Carlton Reid on QR safety

Discussion in 'Mountain Bikes' started by James Annan, Feb 4, 2006.

  1. Jay Beattie

    Jay Beattie Guest

    James Annan wrote:
    > Carlton Reid has a puff piece about a new "Secure QR system" on bikebiz:
    > <http://www.bikebiz.co.uk/daily-news/article.php?id=6427>
    >
    > While promoting this new mechanism as "safer" than the existing system,
    > he also insists that "industry experts say QRs are safe, when used
    > correctly".
    >
    > This assertion is backed up with a quote from "industry expert" Bob
    > Burns (actually Trek's *lawyer*), which is nothing more than a
    > boilerplate denial dating to a few years ago when the QR/disk issue
    > first surfaced.
    >
    > Strangely, alongside this there is no space in his article for these
    > quotes from people who actually have some relevant engineering and
    > technical experience:


    Gee, James, some pretty slick editing on your part, too, being that the
    quote from Bob Burns is three years old, and the last time around, he
    was praised for being the only representative from a large manufacturer
    who was willing to look into your claims. See
    http://www.singletrackworld.com/article.php?sid=1005 . Also, there is
    nothing sinister about Mr. Burns being Trek's *laywer*. General
    counsel usually handles liability claims and knows what claims have
    (and have not) been made. Imagine that -- no *conspiracy*.

    Caveat: I am an evil defense lawyer. Oddly enough, me and my cabal of
    evil defense lawyers here in Oregon -- land of some of the best
    mountain biking in the world -- have yet to see a QR/disc brake claim.
    Not that the design cannot be improved (a question I am not competent
    to answer), but there is no plague of claims as far as I can tell. --
    Jay Beattie
     


  2. James Annan

    James Annan Guest

    Jay Beattie wrote:
    > James Annan wrote:
    > > Carlton Reid has a puff piece about a new "Secure QR system" on bikebiz:
    > > <http://www.bikebiz.co.uk/daily-news/article.php?id=6427>
    > >
    > > While promoting this new mechanism as "safer" than the existing system,
    > > he also insists that "industry experts say QRs are safe, when used
    > > correctly".
    > >
    > > This assertion is backed up with a quote from "industry expert" Bob
    > > Burns (actually Trek's *lawyer*), which is nothing more than a
    > > boilerplate denial dating to a few years ago when the QR/disk issue
    > > first surfaced.
    > >
    > > Strangely, alongside this there is no space in his article for these
    > > quotes from people who actually have some relevant engineering and
    > > technical experience:

    >
    > Gee, James, some pretty slick editing on your part,


    _MY_ part? Did you not realise that the bikebiz article was written by
    Carlton Reid, not me?


    This is a _direct_ cut and paste, completely unedited, from the middle
    of Carlton's article:

    ---
    [...] industry experts say QRs are safe, when used correctly.

    In 2003, Bob Burns, Trek's US-based General Counsel, told BikeBiz.com:
    ---

    James
     
  3. Tim McNamara

    Tim McNamara Guest

    Helen Deborah Vecht <[email protected]> writes:

    > I am neither a physicist nor engineer; many here are.
    >
    > I will leave comments to those more qualified.


    The diagrams are so simple that you can understand the principles
    involved and understand the issue, even if you can't do the math.
    Take responsibility for knowing for yourself.
     
  4. Tim McNamara

    Tim McNamara Guest

    Mike Causer <[email protected]> writes:

    > On Mon, 06 Feb 2006 01:23:22 +0000, jobst.brandt wrote:
    >
    >> I think that calculation accurately states the problem and should
    >> make apparent why this is a bad design.

    >
    > The racing car design world went through this in the 1970s when the
    > argument was that leading calipers loaded the hub bearings under
    > braking but trailing calipers unloaded the hub. So the effect is
    > there without doubt. For racing cars (and modern motorbikes) it
    > turned out that brake cooling was more important than bearing loads.


    FYI that guy you're talking to designed suspensions for Porsche. He's
    probably aware of that.

    >> Forget about GC and other calculations that muddy the straight
    >> forward relationship between downward and upward forces on the
    >> dropout in question.

