Pulsing Disc: Next step?

yeah come back!
good thing the primitive system didn't sling yawl under the bus.
there's a checklist on this disc thingamabob!
more! expletive deleted FI.

Pete Cresswell said:

Feels like there's a slippery spot on the disc: one pulse per

revolution.

It sounds like you have some kind of tenacious contamination on the
rotor. Steel wool won't cut into the disc material to remove the
affected surface-- I would try 400 to 800 grit wet-or-dry sandpaper.
Thoroughly clean off the dust afterwards.

Chalo Colina

"(Pete Cresswell)" <[email hidden]> wrote in message
news:[email hidden]...

Quoted message said:

Rear brake, sudden onset.

Feels like there's a slippery spot on the disc: one pulse per revolution.

Slippery or grabby? Constant braking with a *decrease* in braking power,
once per revolution?
--
Phil, Squid-in-Training

"(Pete Cresswell)" <[email hidden]> wrote in message
news:[email hidden]...

Quoted message said:

Rear brake, sudden onset.

Feels like there's a slippery spot on the disc: one pulse per revolution.

The disk brakes on my bike (not top-end by any means) flex slightly to one
side when I apply the brakes since only the pad on one side moves (not a
floating design like car brakes). Because of that, I would not worry about
slight warping caused by bits of stuff in the mounting area (or caused by
anything else). I think the disc would just flex to a proper alignment. I
could, however, see a change in overall width of the disc as it goes around
being a problem. However, you said it feels like a "slippery spot." Does
this mean you don't feel any pulsing in the brake handle?

If the handle is not pulsing, I think that points to some type of
contamination (as Chalo mentioned).

--
Scott Ehardt
http://www.scehardt.com

Pete wrote: (clip) Comments on reasoning? (clip)
^^^^^^^^^^^^^
One thing I do not see in the list of things you checked is the disk
thickness. Could you mike it to see whether there is a thin or a thick
spot?

Assuming full hydraulic brakes, bleed the line. . . pulsing is caused by air
in the compressionless fluid. . .

"(Pete Cresswell)" <[email hidden]> wrote in message
news:[email hidden]...

Quoted message said:

Rear brake, sudden onset.

Feels like there's a slippery spot on the disc: one pulse per revolution.

So far:

- Rode the rear brake all the way down a hill that's about a mile long -

did

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this several times. Got it to fade to almost nothing, but pulse

returned.

Quoted message said:

- Coated disc with paint remover, washed it off, cleaned it a second time

with

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methyl alcohol, cleaned pads with methyl alcohol. No change.

- Squeezed steel wool against both sides of rotor and cranked the wheel

for

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about five minutes. No change.

- Removed the rotor, checked for gunk/dirt between rotor and mounting

area.

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Scraped off a little loctite film around a couple of holes. No change.

- Held a tool against the chainstay almost touching outer edge of rotor,

could

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see absolutely no deviation as the rotor turned.

- Held flat of same tool against the rotor so it swept most of the

surface,

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*seems* like there's very small amount of irregularity - like .001

inches....not

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sure if this is is within the bounds of normal or not - not even sure if

it's

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there or not....hand tremor and all that.... Maybe it's worth clamping
something together to make it really rigid and re-trying....

Reasoning:

- Seems if the cause is gunk on the rotor that what I've done so far

should at

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least have distributed it more evenly - making the pulse less intense or

making

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it go away entirely....but it seems like the intensity hasn't changed at

all.

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- If the rotor were cocked due to dirt between the rotor and the mounting

area

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of the hub, it seems like holding a tool against the outer braking area

could

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reveal some irregularity...but it didn't.

That leaves the apparent very small irregularity when a flat tool is held
against the braking surface....but it's soooooo small....

One last observation: the rotor is intended for a Hope C2 caliper and

somewhat

Quoted message said:

wider (maybe 5 mils) and thicker than rotors for the Avid mech caliper

that is

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on there now...but that seems moot to me in light of there being no

problem for

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months beforehand with same rotor/caliper.

