×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Why machine brake rotors?
5

Why machine brake rotors?

Why machine brake rotors?

(OP)
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?

RE: Why machine brake rotors?

From my experiance what would cause the pads not to last as long is the fact that after 30k miles or so of use the rotors are grooved and possibly rust pitted. If they are still smooth and in good condition there shouldnt be any premature pad wear, however the rotors should be machined to get rid of the glazing and provide a fresh, clean, smooth surface for the new pads to bed against. Slapping new pads on a set of used rotors will probably give you a much greater stopping distance than installing new pads with new or freshly machined rotors. Machined rotors should have a smooth, non-directional surface finish, i don't think there is a specification or RA number for how rough or smooth they should be.

Most premature pad wear is caused by corroded or improperly lubricated pad sliders and brake hardware.

-Jon

RE: Why machine brake rotors?

BobM3

In my opinion, the main reason for machining rotors is mainly to increase the cash flow for the person replacing the pads. It is a quick and easy job for which they can charge a fairly high price as it "sounds like a major job". It also greatly increases the probability of also being able to stick the customer for a set of new rotors.

I never machine rotors unless there is a noticeable uneven wear on the rotor or deep score marks.

The work hardened surface of the rotor resists wear, and removing it reduces the strength and the life of the rotor.

Rotors quickly work harden or glaze with use, so removing that finish only has a very temporary effect on brake performance. One downside is that the brakes take a little time to bed in, normally like 5 or 10 hard stops from about 80 kilometres per hour. Knowing that, you can take a short and careful drive, and repeatedly left foot brake until the brakes get a bit hot and bed in. You will know when the pedal feels right.

I also never use OEM pads (purely for cost reasons), but I do use premium quality pads from big name aftermarket vendors who are also OEM supplier, like Bendix or Hardie Ferrado. These suppliers might be different in your part of the world. They can normally provide an equal or better compound at a substantially lower cost to OEM.

I do not use semi metallic like "Metal King" as I think they are to abrasive on rotors.

Things to consider when choosing a compound to suite you are:-
Noise.
Pad life.
Rotor wear.
Braking power (friction properties).
Fade resistance.
Dust colour and appearance.
Corrosive nature of dust build up on wheels.

It also always pays to ensure the callipers are free to float easily on the pins, the pads move freely on their mounting blocks, that they have sufficient clearance in the blocks to allow them to move freely even when very hot, and that the calliper pistons move freely and retract properly, so that the brakes do not drag. This is important for avoiding steering pull to one side as the brakes are applied and to get maximum pad life and fuel economy.


 

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.

RE: Why machine brake rotors?

I agree with Pat.  It a money maker for the shop that does the turning.   First, they don’t have to test drive or measure the run-out on the rotors it they machine them.  Second, after machining there is a significant increase in probability that the rotors will need be replaced because they will be too thin to meet minimum specs.  I have even see shops machine new rotors just to be sure they are true.  Why measure the run-out for free when you can charge to turn the new rotor???  I used to work in a shop that always turned rotors and drums, new and reused.

The origin of this practice was with brake drums.  A large lip would develop over years of service.  I used to turn the drums and arc the shoes to match in the old days.  If you did not, a spongy brake peddle would result until the shoes wore in.  If the new friction material was riveted/bonded on slightly to one side, the friction material would hit the lip.  The shoes would wear in quickly and often the automatic adjusters would not catch up with the accelerated wear.  Some cars and trucks did not even have automatic adjusters.  A low brake pedal would result until you back up the car/truck several times while hitting the brakes or returned to the shop for adjustment.  This was not good for customer relations; hence, we always turned the drums to get rid of the lip.  

RE: Why machine brake rotors?

It all depends on the pad composition used previously, driving habits, and of course if the rotor is wearing flat.  I've never once re-machined or replaced rotors on any of my personal cars, but then, if the brakes start squealing or if the build-up pad compound on the rotors starts to delaminate and cause a pulsation, I'm not going to get angry with myself for poor work.  Paying customers are an entirely different story!  The job has to absolutely positively be done right the first time.

Measuring runout doesn't enter into it - you have to measure runout either way.  Properly machining the rotors requires that you measure runout on the vehicle, mark high and low spots, then chuck into the lathe to match, *then* ensure that runout is zero when installed.  Yes, it's a pain in the tail.  No, most people don't do it.  On the other hand, ensuring zero runout on new rotors practically eliminates pulsation problems forever, barring bad castings, so *new* rotors get runout measured and, if necessary, a clean-up machining.