    >
    > I am not questioning the direction of the load, what I _am_
    > questioning is its magnitude in relation to the other loads present.
    > To find the value of the ejection force and the value of the
    > retaining forces we need to know the geometry of the whole bike and
    > rider.


    You're muddying the waters unnecessarily. You need to know the
    orientation of the force compared to the dropout slot, and the
    difference between the distance from the axle to the braking surface
    and from the axle to the tire contact patch. The first will tell you
    whether the reaction force from braking will push the axle out of the
    dropout; the second will tell you how much the force driving the wheel
    will be multiplied. The smaller the brake disk, the higher the
    mutiplication of force- so a wheel with a 6" disk will result in a
    stronger ejection force that an 8" disk (a 26" MTB wheel being
    actually about 24" in diameter, so the difference is 4:1 for a 6" disk
    and 3:1 for an 8" disk).

    The only number you need to add is how much force is resulting from
    forward motion of the bike, and for that you only need velocity and
    mass, not the rest of the stuff you're claiming is necessary.

    >> I think caliper position is an obvious main item for discussion.

    >
    > If the caliper is forward of the axle it will not try to eject the
    > spindle, but it will obstruct cooling of the disc. If the caliper
    > is aft of the axle the cooling will improve, but it will try to
    > eject the spindle. If the cooling _is_ needed are the ejection
    > forces outweighed by the existing retention forces?


    The caliper is behind for fork leg to protect it from hitting rocks,
    bits of tree and undergrowth. The location has nothing to do with
    cooling.
     
  5. Mike Causer writes:

    >> I think that calculation accurately states the problem and should
    >> make apparent why this is a bad design.


    > The racing car design world went through this in the 1970s when the
    > argument was that leading calipers loaded the hub bearings under
    > braking but trailing calipers unloaded the hub. So the effect is
    > there without doubt. For racing cars (and modern motorbikes) it
    > turned out that brake cooling was more important than bearing loads.


    >> Forget about GC and other calculations that muddy the straight
    >> forward relationship between downward and upward forces on the
    >> dropout in question.


    > I am not questioning the direction of the load, what I _am_
    > questioning is its magnitude in relation to the other loads present.
    > To find the value of the ejection force and the value of the
    > retaining forces we need to know the geometry of the whole bike and
    > rider.


    I don't see why. All that is required is what I stated, the ratio of
    disk diameter to tire OD and the position of the caliper. The fore
    that the caliper puts on the fork relative to the wheel is as I
    stated, only caliper location is the matter at hand. You'll note this
    requires no further information about rider position or other bicycle
    dimensions.

    >> I think caliper position is an obvious main item for discussion.


    > If the caliper is forward of the axle it will not try to eject the
    > spindle, but it will obstruct cooling of the disc. If the caliper
    > is aft of the axle the cooling will improve, but it will try to
    > eject the spindle. If the cooling _is_ needed are the ejection
    > forces outweighed by the existing retention forces?


    Well its about time we got rid of the silly holes in the disk and
    realize that in most uses, there isn't much wind for cooling. It's
    not as if we are talking about road bicycle descending from one
    hairpin turn tot he next, coasting at high speed between.

    > As I have not yet seen a bicycle brake disk that looked as if any
    > effort had been made to improve its cooling (funky slots don't do a
    > lot [1]), I suppose that brake overheating is not a significant
    > factor in bike brakes, so the short-duration loading on emergency
    > stops should take precedence and the caliper would preferably be to
    > the front. If I lived in the Alps instead of the Fens I might have
    > a different view.


    > [1] Slots and holes take the "fire-band" off the pad material. To
    > get the pads that hot the disk will be glowing red. To have the
    > slots doing anything to cool the disk there needs to be an airflow
    > _though_ them, which needs air ducts or some other method of moving
    > air at right angles to the direction of travel.


    Holes in the current disks reduce surface area and do nothing positive
    for cooling. As I mentioned earlier, the holes seem to be a holdover
    from auto drum brake mystique.

