Mount front brakes on rear?

On Wed, 07 Nov 2007 21:49:42 -0600, Tim McNamara wrote:

_ wrote:
On Wed, 07 Nov 2007 08:27:51 -0800, wrote:


observation of the dual pivot lever action! and even then, the
ones i have stated. it may indeed be the case that some brakes
don't have this feature, but as i have observed, campy veloce and
record, and 7700 dura-ace, these definitely do!

As long as they remain fixed where? Fix them in a different place
and the deltas disappear or reverse. This is not irrelevant. It
shows that while your observations of pad angle are necessary
conditions to prove your hypothesis of arm twisting, they are not
sufficient. You have not posted any observations of lever action.
Only pad movement, and only movement in directions that point to
more realistic explanations.

And further, since the arms do not twist, there is no support for
this being a reason for not interchanging front and rear brakes ("jim
beam"'s original assertion).

Were it not "jim beam" on the other side of the argument, it would be
amazing that someone could continue to insist that an arm pivoting
on a single bushing can have two degrees of freedom...

It depends on how much slop is in the bushing. The primary motion of
rotating on the pivot would be large, but there are at least two other
potential movements of the brake arm on the pivot: sliding along the
axis of the pivot, and rocking on the pivot. The latter two would be
driven by the rim pulling on the brake pad. These two movements would
be very small, however, and unlikely to be of significant consequence.
Might contribute to brake squeal, though. These motions are especially
noticeable on brakes that mount on cantilever pivots.

"Slop in the pivots" aka worn bushings was indeed suggested as a reason for
"jim beam" seeing two degrees of freedom where geometry will allow but one;
one can but admire the charitable nature of such a poster.

_ wrote:
On Wed, 07 Nov 2007 21:49:42 -0600, Tim McNamara wrote:

_ wrote:
On Wed, 07 Nov 2007 08:27:51 -0800, wrote:


observation of the dual pivot lever action! and even then, the
ones i have stated. it may indeed be the case that some brakes
don't have this feature, but as i have observed, campy veloce and
record, and 7700 dura-ace, these definitely do!

As long as they remain fixed where? Fix them in a different place
and the deltas disappear or reverse. This is not irrelevant. It
shows that while your observations of pad angle are necessary
conditions to prove your hypothesis of arm twisting, they are not
sufficient. You have not posted any observations of lever action.
Only pad movement, and only movement in directions that point to
more realistic explanations.
And further, since the arms do not twist, there is no support for
this being a reason for not interchanging front and rear brakes ("jim
beam"'s original assertion).

Were it not "jim beam" on the other side of the argument, it would be
amazing that someone could continue to insist that an arm pivoting
on a single bushing can have two degrees of freedom...
It depends on how much slop is in the bushing. The primary motion of
rotating on the pivot would be large, but there are at least two other
potential movements of the brake arm on the pivot: sliding along the
axis of the pivot, and rocking on the pivot. The latter two would be
driven by the rim pulling on the brake pad. These two movements would
be very small, however, and unlikely to be of significant consequence.
Might contribute to brake squeal, though. These motions are especially
noticeable on brakes that mount on cantilever pivots.

"Slop in the pivots" aka worn bushings was indeed suggested as a reason

brand new dura-ace. no slop.


for
"jim beam" seeing two degrees of freedom where geometry will allow but one;
one can but admire the charitable nature of such a poster.

found that geometry teacher yet?

