Mount front brakes on rear?

In article
. com,
wrote:

2) You absolutely cannot presume from your measurements of pad
spacing that the brake arms are twisting. This assumes that the front
and rear edges are exactly the same distance from the pivot axis. In
practice, this is going to vary depending on things like fork rake,
rear dropout design, and how well the brakes are set up. It will in
fact vary from front to rear, but not because of any difference in the
calipers.

You seem to be talking about fork offset here.
When the fork rake is varied the fork tip and
fork crown move together; and therefore the
brake pad position wrt the wheel rim remains
invariant.

--
Michael Press

wrote:
2) You absolutely cannot presume from your measurements of pad
spacing that the brake arms are twisting. This assumes that the front
and rear edges are exactly the same distance from the pivot axis. In
practice, this is going to vary depending on things like fork rake,
rear dropout design, and how well the brakes are set up. It will in
fact vary from front to rear, but not because of any difference in the
calipers.

Michael Press wrote:
You seem to be talking about fork offset here.
When the fork rake is varied the fork tip and
fork crown move together; and therefore the
brake pad position wrt the wheel rim remains
invariant.

I assumed he meant the angle of the pad to the arm
--
Andrew Muzi
www.yellowjersey.org
Open every day since 1 April, 1971

On Nov 1, 6:44 am, "James Thomson" wrote:
"jim beam" a écrit:

wrote:
Also, one arm of the dual pivot brake pivots on the
brake bolt and is surely tracking straight, not canted.
indeed, but the other pivot on the short arm isn't. and as it
swings through its arc, it tilts the axis angle for the "c" arm.

There's no mechanism to allow one arm to move the axis of the other. One
pivots about the brake bolt, and the other pivots about an axis that's fixed
with respect to the brake bolt, and parallel to the first.

This horse surely doesn't need to be beaten any more, but
I'll just point out that there are three pieces in a dual pivot
caliper - the two arms and a short link piece - and three
pivot joints. If you think about it for a little, it's very difficult
to see how it could work to have the axes of the brake
arm pivots canted with respect to each other, as the
short link would somehow have to bend or flex.

Ben

_ wrote:
On Thu, 01 Nov 2007 08:11:10 -0700, wrote:

On Nov 1, 5:19 am, jim beam wrote:
A Muzi wrote:
wrote:
Nashbar has a good deal on ultegra brake calipers; I mounted
them on
my front wheel. My rear caliper is kinda corroded, and I was
considering mounting another front caliper on the rear wheel... Is
this a good idea?
jim beam wrote:
as pointed out by others, the bolt length thing is not a big deal.
however, you need to be aware that shimano & campy dual pivot brake
calipers have a pivot action that increases toe on the pad as the
caliper closes. [easily observed for yourself if you have the
caliper
in your hand - simply squeeze and note how the relative angles of
the
pads change as the lever arms move through their arc.] if a front
caliper is used on the rear, the toe is towards the rear of the
pads,
not the front, relative to rim direction.
Gary Young wrote:
According to Shimano spare parts lists
(http://tinyurl.com/2kecg9), the
front and rear brakes are identical except for a bolt, a nut and a
washer
or spacer.
jim beam wrote:
no, that's the parts list for the nuts and bolts, not the caliper
arms.
Gary Young wrote:
How is this magical effect achieved?
jim beam wrote:
"magical"??? how about you ask an objective question rather than a
sarcastic one that presupposes bull**** instead? you could also take
the time to look for the shimano mounting instructions which do indeed
advise against switching front/rear calipers.
the answer is pivot angle. buy a campy or shimano dual pivot caliper
and observe for yourself.
will you notice any effect in practice? maybe some brake squeal,
but
probably not. is it correct in theory? no. and iirc, there's a
warning in the shimano literature to the effect that each caliper is
specific to front or rear application and should not be
interchanged.
wrote:
I took a look at the instructins,
Not to put too fine a point on it, but the instructions actually say
"Brakes designed for use as rear brakes should not be used as front
brakes." Oddly, they do not contain a similar prohibition against
using front brakes as rear brakes...
A Muzi wrote:
I believe that's merely because your average human would forget to
reverse the brake shoes.
jim beam wrote:
andrew, you have stock of dual pivot brakes, and measuring calipers.
please perform the measurements i requested. i can do it, but i have
to strip a bike and that's going to have to wait until the weekend.
Campagnolo are variegated (SP rear) and the Tektro are indeed identical
front to rear except bolt length.
In The Olden Days calipers were bent slightly to correct pad angle, new
calipers have orbital shoe adjustment washers. Am I missing something
here?
repost:

dura-ace, 7700 rear:

open, front tips, 26.2mm, rear tips, 28.8mm, i.e. rear tips further apart.

