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post #1 of 26 Old Jul 10th, 2010, 10:44 am Thread Starter
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Brakes

On our ride I noticed a pulsating in the rear brake. I only use the rear if I have to haul it down fast or at a stop sign.
It is at the low speeds at the stop sign that I notice it. Almost a surging or grab and release.

My suspicion is that the rear rotor is warped and I wonder if this affects the ABS re flashing lights.

I suspect that EBC is the way to go.

Oh well, another job to add to the list.

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post #2 of 26 Old Jul 10th, 2010, 10:59 am
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Re: Brakes

No such thing as a warped rotor, absolutely impossible. Just another shade tree mechanic myth perpetrated down through decades.

What you have is uneven brake pad deposition causing a variation in height on the surface of the rotor - caused by light braking. Even though you only get about 10% of your stopping power from the rear you should always use it in conjunction with the front. I exclusively use it under 5mph.

How to fix it? Take the rotor off and cross hatch sand both sides vigorously with 200 grit Garnet paper, re-install with new pads. Quite a project but it works, unfortunately it will return with continued light braking, no way around it.

Or, perform 10 high speed stops using only the rear brake, you want to make them increasingly harder and faster to build up lots of heat. After your last stop park the bike and let it cool completely WITHOUT touching the rear brake.


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post #3 of 26 Old Jul 10th, 2010, 11:10 am
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Re: Brakes

Not sure the warped rotor thing is purely a myth.

On my old DeVille the brakes needed to be redone so bought ceramic pads and less expensive rotors. NAPA replaced the rotors 3 times that year for same issue. Replaced rotors with most expensive rotors NAPA sold and used the softest brake pads and drove it 2 1/2 years before I gave it to my son. No problems the whole time either of us had it. No visible residue or deposits were noticed on the rotors. Hmmmmmmm.

It would seem that the soft pads would be more likely to succumb to deposition issues than the ceramic pads.

Thoughts?

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post #4 of 26 Old Jul 10th, 2010, 12:00 pm
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Re: Brakes

Quote:
Originally Posted by RonKMiller
...perform 10 high speed stops using only the rear brake, you want to make them increasingly harder and faster to build up lots of heat. After your last stop park the bike and let it cool completely WITHOUT touching the rear brake.
I agree - except that I have always ridden at least 10 minutes non-stop after the 10th hard brake without comng to a complete stop in order to let the brakes cool.

I also say toe-MAY-to, not toe-MAH-to...

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post #5 of 26 Old Jul 10th, 2010, 2:17 pm
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Re: Brakes

Quote:
Originally Posted by RonKMiller
No such thing as a warped rotor, absolutely impossible. Just another shade tree mechanic myth perpetrated down through decades.
Ron, ususally you are right on the money..., but I wish I had 5 bucks for every rotor that I measured with a dial indicator and found to have runout way above the factory specs.

And this was not under a shadetree but as a Porsche Factory Service Engineer. We had the same issue on earlier VW Rabbits and Audis. The last time I saw it as a daily occurence was on early 90's BMW 3series. These days on Mercedes I have not seen a single case of it.

My 99 LT had a rear rotor that was way out of round. I did not measure but holding a screwdriver against the caliper and rotating the wheel you could easily see about a mm of runout. New EBC rotor cured this. Keep in mind the rotor had about 70k on it and went from no problem to severe pulsation during one 100 mile trip.

Not wanting to start a debate or be arumentative, just my experiences.

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post #6 of 26 Old Jul 10th, 2010, 11:43 pm
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Re: Brakes

[QUOTE=Wolfgang]Ron, ususally you are right on the money..., but I wish I had 5 bucks for every rotor that I measured with a dial indicator and found to have runout way above the factory specs.

And this was not under a shadetree but as a Porsche Factory Service Engineer. We had the same issue on earlier VW Rabbits and Audis. The last time I saw it as a daily occurence was on early 90's BMW 3series. These days on Mercedes I have not seen a single case of it.

My 99 LT had a rear rotor that was way out of round. I did not measure but holding a screwdriver against the caliper and rotating the wheel you could easily see about a mm of runout. New EBC rotor cured this. Keep in mind the rotor had about 70k on it and went from no problem to severe pulsation during one 100 mile trip.