Comments on reasoning? Suggestions?
--
PeteCresswell

Hi Pete
I doubt that I or anyone else can see 0.001", unless it's set up very
carefully w/ controlled backlight. And even then I'm somewhat
skeptical. If you can see it I tend to think it's more in the range of
0.010". I know I can see tenths of a tenth of an inch. IE hundredths of
an inch.
IMHO, John

Quoted message said:

Hi Pete
I doubt that I or anyone else can see 0.001", unless it's set up very
carefully w/ controlled backlight. And even then I'm somewhat
skeptical. If you can see it I tend to think it's more in the range of
0.010". I know I can see tenths of a tenth of an inch. IE hundredths of
an inch.

I can see 0.001-inch on the display of electronic or dial calipers.

--
Tom Sherman - Near Rock Island

Per Phil, Squid-in-Training:

Quoted message said:

Slippery or grabby? Constant braking with a *decrease* in braking power,
once per revolution?

Decrease. Slippery, not grabby. I think it's time for Chalo's wet/dry.
--
PeteCresswell

Per [email hidden]:

Quoted message said:

I doubt that I or anyone else can see 0.001", unless it's set up very
carefully w/ controlled backlight. And even then I'm somewhat
skeptical.

I agree - that's why I'm skeptical too. .001 was definately a case of poetic
license. After looking an old automotive leaf gauge, I would revise the .001
to your figure of .010.

--
PeteCresswell

If the "pulse" is at exactly the same place in the rotor, unscrew the
rotor and rotate it 90 degrees and see if that changes where the
"pulse" occurs.

You say that the rotor is "true" when you spin the wheel and watch the
space between the pads and the rotor ... but how about "hop".. the
rotor could be out of round ( doubtful) or the rotor/wheel fit in the
dropouts causes a "hop" .. that would cause a pulse.

also, swap rotors front and back and see if the pulse goes away.

charlieb in ct

ps-- my rotors are far from true and they don't pulse. The rotor is
very flexible and yields to the force of the calipers. Out of true is
really only a problem when the brakes are not applied.

(Pete Cresswell) said:

Rear brake, sudden onset.

Feels like there's a slippery spot on the disc: one pulse per revolution.

Quoted message said:

Comments on reasoning? Suggestions?

Machinist's diagnosis:

Go to tool box, get dial indicator with magnetic base. This is assuming
that it's a steel bike frame, otherwise take wheel out and put in truing
stand, and use THAT for the magnetic base. Put end of dial indicator
against side of rotor. Spin wheel.

Note high/low spots with a dry erase marker.

Mic up high/low spots with a micrometer (NOT DIAL CALIPERS!). If it
measures the same all the way 'round with the micrometer on the high/low
spots that you noted, the rotor is warped. Get a new rotor, because
even going the route of putting it on a surface grinder or B&S
cylindrical grinder to attempt to make it flat and parallel will take a
couple of hours and considering the thickness of the rotor, this will be
an exercise in futility.

If you get variation of less than a thousandth, clean the rotor with
something that leaves no residue, like ether. Outside. Also, replace
the brake pads, because if there was anything on the rotor, it's
definitely embedded in the pads and will be re-deposited on the rotor
and will drive you nuts.

If it still "slips" try resurfacing by bead-blasting (glass beads) or by
lapping it on an iron lapping plate with some Clover compound.

You *do* have all this stuff, right?

It's not in the hydraulics (air in the line), because if it was, it
would be at random spots, not one spot per rev.

--
BMO

Per Boyle M. Owl:

Quoted message said:

Also, replace
the brake pads, because if there was anything on the rotor, it's
definitely embedded in the pads and will be re-deposited on the rotor
and will drive you nuts.

Devil's advocate question: Why isn't it getting re-deposited right now? i.e.
why isn't the slippery spot spreading until there's little or no pulse - just
overall diminished braking ability?

