

    David Reuteler writes:

    >> Forks do flex. At the crown.

    > yea, that's right (as in what i remember the consensus being) .. i can tell you, tho, that the
    > (straight) fork blades certainly do *APPEAR* to flex. but that doesn't mean they are. it'd be nice
    > to see with high-speed photography especially since i'm not willing to subject my fork to enuf
    > force to test otherwise.

    What does high speed photography have to do with fork flex? You can assess that standing still. Just
    lock the front brake and rock the bicycle back and forth while sitting in the saddle while sighting
    over the bars at the front axle. It's not a mystery.

    And by the way, there are evening courses for people for whom English is a second language. They
    usually cover writing and spelling as well as sentence structure.

    Jobst Brandt [email protected] Palo Alto CA
     
  8. Java Man

    Java Man Guest

    In article <[email protected]>, [email protected] says...
    > And by the way, there are evening courses for people for whom English is a second language. They
    > usually cover writing and spelling as well as sentence structure.
    >
    >
    They may cover writing and spelling even better.

    Rick
     
  9. [email protected] wrote:
    : What does high speed photography have to do with fork flex? You can assess that standing still.
    : Just lock the front brake and rock the bicycle back and forth while sitting in the saddle while
    : sighting over the bars at the front axle. It's not a mystery.

    the question was whether it was flexing at the crown or in the blade itself.

    : And by the way, there are evening courses for people for whom English is a second language. They
    : usually cover writing and spelling as well as sentence structure.

    there are also classes in reading comprehension.
    --
    david reuteler [email protected]
     
  10. David Reuteler wrote:
    > [email protected] wrote:
    > : What does high speed photography have to do with fork flex? You can assess that standing still.
    > : Just lock the front brake and rock the bicycle back and forth while sitting in the saddle while
    > : sighting over the bars at the front axle. It's not a mystery.
    >
    > the question was whether it was flexing at the crown or in the blade itself.

    The suggested method allows you to determine this for yourself.

    Some forks have more flex in the blades than others, but the majority of the flex is in the crown
    which is easily assessed by Jobst's method.

    --
    R.

    <> Richard Brockie "Categorical statements <> The tall blond one. always cause trouble." <>
    [email protected]
     
  11. Richard Brockie <[email protected]> wrote:
    : The suggested method allows you to determine this for yourself.
    :
    : Some forks have more flex in the blades than others, but the majority of the flex is in the crown
    : which is easily assessed by Jobst's method.

    god, i realize this. the first thing i did was check the fork flex in the "jobst method" (read my
    other post) and yes, there was flex at the crown. that's static, however. the shudder is around 5hz
    or so and is probably more like 5cm total (2.5cm fore/aft) in amplitude. when you watch the fork
    while riding it *APPEARS* to be flexing the blades (the last 20cm). that may be an optical illusion.
    i would like to see that on film.
    --
    david reuteler [email protected]
     
  12. Tim McNamara

    Tim McNamara Guest

    In article <[email protected]>, "David L. Johnson" <David L. Johnson
    <[email protected]>> wrote:

    > On Sun, 02 Mar 2003 19:30:45 -0500, Mark Hickey wrote:
    >
    > > Hey, whadaya know?
    > >
    > > I'd observed what appeared to be significant amounts of fork shudder when braking with dirty
    > > rims on my road bike - fork deflections that appeared to be the almost an inch in fore/aft
    > > movement... but the r.b.t. consensus seemed to be that there was no reasonable (non-destructive)
    > > way to deflect a fork that much.
    >
    > I don't think that is accurate. The consensus is that the blades themselves do not flex. The
    > assumption of some was that the purpose of a curved fork blade was to provide some "shock
    > absorbsion" or spring, which of course it does not.
    >
    > Forks do flex. At the crown.

    Wouldn't the flex be along much of the length of the blade? Note that fork and aft flex is not the
    same as compression and would absorb no road shock; a "rigid" fork can't compress.

    Now, I have frequently experienced fork flex on bikes with light-gauge forks and cantilever
    brakes. I can't recall it happening under braking on a road bike (but that may just say more about
    my memory).
     