closed, front tips, 4.3mm, rear tips, 0mm [touching], i.e. front tips
further apart.

the change in toe is /clearly/ visible in operation.

Mounting a 'front' brake on a classic rear bridge with a nylock
nut is 'bad' because... ??
But is this because of the calipers or because of the pads? Anyway,
so what? It just means that he will have to wear the pads in to the
correct toe in--they might squeal a little bit for a couple of weeks.
Big deal.

He's said that it is the arms - remember that the origin of his claim for
magic pivots was that the arms were different between front and rear brakes
- and that this difference was what somehow made the arms twist as they
rotated.

no, don't put those words in my mouth.

imagine the following:

^
|
/_________X
/ |

relative to the perspective of the viewer, what happens to the apparent
angle of the slanted end of this arm as it rotates about X?

answer, it decreases as it gets closer to the viewer, and is zero when
"/" is directly above X. no twisting of the arms required.



Not that it's true in the real world, of course...

this is not an ignorance competition. think through the problem.

wrote:
On Nov 1, 5:19 am, jim beam wrote:
A Muzi wrote:
wrote:
Nashbar has a good deal on ultegra brake calipers; I mounted
them on
my front wheel. My rear caliper is kinda corroded, and I was
considering mounting another front caliper on the rear wheel... Is
this a good idea?
jim beam wrote:
as pointed out by others, the bolt length thing is not a big deal.
however, you need to be aware that shimano & campy dual pivot brake
calipers have a pivot action that increases toe on the pad as the
caliper closes. [easily observed for yourself if you have the
caliper
in your hand - simply squeeze and note how the relative angles of
the
pads change as the lever arms move through their arc.] if a front
caliper is used on the rear, the toe is towards the rear of the
pads,
not the front, relative to rim direction.
Gary Young wrote:
According to Shimano spare parts lists
(http://tinyurl.com/2kecg9), the
front and rear brakes are identical except for a bolt, a nut and a
washer
or spacer.
jim beam wrote:
no, that's the parts list for the nuts and bolts, not the caliper
arms.
Gary Young wrote:
How is this magical effect achieved?
jim beam wrote:
"magical"??? how about you ask an objective question rather than a
sarcastic one that presupposes bull**** instead? you could also take
the time to look for the shimano mounting instructions which do indeed
advise against switching front/rear calipers.
the answer is pivot angle. buy a campy or shimano dual pivot caliper
and observe for yourself.
will you notice any effect in practice? maybe some brake squeal,
but
probably not. is it correct in theory? no. and iirc, there's a
warning in the shimano literature to the effect that each caliper is
specific to front or rear application and should not be
interchanged.
wrote:
I took a look at the instructins,
Not to put too fine a point on it, but the instructions actually say
"Brakes designed for use as rear brakes should not be used as front
brakes." Oddly, they do not contain a similar prohibition against
using front brakes as rear brakes...
A Muzi wrote:
I believe that's merely because your average human would forget to
reverse the brake shoes.
jim beam wrote:
andrew, you have stock of dual pivot brakes, and measuring calipers.
please perform the measurements i requested. i can do it, but i have
to strip a bike and that's going to have to wait until the weekend.
Campagnolo are variegated (SP rear) and the Tektro are indeed identical
front to rear except bolt length.
In The Olden Days calipers were bent slightly to correct pad angle, new
calipers have orbital shoe adjustment washers. Am I missing something
here?
repost:

dura-ace, 7700 rear:

open, front tips, 26.2mm, rear tips, 28.8mm, i.e. rear tips further apart.

closed, front tips, 4.3mm, rear tips, 0mm [touching], i.e. front tips
further apart.

the change in toe is /clearly/ visible in operation.

