Re: Can anyone tell the difference between rotors and pads (truthfully)?

Stuart A. Bronstein wrote: snip


It seems to me you've found one article that you want to believe over all the other articles available, as well as the advice from learned technicians, based on their education and experience. Many of these techs I've read in this thread are either experts in the field of brakes or are Master Diagnostic Technicians, which would lead one to believe the majority opinion of these is true. Are you possibly overanalyzing your situation? davidj92
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Hi David,
Actually I found many articles, some with conflicting ideas.
For example, some say we should replace with softer brake pads, others say we should always true brand new rotors, other say never true rotors off the vehicle, others say use the parking brake instead of the pedal when stopped at a light due to cooling differences, etc.
The majority is often wrong, by the way. The majority will tell you, in and of itself, that high octane gas is "better" gas just like the majority of chiropracters will tell you your spine needs "adjustment".
There is only one truth, and I'm simply searching for that one truth.
Here's a quick summary to date of some of those articles Stu
What causes high-speed brake induced shimmy?
Vibration felt in the steering wheel only when the brakes are applied is not a front end alignment problem, but a brake problem. http://www.trustmymechanic.com/besttires.html
The steering wheel is vibrating because the front brake rotors are warped (we call this vibration "shimmy"). http://www.trustmymechanic.com/brakewarp.html
Cold judder occurs primarily as a result of a non-uniform circumferential rotor (friction ring) thickness, which causes a cyclic variation in the brake torque output during braking (1 - 2). These microscopic variations in the cross-sectional thickness of the disc brake rotor, axiomatically referred to as Disc Thickness Variations (D.T.V. or R.T.V. (Rotor Thickness Variation)) may arise during rotor manufacture as a product of the machining process (typical manufactured D.T.V. < 7 m), or, as laboratory and field trial testing have demonstrated, may be generated throughout their lifetime in-service. http://www.eurac-group.com/technote4.htm
Warping can be caused by excessive heat build up, which softens the metal ... however with most ventilated discs ... the sensation of warped brakes (wheel shimmy under braking) most often is a matter of a brake pad material operating outside of its designed temperature range and it has left a thick(er) than normal deposit in one area of the disc surface, creating a "sticky" spot that will grab every revolution of the disk. In cars with automatic transmissions the driver applies brakes when the car is stopped ... the brake pads remain in contact with the disc and the discs will cool unevenly ... leading to warping. http://en.wikipedia.org/wiki/Disk_brake
Wheel shimmy during braking is often caused by thickness variation of the rotor disc. If the rotor has runout, a thin spot will develop by the continuous touch touch touch as the rotor turns while the brakes are not applied. When this thickness variation increases to approximately 0.007 inch, the pulsation can be felt by the driver. http://en.wikipedia.org/wiki/Disk_brake
The accepted cause of brake-induced vibrations is disc thickness variation. Disc thickness variation generated by off-brake running, uneven transfer of lining material to the disc surface, disc corrosion, and distortion of the disc under thermal loading. The variations in rotor disc thickness cause the brake fluid pressure in the caliper to fluctuate, resulting in torque variations. http://support.mscsoftware.com/cgi-bin/kb_files/Ford_Brake_Roughness_ 2001_NAUC.pdf
Pre-loaded wheel bearings have no end play to "absorb" hub and rotor run-out. Hence, almost 100% of any axial run-out of the hub and brake rotor are transmitted to the brake pads. This axial run-out or wobble in the rotor causes the brake pads to wear the rotor unevenly over time, producing two sections of the rotor, 180 degrees apart, where the rotor thickness becomes thinner than the other two sections. This difference in thickness is called Disk Thickness Variation or DTV. http://www.rtitech.com/latheinfo.htm
In general, any run-out greater than 0.002" (50 microns) will lead to an increase in DTV of about 0.0004" (10 microns) in about 3000-5000 miles. In most cars, when DTV reaches 0.0004" (10 microns) or only 4 ten-thousandths of an inch, the driver will complain of pedal pulsation, steering wheel shimmy, or brake shudder. The most important fact to consider here is that the installation of the wheel will almost always increase hub/rotor run-out by 0.001-0.0015" (25-40 microns), even if the lug nuts are carefully torqued. http://www.rtitech.com/latheinfo.htm
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On many vehicles, such as my '94 Jeep Grand Cherokee, it is important to torque the front wheel lugs to a specified point, on my Jeep it is 90 foot/pounds. If this isn't done, my rotors warp in a very short time...

