What is Black Sludge (rim brake question)?

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PH
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Re: What is Black Sludge (rim brake question)?

Postby PH » 15 Feb 2020, 7:16pm

Brucey wrote:I can tell you that when handling standard rims that have not been cleaned, the mess is often unbeleiveable; it gets everywhere, stains clothes badly, and is even difficult to wash off your hands. Yuk!
cheers

Yes, it's this above all else that makes me a fan of the CSS rims.

PDQ Mobile
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Re: What is Black Sludge (rim brake question)?

Postby PDQ Mobile » 16 Feb 2020, 12:32am

There is into this interesting mix, to be put, the factor of the actual interface between block and rim.

If the rim is smoothly polished and no metal shards are embedded in the block wear rates are very very much reduced.

Such a polished rim surface can be promoted by always easing/ stopping braking for a short time as soon as any noise is heard. Feathering front to back on steep descents until the rim has warmed and dried. Once the rim is fully dry any abrasive "sludge" becomes a dust which disperses and the interface stays clean. The sludge is abrasive.
In very wet and dirty conditions this desirable state of affairs is slower and harder to achieve but it can still mostly be done.
Choice of pad is very important.
In my experience some Shimano blocks were designed to wear rims out and achieving such a polish is impossible. Koolstop Salmons polish rims well. I guess there are others.
Ceramic surfaces achieve this polish in another way.

The same applies to discs (I think).
If the disc is polished then wear of the disc will be very reduced given the right quality pads.
Certainly Brucey's proportion of wear figure here seems high for the disc.
My only long term experience of discs is automotive. A set of discs will often outlast 5 sets of pads. Corrosion is the enemy here. Rust on the disc will kill both pads and disc.
So winter salt and standing around unused are big factors.
I suspect a S. Italian vehicle, for example, will demonstrate much higher disc life than a Welsh one.

And maybe S. Italian bike rims last longer too, if treated right and with quality blocks?
All IMHO.

Brucey
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Re: What is Black Sludge (rim brake question)?

Postby Brucey » 16 Feb 2020, 8:27am

re discs and pads; they vary.

On a bicycle, if you are happy with organic pads with a low metal content (BTW I find they are often nearly completely useless when properly wet) then the disc rotors can have a low wear rate. If you use sintered pads you get more consistent braking but higher wear rate. Other pads with an organic (or 'resin') base have various amounts of metal etc added and are variously abrasive/disc wearing.

Shimano appear to be using a rather abrasive pad compound as standard in their current road disc brakes; in combination with their ice-tech rotors the wear rate of the disc seems very high to me; often the discs themselves are at or beyond the allowable wear limits after just one or two sets of pads have been used. I think they have chosen a high friction coefficient so that the brakes feel good (despite relatively small rotor size) at the expense of life.


On normal (roadgoing) cars with disc brakes, for many years the pads could be 'filming' type, whereby in the dry the pad deposited a layer of material on the disc, and subsequently the disc barely wore at all. This worked well unless the brakes got wet or muddy, in which case the film would be disrupted and the brakes would (effectively) need bedding in again. Using such pads, a set of brake discs could outlast the rest of the vehicle, provided they didn't go rusty. In that era, all that was required for good safety on the road was that disc brakes worked as well or better than the brakes on most other vehicles, which at first wasn't that difficult, as there were still plenty of vehicles with rather poor brakes.

However as more vehicles were fitted with disc brakes and technologies such as ABS were introduced, some manufacturers started fitting more abrasive pad compounds. In simplistic terms these worked by keeping the disc face clean, at the expense of disc rotor life. This meant that the brake could be made to have better 'cold bite' and could be made to behave more consistently (important for ABS operation) regardless of brake temperature.

It also meant that the brakes were more resistant to contamination; with 'filming' pads fitted, a couple of greasy fingerprints on the disc could spell disaster. With abrasive pads, not so much. Typically the service interval for brake disc replacement went from 'as needed, maybe never' to ' about 30000 miles (or two sets of pads) if you are lucky' . For the last couple of decades or more pretty much every vehicle sold has been fitted with abrasive pads and brake discs that are expected to be changed several times through the vehicle life. You can still buy aftermarket pads which are kinder to the discs, but they also usually have noticeably poorer 'cold bite' and do not usually perform as well in cold braking tests.