Why is it that there's never enough time to do it right, but always enough time to do it twice?

RE: Why machine brake rotors?

Many shops just aren't competent to machnine rotors and you will end up with a worse rotor than the one you started with. Leave it alone.

RE: Why machine brake rotors?

(OP)
I had thought the glazing of the rotor somehow caused rapid wear of the pads.  Sounds like this isn't so.  Years ago I worked on a machine that used a large disk brake.  Once the disk glazed up the brakes would squeal.  I had to rough up the disk with a grinder to get the noise to go away.  Any idea why a glazed surface would cause a squealing?

RE: Why machine brake rotors?

Machining trues the rotors and maximizes wear surfaces. If the rotor surface is flat, free of grooves, has no noticable wear (radius) on the edges or run out, the pads will last about the same amount of time whether the rotor is machined or not. If you machine rotors that fit all the criteria above, whatever you gain in pad life will be lost in rotor life.

A shop will always machine rotors because they can't afford for things not to work right the first time, every time.  

RE: Why machine brake rotors?

In the 'old' days, that's the 50's and 60's for me, I always turned my brake drums...everytime I put on new shoes...new drums after they were too thin to turn.

With disc brakes, I never machine them and only replace them if they become warped (my '95 Chrysler LHS) or damaged by a broken pad (my 91 Dodge PU), replacing BOTH rotors.  Point of fact, unless there is seriously agressive wear, I will still use them allowing a little extra time to bed in new pads! My old 83 Turbocoupe went well over 200,000 miles on the same discs and my wifes Lincoln is now over 131,000, but will need replacement next time as they have become warped.  On my vintage Mini race car I am still on two year old Brembo discs and now on my fourth set of EBC Greens...on my vintage Lotus Cortina race car, I am still using OEM discs installed in 1988, using the old Repco MetalMaster pads (I still have a stash of them)!!! The big secret is to match the rotor material and mfgr. with the proper pad and if your discs need resurfacing, they need to be REPLACED, IMO!

Rod

RE: Why machine brake rotors?

I do as patprimer, izzums and evelrod, I don't surface my rotors or drums. I also use the cheapest softest pads and shoes. It's a lot cheaper to replace shoes and pads than rotors and drums.  My Dodge diesel dually has 150k on the original brakes and they are like brand new. It will stop a 10,000 pound trailer like it wasn't even back there. Not that I don't use the trailer brakes, but it will stop easily with out them.
My old SuperCoupe had over 200k onthe original rotors too.
However as noted the brake shop can't take a chance on having problems so they must turn rotors and drums. I worked in an auto shop for years and always turned them with the care noted above. The last thing I wanted was a come back for brake problems.  Also if someone has a crash you can rest assured the first thing the lawyer looks at is who serviced the brakes and how. You won't have a chance on thin rotors with grooves in them or slick smooth rotors. The bottom line is if you do them for yourself don't turn them. If you do them for someone else turn them or replace them.

RE: Why machine brake rotors?

Well I have like 60,000 to 70,000 on some brake pads and shoes that were only supposed to last 30,000 or so. And that is on the NONturned drums and rotors.
Also in my flying days, the small planes that I rented had rotors that were grooved and worn worse than anything you would want to use on your car. Don't ask I don't know.
The only reason to have to turn them is if the metal backing has rubbed on them. Or the gooving is way deep.
Otherwise they live just fine.

RE: Why machine brake rotors?

I have to disagree with most all of you, our shop pays the techs the same whether we replace or machine the rotors, so it is not a money maker for the shop to machine them because it takes there techs longer to machine than to replace (less productivity) and the shop doesn't make money off the markup of a new set of rotors. Up here in the northeastern US rotors are always severely rusted and grooved due to our winter road conditions so they are never in a condition where we can just slap pads on them. In fact scoring, grooving, or heat checking on the rotors fails state safety inspection. We measure the rotor thickness before we machine them, based on experience we know how much material we will machine off and whether or not the rotor will go below minimum thickness requirements when we machine them. I really hope that there aren't too many shops out there that aren't competent enough to machine rotors because it certainly isn't brain surgery.

On my personal vehicles at times i have done a complete brake job with new rotors, and other times i have just cleaned and lubed the hardware and slapped pads on. Both ways the vehicle stops safely, however when braking from high speeds (100mph+) you do notice a difference with the grooved unmachined rotors. But as i said before my experience is up here in the rust belt, down south your rotors may last forever.

-Jon

RE: Why machine brake rotors?

