Poor disc brake performance

[Yellow Saddle]

I use cable operated trp spyres....they are awesome in all conditions and inspire full confidence at 40+mph on descents

I agree. In spite of their feel and adjustment problems, in the sweet spot they work as well as any other disc, given similar pads and disc condition.

 

[DWM]

As requested here is a close up of the rotor. Does it look OK?

Cheers

Dave

 

[ColinJ]

I have just checked my CX bike and reminded myself how the brakes are adjusted ...

The pads are very close to the rotors, but not touching. There is not room to get a business card between pad and rotor, so say about half the thickness of the card away.

You want the rotor midway between the pads. Achieve that by slackening the bolts holding the brake on, and then apply the brake. Retighten the bolts while the brake pads are squeezing the rotor. That should have centred everything nicely. (Check by looking down at the brake and observing the gaps either side of the rotor without the brake applied.)

Now you need to take up the slack in the cable. Slacken the cable off so the brake lever can be pulled slightly without operating the brake. Gradually tighten the cable until there is no slack left. At that point, the slightest movement in the lever starts to apply the brake. You should not tighten beyond that point or you will be compromising the ability of the brake to compensate for pad wear. Check that you have not overtightened by pushing in the black spring-loaded knurled pin on the brake assembly and ensuring that it will still screw in. If you can't get it to screw in then you overtightened the cable. Release the pin again.

If you have done all of that and the brake power is still inadequate then you might have contaminated pads or rotor.

The only other thing that occurs to me is that you might have cheapo brake cable outers which are compressing when you apply the brake. If so, replace them with better quality ones.

If the brakes still don't work then I am baffled! I replaced my bad front brake with a Hy/Rd and it stops me fine on our steep local descents.

Good luck!

 

[mythste]

I have just checked my CX bike and reminded myself how the brakes are adjusted ...

The pads are very close to the rotors, but not touching. There is not room to get a business card between pad and rotor, so say about half the thickness of the card away.

You want the rotor midway between the pads. Achieve that by slackening the bolts holding the brake on, and then apply the brake. Retighten the bolts while the brake pads are squeezing the rotor. That should have centred everything nicely. (Check by looking down at the brake and observing the gaps either side of the rotor without the brake applied.)

Now you need to take up the slack in the cable. Slacken the cable off so the brake lever can be pulled slightly without operating the brake. Gradually tighten the cable until there is no slack left. At that point, the slightest movement in the lever starts to apply the brake. You should not tighten beyond that point or you will be compromising the ability of the brake to compensate for pad wear. Check that you have not overtightened by pushing in the black spring-loaded knurled pin on the brake assembly and ensuring that it will still screw in. If you can't get it to screw in then you overtightened the cable. Release the pin again.

If you have done all of that and the brake power is still inadequate then you might have contaminated pads or rotor.

The only other thing that occurs to me is that you might have cheapo brake cable outers which are compressing when you apply the brake. If so, replace them with better quality ones.

If the brakes still don't work then I am baffled! I replaced my bad front brake with a Hy/Rd and it stops me fine on our steep local descents.

Good luck!

Colin makes a good point actually, the actual braking unit not being centered could have an affect, I hadn't realised that it wasn't one size fits all and you do have a degree of "float" on the mounting screws.

 

[Yellow Saddle]

As requested here is a close up of the rotor. Does it look OK?

Cheers

Dave

OK, that confirms it. Your discs are glazed and will require very high pressure to work even reasonably satisfactory. What you need to do now is to sand them until they have the texture of satin. Have a look at a new disc's texture to give you some guidance. It is hard work and not to be attempted with the discs on the bike. You will need some good quality garnet paper, perhaps 100 grit - pretty rough.

Sand the discs on both sides until the new texture is even and pervasive.

Now decide what type of performance you want. If you want high performance at high speeds where the discs run hot most of the time, use sintered metal pads. These provide most grip at high temperatures. They are also noisy but last a long time. At ambient temperate they require a lot of pressure to stop and will make noise.

If you do city type riding where the discs are never really taxed, go for resin pads. These provide most grip at low temperatures. They are quiet but don't last very long. If you overheat them on a very long descent, they will stink and contaminate the disc. Start over again.