Wow, 1mm is insane. That had to be a massive failure of something to happen so quick!

You may be interested in reading this white paper. Carroll Smith had quite a history. He worked for a guy named Shelby and was the race team manager when the GT- 40 won Le Mans in '67.

warped disc brakes


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post #7 of 26 Old Jul 11th, 2010, 1:48 pm
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Re: Brakes

Quote:
Originally Posted by RonKMiller
No such thing as a warped rotor, absolutely impossible. Just another shade tree mechanic myth perpetrated down through decades.

What you have is uneven brake pad deposition causing a variation in height on the surface of the rotor - caused by light braking. Even though you only get about 10% of your stopping power from the rear you should always use it in conjunction with the front. I exclusively use it under 5mph.

How to fix it? Take the rotor off and cross hatch sand both sides vigorously with 200 grit Garnet paper, re-install with new pads. Quite a project but it works, unfortunately it will return with continued light braking, no way around it.

Or, perform 10 high speed stops using only the rear brake, you want to make them increasingly harder and faster to build up lots of heat. After your last stop park the bike and let it cool completely WITHOUT touching the rear brake.
Well, one thing I have learned is that nearly nothing is impossible. However, I agree with you that at least 98% of the time a warped rotor is one that has uneven deposition of pad material. I also mostly agree with your cure, but do have one major disagreement: do NOT immediately park the vehicle while the rotor is still hot. It is not impossible to get some serious pad printing given how close the pad remains to the disk even when not applied. It is certainly worse to stop at a light and keep the brakes applied while the rotor is hot, but I would recommend that after the above procedure you should ride at least 10 miles and let things cool before parking the bike.

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post #8 of 26 Old Jul 11th, 2010, 2:09 pm Thread Starter
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Re: Brakes

Ron,
If this will save me 250 dollars I will do the labour and say thank you very much.

Eric von Laue

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post #9 of 26 Old Jul 11th, 2010, 4:58 pm
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Re: Brakes

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Originally Posted by MountianMama
Ron,
If this will save me 250 dollars I will do the labour and say thank you very much.
I found that stapling the garnet to a wood sanding block that fit my hand makes it very comfortable for scrubbing. I simply propped the rotor between a couple of 2X4's and sanded in a straight line, rotating it a few degrees each time until I had a nice cross hatch over the entire surface. You'll know you've done enough when the garnet doesn't drag anymore and you've removed the invisible deposits. You may want to make a final pass with perhaps an 800 grit, although I didn't bother as it will bed in faster with a more rough cut.

One thing I forgot to mention is to clean the entire surface with Acetone - which means doing it outside with a little breeze and chemical resistant gloves. This includes any holes in the disc. I used a brass/plumbing pipe cleaning brush - with the handle cut off - and chucked in my cordless drill. It has to be spotlessly clean to avoid contaminating the new pads.

The attached picture is from my '79 airhead - same idea. Man those brakes work great!
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post #10 of 26 Old Jul 11th, 2010, 5:26 pm
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Re: Brakes

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Originally Posted by Voyager
Well, one thing I have learned is that nearly nothing is impossible. However, I agree with you that at least 98% of the time a warped rotor is one that has uneven deposition of pad material. I also mostly agree with your cure, but do have one major disagreement: do NOT immediately park the vehicle while the rotor is still hot. It is not impossible to get some serious pad printing given how close the pad remains to the disk even when not applied. It is certainly worse to stop at a light and keep the brakes applied while the rotor is hot, but I would recommend that after the above procedure you should ride at least 10 miles and let things cool before parking the bike.
According to Carroll Smith:
" in more than 40 years of professional racing, including the Shelby/Ford GT 40s one of the most intense brake development programs in history - I have never seen a warped brake disc."

Carroll's list of engineering clients included John Cooper, Mario Andretti, Dan Gurney, A.J. Foyt, Jackie lckx, Steve Millen, Sam Posey, Bobby Rahal, and Peter Revson - just to name a few. Oh yeah, Ferrari too.

Since races are won on braking, I'll bet they believed him when he said he had never seen a warped brake disc. I've never see one either. I guess I just can't join the 2% club right now, but then again I would never join any club that would have me as a member.