    Jobst Brandt
     
  6. In article <[email protected]>,
    Helen Deborah Vecht <[email protected]> wrote:

    > Michael Press <[email protected]>typed
    >
    > > >
    > > >
    > > > The problem is not statistical.
    > > >
    > > > The problem is anecdotal.
    > > >
    > > > Many here in the UK will know of a single catastrophic event that might
    > > > have been the result of wheel ejection.
    > > >
    > > > A single catastrophe does not a statistic make but it does not mean
    > > > there is not a problem.
    > > >
    > > > That IS the problem.

    >
    > > Please comment upon the force diagram for front disc brakes.

    >
    > I am neither a physicist nor engineer; many here are.
    >
    > I will leave comments to those more qualified.


    You owe it to yourself to see for yourself. Jobst Brandt
    has already posted a clear word picture of what is going
    on. The braking force of the disk caliper on the disk
    generates a force. At the fork tips the braking force
    translates into a force on the axle in the direction out
    of the fork tips, and this force is opposed only by the
    clamping of a quick release on the fork tips.

    --
    Michael Press
     
  7. Werehatrack

    Werehatrack Guest

    On 5 Feb 2006 18:08:19 -0800, "Jay Beattie" <[email protected]>
    wrote:

    >
    >Werehatrack wrote:
    >> In some states, the impetus to settle is being reduced by
    >> defendant-friendly changes to statute, often made under the guise of
    >> "ending lawsuit abuse". Sometimes, what's billed as an abuse-control
    >> measure turns out to be a PLI-defense attorney's nightmare...because
    >> the defense lawyers don't get paid the big bucks for doing the
    >> slam-dunk early dismissal filings, they only make the big bucks when
    >> the case goes on long enough to rack up some worthwhile billable
    >> hours.

    >
    >You have been reading too many Grisham novels. Every time some tort
    >reform package is put together, it usually gets smushed like a bug. All
    >I see are statutes creating new claims for relief and not ending them,
    >particularly in the employment field.


    You're not in Texas. Here, the trend is decidedly in the other
    direction, with the effect that it's often difficult to get an
    attorney to take a liability case on contingency anymore. The most
    significant effect is that even a middle-income plaintiff may not be
    able to afford to bring an action for recovery of damages against a
    company.

    Of course, Texas also has the most selectively gullible electorate in
    the nation IMO.
    --
    Typoes are a feature, not a bug.
    Some gardening required to reply via email.
    Words processed in a facility that contains nuts.
     
  8. Tim McNamara writes:

    >>> I think that calculation accurately states the problem and should
    >>> make apparent why this is a bad design.


    >> The racing car design world went through this in the 1970s when the
    >> argument was that leading calipers loaded the hub bearings under
    >> braking but trailing calipers unloaded the hub. So the effect is
    >> there without doubt. For racing cars (and modern motorbikes) it
    >> turned out that brake cooling was more important than bearing
    >> loads.


    > FYI that guy you're talking to designed suspensions for Porsche.
    > He's probably aware of that.


    Yes but that was not an issue. Porsche, in an effort to circumvent
    patents by Girling and Dunlop, designed a peripherally supported disk
    with an inside grasping caliper. This was soon dumped and work
    continued with ATE-Dunlop.

    >>> Forget about GC and other calculations that muddy the straight
    >>> forward relationship between downward and upward forces on the
    >>> dropout in question.


    >> I am not questioning the direction of the load, what I _am_
    >> questioning is its magnitude in relation to the other loads
    >> present. To find the value of the ejection force and the value of
    >> the retaining forces we need to know the geometry of the whole bike
    >> and rider.


    > You're muddying the waters unnecessarily. You need to know the
    > orientation of the force compared to the dropout slot, and the
    > difference between the distance from the axle to the braking surface
    > and from the axle to the tire contact patch. The first will tell
    > you whether the reaction force from braking will push the axle out
    > of the dropout; the second will tell you how much the force driving
    > the wheel will be multiplied. The smaller the brake disk, the
    > higher the multiplication of force- so a wheel with a 6" disk will
    > result in a stronger ejection force that an 8" disk (a 26" MTB wheel
    > being actually about 24" in diameter, so the difference is 4:1 for a
    > 6" disk and 3:1 for an 8" disk).


