Why machine brake rotors? 5

[BobM3]

I know that if I just replace brake pads without machining the rotor the new pads will not last long. What does the machining do to the surface that allows the pads to last longer? Is there a certain roughness that is needed on the machined surface?

 

[JayMaechtlen]

I think it is still the original m/c. Couldn't swear to it, but don't recall changing it.

Jay Maechtlen

http://home.covad.net/~jmaechtlen/

 

[oab]

QUOTE BobM3. Thanks for all the input guys. I finally got around to changing the pads. I saw something I hadn't seen before. There's a small spring attached to the inner
pad that looks like it's supposed to apply a small force or moment to the inner pad. It's so small I can't believe it does much when the pistons are energised. Must be there for when the pistons aren't energised. Any ideas?End Quote.

Bob also mentions about disc squeal. The squeal is caused by the air trapped between the disc and the pad, causing the noise as it is squeezed out. If the disc and the pad surface are smooth, there is no where for the compressed air to go, so it is squeezed out, causing the noise.
If you roughen the surfaces, you provide scratches and pockets for the air to be compressed in, so that it is not squeezed out between the smooth surfaces. The same with
grooving or drilling holes in the disc.
The small spring plates that are fitted between the pad and piston, are to delay the application of the full pressure to the pad, to give time for the air to escape from between the surfaces before the full clamping pressure is applied.

 

[patprimmer]

If they are between the pad and the piston, they might be to take up clearance and prevent rattle. This clearance might be generated by flex in the square section "O" ring calliper bore to calliper piston seal or from knock back from flex between the rotor and the calliper.

If they are between the pad and the pad thrust surface on the calliper, they might be to prevent rattle in the clearance that is necessary to avoid binding when the pads expand as they heat up.

If they are attached to the pad and perpendicular to the friction surface, they might be sounding boards to "ring" to indicate the pads are worn out.



Regards

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[GregLocock]

oab that is a whole new explanation for brake squeal. There again I've only been working on it for 25 years.



Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[oab]

Gee don’t think it is all that new Greg. When Japanese bikes started to use discs on their bikes, Squeal became a real problem. The stainless steel that they used, was not as porous as the cast iron that every body else used. A number of things were done to cure the problem, spring plates, or squeal paste, between the pad and piston, drilled or punched holes in the disc, even to cutting grooves across the pad. All of these were aimed at providing a way of removing the air between the two surfaces, before the rising pressure compressed the air to cause the squeal.
Have I been misled all these years Greg, sound is caused by vibration?

Harvey.

 

[patprimmer]

Sound is caused by vibration

Where is the vibration generated by air between a smooth disk and a brake pad.

Grooves, holes, slots etc are to evacuate gas generated by degradation of the matrix resin when pads become hot. This gas is evacuated to prevent fade as a layer of gas between the pad and the disk reduces contact and causes fade.

I have experienced brake fade with no noise, maybe apart from my expletives at the time.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[drwebb]

I thought perforated rotors on MCs were more to drain water in rainy conditions and reduce unsprung weight and make it look racey?

I also thought air was a potent sound damper, so air at the pad/rotor surface should reduce noise and not increase it??

 

[NormPeterson]

I’m having considerable difficulty with the notion that a few hundred mm^3 of air escaping from between the pads and rotors could cause enough audible noise to elicit complaints. Given that the shear strength of air can’t be much different from that of the products of pad outgassing, I’d expect that poor initial pad ‘bite’ would be the issue instead.

Looking more toward the relative damping capacities of stainless vs CI; a very brief search turned up this page:

http://www.domcastmetals.com/grayiron.html#damping

Quoted in part (boldface mine),
------------------------------------------------------------------------------------------------------------
The exceptionally high damping capacity of gray cast iron is one of the most valuable qualities of this material. For this reason it is ideally suited for machine bases and supports, engine cylinder blocks and brake components. The damping capacity of gray iron is considerably greater than that of steel or other kinds of iron. This behavior is attributed to the flake graphite structure of the gray iron, along with its unique stress-strain characteristics. The relative damping capacity of several different metals is illustrated in Table 4.