I have never seen really rusty rotors, except on a boat trailer.

We do not have ice and snow, therefore no salt.

Daily use cleans of any surface rust well before pitting begins, so your argument is not valid here, although I see your point where salt is used and especially if the rotors see a good dose of salt and are left to stand for a week or two.

I have never felt a difference in the pedal between machined rotors and not machined after 1000 miles of use, as the surface of the machined rotors will have work hardened and glazed just like the non machined ones, and the pads will be totally bedded in on the non machined ones by that 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.

RE: Why machine brake rotors?

I live in the winter salt belt in Indiana and have always just put new pads on, even if the rotors were grooved. I did have a rust related brake failure when the friction material/backing pad interface was undermined by rusting and the pad finally came off during a brake application. Jons999--interesting that the state safety inspection acually requires that the rotors be examined. Any states I lived in with a "safety" inspection considered the brakes OK as long as the car did not crash through the back wall of the shop.

RE: Why machine brake rotors?

Much of the time, the rotors get rust lines or pits to the point that the brakes pulsate violently before the pads are worn out.  The rest of the time, by the time the pads' life is over, they are rusted/grooved badly enough that machining would require removing an excess of material to get to clean metal. nevermind the damage that the rust does to the lathe bits.  It is common to get over 50% of the width of the braking surface covered with rust.

Interestingly, cars with ATE brakes seem immune to this problem, which leads me to believe that pad composition plays a large role in it.  Perhaps someone else could chime in about specific pad compositions, but the compositions that never have rotor glaze/rust problems leave a satiny finish on the rotors, but create copious amounts of brake dust as a downside.

RE: Why machine brake rotors?

3
Sure, different pad materials chew the rotor up to a greater or lesser extent.

It's just another compromise in the black art of pad selection.

Incidentally worst case for rotor wear is often freeway driving, grit builds up on the rotor and then gets scuffed between the pad and the rotor.


Cheers

Greg Locock

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

RE: Why machine brake rotors?

(OP)
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 energized.  Must be there for when the pistons aren't energized.  Any ideas?

RE: Why machine brake rotors?

To stop rattles.

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.

RE: Why machine brake rotors?

After working for an automotive parts distributor (and buying parts at their cost), I don't even question about turning rotors. Turning a rotor for my 98 Dodge Stratus cost $13.00 each at my local parts store. A new replacement rotor (Chinese) cost $7.oo each. Now, multiply these facts times all the brake jobs, local shops, and brake lathe operators, and there goes thousands of US workers made expendable by cheap Chinese imports.Kinda scary

Russell Giuliano

RE: Why machine brake rotors?

BobM3: I've heard there is often a part called an "Anti-rattle spring" or on some cars an "anti-rattle clip"--ironically, it is sometimes the source of rattles to spite its name (happened on my friend's car).

Can anyone confirm or disprove the veracity of this site: http://www.stoptech.com/tech_info/wp_warped_brakedisk.shtml

RE: Why machine brake rotors?

I looked at some of the Chinese rotors around 1992 or so and from a metallurgical standpoint, they were junk. Not every foundry that pour gray iron can make brake rotors.

RE: Why machine brake rotors?

BPM, that article sounds reasonable, but is fixated on friction material buildup on the rotor, which is not generally an important problem with production brakes.

Cheers

Greg Locock

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

RE: Why machine brake rotors?

Greg: Okay, but the next logical question would be if all those things are reasonable for a race brake system, would  "warping" ever occur--at all--on a street car? I'm thinking no considering the differences in brake temperatures observed.

RE: Why machine brake rotors?

I wouldn't say it /never/ occurs, just that I've not had to work on it, ie DTV is practically far more important than runout by itself.

If the callipers are sliding then they will accomodate warping reasonably well, whereas DTV has to be reacted by the hydraulic circuit, or by flexure in the calliper itself.

Cheers

Greg Locock

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

RE: Why machine brake rotors?

In light of the article I feel obliged to correct my termonology... at least re define it as I see it.  
When I refer to a "warped" disc I am refering to that pulsating brake pedal and shake in the steering wheel. The actual cause is technically not a 'warping' but rather a variation in thickness around the pad surface the disc.
I have only had one personal experience on a street car and that was the above mentioned LHS which was eventually traced to metalurgical problems as I recall and not to an actual warped cast iron rotor.  I deal with race brake problems almost exclusively and with racing people and I just "assumed" we were all on the same page which, apparantly, we are not.  Sorry for my part of any confusion.