In bicycle rim brakes the rubber compound can vary and so can the filler in the recipe. If you look at a brake block under a microscope you can sometimes see particles of (potentially abrasive) filler in the material. These brake blocks will always wear the rims; clues that you have this type of brake block include that the brake is noisy at all times (even in the dry with new brake blocks fitted) and that the brakes work fairly well in the wet, even if the brake blocks don't have good slots in them.

However by far the most likely cause of severe rim wear is that particles of rim material (much more likely when using rims with a machined braking surface) and/or dirt get embedded in the surface of the brake block and once this happens it becomes an accelerating process, with positive feedback if you like. Often you can hear the difference when using the brakes; I will stop, investigate and correct if I hear funny noises but many other riders seem completely oblivious to what is happening, until their rims wear out.

As others describe, provided the brake block compound is not too abrasive, they are free from debris, and used in clean conditions, aluminium rims can appear to be polished by the brake blocks, and can last well. However this is often accompanied by less good braking performance, especially in the wet. It is all swings and roundabouts; there is no such thing as a 'perfect brake', just ones that are, for any given task, better than others.

cheers
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531colin
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Re: What is Black Sludge (rim brake question)?

Postby 531colin » 16 Feb 2020, 1:13pm

mercalia wrote:since I rebuilt my wheels with rims with ceramic brake surfaces and discovered Shimano S70C brake blocks I dont see any residue on the rims. The brake blocks dont seem to wear down very much either. no squealing and wet weather preformance not too bad after the water has been removed, before which it is a bit scarey.


I'm using KoolStop salmon on rigida ceramic grizzleys; not exactly the same type of rim coating, but much better in the wet than the Koolstop pads recommended for carbide, which are rock hard. I only used those to smooth the rims when they were new; dreadful in the wet.
Or theres the ones R2 recommends, the name of which escapes me....

Brucey
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Re: What is Black Sludge (rim brake question)?

Postby Brucey » 17 Feb 2020, 11:01am

FWIW I was surprised at how much wear debris my estimations predicted for both rim brakes (which I found fairly believable) and disc brakes. However yesterday I overhauled a caliper from a 'nice clean disc brake' (a BB5 with a seized FPA, no surprise there) and I attach the result;

Image01594.jpg
mucky pup


I think the pads had been changed once before without much cleaning, so the muck represents one and a bit pad set's worth if you like. What is in the picture is a small fraction of the whole, it is just what brushed off the outside of the caliper and fell out when I removed the pads. The caliper did come from a bike which didn't seem to have been cleaned much though.

Much of what was there would stick to a magnet. Since no pad material I have encountered to date seems to stick to a magnet but disc material (typically a 12Cr stainless steel) usually does, I think it contains a lot of disc material.

Needless to say there might be less of it but this finely divided material is just as nasty to deal with as brake dust from rim brakes; it makes everything dirty and I needed to scrub my hands clean afterwards. I also note that this (highly abrasive) stuff is generated in close proximity to the LH hub bearing and the chain/sprockets are not that far away either, in the case of a rear brake. On a FS MTB with disc brakes I had years ago, the LH pivot near the dropout in the four-bar linkage wore out in short order; I reckon brake dust had a lot to do with that.

So yes, disc brakes are cleaner than rim brakes which use standard aluminium rims (esp in wet weather) , but they are not perfecty clean by any means, and indeed may often be less clean than rim brakes using hard-coated rims.

cheers
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mig
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Re: What is Black Sludge (rim brake question)?

Postby mig » 17 Feb 2020, 12:31pm

PDQ Mobile wrote:There is into this interesting mix, to be put, the factor of the actual interface between block and rim.

If the rim is smoothly polished and no metal shards are embedded in the block wear rates are very very much reduced.

