What is Black Sludge (rim brake question)?

[Phileas]

When it is wet and/or the rims are contaminated with black sludge, slotted brake blocks will work much better than once without slots, and it is an easy thing to try , so I'd suggest doing that first of all. You can demonstrate that this makes a difference (or not) by recutting slots in the brake blocks in situ, i.e. without disturbing anything else, even the alignement.

Well I no longer use rim brakes on my commuting bike so it’s academic for me.

[PH]

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 have some Rigida CSS rims and a similar experience. Originally with the purpose made blue Swisstop brake pads and now with Koolstop salmon pads made for aluminium rims. Same brakes and pads - aluminium rims = plenty of sludge. CSS rims = clean.

[horizon]

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 have some Rigida CSS rims and a similar experience. Originally with the purpose made blue Swisstop brake pads and now with Koolstop salmon pads made for aluminium rims. Same brakes and pads - aluminium rims = plenty of sludge. CSS rims = clean.

I don't understand why the rims make the difference if it is the pads that make up the sludge.

[PH]

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 have some Rigida CSS rims and a similar experience. Originally with the purpose made blue Swisstop brake pads and now with Koolstop salmon pads made for aluminium rims. Same brakes and pads - aluminium rims = plenty of sludge. CSS rims = clean.

I don't understand why the rims make the difference if it is the pads that make up the sludge.

I'm convinced by the experience that it's the rims rather than the pads that make up most of it.

[andrew_s]

So am I right in thinking that all this rim wear stuff is a load of nonsense?

No, you're not right.

A rim that's been worn down enough that the flange has bent out under tyre pressure and cracked


A rim that was used with the brake blocks set too low, so it wore through into the cavity.
The original thickness can be seen by the hook

[mercalia]

I have some Rigida CSS rims and a similar experience. Originally with the purpose made blue Swisstop brake pads and now with Koolstop salmon pads made for aluminium rims. Same brakes and pads - aluminium rims = plenty of sludge. CSS rims = clean.

I don't understand why the rims make the difference if it is the pads that make up the sludge.

I'm convinced by the experience that it's the rims rather than the pads that make up most of it.

I suspect its both. My Exal SP19 ceramic rims are mirror like now so dont act like a file on the pads? which is what pads do to simple alloy rims? The nasty rims my Dawes Horizon came with were so soft that braking in fact gouged out bits of the rim and embeded in the pads. The Sputniks are made of harder alloy ( I found that out when widened the valve hole, I practised on the Horizon rims first with a hand drill - easy - had to use a power drill on the Sputniks)

[Brucey]

re proportions of aluminium vs brake block in the sludge; obviously it varies depending on the wear rates of each, but considering one side of a wheel with an aluminium rim, and a 10mm wide brake track/brake block;

Rim wear (of 0.5mm in the brake surface) by volume, in cm^3. ~195 x 0.05 x 1 = 9.75cm^3

Typical brake block wear (3mm in thickness) by volume, in cm^3. ~5.5 x 1 x 0.3 = 1.65cm^3

I guess you might wear out two sets of brake blocks for that amount of rim wear, but even so that still makes the sludge 2/3rds rim debris, 1/3rd brake block, on average.

Obviously CSS rim wear rate is a tiny fraction of that, so there would be hardly any rim debris to speak of.

cheers

[mercalia]

Am right in thinking the only CSS rims are Ryde Grizzlys? - SJS says these are not inended for heavy loaded bikes as only single eyeletted and will thus fail?

[horizon]

So am I right in thinking that all this rim wear stuff is a load of nonsense?

No, you're not right.

No, it seems not. So the brake pad alone theory isn't correct (shame) and the road debris theory certainly isn't so it's rim (with some brake pad). The only thing I wonder here if there is a strong difference between different makes of brake pad - softer might be easier on the rims.

I'm not sure where to go with this now as I have to conclude that:

1. Rims really are vulnerable to pad wear (under all braking conditions, not just wet).
2. The sludge when wet really does reduce braking performance.
3. The sludge builds up in minutes and therefore when wet, braking performance is immediately affected; cleaning after a ride is therefore of little value.

Two things I still cannot equate are (a) my own experience is that rim wear isn't fast enough to be reflected in that amount of sludge - the previous wheels on this bike were 20 years old. (b) how this problem is mitigated in disc brakes but not in rim brakes (I accept the distance from the source of water but is the residue still there?).

[mercalia]

So am I right in thinking that all this rim wear stuff is a load of nonsense?

No, you're not right.

No, it seems not. So the brake pad alone theory isn't correct (shame) and the road debris theory certainly isn't so it's rim (with some brake pad). The only thing I wonder here if there is a strong difference between different makes of brake pad - softer might be easier on the rims.