(I'll have to agree with your recommendation to cool things down after you've gone to all that trouble - especially if you have new pads installed)


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post #11 of 26 Old Jul 12th, 2010, 12:06 am
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Re: Brakes

Quote:
Originally Posted by RonKMiller
According to Carroll Smith:
" in more than 40 years of professional racing, including the Shelby/Ford GT 40s one of the most intense brake development programs in history - I have never seen a warped brake disc."

Carroll's list of engineering clients included John Cooper, Mario Andretti, Dan Gurney, A.J. Foyt, Jackie lckx, Steve Millen, Sam Posey, Bobby Rahal, and Peter Revson - just to name a few. Oh yeah, Ferrari too.

Since races are won on braking, I'll bet they believed him when he said he had never seen a warped brake disc. I've never see one either. I guess I just can't join the 2% club right now, but then again I would never join any club that would have me as a member.

(I'll have to agree with your recommendation to cool things down after you've gone to all that trouble - especially if you have new pads installed)
That doesn't mean it isn't possible. It just means that Carroll never got the right set of conditions.

Also, even the article you quoted earlied put a few constraints in place and I believe these are precisely what causes rotors to warp. Namely, improper torquing of the lugs nuts which places differential stresses on the rotor. Get them hot enough while under stress and warpage most certainly can and does occur. I've measured it with a dial indicator.

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post #12 of 26 Old Jul 12th, 2010, 11:27 am
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Re: Brakes

Quote:
Originally Posted by Voyager
That doesn't mean it isn't possible. It just means that Carroll never got the right set of conditions.

Also, even the article you quoted earlied put a few constraints in place and I believe these are precisely what causes rotors to warp. Namely, improper torquing of the lugs nuts which places differential stresses on the rotor. Get them hot enough while under stress and warpage most certainly can and does occur. I've measured it with a dial indicator.
Well, as Joe Friday used to say: "just the facts mam, just the facts"

The right set of conditions? ...and that would be...

You don't think that over hundreds of thousands of miles of road racing that they got them "hot enough" - as in scarlet red hot - and were so careless that they didn't torque them properly? Racing produces extreme conditions and the absolute hottest the rotors can get is 800 degrees F. I'll even do better and assume that they can ride their brakes down a long, steep hill (for no reason at all) and get them to 1,000 degrees F.

Iron melts at 2,797 degrees F. So in order for it to warp, deform or have any change in structure (IE. to become elastic or ductile) it would need to get within a few hundred degrees of the melt point - lets say 2500 degrees F. At that white hot temperature anything even close to the rotor would have been instantly vaporized a loooong time ago - like the aluminum alloy brake pistons, (melting point 1220 degrees F) fender liners, lubricants, brake lines, tires, etc.

What you've measured - along with EVERYONE else - is thickness variation due to a build up of Cementite. (Fe3C) Cementite is invisible to the naked eye, essentially a ceramic and almost as hard as a diamond. The tips of your dial indicator were sitting on layers of Cementite crystals that form "hills" on a perfectly flat alloy steel or iron plane.

Tell you what - I've got $100 in my PayPal account with your E-mail address on it if you can show me any proof from any automotive engineer or metallurgist (Smith was an automotive engineer, Navy pilot, race car driver and member of the SAE hall of fame) that a rotor can "warp" Not speculation, rumor or old wive's tales: PROOF.

I would also like to know the exact physical forces or mechanism that can cause a thick, hardened iron or stainless disk to get "out of round", bend or have any deviation - except of course at 2500 degrees F. Pure folklore and poppy cock.

Bonus round for another $50: Same thing for cell phones causing gasoline station fires.
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post #13 of 26 Old Jul 12th, 2010, 1:51 pm
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Re: Brakes

I like cake! I like to ride.

You guys are too smart.


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post #14 of 26 Old Jul 12th, 2010, 2:19 pm
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Re: Brakes

Quote:
Originally Posted by RonKMiller
No such thing as a warped rotor, absolutely impossible. Just another shade tree mechanic myth perpetrated down through decades.

What you have is uneven brake pad deposition causing a variation in height on the surface of the rotor - caused by light braking. Even though you only get about 10% of your stopping power from the rear you should always use it in conjunction with the front. I exclusively use it under 5mph.

How to fix it? Take the rotor off and cross hatch sand both sides vigorously with 200 grit Garnet paper, re-install with new pads. Quite a project but it works, unfortunately it will return with continued light braking, no way around it.