Such a polished rim surface can be promoted by always easing/ stopping braking for a short time as soon as any noise is heard. Feathering front to back on steep descents until the rim has warmed and dried. Once the rim is fully dry any abrasive "sludge" becomes a dust which disperses and the interface stays clean. The sludge is abrasive.
In very wet and dirty conditions this desirable state of affairs is slower and harder to achieve but it can still mostly be done.
Choice of pad is very important.
In my experience some Shimano blocks were designed to wear rims out and achieving such a polish is impossible. Koolstop Salmons polish rims well. I guess there are others.
Ceramic surfaces achieve this polish in another way.

The same applies to discs (I think).
If the disc is polished then wear of the disc will be very reduced given the right quality pads.
Certainly Brucey's proportion of wear figure here seems high for the disc.
My only long term experience of discs is automotive. A set of discs will often outlast 5 sets of pads. Corrosion is the enemy here. Rust on the disc will kill both pads and disc.
So winter salt and standing around unused are big factors.
I suspect a S. Italian vehicle, for example, will demonstrate much higher disc life than a Welsh one.

And maybe S. Italian bike rims last longer too, if treated right and with quality blocks?
All IMHO.


car discs rust within hours of stopping. at least mine do. according to a retired mechanic friend of mine they are (now) made out of the cheapest metals in existence. he did fit some NOS discs to my car years ago (made in wales he said) and they've performed beautifully ever since.

the brakes on the commuter bike are working well today despite the floods and associated crud on the road surfaces. this thread might well inspire me to get some old mavic ceramics built up for next winter.

bgnukem
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Re: What is Black Sludge (rim brake question)?

Postby bgnukem » 17 Feb 2020, 1:19pm

It's part of an ongoing trend for things to wear out faster. Shimano have been at it since I've been cycling, thinner chains, thinner sprockets with shallower teeth, 'top swing' front mechs that wear out faster, dreadful rim brake pads that eat rims, the rim manufacturer's themselves producing softer rims with thinner sidewalls (Mavic with 1mm thick rims when new!) and machined braking surfaces that increase swarf pickup by the pads as Brucey says.

'Icetech' disc rotors with stupidly low wear limits, smaller discs that overheat and wear out faster, more abrasive pad materials. Shimano are the masters of built-in obsolesence and equipment that performs well when new but lasts 5 minutes in British cycling conditions. Can't comment on SRAM or Campag as I don't use them.

Modern cars and applicances are not much better, designed to last the warranty period then fail.

Personally with brake pads I've been using Koolstop Salmons to try to eke a little more life out of my chocolate rims, but I had a set that persistently grated on the back of my winter bike and the only reason I could see if that the pads were long V-type and contained about 6 v-shaped grooves. I recently replaced with shorter pads minus grooves as it occurred to me the v-shaped grooves might be collecting and retaining rim wear debris and road grit with which to score the rim.

With discs I noted the Fibrax black semi-metallic pads were much much quieter than the default Shimano sintered which grated horribly and never felt smooth, so I expect the discs to last longer. I use a basic solid steel rotor rather than Icetech but the wear limit is still only around 0.25mm which seems daft to me as the rotor thickness is around 1.5-1.75mm (?) when new and its solid stainless steel.....

nigelnightmare
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Re: What is Black Sludge (rim brake question)?

Postby nigelnightmare » 19 Feb 2020, 12:59pm

I don't have any of the afore mentioned problems with my brakes.
I have Sturmey Archer drum brakes. :mrgreen:
13 years old and still on the original brake shoes.
Good for around 50,000 miles or so.
No rim wear or sludging.
Just consistent reliable braking in all weather conditions. 8)

bgnukem
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Re: What is Black Sludge (rim brake question)?

Postby bgnukem » 19 Feb 2020, 5:47pm

Drum brakes are the future!! :D

While we're at it, can we bring back reliable hub gears, proper (i.e. long enough) mudguards and grease ports in hubs, pedals and bottom brackets???

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horizon
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Re: What is Black Sludge (rim brake question)?

Postby horizon » 19 Feb 2020, 6:24pm

I've been trying to derive some solace from this thread but apart from going down the drums/discs/ceramic rims routes, I've only got the following to go on:

1. Residue from rim/pad wear is the primary cause and is inevitable.
2. Getting the rims wet is the immediate cause as this produces the sludge/slime.
3. Rims will get wet in the rain.
4. The resulting sludge seriously diminishes braking performance.

