Can't get rear canti brake to skid tire. Should it?



A Muzi wrote:
> TomYoung wrote:
>> On Jun 27, 12:41 pm, [email protected] wrote:
>> I
>>> I assume that the straddle cable is out of adjustment or the straddle
>>> cable yoke is bottoming on the cable housing anchor.

>>
>> I don't think the link wire/main cable setup (button/fixed length wire/
>> main cable through button) is grossly out of whack. Picture of pads
>> and rim are here (http://tinyurl.com/6qpprf) and link wire setup here
>> (http://tinyurl.com/64fy3v). I could bring the pads *slightly* closer
>> to the rim but would run up against a wheel that isn't *that* straight
>> as well as lose the "quick release" aspect of the link wire setup.
>> With the brake handle pulled tight against the handlebar the distance
>> from the top of the link wire button to the hanger is 2 - 3
>> centimeters.
>>
>>> Take the wheel out and see if pulling the hand lever can make the
>>> brake pads close to a narrower gap than the width of the rim. Beside
>>> that test, it wouldn't hurt to pull the cable out of the housing and
>>> wipe its entire length in with motor oil before threading it in again
>>> and attaching the straddle cable yoke. Do the same with the front
>>> brake. I don't know in what condition the cables have.

>>
>> With the wheel out the pads pull to within 23 - 24 mm of each other,
>> measured center of pad to center of pad. The rim itself looks to be
>> right at 28mm. The new cables are a Jagwire product with a PVDF
>> coating and advise against lubing. I've used these successfully on
>> other bikes but may try a standard lubed cable.
>>
>>>> A very hard squeeze of the brake handle
>>>> brings the end of the handle to within a 1/4" of the bar.
>>> That doesn't sound good. When the pads make contact with the rim,
>>> about 3/4 of the distance to the hand lever running out of stroke
>>> should be used. If the system is so spongy that the rest of the
>>> stroke is mostly used up, you have an improperly installed cable
>>> housing.

>>
>> Hand lever stroke is just at 6cm. Good solid pad/rim contact is at
>> 3cm. The "hard squeeze" I mentioned was with me squeezing the lever
>> just about as hard as I can.
>>
>> I'd bet you money and give you odds that the cable housing setup when
>> I got the bike was just as it was when the bike was sold, and that's
>> what I copied in my installation. In any case, the arrangement seems
>> right: housing from lever to to front top tube cable stop, then bare
>> cable along the top tube to the rear cable stop, then housing from
>> rear cable stop to hanger via reverse "S" bend with the middle of the
>> "S" secured to the seat tube with a small collar. At this point the
>> only change I can think of is shortening the first length of housing,
>> which is probably a little long.

>
> I don't see any serious issues with the setup


take a second look at the first pic - the pads aren't set at the same
height in the cantilever arm. that means that at least one of the pads
[probably both] isn't sitting right on the rim.


> and the pads are an
> excellent choice.
>
> Oil on rim?
>
> You mention rear only trouble. If you set up the front similarly with
> the same model lever and it works well, we're most likely searching for
> a cable issue of some type.
 
On Jun 27, 2:19 pm, TomYoung <[email protected]> wrote:
> Hi all:
>
> Still working on that bike for my daughter (it was a "freebie" that's
> cost me about $50 in parts and lots of hours of work) and thanks to
> the help I've received here everything seems to be working fine,
> except that I can't get the rear cantilever brake to skid the tire.


You may be able to move the lever all the way as you're squishing the
brake shoes craploads before they grab as they're new. Get the glaze/
mold release off them with some sand paper or a small box plane.
Should grab a lot better.
 
-snip-
>> TomYoung wrote:

-snip-
>>> Picture of pads
>>> and rim are here (http://tinyurl.com/6qpprf) and link wire setup here
>>> (http://tinyurl.com/64fy3v).


> A Muzi wrote:
>> I don't see any serious issues with the setup


jim beam wrote:
> take a second look at the first pic - the pads aren't set at the same
> height in the cantilever arm. that means that at least one of the pads
> [probably both] isn't sitting right on the rim.