Or, perform 10 high speed stops using only the rear brake, you want to make them increasingly harder and faster to build up lots of heat. After your last stop park the bike and let it cool completely WITHOUT touching the rear brake.
ron
what makes brake rotors immune to warpage?
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post #15 of 26 Old Jul 12th, 2010, 2:38 pm
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Re: Brakes

Quote:
Originally Posted by RonKMiller
Well, as Joe Friday used to say: "just the facts mam, just the facts"

The right set of conditions? ...and that would be...

You don't think that over hundreds of thousands of miles of road racing that they got them "hot enough" - as in scarlet red hot - and were so careless that they didn't torque them properly? Racing produces extreme conditions and the absolute hottest the rotors can get is 800 degrees F. I'll even do better and assume that they can ride their brakes down a long, steep hill (for no reason at all) and get them to 1,000 degrees F.

Iron melts at 2,797 degrees F. So in order for it to warp, deform or have any change in structure (IE. to become elastic or ductile) it would need to get within a few hundred degrees of the melt point - lets say 2500 degrees F. At that white hot temperature anything even close to the rotor would have been instantly vaporized a loooong time ago - like the aluminum alloy brake pistons, (melting point 1220 degrees F) fender liners, lubricants, brake lines, tires, etc.

What you've measured - along with EVERYONE else - is thickness variation due to a build up of Cementite. (Fe3C) Cementite is invisible to the naked eye, essentially a ceramic and almost as hard as a diamond. The tips of your dial indicator were sitting on layers of Cementite crystals that form "hills" on a perfectly flat alloy steel or iron plane.

Tell you what - I've got $100 in my PayPal account with your E-mail address on it if you can show me any proof from any automotive engineer or metallurgist (Smith was an automotive engineer, Navy pilot, race car driver and member of the SAE hall of fame) that a rotor can "warp" Not speculation, rumor or old wive's tales: PROOF.

I would also like to know the exact physical forces or mechanism that can cause a thick, hardened iron or stainless disk to get "out of round", bend or have any deviation - except of course at 2500 degrees F. Pure folklore and poppy cock.

Bonus round for another $50: Same thing for cell phones causing gasoline station fires.
The article you referenced says this: "With one qualifier, presuming that the hub and wheel flange are flat and in good condition and that the wheel bolts or hat mounting hardware is in good condition, installed correctly and tightened uniformly and in the correct order to the recommended torque specification..." Why do you think it says this? Because there ARE conditions that can allow the rotors to warp. And the warpage doesn't have to be permanent. The rotor can "potato chip" while hot due to the thermally induced stresses and cause severe brake pulsing and then return to flat once cooled. How many cars do you know that have the lug nuts installed with a torque wrench and using the alternating tightening pattern recommended?

Thermally induced strain can induce extremely large stresses. These stresses can cause even thick parts to locally exceed the yield point and move into the plastic region. At that point, the strain is permanently locked in and it doesn't require the metal to reach anything near the melting point. Steel I-beams yield during earthquakes and they are at ambient temperatures.

You often issue absolute statements (I recall one fairly recently about it being IMPOSSIBLE for a polycarbonate windshield to crack), based on a limited knowledge of the science involved. This is yet another case where you have stated an absolute and that absolute is countered by the same article you referenced.

I know I will never see your money as you are the judge of the argument. Do you care to repeat the offer with the group at large voting on whether I made my case or not? That might make it worth doing the research, but $100 only covers about 35 minutes of my time so it likely still wouldn't be worthwhile.

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post #16 of 26 Old Jul 12th, 2010, 2:50 pm
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Re: Brakes

Quote:
Originally Posted by drjohn55
ron
what makes brake rotors immune to warpage?
Read two posts above!


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post #17 of 26 Old Jul 12th, 2010, 3:16 pm
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Re: Brakes

Quote:
Originally Posted by Voyager
The article you referenced says this: "With one qualifier, presuming that the hub and wheel flange are flat and in good condition and that the wheel bolts or hat mounting hardware is in good condition, installed correctly and tightened uniformly and in the correct order to the recommended torque specification..." Why do you think it says this? Because there ARE conditions that can allow the rotors to warp. And the warpage doesn't have to be permanent. The rotor can "potato chip" while hot due to the thermally induced stresses and cause severe brake pulsing and then return to flat once cooled. How many cars do you know that have the lug nuts installed with a torque wrench and using the alternating tightening pattern recommended?