Damping capacity decreases with increasing strength since the larger amount of graphite present in the lower strength irons increases the energy absorbed. Larger cast section thicknesses increase damping capacity and inoculation usually decreases it. Heat treating can also have an appreciable effect on damping capacity.

Table 4. Relative Damping Capacity
Material x10 to the power of 4
White Iron 2-4
Malleable Iron 8-15
Ductile Iron 5-20
Gray Iron, Fine Flake 20-100
Gray Iron, Coarse Flake 100-500
Eutectoid Steel 4
Armco Iron 5
Aliminum 0.4
Natural Log of the Ratio of Successive Amplitude

------------------------------------------------------------------------------------------------------------

Norm

 

[evelrod]

Thanks for the link, Norm. Some of it is new to me...funny that anything about cast iron should come as a surprise after all these years. It explains in part, why some of the Chinese discs seem to "better" their domestic counterpart (assuming the 'domestic' part is not also Chinese...call me cynical...I've seen 'repackaged' parts before)!

Question---Do any of you know of data similar to this for 'carbon-carbon' brakes?

Trapped air causing squeal? New to me and I've been at it for some 50 odd years. From my experience, the first disc braked bike I rode was a Norton (solid disc) and brake fade was a serious problem. I then had a Japanese bike that had all hydro discs with lots of holes that did not fade but, would sound like a jet plane going over if you put in really hard pads. My last Japanese bike, a Gold Wing, was "normal", no fade, no squeal and, front pad life was nearly 20,000 miles.
I'm back to my old vintage racer (48 Norton) with big drums...What the heck---I don't ride fast any longer.

Rod

 

[GregLocock]

Good to 'see' you again Rod.

The noise is radiated from the rotor, ringing like a bell, but because it is a disc it 'rings' by forming waves like a wavy washer. Typically the wavelength of the waviness in the rotor is about the same as the length of the pad. The usual treatment for brake squeal is to add a constrained damping layer to the back of the pad. This adds damping to the bending induced in the pad's backing plate. The pad bends as it follows (and causes) the waviness in the rotor. So when we damp the backing plate, we damp the waviness in the rotor - no more noise.

When I started work on this we measured the waviness, and the bending of the pad.

The air pumping theory is new to me. I can't discount it completely, but it doesn't seem right. The pad material is in very good contact with the rotor's surface - otherwsie it would not transmit the waviness to the backing plate.

Brake squeal costs the world about $4 per car, so, it is a thoroughly investigated (if not especially well understood) phenomenon. For instance I carefully left out exactly how the pad/rotor interaction causes the waviness.

Drilling holes in the rotor would cost far more than the constrained layer damping, so that may be why I haven't seen it investigated as a solution.



Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[BobM3]

Greg -

I'm interested in the part you left out. I worked with a machine years ago that had a large disc and brake pads. After some usage the squeal would occur. I'd take a disc grinder and sand both sides of the disc and the squeal would go away. I've since wondered what the dynamics were between the disc and the pad that would set off the squeal. I assumed it was a small change in friction coefficient that added (or removed) a bit of damping. Am I close?

 

[evelrod]

Bob, have you ever wet the tip of your finger and rubbed it around the rim of a wine glass?

Greg---Been kinda busy with the race season. Weather has been REALLY hot this year and I don't take the heat like I once did...made lots of rooky mistakes this year, hence the "busy" part. Never fear, I'm still 'lurking' about!

Rod

 

[oab]

Gid’ay Pat,
You say; Where is the vibration generated by air between a smooth disk and a brake pad.?
Well lets put it this way. What is vibrating to produce the high pitched sound? I doubt that the disc can vibrate at that high a frequency, the calliper won’t, but the pad flying on a cushion of air can. This air is the boundary layer of air that is pumped under the pad by the rotating disc, until the clamping pressure is high enough to prevent the entry of this air, the air will be forced under the pad, and keep vibrating out. We will notice that the sound increases with the smoothness of the disc, and the porosity of the pad, hard pads squealed more than soft pads.
We are all old enough to remember that woven linings on drum brakes never squealed, due to the porosity. Moulded linings did.

NormPeterson, the difference is the gas is generated at the pad, not feed in, from the leading edge.

Rod, yes I have been at it for a while, started racing on a BSA Bantam, come on stop the laughing. You would remember the Kawasaki Z1 brakes, King of the squealers.

Harvey.

 

[GregLocock]

BobM3, Our theory at the time was that as the pad grips there is a couple around it's instantaneous rotation of movement, so the pad gets pushed on harder, increasing the moment, etc etc. Finally there must be some non-linear effect that breaks the traction of the pad on the disc (otherwise the pad would be pulled out of the calliper), and it springs forward to the original position where the whole process starts again.

The non-linear circuit-breaker is the key to the excitation, I suspect that modifying the friction coefficient as you did would somehow affect it.

I suppose one obvious non-linear effect is that as the pad moves back the piston rocks, so the normal force distribution alters.

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[evelrod]

He, he---I don't have much room...my first 'big' racer was a '66 BSA Victor. All that means is that I know how to fix a broken chain!!!

My "jet plane" squealer was a Kawasaki. Had to use soft pads which only lasted about 10,000 miles.

Rod

 

[oab]

It looks like we are talking about the same thing Greg, just from different ends. You have found the sound source and followed it back to a disc/pad oscillation. The question is, what started the process?
I believe that it is the layer of air rotating with the disc, that is compressed under the pad. The pressure builds to a point, where it ‘blows’ out from under the pad, to start building up again.I thought that this ‘air explosion’ was the cause of the sound, but your process is the most likely. I think though, that it maybe the root cause of your progression, initiating the pads oscillations.
It would seem that the squeal depends on the amount of air trapped, and the piston pressure applied, as changing the applied pressure alters the conditions necessary for the
air pressure under the pad to oscillate. More or less pedal pressure will usually stop the squeal.
There is not much risk that the air is a major player in the process, as anything that we do to allow the air to escape, from under the pad, will reduce the squeal. Roughening the surface of the disc, allows the air to escape along the scratches. Softer pads allow the air
to escape through the pad. When the spring plates are fitted between the pad and piston, there is a delay, as the pad is pushed to the disc by the spring plate. This allows the pad to scrap the excess air off the surface of the disc, before the piston hits to apply full pressure.

Rod I graduated to falling off Ducatis,
Harvey.

 

[drwebb]

So why don't brakes squeal more when they're wet than dry brakes since there's a lot more water at the interface than air, and water is a much more efficient conductor of sound? This is getting confusing . . .

 

[oab]

drwebb,
Water is non compressable, so the pad just skates on the water till it gets down to the disc. I believe its the air compressed under the pad, that starts the process, that produces the sound.

Harvey.

 

[drwebb]

I predict that if one ran a squeaking brake in a vacuum chamber one could measure similar oscillatory modes as at atmospheric pressure- be they in the disk, the pad, or the caliper. There would be no audible squeal however because there would be no air to propagate the sound.

 

[Tmoose]

Water's Compressibility

http://www.piercecollege.edu/offices/weather/water.html

"Brake fluid has such a low compressibility factor that auto makers use it in brake hydraulic systems. When you hit the brake pedal the brake fluid compresses so slightly you'd think there was no volumetric change at all! If water got in your brake lines (which is VERY dangerous!) you would notice your pedal would feel sluggish and the brakes wouldn't work very well, and may not work at all! This is because water compresses much more relative to brake fluid.

As pressure is added to the water, which is trapped in a brake line, its volume decreases as water molecules are forced closer and closer together. Water at temperatures common in nature has a compressibility factor of around 0.0000034, meaning that a hydrostatic pressure of 6.89 kilopascals (1lb/ sq. in) would reduce unit volume by about 0.0000034 of the original volume.

This compressibility of water is so slight we could never actually see it with our own unaided eyes. We might think water is not compressible. However, if that were the case then the oceans would be about 30 meters higher than they are now, and therefore cover an extra 5 million square kilometers of Earth!"