  13. Mark Hickey

    Mark Hickey Guest

    "David L. Johnson" <David L. Johnson <[email protected]>> wrote:

    >On Sun, 02 Mar 2003 19:30:45 -0500, Mark Hickey wrote:
    >
    >> Hey, whadaya know?
    >>
    >> I'd observed what appeared to be significant amounts of fork shudder when braking with dirty rims
    >> on my road bike - fork deflections that appeared to be the almost an inch in fore/aft movement...
    >> but the r.b.t. consensus seemed to be that there was no reasonable (non-destructive) way to
    >> deflect a fork that much.
    >
    >I don't think that is accurate. The consensus is that the blades themselves do not flex. The
    >assumption of some was that the purpose of a curved fork blade was to provide some "shock
    >absorbsion" or spring, which of course it does not.
    >
    >Forks do flex. At the crown.

    I think you misread my post. I'm talking about fore/aft movement of hub. My testing method doesn't
    identify whether that's happening at the crown, in the legs, or a combination of both. I certainly
    wouldn't suggest that there's any compressive capability in a curved fork (in fact, just wrote
    another post stating just that 5 minutes ago).

    Mark Hickey Habanero Cycles http://www.habcycles.com Home of the $695 ti frame
     
  14. Jon Isaacs

    Jon Isaacs Guest

    >I rubber-banded a gold paint marker onto the rear triangle of my fixed gear bike, aligned so that
    >the marker's tip contacted the rear tire's tread just off-center. Then I locked up the front brake
    >and applied gentle force to the top tube, back and forth with two fingers.

    >The result was 14mm of movement (or plus / minus 7mm) of the rear wheel (I verified the front wheel
    >didn't move at all).

    I threw some numbers off the top of my head into the basic cantilever beam, 1 inch steel steerer
    tube, 0.060 in wall, assumed the entire fork had the same cross-section equation and was 26 inch
    inches long.

    The length is critical because the deformation is the third power of the length. Since most of the
    deflection is caused by the bending of the section near the crown, I figured the tapering probably
    did not change much and that the effective moment on the fork was caused by the contact patch, thus
    L=26inches (should have been 28 inches)

    Then figuring RBTers are a bunch of cyclists, I limited the load to 50 lbs.

    Plug it in, and out pops 12.622 mm as the deflection.

    For a rough and dirty calculation, I would say this is close given the crudeness of the calculation
    but it does indicate that the deflection measured is reasonable.

    Previously it has been suggested to remove the wheel and just push on the fork tips. This reduces
    the effective length and cuts the deflection roughly by a factor of 8 to about 1.5 mm.

    One source of error is the deflection of the brake under the load, some brakes are pretty sloppy.

    jon isaacs
     
  15. On Mon, 03 Mar 2003 09:17:53 -0500, Mark Hickey wrote:

    > "David L. Johnson" <David L. Johnson <[email protected]>> wrote:
    >
    >>On Sun, 02 Mar 2003 19:30:45 -0500, Mark Hickey wrote:
    >>
    >>> Hey, whadaya know?
    >>>
    >>> I'd observed what appeared to be significant amounts of fork shudder when braking with dirty
    >>> rims on my road bike - fork deflections that appeared to be the almost an inch in fore/aft
    >>> movement... but the
    >>> r.b.t. consensus seemed to be that there was no reasonable (non-destructive) way to deflect a
    >>> fork that much.
    >>
    >>I don't think that is accurate. The consensus is that the blades themselves do not flex. The
    >>assumption of some was that the purpose of a curved fork blade was to provide some "shock
    >>absorbsion" or spring, which of course it does not.
    >>
    >>Forks do flex. At the crown.
    >
    > I think you misread my post. I'm talking about fore/aft movement of hub.

    Right.

    > My testing method doesn't identify whether that's happening at the crown, in the legs, or a
    > combination of both. I certainly wouldn't suggest that there's any compressive capability in a
    > curved fork (in fact, just wrote another post stating just that 5 minutes ago).

    Agreed. But your reference to consensus above was that they weren't supposed to do that at all. And
    it is the case that most of the motion you see is coming from the brake, which is very flexy, and
    thus wheel motion allowed by the brake -- along with the tire. Removing the wheel will cut down this
    flex considerably, but some still does occur at the crown.

    --

    David L. Johnson

    __o | Deserves death! I daresay he does. Many that live deserve _`\(,_ | death. And some that die
    deserve life. Can you give it to (_)/ (_) | them? Then do not be too eager to deal out death in
    judgement. -- J. R. R. Tolkein
     
  16. Paul Hays

    Paul Hays Guest

    On Sun, 2 Mar 2003 19:29:51 -0800, [email protected] wrote (in message
    <[email protected]>):

    > And by the way, there are evening courses for people for whom English is a second language. They
    > usually cover writing and spelling as well as sentence structure.
    >
    > Jobst Brandt [email protected] Palo Alto CA

    Do you have a supervisor that slaps you down at work, so that you need to take out your
    frustrations on some random newsgroup poster? I guess that's preferable to your snapping ala
    Michael Douglas in "Falling Down" and laying waste to the general public..
     