Thermally induced strain can induce extremely large stresses. These stresses can cause even thick parts to locally exceed the yield point and move into the plastic region. At that point, the strain is permanently locked in and it doesn't require the metal to reach anything near the melting point. Steel I-beams yield during earthquakes and they are at ambient temperatures.

You often issue absolute statements (I recall one fairly recently about it being IMPOSSIBLE for a polycarbonate windshield to crack), based on a limited knowledge of the science involved. This is yet another case where you have stated an absolute and that absolute is countered by the same article you referenced.

I know I will never see your money as you are the judge of the argument. Do you care to repeat the offer with the group at large voting on whether I made my case or not? That might make it worth doing the research, but $100 only covers about 35 minutes of my time so it likely still wouldn't be worthwhile.
I'm up for that in a heartbeat - anybody can join in! $100. But lets make it even more interesting and have you offer up $100 to match and we'll make it a bet. I mean, if you're so SURE...

I put my money where my mouth is, period. You are still speculating - and getting out even farther on a limb with theory, postulation (thermally induced stress?) and just plain silly observations (earthquakes?) that have nothing at all to do with the specific topic. Bring me someone who is qualified and can back up the science behind your way out theories - like an automotive engineer or metallurgist - and I'll listen.

..and you will recall that the reason the polycarbonate cracked is because super glue was introduced into the equation. A foreign element that never should have been there in the first place. Under normal use polycarbonate won't crack, period. We even had a plastics engineer chime in to verify that.

Introducing a catastrophic failure point caused by a human - such as poor mounting technique - isn't germane to the discussion. A nuclear bomb could cause a rotor to warp - so what?

I still want to know the physical forces or mechanism at work.

Show me the science - but most importantly - show me the money!


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post #18 of 26 Old Jul 12th, 2010, 3:22 pm
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Re: Brakes

Quote:
Originally Posted by RonKMiller
I'm up for that in a heartbeat - anybody can join in! $100. But lets make it even more interesting and have you offer up $100 to match and we'll make it a bet. I mean, if you're so SURE...

I put my money where my mouth is, period. You are still speculating - and getting out even farther on a limb with theory, postulation (thermally induced stress?) and just plain silly observations (earthquakes?) that have nothing at all to do with the specific topic. Bring me someone who is qualified and can back up the science behind your way out theories - like an automotive engineer or metallurgist - and I'll listen.

..and you will recall that the reason the polycarbonate cracked is because super glue was introduced into the equation. A foreign element that never should have been there in the first place. Under normal use polycarbonate won't crack, period. We even had a plastics engineer chime in to verify that.

Introducing a catastrophic failure point caused by a human - such as poor mounting technique - isn't germane to the discussion. A nuclear bomb could cause a rotor to warp - so what?

I still want to know the physical forces or mechanism at work.

Show me the science - but most importantly - show me the money!
After 28 years of engineering, I am no longer sure about anything and that is why you won't see me making the sort of absolute statements that you are so fond of. Everyone thought Newton had it nailed until Einstein came along. How many times have scientists claimed to have found the smallest particle of matter/energy, only to have someone else find evidence of something even smaller.

Sorry, you won't see me entering into absolutes as the world is just too complex for that. There is always one more factor that we don't know of or failed to consider. The only absolute I will state is that anyone who states absolutes is absolutely wrong most of the time. :-)

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post #19 of 26 Old Jul 12th, 2010, 3:33 pm
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Re: Brakes

Quote:
Originally Posted by RonKMiller
I'm up for that in a heartbeat - anybody can join in! $100. But lets make it even more interesting and have you offer up $100 to match and we'll make it a bet. I mean, if you're so SURE...

I put my money where my mouth is, period. You are still speculating - and getting out even farther on a limb with theory, postulation (thermally induced stress?) and just plain silly observations (earthquakes?) that have nothing at all to do with the specific topic. Bring me someone who is qualified and can back up the science behind your way out theories - like an automotive engineer or metallurgist - and I'll listen.