On Nov 7, 11:26 pm, jim beam wrote:
wrote:
On Nov 6, 11:32 pm, jim beam wrote:
wrote:
On Nov 4, 12:23 pm, jim beam wrote:
wrote:
On Nov 3, 7:27 pm, jim beam wrote:
wrote:
On Nov 2, 9:18 pm, jim beam wrote:
wrote:
On Nov 2, 12:53 am, jim beam wrote:
_ wrote:
On Thu, 1 Nov 2007 14:08:07 +0100, James Thomson wrote:
What should I be measuring?
"jim beam" a ?crit:
distance between the front tips of the brake pads with the caliper
open vs. caliper closed, and for the rear tips, open vs closed. when
open, the front tips are further apart than the rears. when closed,
the front tips are closer than the rears. [front caliper]
I'm using the moulding seam of the brake block at the point it touches the
holder as a reference point on the Ultegra 6500 brakes, and (in the absence
of a convenient moulding mark) trying to pick a consistent point on the
Centaur pad holder. The measurements are repeatable to within about 1mm, and
I can't detect any sign of the effect you say is there.
James Thomson
Either beam's bushings are worn or the arms are loose on the pivots - that
was already pointed out.
no, these are new calipers - in perfect condition.
It's impossible for an arm to pivot on a
cylindrical bushing and change the axis of rotation without another pivot
(which is, in essemce, what beamboy is claiming).
no it's not. it's a simple geometry problem. you not figuring it out
doesn't mean it's impossible - after all, it is observed to be happening.
Yes, it's a very simple geometry problem. Far simpler than your
explanation of magic pivots describes. The brake pads are adjustable
in a number of directions, one of which is rotation about the pad
fixing bolt.
duh. and when that bolt is tightened, they remain fixed. from that
point on, it doesn't matter /what/ you do, the /delta/ measurements i
did, remain the same.
Look at the brake from the side with the mounting bolt
horizontal, and if the brake is set up to go on the rear, the rear
ends of the pads are going to be sitting lower than the front ends.
Assuming everything is set up square and parallel when the pads are
about a rim width apart, this will will give the appearance of the
front ends of the pads pointing inwards when you open the caliper all
the way. The rear ends of the pads moved farther out horizontally
because they are father from the pivot axes. Close the caliper all
the way and the front ends of the pads will be pointed out, because
they traveled a shorter horizontal distance for the same angle. Re-
adjust the pads so that the front ends are sitting lower than the rear
when viewed from the side, and you get the exact opposite behavior.
Adjust them level, and you're back to boring old parallel motion,
which is what everyone who actually wasted their time obeying your
command to measure their brakes observed. This is why you need to
prove your magic pivot theory by measuring from the arms not the
pads. The pads may be fixed, but they are not always fixed in a
position that supports your theory. If the second pivot really is
angled, there will also be some fore-aft translation of the arm
attached to it. Another reason why common sense dictates that this
would be an incredibly stupid way to design a brake.
says the guy that doesn't understand the concepts.
And what concepts would those be? That the distance traveled by a
point on a rigid rotating body does not depend on that point's
distance from the axis of rotation? Because that's what you're trying
to claim, and you're exactly right that I don't understand it. You
would be hard pressed to find anyone with a passing knowledge of
geometry who would buy that one. If you change the relative distances
of the pad ends from the pivot axes (in whichever direction you
believe them to be pointing) the delta measurements you performed will
change. Front or rear, and I'm looking at it right now on a single
pivot caliper.
eh? you're basing your entire comment on single pivot???? holy carp.
The ends of the pads farthest from the pivot will
always be farther apart when the caliper is open, and closer together
when it's closed. How do you suppose it is that I can observe
something that you say absolutely has to be the result of one of a two
degree of freedom mechanism on a brake with only one degree of freedom?
forget it. if you can't be bothered, nor can i.
No, I'm basing it on single pivot, dual pivot, and common sense.
no, you're basing it on single pivot and underinformed presumption.
And
if your observation of changing brake pad distances can be observed in
a single pivot brake, then it absolutely cannot be proof of out of
plane rotation.
rubbish.
Please tell me what it is that I'm presuming. I've duplicated your
progressive toe in measurements on both single and dual pivot brakes,
and I've changed it to a progressive toe out simply by changing the
pad adjustment. If either of those calipers has an arm that's
rotating out of plane, brake pad measurements simply are not
sufficient proof. Seeing how you're the only person on the planet
that believes that this rotation is happening, you're going to have to
do a lot more than stomp your feet and yell rubbish to convince me.
i'm not trying to "convince" you - i simply want you to stop confusing
yourself with irrelevancies about single pivot and pad adjustments [as
long as they remain fixed, the deltas remain the same] and focus on
observation of the dual pivot lever action! and even then, the ones i
have stated. it may indeed be the case that some brakes don't have this
feature, but as i have observed, campy veloce and record, and 7700
dura-ace, these definitely do!