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Hi Emmo, I agree and disagree (now that I've done some research). I agree one should do a three pass tightening of the lug nuts, which, in the case of the six-lug-nut 1998 Toyota 4Runner, is 84 foot pounds.
FIRST PASS: While the truck is in the air, tighten three lug nuts by hand in a triangle pattern; then tighten the other three in a reverse triangle pattern.
SECOND PASS: Back on the ground, tighten the second lug nut triangle to 1/2 the recommended torque (42 foot pounds for a 1998 Toyota 4Runner); then do the same for the first triangle set of lug nuts.
THIRD PASS: Finally, tighten the first triangle to full torque; and then the second triangel to full torque.
Of course, this assumes the hub was cleaned of all rust and was greased with synthetic Mobil 1 grease to prevent rust creep (pushing out of the rotors like tree roots push out a sidewalk or curb).
However, now that I've learned enough to be dangerous, I will disagree with your assertion that the rotors will "warp" if you tighten lug nuts incorrectly. Many articles have shown that brake discs almost never warp. What people call warp is really disk thickness variation or rotor thickness variation caused by the pad deposition being different on various parts of the rotor.
See a more detailed explanation in the following reference: http://www.stoptech.com/tech_info/wp_warped_brakedisk.shtml #
Stu
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Dear Stuart, I know for a fact, that incorrectly tightening lugnuts will warp rotors. (even brand new rotors). I can prove it on just about any vehicle. (not just hubless types) Those websites are just playing on words mostly and also must not know that even a 0 thickness variation can still have a warp in the rotor if the wheels are not properly tightened. You really should not listen to websites as if they are law.. Since if you do. There are websites that have proof of many other things that are far more silly than the "rotors can not warp" bologna you are reading.
-- James M Driscoll Jr Spaceman
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. . ==============Spaceman wrote:

====================== I agree with Stuart, he has learned enough to be dangerous.
I agree with Spaceman, it is very easy to warp a rotor during tightening of lug nuts.
I've done it. More than once. Not recently. Not in the last year. Not in the last 15-20 years. But..I've done it.
why....??...... because I too was onced learn'ed enough to be dangerous.
~:~ MarshMosnter ~wonders why it takes 3 threads to diagnose a symptom~
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Hi Marsh Monster,
The three related threads were a result of each other.
THREAD #1: Why does my 4Runner vibrate when I apply the brakes? THREAD #2: What replacement pads should we use for this 4Runner? THREAD #3: How do we test piston retraction?
The actual titles of those only somewhat related threads are: #1: Bad shimmy upon heavy braking Toyota 4Runner (why?) #2: Can anyone tell the difference between rotors and pads (truthfully)? #3: Can anyone check whether brake pistons move freely?
What I found out, thanks to you, from each thread was: #1: I have DTV which is being felt in the front steering & pedal. #2: A DIY can not tell the difference between pads so go OEM. #3: There is no tool that can test brake piston movement.
As stated, I've learned a lot (enough to be argumentative :) ... but I still don't understand what's wrong with the referenced method of torquing the lug nuts half way first.
What is wrong with torquing them to 1/2 torque in a triangle pattern in two passes and then repeating the two pass triangle pattern for the full torque value?
Is that wrong?
Stu
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==================Stuart A. Bronstein wrote:

====================Stu, addressing your last question....... I find nothing wrong with that tightening proceedure.
Personally, I jest stick my Ingersol on em and have at it. every day. every week. every year. for many,many,many years.
and...like I stated earlier, i don't warp rotors. use to. sometimes. once or twice. on occasion. but.....not for the last 15-20 years.
The proceedure you listed will work. It's slow as snot run'n up a sand hill. But...it'll work.
~:~ Marsh ~:~
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On Sun, 05 Mar 2006 18:21:06 GMT, "Stuart A. Bronstein"

Generally the last 3 articles pretty well nail it, while many of the others contain truth.
Rotor quality is one BIG variable - and todays rotors are not "aged" before finishing. The "green" castings relieve after machining - so often arive out of true from the factory. Warpage, in itself, seldom causes pedal pulsation OR steering shimmy unless it is quite sizeable. Thickness variation causes problems even if very close in tolerance. Vented rotors often have "callapse" problems, where several "fins" move in tolerance as the rotor ages, or in use. Particularly in the "salt belt" todays pads often cause a "glaze" on the rotors, which traps moisture and pits the rotor away. When it pits, the oxide layer expands behind the glaze, clausing "blisters" that cause pulsations.
Machining the rotor temporarily solves the problem, but reduced thickness and mass makes warpage more likely.
As for surfacing new rotors, SOME need surfacing. On Car machining will usually give you a truer disk - but since runout is NOT the big problem, off-car machining on a good lathe will sometimes give better parallellism than on-car machining, where vibrations can have an effect. Pad composition is one thing you CAN change to prevent or greatly reduce break pulsation problems. Anything with an iron metallic is going to cause problems in salt-belt areas. Brass metallic is MUCH better, from early Toyota experience, and carbon metallic or ceramic pads have proven to be a great improvement in my experience with later model Ford and GM vehicles.
Carbon Metallic pads on my 90 Aerostar improved rotor life by over 400%, and pad life by 100% over factory parts. Brake effectiveness was also GREATLY improved - allowing me to actually lock the front wheels on dry pavement, which was absolutely impossible with the factory pads.

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