..and you will recall that the reason the polycarbonate cracked is because super glue was introduced into the equation. A foreign element that never should have been there in the first place. Under normal use polycarbonate won't crack, period. We even had a plastics engineer chime in to verify that.

Introducing a catastrophic failure point caused by a human - such as poor mounting technique - isn't germane to the discussion. A nuclear bomb could cause a rotor to warp - so what?

I still want to know the physical forces or mechanism at work.

Show me the science - but most importantly - show me the money!
It wasn't super glue it was loctite. I agree its the uneven transfer of brake pad material to the rotors causing uneven frictions that make it feel like the rotor is warped.

Just Go
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post #20 of 26 Old Jul 12th, 2010, 4:34 pm
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Re: Brakes

Quote:
Originally Posted by mwnahas
It wasn't super glue it was loctite. I agree its the uneven transfer of brake pad material to the rotors causing uneven frictions that make it feel like the rotor is warped.
Pretty much the same thing - cyanoacrylate (super glue) is the major component of loctite.


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post #21 of 26 Old Jul 12th, 2010, 5:46 pm
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Re: Brakes

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Originally Posted by Voyager
After 28 years of engineering, I am no longer sure about anything and that is why you won't see me making the sort of absolute statements that you are so fond of. Everyone thought Newton had it nailed until Einstein came along. How many times have scientists claimed to have found the smallest particle of matter/energy, only to have someone else find evidence of something even smaller.

Sorry, you won't see me entering into absolutes as the world is just too complex for that. There is always one more factor that we don't know of or failed to consider. The only absolute I will state is that anyone who states absolutes is absolutely wrong most of the time. :-)
Well, wait just a cotton pickin' minute - you're not gonna' bail on me THAT easy - especially if you're an ENGINEER - right? I want my $100. I mean, after all, I'm just a lowly chemist. You know - better living through chemistry? It's not really ALL that complex. I'll even attempt to distill it down to layman's terms.

How about some show and tell? Pictures always seem to work.

Attached are a couple of thermal images of disc brake rotors and the associated hot spotting caused by thickness variation from cementite crystals on the surface. Other various forms of ceramic crystals can also form depending on temperature and alloy - like martensite which can be responsible for the "chipping" you mentioned. It is VERY brittle.

The first picture shows what a nice smooth disc brake feels like when the pads clamp down on the rotor. Notice the continual white area. Nice even pressure, temperature and no hot spots. A nice smooth stop.

The second picture shows what "brake judder" (what is commonly and mistakenly referred to as the warped disc brake myth) feels like when the pads clamp down on the rotor. You can actually SEE the high spots as much brighter white areas. Those bright white spots are the "Cementite hills on the plane" I was alluding to earlier. You're gonna have a whole lot o' shakin' going on here - and I'm not talkin' about listening to Jerry Lee on the box.

BUT EVEN MORE TELLING is the difference in the hot spotting between the inner and outer surfaces, left versus right. They are out of phase, meaning that the pads are riding up and down and contacting the disc surface at different times - and actually skipping over the tops - of the "hills" caused by the depositions. That feeling, translated to the brake pedal is a distinct pulsing. The more out of phase the deeper the pulsing sensation. It is a vicious downward spiral - it just gets worse and worse - unless you remove the hills with a very hard abrasive that will not leave any residue behind - aka garnet paper. You can also blanchard grind, but that's only for advanced cases where the hills have literally turned into mountains. If you catch it early enough it can be easily removed - precisely when you start to feel the judder. Unfortunately grinding is not an option on our stainless motorcycle discs. It might be available somewhere but it is certainly not common nor inexpensive compared to just buying a new rotor and slapping it on.

Interestingly enough, the thickness of the pads and the composition of the pads play a role in this as well - separate from the depositions. A thick pad has more ability to absorb the hit when slamming into the peak of the hill, a thin pad transfers more energy since it has less material to dissipate the energy. The composition is important too - racing pads (hard) will bounce off the hills, while street pads (soft) more or less skip off the top.

Now can I have my money?

Not yet - because - wait, there's more!

...we still have to discuss the possibility that some sort of magical thermal stress has happened to the rotor to cause it to distort.