PS: My current, ongoing problem with the wifes Lincoln turned out to be a rather sever runout problem on the left front...I already bought new Brembo discs and EBC Greens so I will eventually change then anyway...Oh well, never too old to learn to 'look before you leap', I guess.

Rod

RE: Why machine brake rotors?

"When I refer to a "warped" disc I am refering to that pulsating brake pedal and shake in the steering wheel. The actual cause is technically not a 'warping' but rather a variation in thickness around the pad surface the disc.
"

Exactly. Disc Thickness Variation. Here's a paper written by my partner in crime, Mr Leslie, aka Bubba:

Mathematical Model of Brake Caliper to Determine Brake Torque Variation
Associated with Disc Thickness Variation (DTV) Input
AU LESLIE A C, (Ford Motor Co. Australia Ltd.)
SO SAE Tech Pap Ser (Soc Automot Eng) SAE-2004-01-2777

Cheers

Greg Locock

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

RE: Why machine brake rotors?

I worked with an ASE mechanic around '98 and recall him tossing some fairly new chinese rotors someone else had installed and claiming they 'always warp.' The same conclusion as swall from different data . . .

RE: Why machine brake rotors?

Rings are quite hard.

Brake pads are reasonably soft by comparison, so the bore wears to the rings, but the pads wear to the rotor. In that case your logic suggests you should hone the pads, not the rotors.

I don't see how work hardened rotors causes pull, unless the rotors are obviously damaged from metal to metal contact.

Knock back and shudder can result from warped disks, but that would be obvious from a road test before the pads were replaced.

Bedding in pads takes about 2 minutes of road test while left foot braking. I think that is faster than aching, and the cost of the job is less than half, or cost of job is the same, but you use premium pads.

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.

RE: Why machine brake rotors?

Pat, even severely damaged disc usually won't pull once the pads are bedded in. Knockback is usually a 'runout' problem (or an engineering problem as we have noted in other threads).  The 'shudder' is probably DTV in origin.  I'll stick on message...If it's bad enough to need machining, it's bad enough to need replacement.

Rod

RE: Why machine brake rotors?

Typo in my last post

"Bedding in pads takes about 2 minutes of road test while left foot braking. I think that is faster than (aching should be machining), and the cost of the job is less than half, or cost of job is the same, but you use premium pads."

Rod

I agree entirely. If the groves are so deep that you cannot bed the pads via driving while left foot braking in less time than it takes to machine the rotors, the rotors need replacing.

I think that someone who uses A$15.00 a set pads is not going to replace rotors. My pads for my Honda Integra  cost about A$40.00 with trade discount.

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.

RE: Why machine brake rotors?

I'm with Pat and others, in recommending that rotors never be replaced unless they have an obvious and severe defect, or if the brake pedal pulsates when applied:  that is a sign of rotor thickness variation (NOT runout) and will be caused by variations in thickness that are undetectible with a micrometer.

Having worked in brake engineering at a *former* Big Three firm, I saw a number of pedal pulsation complaints caused by thickness variation.
This thickness variation problem cropped-up most frequently in cars that were built, driven briefly,  and then sat on a storage lot for several weeks or months before being sold. The rotors got wet and/or salty,  then rusted everywhere except where the pads were.  Once they were driven for a few thousand miles the softer rust wore down leaving high spots where the pads had been parked.
 
I also frequently saw newly-turned rotors that the technician only succeeded in making worse,  by introducing runout and/or thickness variation and creating a noise or vibration problem where none had existed before.

With my own vehicle, one having grossly under-sized brakes,  the front pads wore out at 15,000 mile intervals, so I got quite good at replacing them, and could do the job in 20 minutes, working in my driveway.  The rotors finally required replacement at about 160,000 miles (and 20 years).

RE: Why machine brake rotors?

(OP)
Twenty minutes?  I hope that's per wheel.  I think I took about 1.5 hours per wheel!! (Plus an hour to clean off)

RE: Why machine brake rotors?

20 min for the full job sounds about right for fronts to me. A\ little longer for rears if they have to be wound in for the park brake mechanism

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.

RE: Why machine brake rotors?

Twenty minutes sounds about right for an experienced tech.
I just replaced the front discs (Brembo) and all four wheel pads (EBC Greens) on the 01 Lincoln LS @ 134,000 miles...took me about an hour, total, but I have a full shop with all (well, mostly all) the proper tools (finally, after 50 years)... smile

Pat, a little fab time, a 3/4 nut and long bolt, a little plate steel...voila...neat little gizmo for winding in rear pads in about 30 seconds.