-snip-

Yes you're right. And there are small details besides but not 'serious'
ones. As you know, braking is primarily about swept area rather than
puck size and even 1/2 pad contact should skid that wheel.

--
Andrew Muzi
<www.yellowjersey.org/>
Open every day since 1 April, 1971
** Posted from http://www.teranews.com **
 
A Muzi wrote:
> -snip-
>>> TomYoung wrote:

> -snip-
>>>> Picture of pads
>>>> and rim are here (http://tinyurl.com/6qpprf) and link wire setup here
>>>> (http://tinyurl.com/64fy3v).

>
>> A Muzi wrote:
>>> I don't see any serious issues with the setup

>
> jim beam wrote:
>> take a second look at the first pic - the pads aren't set at the same
>> height in the cantilever arm. that means that at least one of the
>> pads [probably both] isn't sitting right on the rim.

> -snip-
>
> Yes you're right. And there are small details besides but not 'serious'
> ones. As you know, braking is primarily about swept area


typical r.b.t. misunderstanding - it's not that simple. imagine a pad
of standard width but only 1mm long - same swept area, negligible
braking capability.


> rather than
> puck size and even 1/2 pad contact should skid that wheel.


nope, try it. especially on new pads. and the op's not getting 10%
contact, let alone 50%.
 
On Jun 27, 3:51 pm, TomYoung <[email protected]> wrote:
> On Jun 27, 2:34 pm, M-gineering <[email protected]> wrote:
>
> > Is the V of the transverse cable to acute? This give a nice sturdy lever
> > feel, but all you are accomplishing is you're trying to pull the brakes
> > off the posts. Slide the posts of the blocks as far out as they will go
 
On Fri, 27 Jun 2008 12:19:05 -0700, TomYoung wrote:


> So... I've put on new cables and new housings and new housing
> ferrules. Housing ends properly prepared as laid out at Sheldon
> Brown's site. New brake pads (Ritchey Logic red) and new tires.
> Brake pads properly toed in and positioned equally close to either
> side of the rim.

Try using no toe in and tighten the cable as much as possible with out the
pads touching the rim.

If you lubricated the chain with some teflon spray, you might
accidentially have sprayed some of it on the rim. If so, clean rim and
brake pads with some kind of solvent.


Tomas
 
On Jun 27, 6:20 pm, TomYoung <[email protected]> wrote:
> On Jun 27, 2:34 pm, M-gineering <[email protected]> wrote:
>
>
>
> > TomYoung wrote:
> > > Hi all:

>
> > > Still working on that bike for my daughter (it was a "freebie" that's
> > > cost me about $50 in parts and lots of hours of work) and thanks to
> > > the help I've received here everything seems to be working fine,
> > > except that I can't get the rear cantilever brake to skid the tire.
> > > I'm not sure if being able to skid the rear tire really means anything
> > > in terms of fast and safe stops, but it seems odd that I can't
> > > accomplish this with this bike.

>
> > > The bike is an older mountain bike (26" wheels) hung with Shimano
> > > Exage 300LX parts.  The brakes themselves just read "Shimano" but I
> > > assume they're either Exage 300LX or similar.

>
> > > So... I've put on new cables and new housings and new housing
> > > ferrules.  Housing ends properly prepared as laid out at Sheldon
> > > Brown's site.  New brake pads (Ritchey Logic red) and new tires.
> > > Brake pads properly toed in and positioned equally close to either
> > > side of the rim.  (Remarkably, the wheels were quite true for such an
> > > old bike.)  Because of the highly sloped top tube the rear brake
> > > housing makes a fairly long reverse "S" bend to get up from the top
> > > tube to the rear brake hanger, but friction through the system doesn't
> > > seem too bad.  A very hard squeeze of the brake handle brings the end
> > > of the handle to within a 1/4" of the bar.  The only thing I haven't
> > > done at this point is clean the rim sidewalls, but the sidewalls
> > > aren't particularly glazed with old brake pad material.

>
> > > Anything else to be done, or is this as good as it gets?