The third image shows the thickness of a rotor prior to contact with the pads, during contact, and again after contact. Not surprisingly it DOES change thickness during contact - but literally snaps back to the previous thickness immediately. This happens probably a hundred thousand times during the normal life of rotor and pads. The interesting thing is that the difference in thickness here is measured in angstroms on an atomic level - which is pretty much a defunct method of measuring things these days. The variation is so small it's barely measurable, certainly not enough to cause any feeling at the brake pedal, and besides, this is only a temporary condition at the temperatures involved. There just isn't enough energy here to cause a permanent change in the thickness. No distortion, no "warping".

OK, now I'm done... just how much does a box of good ceegars cost these days? Why, about $100!. As Jesse James was so fond of saying: "Pay Up Sucker."

...but I ain't holding my breath.
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post #22 of 26 Old Jul 12th, 2010, 11:38 pm
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Re: Brakes

Quote:
Originally Posted by RonKMiller
Well, wait just a cotton pickin' minute - you're not gonna' bail on me THAT easy - especially if you're an ENGINEER - right? I want my $100. I mean, after all, I'm just a lowly chemist. You know - better living through chemistry? It's not really ALL that complex. I'll even attempt to distill it down to layman's terms.

How about some show and tell? Pictures always seem to work.

Attached are a couple of thermal images of disc brake rotors and the associated hot spotting caused by thickness variation from cementite crystals on the surface. Other various forms of ceramic crystals can also form depending on temperature and alloy - like martensite which can be responsible for the "chipping" you mentioned. It is VERY brittle.

The first picture shows what a nice smooth disc brake feels like when the pads clamp down on the rotor. Notice the continual white area. Nice even pressure, temperature and no hot spots. A nice smooth stop.

The second picture shows what "brake judder" (what is commonly and mistakenly referred to as the warped disc brake myth) feels like when the pads clamp down on the rotor. You can actually SEE the high spots as much brighter white areas. Those bright white spots are the "Cementite hills on the plane" I was alluding to earlier. You're gonna have a whole lot o' shakin' going on here - and I'm not talkin' about listening to Jerry Lee on the box.

BUT EVEN MORE TELLING is the difference in the hot spotting between the inner and outer surfaces, left versus right. They are out of phase, meaning that the pads are riding up and down and contacting the disc surface at different times - and actually skipping over the tops - of the "hills" caused by the depositions. That feeling, translated to the brake pedal is a distinct pulsing. The more out of phase the deeper the pulsing sensation. It is a vicious downward spiral - it just gets worse and worse - unless you remove the hills with a very hard abrasive that will not leave any residue behind - aka garnet paper. You can also blanchard grind, but that's only for advanced cases where the hills have literally turned into mountains. If you catch it early enough it can be easily removed - precisely when you start to feel the judder. Unfortunately grinding is not an option on our stainless motorcycle discs. It might be available somewhere but it is certainly not common nor inexpensive compared to just buying a new rotor and slapping it on.

Interestingly enough, the thickness of the pads and the composition of the pads play a role in this as well - separate from the depositions. A thick pad has more ability to absorb the hit when slamming into the peak of the hill, a thin pad transfers more energy since it has less material to dissipate the energy. The composition is important too - racing pads (hard) will bounce off the hills, while street pads (soft) more or less skip off the top.

Now can I have my money?

Not yet - because - wait, there's more!

...we still have to discuss the possibility that some sort of magical thermal stress has happened to the rotor to cause it to distort.

The third image shows the thickness of a rotor prior to contact with the pads, during contact, and again after contact. Not surprisingly it DOES change thickness during contact - but literally snaps back to the previous thickness immediately. This happens probably a hundred thousand times during the normal life of rotor and pads. The interesting thing is that the difference in thickness here is measured in angstroms on an atomic level - which is pretty much a defunct method of measuring things these days. The variation is so small it's barely measurable, certainly not enough to cause any feeling at the brake pedal, and besides, this is only a temporary condition at the temperatures involved. There just isn't enough energy here to cause a permanent change in the thickness. No distortion, no "warping".

OK, now I'm done... just how much does a box of good ceegars cost these days? Why, about $100!. As Jesse James was so fond of saying: "Pay Up Sucker."

...but I ain't holding my breath.
Your short-term memory is really short! I never raised the issue of money or a bet and never offered to pay anything for anything. Is your reading comprehension really this weak?