Is the iron lapping plate something I can purchase at a reasonable price? If
so, what kind of store? I'd guess it's just a hunk of iron or steel that has a
really flat face and I'd smear some sort of abrasive compound ("Clover
compound"?) on it and then rub the disk on it. (?) If so, would coarse
automotive polishing compound and a sheet of glass suffice for a one-time
attempt?

I'm thinking that the lapping routine would be a good no-brainer action whenever
there's a problem. i.e. don't look further; lap the thing and see if the
problem goes away. If not, look further. If it goes away, only to recur in a
few hours of riding, replace the pads and lap the disc one more time.

I'm kind of dancing around the dial indicator thing bc I don't have one and
can't imagine they're exactly cheap....I've put it in a truing stand and done
something functionally-similar just eyeballing/listening to the flat end of a
tool against the rotor surface with the wheel spinning - but can't perceive any
irregularity.

In all the threads about rim vs disc brakes, the idea of discs being
high-maintainence in this respect doesn't seem to come up - but I think it's a
valid pro/con point...not so much the idea of noise or pulsing per se (which
comes up in relation to both types of brakes)...but the elusiveness of the root
cause when it's a disc brake.
--
PeteCresswell

Pulsing brakes on a car mean the rotors are not round. Maybe you hit it.
Maybe it got hot and warped when cooling.

You're probably better off replacing the rotor than machining it flat. The
costs are bound to be similar and there isn't a lot of metal to remove on a
bike brake disc.

-Dion

"(Pete Cresswell)" <[email hidden]> wrote in message
news:[email hidden]...

Quoted message said:

Rear brake, sudden onset.

....

Quoted message said:

Comments on reasoning? Suggestions?
--
PeteCresswell

Pulsing brakes on a car mean the rotors are not round. Maybe you hit it.
Maybe it got hot and warped when cooling.

You're probably better off replacing the rotor than machining it flat. The
costs are bound to be similar and there isn't a lot of metal to remove on a
bike brake disc.

-Dion

"(Pete Cresswell)" <[email hidden]> wrote in message
news:[email hidden]...

Quoted message said:

Rear brake, sudden onset.

....

Quoted message said:

Comments on reasoning? Suggestions?
--
PeteCresswell

Per Dion Dock:

Quoted message said:

You're probably better off replacing the rotor than machining it flat. The
costs are bound to be similar and there isn't a lot of metal to remove on a
bike brake disc.

Agreed: the no-brainer is to just replace the rotor...but this has happened to
me 2-3 times in the past few years and I was holding out some small hope of
getting the debugging process wired to the point where a future occurrance would
just trigger some SOP and the problem could be fixed within an hour or so...

The rotor in question, however, is on a Rohloff hub. Removal is not trivial
like it is on a regular hub - you have to pull out part of Roholoff's shifting
mech and every time that's done, the chance for an oil leak increases; so even
if I were to have a foolproof fix, if it involved removing the rotor it wouldn't
be so foolproof...

I'm still gonna find out what's causing this - just out of
curiosity/stubborness...

But after today's ride, I installed a v-brake on the rear. Wanted that on my
old Isis anyhow, but on that frame my calf touched the brake on every stroke.
Now I've got a different frame and I can run a v on the rear without my leg
hitting it. I'll leave the disc caliper/rotor in place until I'm satisifed
that the v works for me (highly likely...) and then just remove the unused
hardware.
--
PeteCresswell

"Sven Longren" <[email hidden]> wrote in message
news:[email hidden]...

Quoted message said:

Assuming full hydraulic brakes, bleed the line. . . pulsing is caused by

air

Quoted message said:

in the compressionless fluid. . .

care to explain this??

(Pete Cresswell) said:

Per Boyle M. Owl:

Quoted message said:

Also, replace
the brake pads, because if there was anything on the rotor, it's
definitely embedded in the pads and will be re-deposited on the rotor
and will drive you nuts.

Devil's advocate question: Why isn't it getting re-deposited right now? i.e.
why isn't the slippery spot spreading until there's little or no pulse - just
overall diminished braking ability?