As long as they remain fixed where? Fix them in a different place and
the deltas disappear or reverse. This is not irrelevant. It shows
that while your observations of pad angle are necessary conditions to
prove your hypothesis of arm twisting, they are not sufficient. You
have not posted any observations of lever action. Only pad movement,
and only movement in directions that point to more realistic
explanations.

read the thread again. i explained how it works back on 11/1 in a reply
to "_".

You didn't explain how it works. You explained how it could work, but
your observations are not sufficient to support your explanation.
Your observation of changing relative brake pad distances may be
explained by either the orientation of the brake pads (which you
refuse to accept) or an out of plane orientation of the second pivot
(which the rest of us refuse to accept). The tie breaker is fore-aft
pad translation. One arm of the dual pivot caliper cannot twist.
It's rotating about the mounting bolt, with one degree of freedom. If
there is something special about that second pivot, which
differentiates the caliper as front or rear by making the arm twist
one way or another, then the pad attached to it will be moving fore
and aft relative to the other pad as the caliper opens and closes.
Since you have such a great geometry teacher, this should be obvious.
This is not happening though, and nobody has claimed to have observed
it happening, including yourself.

wrote:

snip much verbiage

On Nov 7, 11:26 pm, jim beam wrote:

read the thread again. i explained how it works back on 11/1 in a
reply to "_".

You didn't explain how it works. You explained how it could work,
but your observations are not sufficient to support your explanation.
Your observation of changing relative brake pad distances may be
explained by either the orientation of the brake pads (which you
refuse to accept) or an out of plane orientation of the second pivot
(which the rest of us refuse to accept). The tie breaker is fore-aft
pad translation. One arm of the dual pivot caliper cannot twist.

If that was the case, then brake squeal would not happen. The caliper
arm can flex in at least two ways; the longer the distance from the
pivot to the brake pad, the greater this is an issue.

It's rotating about the mounting bolt, with one degree of freedom.

That's assuming a perfect bearing, which doesn't exist. There are at
least three degrees of freedom there. One is large, the other two are
(or should be) very small.

If there is something special about that second pivot, which
differentiates the caliper as front or rear by making the arm twist
one way or another, then the pad attached to it will be moving fore
and aft relative to the other pad as the caliper opens and closes.
Since you have such a great geometry teacher, this should be obvious.
This is not happening though, and nobody has claimed to have
observed it happening, including yourself.

Grab a brake lever and squeeze tight. Now rock the bike back and forth.
Do the brake pads move relative to the fork blades or seatstays? On
my bikes I can detect a very small fore and aft movement by placing my
finger between the brake and the frame. There is no visual movement of
the pads vis a vis the frame on my bikes, except on one with
cantilever brakes- there's much more play in the pivots as well as twice
as many pivots. On all my bikes the movement is more pronounced for the
front brake, which is probably due to fork flex. Note that the forces
in this little experiment are much smaller than actual braking forces
when riding. In real life, my guess (and it is a guess) is that frame
flex would account for more undesirable movement of the brake pads than
odd motions in the pivot.

Strain gauges on the brake caliper arms might answer the question.
Anybody here got some? Maybe a project for Fogel Labs!

On Nov 9, 10:10 am, Tim McNamara wrote:
wrote:

snip much verbiage

On Nov 7, 11:26 pm, jim beam wrote:

read the thread again. i explained how it works back on 11/1 in a
reply to "_".

You didn't explain how it works. You explained how it could work,
but your observations are not sufficient to support your explanation.
Your observation of changing relative brake pad distances may be
explained by either the orientation of the brake pads (which you
refuse to accept) or an out of plane orientation of the second pivot
(which the rest of us refuse to accept). The tie breaker is fore-aft
pad translation. One arm of the dual pivot caliper cannot twist.

If that was the case, then brake squeal would not happen. The caliper
arm can flex in at least two ways; the longer the distance from the
pivot to the brake pad, the greater this is an issue.

