Mount front brakes on rear?

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...

I believe that's merely because your average human would forget to
reverse the brake shoes.

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.

wrote:
On Oct 30, 11:48 pm, jim beam wrote:
Gary Young wrote:
On Sun, 28 Oct 2007 20:01:29 -0700, jim beam 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?
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.
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.
no, that's the parts list for the nuts and bolts, not the caliper arms.

How is this magical effect achieved?
"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.

No, the answer is that either your pivot bolts are loose or the
bushings are worn. The effect you're describing is geometrically
impossible with fixed pivots no matter what angle they're at. There's
slop in the pivots, and it looks like the arms are twisting because
the cable attachments and springs are out of plane. The mechanism was
most certainly not designed to move that way.


brand new dura-ace, i.e. no loose pivots or worn bushings, says you need
to check your facts. supposition or disbelief are no a basis for
"certainly" statements.

_ wrote:
On Wed, 31 Oct 2007 15:33:32 +0100, James Thomson wrote:

"jim beam" a �crit:

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.]
since you presumably have these things laying about your store,
new in box, why not dig out both brake calipers and measuring
calipers and measure what i describe?
What should I be measuring? I have new (previous generation) Campag Centaur
and Shimano Ultegra calipers here on my desk, and I can't see or measure any
change in angle between the pads relative to the plane of the rim. In fact I
can't readily see how that would be possible, as the pivot of one arm is
co-axial with the brake bolt in each case (and that's true of all dual-pivot
brakes I've seen).


Exactly - as another poster has said, it is geometrically impossible to
produce two degrees of freedom with one pivot axis.

and with a _dual pivot_ brake caliper you have, wait for it, _two_ pivot
axes! now, that gives /how/ many degrees of freedom?

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...

I believe that's merely because your average human would forget to
reverse the brake shoes.

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.

ok, i found a caliper still in a box. 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.

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... ??

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? Mounting a 'front' brake on a classic rear bridge with a nylock
nut is 'bad' because... ??
--
Andrew Muzi
www.yellowjersey.org
Open every day since 1 April, 1971

On Oct 31, 8:55 pm, jim beam wrote:
jim beam wrote:
ok, i found a caliper still in a box. 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.

I tried this with two types of Shimano dual pivot brakes
and I can't see any cammed toe-in effect, either on or
off a bike. I think I might be able to get one end to
touch first on the off-bike pair if the pads are
rotated up (no longer parallel to the brake bolt)
but that isn't how they are usually used.

Further, in usage, brakes are never closed to 0mm, since
rims are of order 18-20mm wide. Also, one arm of the
dual pivot brake pivots on the brake bolt and is surely
tracking straight, not canted.

I can't see this as a valid safety argument against
exchanging rear and front brake. You just have to
be careful to change around cartridge pads if you
have them to avoid pad ejection. And to make sure
the nut engages enough threads on the bolt, of course.
Naturally, when mounting any brake, you should
try to get the toe-in correct as the pads hit the
rim. The toe-in when the pads are off the rim only
affects brake clearance.

Ben

On Wed, 31 Oct 2007 20:37:23 -0700, jim beam wrote:

_ wrote:
On Wed, 31 Oct 2007 15:33:32 +0100, James Thomson wrote:

"jim beam" a �crit:

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.]
since you presumably have these things laying about your store,
new in box, why not dig out both brake calipers and measuring
calipers and measure what i describe?
What should I be measuring? I have new (previous generation) Campag Centaur
and Shimano Ultegra calipers here on my desk, and I can't see or measure any
change in angle between the pads relative to the plane of the rim. In fact I
can't readily see how that would be possible, as the pivot of one arm is
co-axial with the brake bolt in each case (and that's true of all dual-pivot
brakes I've seen).


Exactly - as another poster has said, it is geometrically impossible to
produce two degrees of freedom with one pivot axis.

and with a _dual pivot_ brake caliper you have, wait for it, _two_ pivot
axes! now, that gives /how/ many degrees of freedom?

One for each arm. You were saying that each arm changes angle as it
pivots; as another poster has stated, that is geometrically impossible with
a single axis.

Time to weasle again, beamboy.

wrote:
On Oct 31, 8:55 pm, jim beam wrote:
jim beam wrote:
ok, i found a caliper still in a box. 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.

I tried this with two types of Shimano dual pivot brakes
and I can't see any cammed toe-in effect, either on or
off a bike.

well, i've observed this on both campy and shimano. hence the above
measurements.


I think I might be able to get one end to
touch first on the off-bike pair if the pads are
rotated up (no longer parallel to the brake bolt)
but that isn't how they are usually used.

Further, in usage, brakes are never closed to 0mm, since
rims are of order 18-20mm wide.

full open to full close exaggerates the effect for sure, but the point
is that this operation is a design feature. i can only presume that
it's for increasing toe in operation.


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.



I can't see this as a valid safety argument against
exchanging rear and front brake. You just have to
be careful to change around cartridge pads if you
have them to avoid pad ejection.

well /that/ is a given - and not relevant to pivot angles.


And to make sure
the nut engages enough threads on the bolt, of course.
Naturally, when mounting any brake, you should
try to get the toe-in correct as the pads hit the
rim.

my personal experience of this is that toe simply acts as a trap for
grit - all my bikes have orbital pad holders and i take the toe out.
and the caliper having its own toe action makes it even more unnecessary
imo.


The toe-in when the pads are off the rim only
affects brake clearance.

Ben

On Oct 31, 11:37 pm, jim beam wrote:
wrote:
On Oct 30, 11:48 pm, jim beam wrote:
Gary Young wrote:
On Sun, 28 Oct 2007 20:01:29 -0700, jim beam 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?
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.
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.
no, that's the parts list for the nuts and bolts, not the caliper arms.

How is this magical effect achieved?
"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.

No, the answer is that either your pivot bolts are loose or the
bushings are worn. The effect you're describing is geometrically
impossible with fixed pivots no matter what angle they're at. There's
slop in the pivots, and it looks like the arms are twisting because
the cable attachments and springs are out of plane. The mechanism was
most certainly not designed to move that way.

brand new dura-ace, i.e. no loose pivots or worn bushings, says you need
to check your facts. supposition or disbelief are no a basis for
"certainly" statements.

Allow me to clarify, since there seem to be some differences in the
ways that engineers and pretend metallurgists approach problems. A
few of the things that I considered.
1) In all of the press releases, web copy, and magazine reviews I've
ever seen, all filled with outlandish claims of technical achievement,
neither Campy nor Shimano has ever said anything about variable toe
in. This is not terribly surprising, as I'm looking at the Campy
installation manual right now, and they don't seem to believe that the
pads should be toed in to begin with. Also, despite all manner of
uptight safety warnings, there's no mention of not interchanging front
and rear calipers.
2) Two pivots do not always mean two degrees of freedom. This is the
kind of thing that we teach the real engineers to stop and think about
before they rip machines apart to see how they work. We teach it to
them before they reach legal drinking age, so I'll understand if
you're a bit behind the curve. In a dual pivot brake, one of the arms
is only rotating around one axis. If the second pivot were somehow
being used to twist one arm relative to one that's fixed in plane, the
result would be bad for pad wear, and potentially bad for the rim.
Now on to fact checking and supposition of fact as a basis for
"certainly" statements
1) My assumption of worn/loose pivots was based on my assumption that
you understand what you're talking about. I promise not to make that
mistake again.
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. Now, before instructing other people to take their bikes
apart to make the same useless measurements, perhaps you should take
the pads off of those shiny new Dura-Ace brakes and make some
measurements that actually prove some caliper twist that's different
between front and rear.