    Dropout slot orientation is a secondary consideration, entirely
    missing the first problem which is that the disk brake causes
    reversing (up and down loads) on the axle. If you use a motorcycle
    type clamped axle, as some BMX bicycles do, then the problem goes away
    entirely.

    > The only number you need to add is how much force is resulting from
    > forward motion of the bike, and for that you only need velocity and
    > mass, not the rest of the stuff you're claiming is necessary.


    No you don't. You only need to know the parameters I gave. If the
    wheel skids, you have the force.

    >>> I think caliper position is an obvious main item for discussion.


    >> If the caliper is forward of the axle it will not try to eject the
    >> spindle, but it will obstruct cooling of the disc. If the caliper
    >> is aft of the axle the cooling will improve, but it will try to
    >> eject the spindle. If the cooling _is_ needed are the ejection
    >> forces outweighed by the existing retention forces?


    > The caliper is behind for fork leg to protect it from hitting rocks,
    > bits of tree and undergrowth. The location has nothing to do with
    > cooling.


    That's an excuse. You don't need to believe these convenient excuses.

    Jobst Brandt
     
  9. [email protected] wrote:
    > Mike Causer writes:


    >>>Forget about GC and other calculations that muddy the straight
    >>>forward relationship between downward and upward forces on the
    >>>dropout in question.

    >
    >
    >>I am not questioning the direction of the load, what I _am_
    >>questioning is its magnitude in relation to the other loads present.
    >>To find the value of the ejection force and the value of the
    >>retaining forces we need to know the geometry of the whole bike and
    >>rider.

    >
    >
    > I don't see why. All that is required is what I stated, the ratio of
    > disk diameter to tire OD and the position of the caliper. The fore
    > that the caliper puts on the fork relative to the wheel is as I
    > stated, only caliper location is the matter at hand. You'll note this
    > requires no further information about rider position or other bicycle
    > dimensions.


    Actually, if you're going to investigate the forces that could be
    responsible for a wheel ejection, you need the more detailed level of
    information that was indicated: that's how you isolate a force that
    could cause an ejection and see if it has sufficient force acting in the
    proper direction to yield the result in question...at least that's how
    we looked at resolving forces in all my engineering classes and in my
    professional practice as an engineer.

    Now, have there been a bunch of non-walmart related incidents that I
    haven't seen posted on the bicycle usnet groups, or is this really
    related to all the wallyworld incidents? From what I've seen and heard,
    most incidents have been related to uninformed people getting less than
    ideal components and no user information (i.e. manuals) from a large
    chain store (causing user error) rather than the one-on-one information
    and support that goes with quality components and a "real" bike shop
    (possibly indicative of a design flaw).

    Myself, I've been running disc brakes for a fair bit of time now and
    have had absolutely no issues with QR's becoming loose, wheels becoming
    misaligned or wheels dropping out of the forks.

    >
    > Holes in the current disks reduce surface area and do nothing positive
    > for cooling. As I mentioned earlier, the holes seem to be a holdover
    > from auto drum brake mystique.
    >
    > Jobst Brandt


    Actually, vented rotors can break up the boundry air flow along the
    surface of the disc, causing an improvement in the turbulent airflow
    near the disc surface and better air movement around the disc (and
    better cooling as a result). They also allow any mud that may get
    caught in the caliper area a chance to escape that isn't present in a
    solid disc, improving the wear of the braking system. If you're in doubt
    about the airflow, it is essentially the same reason that golfballs
    aren't smooth.

    Psycho Mike
     
  10. James Annan

    James Annan Guest

    Werehatrack wrote:

    > It is very hard to convince people that a problem is both real *and
    > serious* when you don't have anything but math and a few isolated
    > phenomena to offer as evidence. That the problem is real they may
    > accept if they are math-literate, but since nearly everything has risk
    > of one sort or another, it's also necessary to convince them that the
    > problem is serious enough (not just in terms of potential harm should
    > it occur, but more specifically in terms of the potential for the harm
    > to come *to them* at all) before they will be persuaded that action is
    > warranted or necessary. The paucity of demonstrated failures speaks
    > volumes to the masses.