>
> > > TIA

>
> > > Tom Young

>
> > Is the V of the transverse cable to acute? This give a nice sturdy lever
> > feel, but all you are accomplishing is you're trying to pull the brakes
> > off the posts. Slide the posts of the blocks as far out as they will go
 
TomYoung wrote:
> On Jun 27, 6:20�pm, TomYoung <[email protected]> wrote:
>> On Jun 27, 2:34�pm, M-gineering <[email protected]> wrote:
>>
>>
>>
>>> TomYoung wrote:
>>>> Hi all:
>>>> Still working on that bike for my daughter (it was a "freebie" that's
>>>> cost me about $50 in parts and lots of hours of work) and thanks to
>>>> the help I've received here everything seems to be working fine,
>>>> except that I can't get the rear cantilever brake to skid the tire.
>>>> I'm not sure if being able to skid the rear tire really means anything
>>>> in terms of fast and safe stops, but it seems odd that I can't
>>>> accomplish this with this bike.
>>>> The bike is an older mountain bike (26" wheels) hung with Shimano
>>>> Exage 300LX parts. �The brakes themselves just read "Shimano" but I
>>>> assume they're either Exage 300LX or similar.
>>>> So... I've put on new cables and new housings and new housing
>>>> ferrules. �Housing ends properly prepared as laid out at Sheldon
>>>> Brown's site. �New brake pads (Ritchey Logic red) and new tires.
>>>> Brake pads properly toed in and positioned equally close to either
>>>> side of the rim. �(Remarkably, the wheels were quite true for such an
>>>> old bike.) �Because of the highly sloped top tube the rear brake
>>>> housing makes a fairly long reverse "S" bend to get up from the top
>>>> tube to the rear brake hanger, but friction through the system doesn't
>>>> seem too bad. �A very hard squeeze of the brake handle brings the end
>>>> of the handle to within a 1/4" of the bar. �The only thing I haven't
>>>> done at this point is clean the rim sidewalls, but the sidewalls
>>>> aren't particularly glazed with old brake pad material.
>>>> Anything else to be done, or is this as good as it gets?
>>>> TIA
>>>> Tom Young
>>> Is the V of the transverse cable to acute? This give a nice sturdy lever
>>> feel, but all you are accomplishing is you're trying to pull the brakes
>>> off the posts. Slide the posts of the blocks as far out as they will go .
>>> Many bikes come with the stock transverse cable doohicky in too big a
>>> size. Reverting to the old fashioned cable and cablehanger gives you
>>> more options

>> You may have hit the nail on the head here.
>>
>> One thing I didn't mention, because it just didn't seem relevant, was
>> that the bike has front and rear reflectors mounted to the fork crown
>> and rear brake bridge. �I ended up bending both of them down to give
>> me a little more room for the link wire button when the brake was
>> released but I didn't think a lot about the issue because my reasoning
>> was "that's how they sold the bikes and the brakes must have worked OK
>> when they sold them." �However, in looking more closely at the front
>> and rear brakes (the fronts seem to work OK) was that the brake arms
>> up front formed a wider angle, by a good margin, then the brake arms
>> in the rear. �The limiting factor for the narrow angle in the rear is
>> that rear reflector.
>>
>> Tomorrow I'll try again, either bending that rear reflector a heck of
>> a lot more or removing it all together and widen the angle of the
>> brake arms.
>>
>> Tom Young

>
> Bingo! By moving the arms of the brakes markedly away from the shoes


and thereby re-setting the brake pads...


> and lowering the main wire/button/link wire junction to compensate, I
> can now skid the rear tire. The problem was lack of mechanical
> advantage in the system which should have been fairly obvious, in
> retrospect, when I stated that the brake pads made "firm" contact with
> the rim with the brake levers pulled halfway through their swing.
> Thanks for the suggestion.
>
> Of course, now there's considerable squealing coming from the rear
> brakes, a noise that so far I haven't been able to adjust out. I may
> try to negotiate a compromise position between effective braking and
> silent braking, but at least now I understand why the brakes weren't
> working as they should.
>
> Tom Young
 
On Jun 28, 6:49 pm, jim beam <[email protected]> wrote:
> TomYoung wrote:
> > On Jun 27, 6:20�pm, TomYoung <[email protected]> wrote:
> >> On Jun 27, 2:34�pm, M-gineering <[email protected]>wrote:

