Mount front brakes on rear?

[jim beam]

_ wrote:
> 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" <[email protected]> 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.

1. you need to find a better math teacher.
2. go find yourself a dual pivot caliper and observe for yourself.

 

[jim beam]

[email protected] wrote:
> On Oct 31, 11:37 pm, jim beam <[email protected]> wrote:
>> [email protected] wrote:
>>> On Oct 30, 11:48 pm, jim beam <[email protected]> wrote:
>>>> Gary Young wrote:
>>>>> On Sun, 28 Oct 2007 20:01:29 -0700, jim beam wrote:
>>>>>> [email protected] 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 ******** 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.
>

that "argument" is pure presumption. get a caliper and check for yourself.

 

[James Thomson]

>> What should I be measuring?

"jim beam" <[email protected]> a écrit:

> 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

 

[[email protected]]

On Nov 1, 9:08 am, "James Thomson" <[email protected]> wrote:
> >> What should I be measuring?
>
> "jim beam" <[email protected]> a écrit:
>
> > 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

Just for laughs, try it again with the pads rotated one way or another
about the pad fixing bolts. You'll be able to reproduce jim's magical
toe in mechanism in either direction with the same caliper. The end
of the pad that's closest to the pivot axis is going to travel along a
shorter arc giving you a wonderful optical illusion that both arms of
the caliper are twisting in ways that they are physically unable to.

 

[James Thomson]

"jim beam" <[email protected]> a écrit:

> [email protected] 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.

James Thomson

 

[_]

On Thu, 1 Nov 2007 14:08:07 +0100, James Thomson wrote:

>>> What should I be measuring?
>
> "jim beam" <[email protected]> a écrit:
>
>> 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. 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).

 

[_]

On Thu, 01 Nov 2007 06:03:00 -0700, [email protected] wrote:

In re. jim beam's assertion of magic pivots in shimano brakes:

> 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.

Ten cents says jim beam will now start using insults as his strongest
support for this impossible claim.

 

[[email protected]]

On Nov 1, 5:19 am, jim beam <[email protected]> wrote:
> A Muzi wrote:
> >>>>>>> [email protected] 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 <[email protected]> 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 <[email protected]> wrote:
> >>>>> "magical"??? how about you ask an objective question rather than a
> >>>>> sarcastic one that presupposes ******** 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.
>
> >>> [email protected] 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.

 

[_]

On Thu, 01 Nov 2007 08:11:10 -0700, [email protected] wrote:

> On Nov 1, 5:19 am, jim beam <[email protected]> wrote:
>> A Muzi wrote:
>>>>>>>>> [email protected] 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 <[email protected]> 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 <[email protected]> wrote:
>>>>>>> "magical"??? how about you ask an objective question rather than a
>>>>>>> sarcastic one that presupposes ******** 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.
>>
>>>>> [email protected] 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.

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

 

[A Muzi]

> On Thu, 01 Nov 2007 06:03:00 -0700, [email protected] wrote:
> In re. jim beam's assertion of magic pivots in shimano brakes:
>> 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.

_ wrote:
> Ten cents says jim beam will now start using insults as his strongest
> support for this impossible claim.

At that point just read "******" for "f**king moron f**ktard" and we can
close this.
--
Andrew Muzi
www.yellowjersey.org
Open every day since 1 April, 1971

 

[Michael Press]

In article
<[email protected]>,
[email protected] 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

 

[A Muzi]

> [email protected] 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

 

[[email protected]]

On Nov 1, 6:44 am, "James Thomson" <[email protected]> wrote:
> "jim beam" <[email protected]> a écrit:
>
> > [email protected] 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

 

[jim beam]

_ wrote:
> On Thu, 01 Nov 2007 08:11:10 -0700, [email protected] wrote:
>
>> On Nov 1, 5:19 am, jim beam <[email protected]> wrote:
>>> A Muzi wrote:
>>>>>>>>>> [email protected] 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 <[email protected]> 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 <[email protected]> wrote:
>>>>>>>> "magical"??? how about you ask an objective question rather than a
>>>>>>>> sarcastic one that presupposes ******** 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.
>>>>>> [email protected] 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.

 

[jim beam]

[email protected] wrote:
> On Nov 1, 5:19 am, jim beam <[email protected]> wrote:
>> A Muzi wrote:
>>>>>>>>> [email protected] 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 <[email protected]> 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 <[email protected]> wrote:
>>>>>>> "magical"??? how about you ask an objective question rather than a
>>>>>>> sarcastic one that presupposes ******** 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.
>>>>> [email protected] 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.

 

[jim beam]

[email protected] wrote:
> On Nov 1, 6:44 am, "James Thomson" <[email protected]> wrote:
>> "jim beam" <[email protected]> a �crit:
>>
>>> [email protected] 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.

 

[jim beam]

_ wrote:
> On Thu, 1 Nov 2007 14:08:07 +0100, James Thomson wrote:
>
>>>> What should I be measuring?
>> "jim beam" <[email protected]> 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.

 

[[email protected]]

On Nov 1, 9:52 pm, jim beam <[email protected]> wrote:
> [email protected] wrote:
> > On Nov 1, 6:44 am, "James Thomson" <[email protected]> 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:

>> [email protected] 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".