Fit the new pads and follow a proper break-in procedure. If you think your old pads are still good and you know they have not been contaminated by solvents or oil, you can reuse them but sand them flat first. Put some sandpaper on a very flat surface like glass, place the pads on top, face down and move them in a figure 8 pattern until the surface is dull. The figure 8 pattern helps prevent sanding the pads with two tapered edges.

Don't swap pads to the other type without going through the sanding procedure again.

Edit: Please note that not all discs are metal-pad compatible. It is usually stamped on the disc as Resin-Only. If there are no markings, then assume it is resin and metal-compatible. Resin-only discs are stamped and you'll see that they have a slightly rounded edge on the one side and sharp on the other. Metal discs are laser cut and both edges will be sharp.

 

[mythste]

OK, that confirms it. Your discs are glazed and will require very high pressure to work even reasonably satisfactory. What you need to do now is to sand them until they have the texture of satin. Have a look at a new disc's texture to give you some guidance. It is hard work and not to be attempted with the discs on the bike. You will need some good quality garnet paper, perhaps 100 grit - pretty rough.

Sand the discs on both sides until the new texture is even and pervasive.

Now decide what type of performance you want. If you want high performance at high speeds where the discs run hot most of the time, use sintered metal pads. These provide most grip at high temperatures. They are also noisy but last a long time. At ambient temperate they require a lot of pressure to stop and will make noise.

If you do city type riding where the discs are never really taxed, go for resin pads. These provide most grip at low temperatures. They are quiet but don't last very long. If you overheat them on a very long descent, they will stink and contaminate the disc. Start over again.

Fit the new pads and follow a proper break-in procedure. If you think your old pads are still good and you know they have not been contaminated by solvents or oil, you can reuse them but sand them flat first. Put some sandpaper on a very flat surface like glass, place the pads on top, face down and move them in a figure 8 pattern until the surface is dull. The figure 8 pattern helps prevent sanding the pads with two tapered edges.

Don't swap pads to the other type without going through the sanding procedure again.

@Yellow Saddle - What causes a disk to glaze? Is it a contaminant?

 

[Yellow Saddle]

@Yellow Saddle - What causes a disk to glaze? Is it a contaminant?

Yes and no. This is one of those complicated things that can't be explained in one or two sentences.

It is a contaminant in that the disc is contaminated by the pad.

Friction is a strange animal and things don't really work like they teach us at school. At school they would have taught you that there is two types of friction - static and kinetic (sliding) friction. This is a simplification. Since we are talking brake pads which are involved in sliding friction, lets focus on that alone. When two surfaces slide over each other without external lubrication, then the lubrication is provided by one of the two surfaces melting locally to provide the required lubrication. For instance, this morning I had to cut a piece of leather in a straight line and I used an aluminium straight edge and a steel knife. Here is the knife blade in the region of where it slid against the alu straight edge. The scale is such that the length of blade you can see is 8mm long along the sharp edge. You can clearly see how the aluminium melted under even moderate force where it slid against the knife. The fact that I held the knife at different angles is evident fro the coverage of alu, spanning probably 6mm of the total 8mm. You can deduce the direction of the knife's travel by the shape of the aluminium blobs. These lie in a direction parallel to the blade edge.

This local melting happens between most materials but obviously some don't melt - wood or cork, for instance. However, aluminium melts on rim brakes and leaves molten metal embedded in the rubber. Nevertheless, material transfers from one surface to the other and migrates back again. In a healthy brake system this back and forth migration is even and results in an even layer of material on the harder of the two surfaces.

The same happens with brake pads. Material transfers from the pad to the disc and back all the time. However, for the brakes to work optimally, we need an initial transfer of pad material that is even and consistent. This "base layer", if you like, is what keeps the system working smoothly. Apply the base layer to thick, and the brakes underperform. Apply the base layer unevenly and the brakes shudder and vibrate and also underperform. Apply the wrong base layer by either mixing pads or by making pads heat up too quickly from poor bedding in, and the base layer will deposit contaminants and unevenly.

A glazed disk is one where the base layer is too thick or made up from foreign material such as overheated resin. This layer is impossible to remove chemically and has to be abraded away. It is very hard, in the brake industry they call it cementite. That alone gives you a clue as to its hardness and tenacity. On cars it deposits in clumps so thick people think the discs are warped. They aren't, the deposits are uneven.

