Cycling UK Forum

Cugel wrote: ↑1 Mar 2023, 12:13pm It would be interesting to have a better understanding of what it is that causes disc brake squealing at the most fundamental level rather than in terms of "changing this component or that of the system eliminates the squeal".

Squealing is caused by simple stick-slip motion.

When you push two surfaces to slide them one over the other, at first the force is insufficient to overcome the stiction (static friction), so the force deforms the components due to the modulus of the materials they’re made of. Eventually the materials have flexed enough that the force will overcome stiction, and they start to move, but at this point you now have the dynamic coefficient of friction, which is lower than the static coefficient. This means that there is an excess force over and above that required to get the motion started, so one surface accelerates over the other.

What happens now depends on how fast the motion is. If the movement is slow, the acceleration will be sufficient for the moving surface to catch up with the motion, so that the relative movement between the surfaces is again zero, and again the static coefficient pertains. Since some of the stress and strain accumulated at the start has now been released, we are back to the situation with insufficient force to overcome stiction, and the whole cycle repeats again as above. The result of this effect is that whilst the motion applied is smooth and continuous, the movement between the slipping surfaces is a series of stop-start lurches, and when these occur rapidly many times a second, the result is heard as a squeal.

That’s when motion is relatively slow. If you move the surfaces faster, what happens then is that once the stiction is broken at the start, the acceleration of the surfaces will be insufficient for the movement between them to catch up with the applied motion, and so the relative speed never returns to zero, and the stiction is never re-established. In this case, the surfaces slide over each other smoothly, and there is no squeal.

It’s very easy to demonstrate, here is a video with the modulus of the materials modelled with an elastic band, and the surfaces modelled by my diary on the dining room table. See how the movement progresses in a series of lurches when pulled slowly, and slides smoothly when pulled fast.

Here is exactly the same effect from the hinge on my lounge door, when I move the door as slowly as I can, you can hear each individual slip as a click, then as the door moves faster the clicks merge into a squeal with ever increasing pitch, and when it moves fast enough the squeal stops.

axel_knutt wrote: ↑10 Mar 2023, 10:06pm

Cugel wrote: ↑1 Mar 2023, 12:13pm It would be interesting to have a better understanding of what it is that causes disc brake squealing at the most fundamental level rather than in terms of "changing this component or that of the system eliminates the squeal".

Squealing is caused by simple stick-slip motion.

When you push two surfaces to slide them one over the other, at first the force is insufficient to overcome the stiction (static friction), so the force deforms the components due to the modulus of the materials they’re made of. Eventually the materials have flexed enough that the force will overcome stiction, and they start to move, but at this point you now have the dynamic coefficient of friction, which is lower than the static coefficient. This means that there is an excess force over and above that required to get the motion started, so one surface accelerates over the other.

What happens now depends on how fast the motion is. If the movement is slow, the acceleration will be sufficient for the moving surface to catch up with the motion, so that the relative movement between the surfaces is again zero, and again the static coefficient pertains. Since some of the stress and strain accumulated at the start has now been released, we are back to the situation with insufficient force to overcome stiction, and the whole cycle repeats again as above. The result of this effect is that whilst the motion applied is smooth and continuous, the movement between the slipping surfaces is a series of stop-start lurches, and when these occur rapidly many times a second, the result is heard as a squeal.

That’s when motion is relatively slow. If you move the surfaces faster, what happens then is that once the stiction is broken at the start, the acceleration of the surfaces will be insufficient for the movement between them to catch up with the applied motion, and so the relative speed never returns to zero, and the stiction is never re-established. In this case, the surfaces slide over each other smoothly, and there is no squeal.

It’s very easy to demonstrate, here is a video with the modulus of the materials modelled with an elastic band, and the surfaces modelled by my diary on the dining room table. See how the movement progresses in a series of lurches when pulled slowly, and slides smoothly when pulled fast.

Here is exactly the same effect from the hinge on my lounge door, when I move the door as slowly as I can, you can hear each individual slip as a click, then as the door moves faster the clicks merge into a squeal with ever increasing pitch, and when it moves fast enough the squeal stops.

This is a fine start, surely, on the way to understanding what it is exactly that causes disc brake squeaks from bicycle disc brakes. The next step would be to try and identify the components that are acting together to produce the squeals and squeaks in the manner you so well describe.

The obvious first candidate would be a high frequency slip-stick between the brake pad and disc. A now educated guess would suggest that squealing is more common when the brake is damp or operating in a damp environment because the water between the pad and disc presumably exacerbates the stick-slip effect, perhaps both in terms of how often it happens (giving a higher frequency) and the amount of energy involved in the stick-slip cycle (giving more amplitude to the squeak) ..... ?

I've noticed that a disc brake always makes a noise, wet or dry. The low frequency noise is generally also low in volume and might be termed "normal" or "just what all disc brakes do. Does anyone have a truly silent disc brake?

Sometimes my disc brakes will squeal no matter what, if there's just very damp air rather than rain or road water thrown up on to the brakes. If such conditions are the greatest test of disc brakes' inclination to squeal, what could be done to reduce the effect? Is there a wipe-on something for the discs, to reduce the inclination to slip-stick even when there's damp, yet still allow good braking?

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Other conditions besides dampness might have similar effects perhaps? A slightly buckled disc, meaning that the pad pressure on it when braking varies in a fashion causing stick-slips? A patch of contamination on the disc having a similar effect?

For pad/disc-caused squeals and squeaks, then, the trick would be to somehow find a way of making the disc very smooth and flat yet with a surface that can be effectively gripped by the pads so the brakes actually work. The pads themselves would perhaps need to be more grippy, then - but still with surface qualities that reduce the chance of slip-stick happening. Is this why softer "organic" pads are generally less likely to squeal than sintered or metallic pads?

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There are probably other conditions of the whole braking mechanism (including calliper & pistons, condition of the hydraulics or cables and even the nature of the forks to which a brake is mounted) which contribute to a slip-stick effect at the pad/disc interface. A loose piston, perhaps? A cable/fork arrangement that adds its own variation of cable-transmitted force to the brake as a high frequency judder? And so forth.

But the pad-disc interface seems the most likely culprit and therefore the features of a disc brake that should be fettled first if there's squeaking or squealing.

Cugel

I think another factor to consider is the resonant frequency of the forks/frame which can lead to amplification of the noise produced, if it should hit that frequency or multiples thereof.