    Do you think that Shimano were wrong to recall their brake cables?

    http://www.bikebiz.com/daily-news/article.php?id=4933

    ----
    A statement from Shimano said:

    "It is possible that the tensile strength of the joint between the
    cable and the cable end (nipple) may not meet Shimano's usual standards
    and that therefore the nipple, when under stress during application of
    the brake, could pull loose or detach from the cable. This could lead
    to brake failure.

    "Shimano is not aware of any case in which the nipple has separated
    from one of these cables during use on a bicycle."
    ----

    Note that not only was there not a single injury as a result of this
    fault, there wasn't even a single failure in use. Numerous recalls are
    made on a similar basis - this was just the first I googled. I question
    whether you are aware of the relevant laws on the matter.

    James
     
  11. Clive George

    Clive George Guest

    "Michael Press" <[email protected]> wrote in message
    news:[email protected]

    >> > Please comment upon the force diagram for front disc brakes.

    >>
    >> I am neither a physicist nor engineer; many here are.
    >>
    >> I will leave comments to those more qualified.

    >
    > You owe it to yourself to see for yourself.


    Micheal, you seem to be confusing comment with understanding. Understanding
    the force diagram is easy enough. Being able to comment on it in a useful
    manner, especially without repeating what has already been said, is entirely
    different.

    Or do you feel "me too" is an acceptably useful post?

    clive
     
  12. Simon Brooke

    Simon Brooke Guest

    in message <[email protected]>,
    [email protected] ('[email protected]')
    wrote:

    > Dropout slot orientation is a secondary consideration, entirely
    > missing the first problem which is that the disk brake causes
    > reversing (up and down loads) on the axle. If you use a motorcycle
    > type clamped axle, as some BMX bicycles do, then the problem goes away
    > entirely.


    FWIW downhillers now usually use 'through axle' designs as well, as do
    seom 'freeride' bikes. The Cannondale 'lefty' and USE 'SUB' monoblade
    hubs are also immune to the ejection problem.

    --
    [email protected] (Simon Brooke) http://www.jasmine.org.uk/~simon/

    ;; not so much a refugee from reality, more a bogus
    ;; asylum seeker
     
  13. Mike Causer

    Mike Causer Guest

    On Mon, 06 Feb 2006 04:12:58 +0000, Michael Press wrote:

    > You owe it to yourself to see for yourself. Jobst Brandt has already
    > posted a clear word picture of what is going on. The braking force of the
    > disk caliper on the disk generates a force. At the fork tips the braking
    > force translates into a force on the axle in the direction out of the fork
    > tips,


    This is correct if the caliper is behind the fork and the fork slots
    are vertical.


    > and this force is opposed only by the clamping of a quick release
    > on the fork tips.


    Incorrect. There is the weight of the bike and rider as well.



    Mike
     
  14. David Martin

    David Martin Guest

    Mike Causer wrote:
    > On Mon, 06 Feb 2006 04:12:58 +0000, Michael Press wrote:
    >
    > > You owe it to yourself to see for yourself. Jobst Brandt has already
    > > posted a clear word picture of what is going on. The braking force of the
    > > disk caliper on the disk generates a force. At the fork tips the braking
    > > force translates into a force on the axle in the direction out of the fork
    > > tips,

    >
    > This is correct if the caliper is behind the fork and the fork slots
    > are vertical.
    >
    >
    > > and this force is opposed only by the clamping of a quick release
    > > on the fork tips.

    >
    > Incorrect. There is the weight of the bike and rider as well.


    Unless the point of contact is behind the caliper (eg the bike is
    rolling off a rock) in which case the weight of the rider + bike will
    add to the ejection force rather than reduce it.

    Given that the few reports we have of incidents are from people
    descending rocky terrain at speed, the simplistic testing that has been
    done, doesn't appear to be particularly realistic.

    ...d
     
  15. Mike Causer

    Mike Causer Guest

    On Mon, 06 Feb 2006 04:21:25 +0000, jobst.brandt wrote:


    > Yes but that was not an issue. Porsche, in an effort to circumvent
    > patents by Girling and Dunlop, designed a peripherally supported disk with
    > an inside grasping caliper. This was soon dumped and work continued with
    > ATE-Dunlop.