This is a simplification of what goes on in brakes. Resin brakes are mostly adherent and metal pads mostly abrasive. Neither are purely abrasive or adherent. You need some abrading effect to keep the layer even and you need some adherent to coat the disc.

This passage from a book on the subject I'm busy with right now, goes into a bit more detail. Skip it if you have had enough.

Static friction is in a way easy to understand. The two objects are glued to each other by way of Van der Waals attraction and held together so tightly, they cannot move from the available force. It is when that bond is overcome and goes into sliding friction that things become a bit more muddy. We now understand how two close and adjacent molecules polarize to present each other with an opposing and thus attracting polarity and thus a small attraction. This is easy to visualize but what happens when the objects start to slide? Do the bonds break and hand over to the next available molecule as in the model of a cellular phone network? What happens if there are millions of adjacent molecules sliding over millions of adjacent molecules? Do some grip whilst others let go? Do they all grip and let go at the same time? Do they take turns?

We have to turn to tribology to get the answer to this. This field of study tells us that two materials sliding against each other without external lubrication melt or break off at the sliding interface.

If the material primarily breaks off, it is referred to as abrasive friction, one type of kinetic friction if you like. Although both surfaces lose material, the softer loses the most. The lost material isn’t all expelled in the form of dust or gas, some of it is transferred across to the adjacent surface and coats it in a random and uneven pattern.

If the material primarily melts, it is called adherent friction (a type of kinetic friction). In this mode the material melts locally to provide sliding lubrication and, it migrates to the adjacent surface and coats it. With further passes the effect is that two surfaces of the same material slide over each other with free migration of material back and forth.

Unless the softer material is completely uniform as in rubber on asphalt, the mode is neither purely abrasive or adherent friction but a combination of both. Some material breaks off and settles on the adjacent surface and some melt and migrate to the adjacent surface. Disc brake pads are designed so that there is always some adherent action present to keep the transferred layer even and uniform. If the pad is purely abrasive[1] (resin or organic pads), then the transferred layer will be lumpy and spotty, causing uneven sliding and pulsing, squealing and underperforming. The pad requires some adhesive qualities to smooth out the deposits.

Melting of a friction surface is evident when tyres slide over asphalt. A layer of molten rubber is laid down on the road and is quite visible. Molten material is not all that visible on disc brakes (but it is there nevertheless and carries with it lots of implications as we’ll see later on). However, the mere fact that objects which are sliding over each other eventually wear away, demonstrates the melting and material break-off theory. An added lubricant (which has to be thicker than the surface roughness) reduces wear but in the absence of such a lubricant the melting material offers the required lubrication. Many people find it easy to envisage this localized melting when thinking of rubber sliding against metal or asphalt but when it comes to two metals sliding over each other, they find it difficult to envisage melting at the surface, especially if there is no smoke and flames involved and the action seems so benign.



[1]Note that the terms abrasive and adherent seem contradictory in this context. Abrasive refers to the fact that material is broken (abraded) off and adherent refers to the depositing (adhesion) of material on the opposite surface by way of transferring molten material.

 

[DWM]

@Yellow Saddle - Wow. Thank you very much for your detailed response. I think I understand the process! What I don't understand (forgive me if I missed something) is why this has happened. Pads not bedded in enough? Too much? Brakes being dragged?

This rotors in question were cheap Wiggle own brand ones, so I will probably not bother with them. I obviously want to make sure it doesn't happen with my new (more expensive) rotors. Is it more likely with softer (resin?) pads where presumably more materials can melt?

This bike is almost exclusively used for commuting (11 miles each way, 500 ft of climbing) so not really taxed at all.

Thanks again.

Dave

 

[Siclo]

I'm pleased that a number of you have reported that your HyRd's are effective - that gives me hope of finding a solution.

@ColinJ - the brakes work, just not very well! I know HyRd's have a long lever pull, but I can pull the lever all the way back to the bar without any fear of locking up/going over the handlebars. The rear brake is perhaps a little better than the front, but nowhere near as good as I think it should be.

I think the only option is to try new rotors and pads and see what happens. I will report back.

Thanks again for all the comments.

Dave

My bold

What levers are you using?