So, do you trust a university such as LSU? This article mentions at least two causes of brake pulsing/judder and only ONE of them is due to mu variation (cementite or uneven pad material transfer). The other is thermally induced variation as I mentioned earlier.

http://www.cerom.lsu.edu/serhan.htm

Do you consider Bembo to be a reliable source of information? If you do, read the following: http://www.brembo.com/US/Cars/Dual-Cast/

Note comments such as "The rotor function effectively as an integral disc at low temperatures; then as a floating disc at high temperatures, when maximum performance is needed and distortion tends to occur." Hmm. Distortion at high temperatures almost sounds like something I wrote earlier. Hmm.

Also note "As a result, incidences of both transient and permanent thermal distortion can be significantly reduced. Transient deformation events occur typically when the brakes are applied, and the discs run hot; whereas permanent disc distortion is the main cause of vibration during braking in the vehicle." Hmm. Both transient AND PERMANENT thermal distortion. Hmm. I suspect that PERMANENT thermal distortion could be measured with a dial indicator as I have PERSONALLY done. Hmm.

Then again, what does Brembo know about brakes. They have only been making them since 1961 so what would they know?

And the SAE would not know much about brake systems, yet they have papers like this that talk about THERMAL DISTORTION of brake rotors: http://papers.sae.org/2000-01-0445/ Unfortunately, to get the full article requires buying it, but the title is pretty clear as to the topic. But then you claimed the thermal distortion wasn't possible, right?

Here is a good paper on thermoelastic instability: http://www-personal.umich.edu/~jbarber/TEI.html Nothing to do at all with uneven material transfer to the disk.

Note discussion about "high temperature judder": http://www.hitachi.com/ICSFiles/afie...009_07_107.pdf

Note comments about TEI and thermally induced buckling of the rotor: http://www.fkm.utm.my/~arahim/jacobsson.pdf

You have fixated on ONE cause of brake pulsing/judder and claimed it is the ONLY cause. This is simply incorrect and there are MANY solid reference papers that demonstrate this. I can find many more, but I think the above is more than sufficient for any open-minded researcher.

I know I will never see your $100, but that doesn't change that fact that your absolute statement is absolutely wrong.

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post #23 of 26 Old Jul 13th, 2010, 9:20 am
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Re: Brakes


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post #24 of 26 Old Jul 13th, 2010, 10:46 am
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Re: Brakes

Nothing more compelling than a technical discussion complete with cites!

It would seem that Matt and Ron are both certain that their positions on the warped rotor are correct and that neither is likely to change their position on the matter.

One of the them is right which means the other is at least partly wrong but, as a collector of quotes, I snagged (pilfered, borrowed, etc) from a member of this site the following:

"Sometimes you can get so fixated on the fact that you are right that you lose sight of the fact that it doesn't matter"

I think this topic has reached this point.

I apologize to the individual I got this from as I don't remember whose signature it was in.

I know which individual I would put my money on but who cares? Certainly not me enough to cast my vote in public.

Go in peace,

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post #25 of 26 Old Jul 13th, 2010, 2:30 pm
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Re: Brakes

Quote:
Originally Posted by wa1200lt
Nothing more compelling than a technical discussion complete with cites!

It would seem that Matt and Ron are both certain that their positions on the warped rotor are correct and that neither is likely to change their position on the matter.

One of the them is right which means the other is at least partly wrong but, as a collector of quotes, I snagged (pilfered, borrowed, etc) from a member of this site the following:

"Sometimes you can get so fixated on the fact that you are right that you lose sight of the fact that it doesn't matter"

I think this topic has reached this point.

I apologize to the individual I got this from as I don't remember whose signature it was in.

I know which individual I would put my money on but who cares? Certainly not me enough to cast my vote in public.

Go in peace,

Loren
I will always change my position if evidence is produced to show that my current position is incorrect. I just have yet to see such evidence.

The reason I provided a range of citations is so that you and others could read for yourselves and draw your own conclusion. Whether you adopt my conclusion or not is of no import to me.

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post #26 of 26 Old Jul 13th, 2010, 3:18 pm
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Re: Brakes

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Originally Posted by RonKMiller
Read two posts above!
now i'm confused.
how come a cast iron cylinder head can warp?
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