True... I said "if anything was on the rotor"...so if there isn't
anything on the rotor, then there isn't on the pads. And yeah, it would
be smeared all over the rotor.

Quoted message said:

Is the iron lapping plate something I can purchase at a reasonable price?

Actually, you can. All it is, is an iron plate ground flat with grooves
cut into it every half inch (or however you want it), for the lubricant
(oil, kerosene, water, you pick...clover compound has oil)

Quoted message said:

If so, what kind of store?

MSC or Grainger. Have them ship you the catalogs. Your postman will
kill you, though. They're heavy.

http://www1.mscdirect.com/cgi/nnsrhm
http://www.grainger.com/Grainger/wwg/start.shtml

Quoted message said:

I'd guess it's just a hunk of iron or steel that has a
really flat face and I'd smear some sort of abrasive compound ("Clover
compound"?)

Yup.

And for good measure, take a piece of round steel and roll it over the
surface, to embed the grit into the iron.

Quoted message said:

on it and then rub the disk on it. (?) If so, would coarse
automotive polishing compound and a sheet of glass suffice for a one-time
attempt?

I don't know. I don't know how hard the grit is. Also, automotive
polishing compound is water soluble, and after a few drags over the
glass, the rotor will likely stick to the glass pretty hard as the
compound has dried up.

Clover compound is aluminum oxide in an oil. Stir it up with a
screwdriver or something before sticking your finger in there (YUMMY!)
and smear some on the lapping plate. If you need to thin it out, use a
little bit (a few drops) of kerosene on the plate. This will make it
cut more agressively. As you lap the rotor, take it off from time to
time and check with a micrometer for high spots and put fingertip
pressure on the high spot.

Clover compound comes in little green pint cans. One pint can will last
you 20 years, and it's cheap.

Quoted message said:

I'm thinking that the lapping routine would be a good no-brainer action whenever
there's a problem. i.e. don't look further; lap the thing and see if the
problem goes away. If not, look further. If it goes away, only to recur in a
few hours of riding, replace the pads and lap the disc one more time.

That's fine. You're only removing less than a thousandth at a time anyway.

Quoted message said:

I'm kind of dancing around the dial indicator thing bc I don't have one and
can't imagine they're exactly cheap...

If cost is an issue, buy the chinese ones, dirt cheap. Under 20 bux.
They're not Federals (http://www.mahr.com/index.php?Language=enus),
which are my faves (and they'd actually outlive you if taken care of).
Buy a 0 to 1 inch that has .001 inch graduations and get a magnetic
base, so you can stick it to your wheel stand. You can use this for
truing your wheels, too, instead of using the feelers.

BTW, I've got a Federal indicator that's pre WWII. It works better than
the new chinese ones.

BTW, I saw a Starrett dial caliper the other day. It didn't say "Athol,
Massachussetts" on it, it said "China"

This may or may not make you seriously angry.

Quoted message said:

.I've put it in a truing stand and done
something functionally-similar just eyeballing/listening to the flat end of a
tool against the rotor surface with the wheel spinning - but can't perceive any
irregularity.

Then it probably needs resurfacing/lapping. It's like the disk brakes
in your car. When they pulse, the mechanic will turn them and make them
flat again, if they're not grooved too much.

Quoted message said:

In all the threads about rim vs disc brakes, the idea of discs being
high-maintainence in this respect doesn't seem to come up - but I think it's a
valid pro/con point...not so much the idea of noise or pulsing per se (which
comes up in relation to both types of brakes)...but the elusiveness of the root
cause when it's a disc brake.

Rim brakes are simpler mechanisms, so there's less that can go wrong
with them. Therefore, debugging is easier.

Please note that I'm a machinist and toolmaker, and I tried to give a
different geek view on this. I've got access to all of this stuff, so
it's easy for me to say "lap the rotor". It may be simply better for
you to get a new rotor if it's too much trouble/expense, and the rotor
is pissing you off enough.

--
BMO