Mounting a 'front' brake on a classic rear bridge with a nylock
nut is 'bad' because... ??

But is this because of the calipers or because of the pads?

calipers - the pads are fixed.

Anyway,
so what? It just means that he will have to wear the pads in to the
correct toe in--they might squeal a little bit for a couple of weeks.
Big deal.

that's right, trivialize what is not understood. this is r.b.t.

wrote:
On Nov 1, 6:44 am, "James Thomson" wrote:
"jim beam" a �crit:

wrote:
Also, one arm of the dual pivot brake pivots on the
brake bolt and is surely tracking straight, not canted.
indeed, but the other pivot on the short arm isn't. and as it
swings through its arc, it tilts the axis angle for the "c" arm.
There's no mechanism to allow one arm to move the axis of the other. One
pivots about the brake bolt, and the other pivots about an axis that's fixed
with respect to the brake bolt, and parallel to the first.

This horse surely doesn't need to be beaten any more, but
I'll just point out that there are three pieces in a dual pivot
caliper - the two arms and a short link piece - and three
pivot joints. If you think about it for a little, it's very difficult
to see how it could work to have the axes of the brake
arm pivots canted with respect to each other, as the
short link would somehow have to bend or flex.

not at all. the axis on the end of the short arm is skewed. as the
short arm rotates about the central pivot, the angle of the outer pivot
rotates.

_ 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.

On Nov 1, 9:52 pm, jim beam wrote:
wrote:
On Nov 1, 6:44 am, "James Thomson" wrote:
There's no mechanism to allow one arm to move the axis of the other. One
pivots about the brake bolt, and the other pivots about an axis that's fixed
with respect to the brake bolt, and parallel to the first.

This horse surely doesn't need to be beaten any more, but
I'll just point out that there are three pieces in a dual pivot
caliper - the two arms and a short link piece - and three
pivot joints. If you think about it for a little, it's very difficult
to see how it could work to have the axes of the brake
arm pivots canted with respect to each other, as the
short link would somehow have to bend or flex.

not at all. the axis on the end of the short arm is skewed. as the
short arm rotates about the central pivot, the angle of the outer pivot
rotates.

Okay, now that I'm home and have a brake in my hand
to play with, I see that that pivot _could_ be canted
without causing the brakes to bind. However, on my
brakes (2 diff kinds of Shimano), it isn't canted.

Furthermore, if one canted that pivot, it would not
cause the amount of toe-in to change as the brakes
close. The brake pad on that side would be toed
in by the axis cant angle in open or closed position.
What would change is that the pad would move
slightly fore-aft as the brakes closed. I can't
see what purpose that would serve.

So either there's something unique about your model of
brake arms, or the pads are rotated in such a way that
the tips at one end are describing a smaller circle
and touching earlier, as another poster suggested.

Anyway, I have a Tektro rear DP brake mounted on the
front of a bike, with the brake pads swapped,
and it hasn't killed me yet. So either Tektro didn't
get the memo for this feature, or I am doomed to die
and am passing up my chance to live forever with
this brake miscegenation.

Ben

On Thu, 01 Nov 2007 21:53:09 -0700, jim beam wrote:


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.

Only in your world. In the real world, others have measured and found no
such change.

On Thu, 01 Nov 2007 19:27:33 -0600, A Muzi wrote:

wrote:
2) You absolutely cannot presume from your measurements of pad
spacing that the brake arms are twisting. This assumes that the front
and rear edges are exactly the same distance from the pivot axis. In
practice, this is going to vary depending on things like fork rake,
rear dropout design, and how well the brakes are set up. It will in
fact vary from front to rear, but not because of any difference in the
calipers.

Michael Press wrote:
You seem to be talking about fork offset here.
When the fork rake is varied the fork tip and
fork crown move together; and therefore the
brake pad position wrt the wheel rim remains
invariant.

I assumed he meant the angle of the pad to the arm

You were correct. The following jim beam quote makes this clear:

"...shimano & campy dual pivot brake
calipers have a pivot action that increases toe on the pad as the
caliper closes."

He's not yet explained[1]:

a) why nobody else can measure see this; and/or

b) just how these pivots change their axis of rotation

[1] this assumes that the standard beamboy response of the form "f**king
moron f**ktard" does not qualify as "explanation".