Hy-Rd's require a lever with a 2.5 x pull ratio, latest 11 speed has this ratio, with 10 speed you will need to find the series number to figure out the pull ratio. If you only have 2 x pull ratio the lever will travel all the way to the bars and the brake will feel 'spongy' and lack stopping power.

 

[Yellow Saddle]

@Yellow Saddle - Wow. Thank you very much for your detailed response. I think I understand the process! What I don't understand (forgive me if I missed something) is why this has happened. Pads not bedded in enough? Too much? Brakes being dragged?

This rotors in question were cheap Wiggle own brand ones, so I will probably not bother with them. I obviously want to make sure it doesn't happen with my new (more expensive) rotors. Is it more likely with softer (resin?) pads where presumably more materials can melt?

This bike is almost exclusively used for commuting (11 miles each way, 500 ft of climbing) so not really taxed at all.

Thanks again.

Dave

It happened for a variety of reasons but I need more detail to pinpoint it.

The primary reason for most non-performance riders is that the pads were never bedded in properly. The bedding in process is the final process in the curing of the resins used in the pads. If they are flash heated right at the beginning they outgas some stuff that settles on the disc and remains there for ever or until you sand it off. The second reason is overheating of resin pads in dragging them down long hills. You can do this with metal pads but not resin ones. This usually happens when inexperienced riders drag their rear brakes. Front brakes are more effective (another essay in this is required but not now) and will generate less trouble with the same amount of stopping.

Then, willy-nilly swapping pads. The two are just not compatible without a resurfacing between swap-outs. Keep in mind that discs come in two types - heat treated and non-heat treated. The latter can only work with resin pads (low tempertures) whilst the former is compatible with both pads. They have different coefficients of friction and even more complicated, different variance of coefficients of friction at higher temperatures. You can already see the minefield manufacturers have to wade through to get a satisfactory product to the consumer.

The third reason is coming to rest with hot pads and discs at say a stop street, but keeping the brake clamped on. This causes the pad material to migrate unevenly. The area where the pad now rests and cools down slowly suddenly receives a lump of cementite. This causes vibration and vibration on sliding friction brings down the coefficient of friction and translates as poor braking performance.

I predict the mass onset of disc brakes on road bikes and with that, mass dissatisfaction of disc brakes because of the mass ignorance by bike mechanics to deal with the problem. In their defense, how do you educate a public on something so complicated? Even worse, how do you educate a public that buys pads off-the-peg in self-help supermarkets or online without really knowing the consequences of swapping or not bedding them in.

For your riding style I would recommend resin pads. Have I mentioned that proper bedding in is important? No, I don't think so, so let me say it. Proper bedding in is important.

 

[Yellow Saddle]

My bold

What levers are you using?

Hy-Rd's require a lever with a 2.5 x pull ratio, latest 11 speed has this ratio, with 10 speed you will need to find the series number to figure out the pull ratio. If you only have 2 x pull ratio the lever will travel all the way to the bars and the brake will feel 'spongy' and lack stopping power.

I disagree that a lever with the wrong ration will feel spongy. If the pull ratio is wrong you may well end up with brakes that cannot give you enough disc clearance yet cannot clamp full before bottoming out. The spongy feeling is a function of air in the system or, a too-long and convoluted cable route.

 

[Siclo]

Ok, but that's how it feels to me when I set the same Hy-Rd caliper up with different pull ratio shifters. YMMV.

 

[Yellow Saddle]

Ok, but that's how it feels to me when I set the same Hy-Rd caliper up with different pull ratio shifters. YMMV.

I just don't see how the different pull ratio can introduce flex.

 

[Siclo]

I just don't see how the different pull ratio can introduce flex.

Maybe spongy is the wrong term, lets go with feels bleeding awful. Certainly worth the OP checking.

 

[mythste]

@Yellow Saddle, as always, thanks for such a comprehensive reply!

It's an interesting point you make about holding on a hot brake and that (if I've read correctly) that could introduce some vibration - or shudder(?) into the equation. I've noticed an increase in shudder over the last month or so and I do typically hold the front brake on at stop signs, lights etc. Not a problematic amount but I'll ensure that the next pad change is a more thorough one and I'll report back.

It is my experience that Hy-Rd's feel a bit "spongy" compared to most other brake systems I've tried. It's neither here nor there for me, I have plenty of modulation and I have about 1 cm between bar and lever at full lockup.