>
> >>> TomYoung wrote:
> >>>> Hi all:
> >>>> Still working on that bike for my daughter (it was a "freebie" that's
> >>>> cost me about $50 in parts and lots of hours of work) and thanks to
> >>>> the help I've received here everything seems to be working fine,
> >>>> except that I can't get the rear cantilever brake to skid the tire.
> >>>> I'm not sure if being able to skid the rear tire really means anything
> >>>> in terms of fast and safe stops, but it seems odd that I can't
> >>>> accomplish this with this bike.
> >>>> The bike is an older mountain bike (26" wheels) hung with Shimano
> >>>> Exage 300LX parts. �The brakes themselves just read "Shimano" but I
> >>>> assume they're either Exage 300LX or similar.
> >>>> So... I've put on new cables and new housings and new housing
> >>>> ferrules. �Housing ends properly prepared as laid out at Sheldon
> >>>> Brown's site. �New brake pads (Ritchey Logic red) and new tires.
> >>>> Brake pads properly toed in and positioned equally close to either
> >>>> side of the rim. �(Remarkably, the wheels were quite true for such an
> >>>> old bike.) �Because of the highly sloped top tube the rear brake
> >>>> housing makes a fairly long reverse "S" bend to get up from the top
> >>>> tube to the rear brake hanger, but friction through the system doesn't
> >>>> seem too bad. �A very hard squeeze of the brake handle brings the end
> >>>> of the handle to within a 1/4" of the bar. �The only thing Ihaven't
> >>>> done at this point is clean the rim sidewalls, but the sidewalls
> >>>> aren't particularly glazed with old brake pad material.
> >>>> Anything else to be done, or is this as good as it gets?
> >>>> TIA
> >>>> Tom Young
> >>> Is the V of the transverse cable to acute? This give a nice sturdy lever
> >>> feel, but all you are accomplishing is you're trying to pull the brakes
> >>> off the posts. Slide the posts of the blocks as far out as they will go .
> >>> Many bikes come with the stock transverse cable doohicky in too big a
> >>> size. Reverting to the old fashioned cable and cablehanger gives you
> >>> more options
> >> You may have hit the nail on the head here.

>
> >> One thing I didn't mention, because it just didn't seem relevant, was
> >> that the bike has front and rear reflectors mounted to the fork crown
> >> and rear brake bridge. �I ended up bending both of them down to give
> >> me a little more room for the link wire button when the brake was
> >> released but I didn't think a lot about the issue because my reasoning
> >> was "that's how they sold the bikes and the brakes must have worked OK
> >> when they sold them." �However, in looking more closely at thefront
> >> and rear brakes (the fronts seem to work OK) was that the brake arms
> >> up front formed a wider angle, by a good margin, then the brake arms
> >> in the rear. �The limiting factor for the narrow angle in the rear is
> >> that rear reflector.

>
> >> Tomorrow I'll try again, either bending that rear reflector a heck of
> >> a lot more or removing it all together and widen the angle of the
> >> brake arms.

>
> >> Tom Young

>
> > Bingo!  By moving the arms of the brakes markedly away from the shoes

>
> and thereby re-setting the brake pads...
>
> > and lowering the main wire/button/link wire junction to compensate, I
> > can now skid the rear tire.  The problem was lack of mechanical
> > advantage in the system which should have been fairly obvious, in
> > retrospect, when I stated that the brake pads made "firm" contact with
> > the rim with the brake levers pulled halfway through their swing.
> > Thanks for the suggestion.

>
> > Of course, now there's considerable squealing coming from the rear
> > brakes, a noise that so far I haven't been able to adjust out.  I may
> > try to negotiate a compromise position between effective braking and
> > silent braking, but at least now I understand why the brakes weren't
> > working as they should.

>
> > Tom Young


I guess you're suggesting the faces of the brake pads weren't making
good contact in my 1st setup? Before resetting the brake pads further
away from the arms I removed them from the brake arms all together and
inspected their faces for evidence of rim contact. From my inspection
it appears that all parts of both faces were making contact with the
rim. I'm confident the increased MA and not more effective pad/rim
contact is the difference I'm experiencing.

Tom Young