    IIRC some aircraft brakes use this layout. Possibly because they can get
    the maximum disk diameter for overall package size. You and I were in
    competition then, because I used to design brakes at Girling in the early
    1970s.



    Mike
     
  16. Mike Causer

    Mike Causer Guest

    On Mon, 06 Feb 2006 04:05:23 +0000, jobst.brandt wrote:

    >> I am not questioning the direction of the load, what I _am_ questioning
    >> is its magnitude in relation to the other loads present. To find the
    >> value of the ejection force and the value of the retaining forces we
    >> need to know the geometry of the whole bike and rider.

    >
    > I don't see why. All that is required is what I stated, the ratio of disk
    > diameter to tire OD and the position of the caliper. The fore that the
    > caliper puts on the fork relative to the wheel is as I stated, only
    > caliper location is the matter at hand.


    Except that there is a maximum force that be generated in this way, and
    to find the maximum we need to consider the factors I've mentioned.

    Assuming a conventional upright bike, with wheelbase a little over 1
    metre, the maximum braking effort is found when the back wheel lifts, at
    which point the retardation will be about 0.65g. The friction coefficient
    tyre-ground needs to be 0.65 or better. Higher friction won't gain any
    more retardation. At this point the whole of the weight of bike and rider
    is carried by the front axle,

    The worst case of caliper location is to have it on the horizontal axis
    of the spindle, and the rejection force is going to be the Gee times the
    rider+bike weight times the ratio of tyre diameter to effective disk
    diameter. I do not have a disk-braked bike to measure up, so I'll take
    Tim McNamara's assertion that a disk could be 6" or 8" -- although the
    26" tyres I've just measured were 25" (slick) and 26" (knobbly) in
    diameter. This gives a ratio of 4.3:1 using the worst case of smallest
    disk and largest wheel.

    So the rejection force is going to be:
    0.65 * W * 4.3 = 2.8 * W

    Opposing this we have 1.0 * W the full weight of rider + bicycle,
    because at maximum retardation the back wheel is off the ground.

    Therefore the spindle clamping forces actually need to handle 1.8 * W,
    a reduction of over one third on the calculation that didn't take the
    weight factor into account.


    Mike
     
  17. David Martin

    David Martin Guest

    Mike Causer wrote:
    > On Mon, 06 Feb 2006 04:05:23 +0000, jobst.brandt wrote:
    >
    > >> I am not questioning the direction of the load, what I _am_ questioning
    > >> is its magnitude in relation to the other loads present. To find the
    > >> value of the ejection force and the value of the retaining forces we
    > >> need to know the geometry of the whole bike and rider.

    > >
    > > I don't see why. All that is required is what I stated, the ratio of disk
    > > diameter to tire OD and the position of the caliper. The fore that the
    > > caliper puts on the fork relative to the wheel is as I stated, only
    > > caliper location is the matter at hand.

    >
    > Except that there is a maximum force that be generated in this way, and
    > to find the maximum we need to consider the factors I've mentioned.
    >
    > Assuming a conventional upright bike, with wheelbase a little over 1
    > metre, the maximum braking effort is found when the back wheel lifts, at
    > which point the retardation will be about 0.65g. The friction coefficient
    > tyre-ground needs to be 0.65 or better. Higher friction won't gain any
    > more retardation. At this point the whole of the weight of bike and rider
    > is carried by the front axle,
    >
    > The worst case of caliper location is to have it on the horizontal axis
    > of the spindle, and the rejection force is going to be the Gee times the
    > rider+bike weight times the ratio of tyre diameter to effective disk
    > diameter. I do not have a disk-braked bike to measure up, so I'll take
    > Tim McNamara's assertion that a disk could be 6" or 8" -- although the
    > 26" tyres I've just measured were 25" (slick) and 26" (knobbly) in
    > diameter. This gives a ratio of 4.3:1 using the worst case of smallest
    > disk and largest wheel.
    >
    > So the rejection force is going to be:
    > 0.65 * W * 4.3 = 2.8 * W
    >
    > Opposing this we have 1.0 * W the full weight of rider + bicycle,
    > because at maximum retardation the back wheel is off the ground.
    >
    > Therefore the spindle clamping forces actually need to handle 1.8 * W,
    > a reduction of over one third on the calculation that didn't take the
    > weight factor into account.