I'm not sure where to go with this now as I have to conclude that:

1. Rims really are vulnerable to pad wear (under all braking conditions, not just wet).
2. The sludge when wet really does reduce braking performance.
3. The sludge builds up in minutes and therefore when wet, braking performance is immediately affected; cleaning after a ride is therefore of little value.

Two things I still cannot equate are (a) my own experience is that rim wear isn't fast enough to be reflected in that amount of sludge - the previous wheels on this bike were 20 years old. (b) how this problem is mitigated in disc brakes but not in rim brakes (I accept the distance from the source of water but is the residue still there?).

DIsc brakes- the discs are steel not alloy & the pads presumably some thing similar to my m/c ones either organic or scintered? I dont think rims would last long with scintered blocks

[PH]

Am right in thinking the only CSS rims are Ryde Grizzlys? - SJS says these are not inended for heavy loaded bikes as only single eyeletted and will thus fail?

I think Rigida/Ryde offered the CSS on several rims, though it's academic as they've been discontinued for a while and anything available is old stock. There's been rumors of them making a comeback, but I haven't seen anything to back that up.
I have three, a derailleur rear and dynamo front built by Spa about ten years ago, around 20,000 miles on a touring bike, about a quarter of that loaded touring, half commuting with at least one pannier and the rest a mixture of leisure rides. I also have a SJS built dynamo front, a year younger, double the mileage, mixed usage, no issues. it originally had a matching Rohloff rear, the eyelets split, replaced under warranty and split again, replaced with a Sputnik, no issues.
Make of that what you will.

[PH]

Obviously CSS rim wear rate is a tiny fraction of that, so there would be hardly any rim debris to speak of.cheers

I'm not daft enough to think the pad wear disappears into thin air, but whatever it is and wherever it goes, the rims still look reasonably clean after a day in the rain and there's no sludge. There's certainly no noticeable muck from the brakes on the frame or forks.

[Brucey]

re disc brake wear, a similar calculation;

Disc wear (165mm disc with 20mm track, 0.25mm wear) in cm^3 14.5 x 3.14 x 2 x 0.025 = 2.28 cm^3

Pad wear (20mm x 20mm pad, 1.25mm wear) in cm^3 2 x 2 x 0.125 = 0.5 cm^3

Again you might wear two sets of pads for that amount of disc wear, making the debris about 70:30 disc/pad on average.

However I would say that, since many disc brakes

a) barely work at all when the discs are actually properly wet and
b) soon dry themselves in use, unless the rain is really tipping down

most of the time the brake is wearing/working dry, and any debris is ejected as a dry powder, relatively little of which gets the chance to stick to the bike. There is less of it anyway (although the masses of worn material concerned are much closer).

Rim brakes (even front brakes) tend to pick up some road dirt in the wet; riding slowly and having mudguards with brackets that encourage dribbles of crud onto the wheel make that more likely.

cheers

[mercalia]

Obviously CSS rim wear rate is a tiny fraction of that, so there would be hardly any rim debris to speak of.cheers

I'm not daft enough to think the pad wear disappears into thin air, but whatever it is and wherever it goes, the rims still look reasonably clean after a day in the rain and there's no sludge. There's certainly no noticeable muck from the brakes on the frame or forks.

maybe sludge is a combination of wear due to braking and wear due to abrasive action of the rim on the pads? maybe two differnt things? And maybe most of the sludge is due to abrasion, with alloy rims? Also if there is alloy particles in the sludge when wet it becomes a grinding paste?

[Brucey]

Obviously CSS rim wear rate is a tiny fraction of that, so there would be hardly any rim debris to speak of.cheers

I'm not daft enough to think the pad wear disappears into thin air, but whatever it is and wherever it goes, the rims still look reasonably clean after a day in the rain and there's no sludge. There's certainly no noticeable muck from the brakes on the frame or forks.

the wear debris would 'normally' (roughly, on average) be 2/3 rim and 1/3 brake block on a standard rim. In wet conditions it might be a different ratio to that. Also on CSS rims the brake blocks may not wear more quickly in the same way as brake blocks do on standard rims. Finally the particles of debris migth be a very different size and have different abilities to 'stick' to surfaces (e.g. have you ever tried cleaning up soot? -like that).

So with CSS rims you might expect 1/3rd the debris even if the wear rate of the brake block is the same, and if it is different it might be 1/3rd of that. And the debris itself might be much less (say x5 less) likely to stick to the bike and make an unsightly mess. This is not proven by any means but potentially this could explain why you see a tiny fraction of the mess.

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