  17. David Reuteler wrote:
    > god, i realize this. the first thing i did was check the fork flex in the "jobst method" (read my
    > other post) and yes, there was flex at the crown. that's static, however. the shudder is around
    > 5hz or so and is probably more like 5cm total (2.5cm fore/aft) in amplitude. when you watch the
    > fork while riding it *APPEARS* to be flexing the blades (the last 20cm). that may be an optical
    > illusion. i would like to see that on film.

    Compared to the resonant frequency, 5Hz is rather low, so the static behaviour is a good
    approximation of what is going on. However, I agree that it would be an interesting thing to
    view a film.

    --
    R.

    <> Richard Brockie "Categorical statements <> The tall blond one. always cause trouble." <>
    [email protected]
     
  18. Mark Hickey

    Mark Hickey Guest

    "David L. Johnson" <David L. Johnson <[email protected]>> wrote:

    >On Mon, 03 Mar 2003 09:17:53 -0500, Mark Hickey wrote:

    >> My testing method doesn't identify whether that's happening at the crown, in the legs, or a
    >> combination of both. I certainly wouldn't suggest that there's any compressive capability in a
    >> curved fork (in fact, just wrote another post stating just that 5 minutes ago).
    >
    >Agreed. But your reference to consensus above was that they weren't supposed to do that at all.

    I recall lots of people being told that it was an optical illusion that their hub was moving back
    and forth significantly under uneven braking.

    > And it is the case that most of the motion you see is coming from the brake, which is very flexy,
    > and thus wheel motion allowed by the brake -- along with the tire. Removing the wheel will cut
    > down this flex considerably, but some still does occur at the crown.

    I considered this, but agree with Jon's analysis. If the brake was flexing, there would be some
    rotation of the front tire. If the tire were flexing, you'd see movement of the rim relative to
    the ground. Yes, there may be some extremely small amount of movement from both, but it's very,
    very small - and the rotation of the rear tire is very, very easy to see. Try it and you'll see
    what I mean.

    Mark Hickey Habanero Cycles http://www.habcycles.com Home of the $695 ti frame
     
  19. Richard Brockie <[email protected]> wrote:
    : Compared to the resonant frequency, 5Hz is rather low, so the static behaviour is a good
    : approximation of what is going on. However, I agree that it would be an interesting thing to
    : view a film.

    5hz is a pretty rough guess, obviously. when watching it shudder roughly a third the blade from the
    brake boss to the hub appears to be relatively fixed while the last two-thirds appear to flex the
    distance (2.5cm or more). it's this very difference between what i can see statically and what my
    eyes seem to be showing me when it's shuddering that makes me want to see it on film. i think you
    get that, however.

    adding a brake booster increased the frequency (not sure about the amplitude) noticeably maybe as
    much as twice. and tire pressure also (at least with wider tires, 32mm) makes a difference. high
    pressure and the fork will tend to just flex backwards (no shudder), if i lose pressure (get a slow
    leak) it'll start to shudder. it's always dependent on the power of the brake with more being worse.
    --
    david reuteler [email protected]
     
  20. Ed Ness

    Ed Ness Guest

    > >On Mon, 03 Mar 2003 09:17:53 -0500, Mark Hickey wrote:
    >
    > >> My testing method doesn't identify whether that's happening at the crown, in the legs, or a
    > >> combination of both. I certainly wouldn't suggest that there's any compressive capability in a
    > >> curved fork (in fact, just wrote another post stating just that 5 minutes ago).
    > >
    > Mark Hickey Habanero Cycles http://www.habcycles.com Home of the $695 ti frame

    My experience indicates, to me at least, that there is a fair amount of flex that occurs in the
    blades themselves. I have a couple of forks that are identical in all aspects except for the fork
    blades. Both use a Henry James fork crown and the same True Temper steerer tube. One fork uses RCX
    blades whereas the other uses thicker OXRCX blades. Of these two forks, the RCX fork is quite a bit
    more flexable. Clearly the thicker fork blades make a difference in overall fork stiffness. I think
    this is why the tube manufactuers make different models of fork blades with a varity of thickness;
    so the builder can taylor the ride. No different than building a frame.

    Ed
     
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