    Except that it is perfectly possible to set up a *stationary* bicycle
    such that increases in rider weight will increase the ejection force.

    ASCII art is difficult to represent this with so here is a set of rough
    coordinates. We assume the rear tyre is locked. Makes no differnece
    either way.

    0,0 is the contact patch of rear tyre on ground. (10,3) is the front
    hub. (9,4) is the brake.
    The key thing now is the contact patch. If it is flat level ground then
    the contact patch for the front tyre will be at about (10.1, 0) , ie in
    front of the front hub. As long as the contact patch is in front of the
    brake, the force from the rider will act through the brake mounting
    against an ejection force. As soon as the contact patch moves behind
    the brake, the riders weight adds to the ejection force.

    So your simplistic calculation is only correct for a normal, stable
    bicycle where the contact patch is as intended on a flat road. On rough
    terrain the forces can be vastly different and that is before starting
    to consider peak loading. To take this to the extreme, heading down a
    rocky descent the front wheel becomes unweighted but the brakes are on
    and binding the wheel. The wheel comes back in contact with the ground
    but it is the back edge of the wheel which strikes first, providing a
    peak load amplified by the riders weight which pivots around the brake.
    The force able to be applied has a mechanical advantage of 3-4 times
    (ratio of the distance of contact patch to pivot vs pivot to hub.

    ...d
     
  18. Tony Raven

    Tony Raven Guest

    Mike Causer wrote:
    > On Mon, 06 Feb 2006 04:12:58 +0000, Michael Press wrote:
    >
    >> You owe it to yourself to see for yourself. Jobst Brandt has already
    >> posted a clear word picture of what is going on. The braking force of the
    >> disk caliper on the disk generates a force. At the fork tips the braking
    >> force translates into a force on the axle in the direction out of the fork
    >> tips,

    >
    > This is correct if the caliper is behind the fork and the fork slots
    > are vertical.
    >


    Wrong. Provided the caliper is not mounted in line with the centre line
    of the fork slot there will be a component of the force along the centre
    line of the slot. If the caliper is behind the centre line the force
    component will be out of the slot, in front and its into the slot.

    No one has yet commented on how the QR gets over the lawyers lips
    without anyone noticing how loose the wheel has become in the forks and
    the disc rubbing on the pads as the wheel flops from side to side.

    --
    Tony

    "Man is a credulous animal, and must believe something; in the absence
    of good grounds for belief, he will be satisfied with bad ones."
    - Bertrand Russell
     
  19. Mike Causer

    Mike Causer Guest

    On Mon, 06 Feb 2006 05:53:44 -0800, David Martin wrote:

    > Except that it is perfectly possible to set up a *stationary* bicycle such
    > that increases in rider weight will increase the ejection force.


    Sure, make the dropout point anywhere _above_ the horizontal.


    > As soon as the contact patch moves behind the
    > brake, the riders weight adds to the ejection force.


    As the contact patch moves either forward or backward from directly under
    the spindle the anti-ejection force due to weight of rider + bike will
    diminish until it becomes zero when the contact patch is horizontal with
    the spindle. As this force diminishes so does the braking power
    available, until it also becomes zero when contact patch is horizontal.



    Mike
     
  20. Quoting Andy H <[email protected]>:
    >Life sucks and shit happens, our (my anyway) pastime is rife with risk.


    That doesn't really create a desire for extra risk without any extra fun.

    >If
    >the design is inherently flawed why have we not all been maimed by our
    >disk/qr problems?


    Hyperbole. A design can be unsafe relative to other designs without the
    failure rate being such as to injure every user.
    --
    David Damerell <[email protected]> Kill the tomato!
    Today is Olethros, February - a weekend.
     
Loading...
Loading...