Cycling UK Forum

I did a test yesterday riding in heavy rain and wind, actually the calm before Storm Dennis hits. The roads were running over with water and it rained incessantly. So a good time for this experiment:

Before I set off I cleaned and dried my rims. There was a black residue from a previous ride. After a couple of miles I stopped in a shop doorway and cleaned them again - thick black soft sludge. I did this about three times after just a couple of miles each time. After the last time I left it longer and I started to hear grinding noises. Now I know that this proves the case for disc brakes but I have to say that the brakes worked well after cleaning, even down long steep hills in heavy rain. Actually I was pleased and surprised by this as by then on this trip I had written off rim brakes as useless. What did shock me though was the speed at which they sludged up again after cleaning. My conclusion was that rim brakes are fine in the wet: it is the sludge, not water, that is the problem.

So my question is: what really is this sludge and how does it get on the rims? I don't think it is either rim material (though that might come later after grinding) or brake blocks - these were still good. My presumption is that it is other people's car tyre debris from the road itself.

Any thoughts?

I thought it was a mix of brake blocks and road dirt including car tyre and brake dust, but I've not done any chemical testing or mass spectroscopy to confirm that.

It's rim material. If it was pollution from traffic, then it would be all over the bike rather than just the rims, and concentrated around the brake pads,

I always used to get this during wintry weather and I assumed it was linked to something added to the road grit (molasses?) to extend its usefulness. I’ve had a few hairy moments when my brakes failed to completely stop me on downslopes when the rims had this sort of black slime on them.

The sludge is mostly a mixture of brake block and rim material. You get more road dirt mixed in with it on a rear brake, but it doesn't look much different to the stuff that comes off front brakes, which usually contains very little road dirt indeed. Note that most brake blocks do wear a lot faster in wet weather.

But surely finely divided aluminium should look silvery? Or the sludge should be brake block colour? Nope, it doesn't work like that; finely divided stuff quite often looks black in good part because the particles are smaller than the wavelength of light. You can see this very clearly when using metal polish; you can start with clean metal and white metal polish, and very soon end up with black sludge.

FWIW the one simple thing that makes the biggest difference to wet weather braking is that the brake blocks (still) have slots in them, and that the slots don't get clogged with crud. The slots often reduce the dry weather brake performance, but the wet braking performance can be utter rubbish without them. Thicker brake blocks can be recut when half-worn, so that the wet braking is still OK; lots of brake blocks just don't have enough slots in from the start.

[edit; one reason slots can make dry braking worse is that the whole brake block can flex under load and this can means that the loading on the brake block is no longer uniform, leading to 'hot spots' in the brake block. Hence slots are usually not cut more than ~2mm into the face of the friction material, even if there is 4mm to wear away; if they were then the dry braking with unworn brake blocks could be rather underwhelming. However this does mean that unless you recut the slots, you won't get good life and good braking from your brake blocks.]

cheers

It’s mostly brake pad material. It has to go somewhere when it wears away. The rim may look smooth but the metal has thousands of pits and imperfections in its surface that act like a microscopic cheese grater.

I often recut slots in my brake pads. It also helps to indicate how much brake pad material remains and when you will need to renew the blocks, often well past the limit marks engraved on them.

Brucey wrote:The sludge is mostly a mixture of brake block and rim material.

cheers

I'm thinking that there is a lot more of it in the wet - so am I wrong on this (I must be)? And am I right in thinking that it's all there in the dry but doesn't affect braking because it isn't slimey?

And if it isn't from the road, then why are disc brakes not affected in the same way?

Particulates that, because it was wet, have not ended up in the air.

peetee wrote:It’s mostly brake pad material. It has to go somewhere when it wears away. The rim may look smooth but the metal has thousands of pits and imperfections in its surface that act like a microscopic cheese grater.

So am I right in thinking that all this rim wear stuff is a load of nonsense? It is the rims that wear the blocks not the other way round. My experience is that my rims don't wear much from braking very much but I do get through blocks - fine by me.

Further to my earlier post, from this article http://www.compassmineralsuk.com/news/f ... d-deicing/
Thawrox+ is rock salt with a molasses-based additive called Safecote®, which improves the spread pattern and resilient properties of standard salt, allowing a reduction in the amount of salt used

As I said,I only used to get this black slime on my rims following a snow thaw. YMMV

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.

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

I think you answered it just there

Phileas wrote:Further to my earlier post, from this article http://www.compassmineralsuk.com/news/f ... d-deicing/
Thawrox+ is rock salt with a molasses-based additive called Safecote®, which improves the spread pattern and resilient properties of standard salt, allowing a reduction in the amount of salt used

As I said,I only used to get this black slime on my rims following a snow thaw. YMMV

The rate of wear of aluminium rims and brake blocks is always increased whenever there is road salt present; essentially there is chemistry going on when the brake blocks contact the rims. Normally the Aluminium surface which is freshly exposed (by the slightest wear) immediately oxidises again to Aluminium oxide, which is hard and unreactive (were this not to occur, aluminium rims would last about a week). When there is just water present, the reaction is likely to be slightly different, but the increase in wear rate arises mainly because the water acts as a cutting lubricant.

However when there is even a small quantity of salt present, there are other reactions which can occur at the asperities of the rim surface, and this would be expected to lead to higher wear rates of both rim and brake block.

I'm unsure of the exact effect of molasses here (I don't think it is used local to me, leastways I have not noticed), but IME wet makes lots of black crud and wet + road salt makes much more black crud than that. [NB Snow thaw on gritted roads can lead to very salty water/slush on the road, whereas when it rains, it soon washes most of the salt away.]

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.

cheers

Bonefishblues wrote:

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

I think you answered it just there

The brake blocks I have on the bike aren't my usual ones so I'm going to check in future when I'm on a different bike to see if there is a difference between blocks.

Thanks all BTW, this has been a very enlightening thread!