The workarounds are:

1. Clean the rims before AND at intervals during the ride if rain persists.
2. Try out different pads (I'm going to experiment)

One workaround I'm less convinced about now is simply adjusting riding speed to braking distance: the brakes are deteriorating quickly in wet conditions and building up a grinding paste. This isn't a problem of stopping distance in the wet (which can be allowed for) - this is a problem of the brakes themselves becoming inoperative (and worse) due to the sludge/slime build up.

I feel a lot better knowing more about the cause of the problem. I might try out discs on my Sardar but I am otherwise wedded to (and mostly very happy) with rim brakes. What I would choose for a new bike, I am not sure but probably mechanical discs.
When the pestilence strikes from the East, go far and breathe the cold air deeply. Ignore the sage, stay not indoors. Ho Ri Zon 12th Century Chinese philosopher

Brucey
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Re: What is Black Sludge (rim brake question)?

Postby Brucey » 19 Feb 2020, 11:45pm

you missed the benefits of slotted brake blocks; IME an angled (slash cut) slot (to throw debris forwards and outwards) is better than a V shaped slot. Preferably the slots should be tapered too, such that they get wider in the forwards/outwards direction.

More slots = more chances to scrape the crud off the rim.

cheers
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horizon
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Re: What is Black Sludge (rim brake question)?

Postby horizon » 20 Feb 2020, 1:10am

Brucey wrote:you missed the benefits of slotted brake blocks; IME an angled (slash cut) slot (to throw debris forwards and outwards) is better than a V shaped slot. Preferably the slots should be tapered too, such that they get wider in the forwards/outwards direction.

More slots = more chances to scrape the crud off the rim.

cheers


Thanks Brucey. I should also have added checking blocks for sharps and checking blocks for filling which you mentioned above (and which may mean buying different blocks). I'm quite keen to start looking at different blocks now as well as keeping on top of the problem generally.
When the pestilence strikes from the East, go far and breathe the cold air deeply. Ignore the sage, stay not indoors. Ho Ri Zon 12th Century Chinese philosopher

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horizon
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Re: What is Black Sludge (rim brake question)?

Postby horizon » 26 Feb 2020, 2:43pm

I've ordered a pair of Koolstop MTB Salmons as a trial run. I'm not certain they have the best slots but I'm going to give them a try (all the other advice notwithstanding).
When the pestilence strikes from the East, go far and breathe the cold air deeply. Ignore the sage, stay not indoors. Ho Ri Zon 12th Century Chinese philosopher

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Re: What is Black Sludge (rim brake question)?

Postby PDQ Mobile » 26 Feb 2020, 10:57pm

horizon wrote:I've ordered a pair of Koolstop MTB Salmons as a trial run. I'm not certain they have the best slots but I'm going to give them a try (all the other advice notwithstanding).

I don't think you'll be disappointed.
Come back and tell us after they have given a bit of service.
I have a bike with Alesa touring rims, they are nice and strong. In my ownership they have only seen Koolstops.
I always have tried to keep the braking surfaces polished by judicious use of alternating the brakes as soon as "noise" is heard.
The rims have done 15 or 20 thousand miles at a guess and show very little thickness loss on the braking surfaces.
I wiped them round with Meths yesterday, after a quick lube service, to remove any lube (GT 85) overspray and dirty oil spots on the rear from the chain. There is black on the rag but the rims really are almost as new.

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horizon
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Re: What is Black Sludge (rim brake question)?

Postby horizon » 27 Feb 2020, 10:04am

PDQ Mobile wrote:I don't think you'll be disappointed.

The rims have done 15 or 20 thousand miles at a guess and show very little thickness loss on the braking surfaces.



PDQ: many thanks for that. That's quite incredible (in a good way) as it starts to question whether rim wear need be an determinant issue in brake choice. Wet weather performance is obviously the issue on this thread but rim wear is often quoted as the problem with rim brakes. My own experience matches yours in terms of how long rims last.
When the pestilence strikes from the East, go far and breathe the cold air deeply. Ignore the sage, stay not indoors. Ho Ri Zon 12th Century Chinese philosopher