Rob, I think I had one of those cars from Canada...I know the brakes went bad after only a few thousand miles, unacceptable for the 'flagship' of the company.

Rod

RE: Why machine brake rotors?

Yep, 20 minutes for the whole job, from walking out my backdoor with new pads in one hand and toolbox in the other;  but as I said,  I got a *lot* of practice at it!

High-Lift jack under the brushguard ('roo bar, to you, Greg!) lifts both front wheels off the ground, removing one bolt gets the caliper off the slider after prying the piston all the way back into its bore. Wirebrush the rust off the slider ways,  file the paint off the edges of the pads, drop 'em in, bolt the caliper down and you're done.

The only hazard is if you forget where you put your beer down and kick it over!

Cheers!

RE: Why machine brake rotors?

I have found that an inverted brake drum placed on the ground prevents your beer from getting knocked over.

RE: Why machine brake rotors?

I found that insisting who ever owned the car, or from time to time wanted to borrow my stood there and held the beer for me, passing it to me on demand. This removed certain risks.

I also strategically placed the wheels under the car, so I had half a chance if the jack failed.

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.

RE: Why machine brake rotors?


Almost 85k miles ago I put a set of chinese made rotors on a Grand Cherokee. They were much heavier units than the stock one's, quite a bit thicker. The dudes at the parts house measured the factory ones and said, yep, they were original and never been turned and couldn't be turned.

The point I'm making is, the factory units lasted about 40k. The chinese units have over twice the mileage on them, and the brakes don't fade under constant load like they did with the factory units. They factory units were replaced because they had small cracks in them (from the heat).

That said, I don't drink beer below the car. I make up for it when I get back up. The brake drum idea is a good one. I often use a piece of tree trunk for a secondary safety stand.

Of course, my F-body is real sensitive about where I "stand" it.

RE: Why machine brake rotors?

I've had brake pedal pulsation and steering wheel shake from brakes on about 2/3 of the cars I've had.
Maybe I drove them too hard, or too inconsistantly?
'89 Aerostar- don't recall mileages, but definately had rotors turned to alleviate the problem. Usually helps for quite a while.
Ditto '89 and '93 Buicks. All the above have kinda puny brakes for how I drove them.
No probs yet with 03 Regal- original pads went 75k miles, don't recall if I turned the rotors or not. Probably did out of paranoia.
Now that rotors are pretty cheap, replacing them makes a lot more sense. The older cars, where the rotor included the wheel hub, were much more expensve to replace the rotors.
The worst fade I ever had was going down a steep fire road in the Aerostar. Had to ride the brakes, because even first gear wasn't low enough to slow the vehicle. Wasn't going fast enough to get any air flow through the rotors. Bad news- just barely got stopped (still on the hill) before they faded completely.
Could have used an electric cooling blower on each corner!

cheers
Jay

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

RE: Why machine brake rotors?

Not a rotor issue, but did you have any problems with the master cylinder on the Aerostar?

RE: Why machine brake rotors?


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.      

RE: Why machine brake rotors?

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

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.

RE: Why machine brake rotors?

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.

RE: Why machine brake rotors?


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.

      

RE: Why machine brake rotors?

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.

RE: Why machine brake rotors?

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??

RE: Why machine brake rotors?

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

RE: Why machine brake rotors?

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

RE: Why machine brake rotors?

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.

RE: Why machine brake rotors?

(OP)
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?

RE: Why machine brake rotors?

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

RE: Why machine brake rotors?




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.

RE: Why machine brake rotors?

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.

RE: Why machine brake rotors?

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

RE: Why machine brake rotors?

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.

RE: Why machine brake rotors?

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 . . .

RE: Why machine brake rotors?

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.   

RE: Why machine brake rotors?

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.

RE: Why machine brake rotors?

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!"


RE: Why machine brake rotors?

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

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.

RE: Why machine brake rotors?


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.

RE: Why machine brake rotors?

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

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.

RE: Why machine brake rotors?

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 (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)

RE: Why machine brake rotors?

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

RE: Why machine brake rotors?

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

RE: Why machine brake rotors?

"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

RE: Why machine brake rotors?

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.

RE: Why machine brake rotors?

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?

RE: Why machine brake rotors?

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!

RE: Why machine brake rotors?

Quote:

...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.

RE: Why machine brake rotors?

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.

RE: Why machine brake rotors?

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.)

RE: Why machine brake rotors?

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.

RE: Why machine brake rotors?

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!

RE: Why machine brake rotors?

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.

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources