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?

 

[patprimmer]

Probably technically correct and precise, but how many noughts was that in front of the 34. I counted 5 without my glasses on. Hardly comparing to the combined gas laws to the same degree that air would.

I think there is only one contributor that thinks air entrapment and compression sets up the oscillation and at least 5 who think it does not. I think the nays have it.

But what does cause it seems open to question. Maybe the harsh environment around a calliper causes enough friction on the surface of the pad thrust bearings and calliper pins, slides or whatever, that one pad hits first thus setting up an oscillation in the rotor hub assembly, or one edge of the pad hits first, thus setting up an oscillation in the pad.

I personally have only noticed this squeal only in metallic compound pads, but my experience is limited.

I wonder if the oscillation is generated from metal particles in the pad matrix, whereby the matrix does not fully support the metal particle, so the metal in the pad grabs micro groves in the rotor, moves a few microns in a void in the matrix, hits solid matrix then rebounds. This would be a very low amplitude high frequency vibration. I have no idea if it might be in the audible range as this is a long way from my field of experience or study.

Regards

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[oab]

This problem is not only confined to disc brakes, we had the same problem with drum brakes. The wider the shoes, or the harder the linings, the worse the squeal.

Why did hacksaw cuts across the brake shoe stop the squeal????

Well Pat, I don't mind being different. Can somebody else supply an explanation for the cause.

Harvey.

 

[patprimmer]

In my experience, squeal in drum brakes is mainly due to dust. Remove the dust, the squeal goes with it.

Regards

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

 

[NickE]

I think its stick slip.

Thats why tapering the leading contact of the pad to rotor works, it changes the friction at the interface.

Teh CRC-Brand viscoelatic compounds work really good.

There is also a contribution from the rotor. Lower strength irons with their correspondingly large volume fraction of graphite will damp the vibration more.

That said I generally get 20-35 k miles from pads and twice that from a set of rotors.

I did buy the cheap Chinese rotors last time, and they (using another new set of the same pads) squeak during highway travel and also when lightly using thme and they're hot. While with US made rotors that had been cryo-treated (

http://www.frozenrotors.com)

I never heard the pads at all. Of course teh cryo rotor's didn't last any longer than the stock rotors did.

(PADS: Ferrodo DS 2500)

 

[swall]

NickE--Interesting to hear the results of your cryo rotor treatment trial!

 

[swall]

Tmoose--looks like the professor giving the water physics lecture swerved off road into something he knows absolutely nothing about--brakes.

 

[Surestick]

"I think its stick slip"

Agreed.
If it was air escaping, given the tiny amount of air possible between the pad & rotor, the squealing should fade away after the brakes are applied for more than a few seconds which is usually not the case.

The stick-slip mechanism occurs in mountain bike rim brakes as well & causes squealing though at a slighty lower frequency.
Due to the relatively large amount of flex in the system the pads have to be toed-in so that the trailing edge of the pad (the end towards the front of the bike) meets the rim first (usually adjusting the pads with a quarter under the leading edge gives the perfect amount of toe-in). On cheap department-store type bikes the flex can be so bad you can actually see the brake arms vibrating when squeal occurs

 

[oab]

Surestick,
It is not just the small amount of air escaping from under the pad, that produces the sound. If that was the case, the noise would be there without the disc rotating. It is the boundary layer of air, that rotates with the disc, that is being forced under the pad, that causes the sound to continue.

Harvey.

 

[Surestick]

If that was the case wouldn't there always be less wear on the leading edge of the pads due to it being held off the rotor by the layer of air?

 

[NickE]

Also if the boundry layer of air (that is spinning at the same speed as the disc, and staionary relative to the disk) was forced under the pad then the braking force woudl go to zero evey time the pads lifted off, then return to normal when they fall back to the surface.

If I pull a threshold stop from 110mph I should feel this lift then settle in the brake pedal. I dont. And my brakes squeak like no tomorrow.

Surestick -- I used to work in a bike shop, and completly understand the toe-in of rim brakes. (Crecent wrench anyone?)

I agree, its likely flex in the caliper bracket relative to the caliper and rotor that cause it. (At least in my perception.)


Nick
I love materials science!

 

[Fabrico]

...I agree, its likely flex in the caliper bracket relative to the caliper and rotor that cause it. (At least in my perception.)

That goes along with my thoughts. If it were related to air, most if not all disc brakes would squeal. In actuallity, very few disc brakes squeal. So much so, that many mechanics don't bother doing anything to prevent it.

 

[GregLocock]

Most production disc brakes don't squeal because somebody has spent several months making sure that they don't.

Cheers

Greg Locock

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

 

[NickE]

greg- eliminating the various causes discussed here?

(Actually seems that you would modify the different charecteristics of the interface. Pad / Rotor, shims and such before going to a re-design of the castings.)

 

[GregLocock]

Well, it depends on what the path of least resistance is. Often the compound is the easiest thing to change.

Cheers

Greg Locock

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

 

[tripleZ]

For racers, there have been some advancements made in the air category through the introduction of wave rotors, or so the aftermarket companies would like me to believe. I think they're running standard now on Yamaha R6s and R1s...and of course GP bikes. Same concept as the holes in the rotor concept. Sportrider magazine did a review and analysis of these rotors a while back. I think it was September of '05 (Triumph Daytona 675 first ride report issue...I remember that at least). I believe Galfer in Spain was doing the R & D. From what I remember of the explanation, the holes and shaped designs had a lot to do with heat dissipation at high speeds (100MPH+), moreso than noise at least. Performance of these rotors seemed to be noticeable at high GP-style speeds, but were negligible for slow-pokes like me.

http://forums.sportbikes.net/forums/showthread.php?t=3038

The above link gives Ferodo's perspective on sintered pads and ductile vs. gray iron rotors. Not entirely related to the discussion, but a slightly informative read on materials.

Keep the information coming!

 

[Knap]

why don't brakes squeal more when they're wet than dry brakes

My Front left squeals more when wet, tried all the various potions including stick on which should eliminate piston vibration. If I change pads over still left squeal but change to old pad compound no squeal and if change to carbon metallic both sides squeal.