It's rotating about the mounting bolt, with one degree of freedom.

That's assuming a perfect bearing, which doesn't exist. There are at
least three degrees of freedom there. One is large, the other two are
(or should be) very small.

If there is something special about that second pivot, which
differentiates the caliper as front or rear by making the arm twist
one way or another, then the pad attached to it will be moving fore
and aft relative to the other pad as the caliper opens and closes.
Since you have such a great geometry teacher, this should be obvious.
This is not happening though, and nobody has claimed to have
observed it happening, including yourself.

Grab a brake lever and squeeze tight. Now rock the bike back and forth.
Do the brake pads move relative to the fork blades or seatstays? On
my bikes I can detect a very small fore and aft movement by placing my
finger between the brake and the frame. There is no visual movement of
the pads vis a vis the frame on my bikes, except on one with
cantilever brakes- there's much more play in the pivots as well as twice
as many pivots. On all my bikes the movement is more pronounced for the
front brake, which is probably due to fork flex. Note that the forces
in this little experiment are much smaller than actual braking forces
when riding. In real life, my guess (and it is a guess) is that frame
flex would account for more undesirable movement of the brake pads than
odd motions in the pivot.

Strain gauges on the brake caliper arms might answer the question.
Anybody here got some? Maybe a project for Fogel Labs!

All very good points, but the ways in which a caliper deflects under
load aren't the issue here. There's a claim here that there's an arm
twisting under no load that's a result of a design feature. There
has been no credible evidence to support this, only optical tricks.

wrote:
On Nov 9, 10:10 am, Tim McNamara wrote:
wrote:

snip much verbiage

On Nov 7, 11:26 pm, jim beam wrote:
read the thread again. i explained how it works back on 11/1 in a
reply to "_".
You didn't explain how it works. You explained how it could work,
but your observations are not sufficient to support your explanation.
Your observation of changing relative brake pad distances may be
explained by either the orientation of the brake pads (which you
refuse to accept) or an out of plane orientation of the second pivot
(which the rest of us refuse to accept). The tie breaker is fore-aft
pad translation. One arm of the dual pivot caliper cannot twist.
If that was the case, then brake squeal would not happen. The caliper
arm can flex in at least two ways; the longer the distance from the
pivot to the brake pad, the greater this is an issue.

It's rotating about the mounting bolt, with one degree of freedom.
That's assuming a perfect bearing, which doesn't exist. There are at
least three degrees of freedom there. One is large, the other two are
(or should be) very small.

If there is something special about that second pivot, which
differentiates the caliper as front or rear by making the arm twist
one way or another, then the pad attached to it will be moving fore
and aft relative to the other pad as the caliper opens and closes.
Since you have such a great geometry teacher, this should be obvious.
This is not happening though, and nobody has claimed to have
observed it happening, including yourself.
Grab a brake lever and squeeze tight. Now rock the bike back and forth.
Do the brake pads move relative to the fork blades or seatstays? On
my bikes I can detect a very small fore and aft movement by placing my
finger between the brake and the frame. There is no visual movement of
the pads vis a vis the frame on my bikes, except on one with
cantilever brakes- there's much more play in the pivots as well as twice
as many pivots. On all my bikes the movement is more pronounced for the
front brake, which is probably due to fork flex. Note that the forces
in this little experiment are much smaller than actual braking forces
when riding. In real life, my guess (and it is a guess) is that frame
flex would account for more undesirable movement of the brake pads than
odd motions in the pivot.

Strain gauges on the brake caliper arms might answer the question.
Anybody here got some? Maybe a project for Fogel Labs!

All very good points, but the ways in which a caliper deflects under
load aren't the issue here. There's a claim here that there's an arm
twisting under no load that's a result of a design feature. There
has been no credible evidence to support this, only optical tricks.


er, you said:

"If there is something special about that second pivot, which
differentiates the caliper as front or rear by making the arm twist one
way or another, then the pad attached to it will be moving fore and aft
relative to the other pad as the caliper opens and closes."

and my pics just so happen to show precisely that. but now you're
back-tracking saying it's some kind of deception? that's ridiculous.