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Old 10-30-2014, 04:09 PM   #43
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Also, relative to the "discussion" upthread about rotor warping vs. thickness variation, while looking up the burnishing procedure, I landed upon a bulletin covering proper brake diagnosis and repair.

This about brake pulsation:

"Brake pulsation is caused by brake rotor thickness variation. Brake rotor thickness variation causes the piston in the brake caliper, when applied, to “pump” in and out of the caliper housing. The “pumping” effect is transmitted hydraulically to the brake pedal. Brake pulsation concerns may result from two basic conditions:

 1. Thickness Variation Pulsation is Caused by Lateral Run Out (LRO). LRO on a brake corner assembly is virtually undetectable unless measured (with a dial indicator after the brake service) and will not be detected as brake pulsation during an after brake service test drive. If the brake corner is assembled with excessive LRO (greater than 0.050 mm (0.002 in), thickness variation will develop on the brake rotor over time and miles. Excessive LRO will cause the brake pads to wear the brake rotors unevenly, which causes rotor thickness variation. Pulsation that is the result of excessive Lateral Run Out usually develops in 4,800-16,000 km (3,000-10,000 mi). The more excessive the LRO, the faster the pulsation will develop. LRO can also be induced when uneven torque is applied to wheel nuts (lug nuts). Improper wheel tightening after tire rotation, spare tire usage, brake inspection, etc. can be the cause of brake pulsation. Again, it usually takes 4,800-16,000 km (3,000-10,000 mi) AFTER the service event for the condition to develop. The customer does not usually make the connection between the service event and the awareness of the pulsation. The proper usage of torque wrenches (torque limiting sockets) will greatly reduce or eliminate the pulsation conditions after wheel service events. The improper use of impact wrenches on wheel nuts greatly increases the likelihood of pulsation after wheel service.
 2. Thickness Variation Pulsation Caused by Brake Rotor Corrosion — Rotor corrosion is another form of thickness variation, which can cause a pulsation concern and can be addressed as follows:
◦Cosmetic Corrosion:
In most instances rotor corrosion is cosmetic and refinishing the rotor is unnecessary.

◦Corrosion — Pulsation Caused by Thickness Variation (Lot Rot / Low Miles — 0–321 km (0–200 mi):
At times more extensive corrosion can cause pulsation due to thickness variation. This usually happens when the vehicle is parked for long periods of time in humid type conditions and the braking surface area under the pads corrodes at a different rate compared to the rest of the braking surface area. Cleaning up of braking surfaces (burnishing) can be accomplished by 10 – 15 moderate stops from 56– 64 km/h (35 – 40 mph) with cooling time between stops. If multiple moderate braking stops do not correct this condition, follow the “Brake Rotor Clean-Up Procedure” below.

◦Corrosion — Pulsation Caused by Thickness Variation (without rotor flaking / higher mileage — 3,200-8,000 km (2,000-5,000 mi):
In some cases, more extensive corrosion that is not cleaned up by the brake pad over time and miles can cause the same type of pulsation complaint due to thickness variation. In these cases, the rotor surface is usually darker instead of shiny and a brake pad foot print can be seen against the darker surface. This darker surface is usually due to build-up, on the rotor material surface, caused by a combination of corrosion, pad material and heat. To correct this condition, follow the “Brake Rotor Clean-up Procedure” below.

◦Corrosion — Pulsation Caused by Thickness Variation (with rotor flaking / higher mileage — 8,000 + km (5,000 + miles):
Important: In some flaking instances, cleaning-up this type of corrosion may require more rotor material to be removed then desired. "
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Old 10-30-2014, 07:12 PM   #44
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Could it be the back rotors if you feel pulsating in the pedal, but not in the steering wheel?
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Old 10-30-2014, 07:22 PM   #45
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Originally Posted by m.hony View Post
Could it be the back rotors if you feel pulsating in the pedal, but not in the steering wheel?
Yes, if it's back only, it will not be felt in the wheel. But, front thickness variation typically won't be felt in the wheel much, if at all either. Wheel shimmy is typically a truly warped rotor due to severe overheat.

However, a low mile vehicle (a few thousand miles) could be, as Perry alluded to, due to a "green" casting (of poor quality) aging into a set. Most mfrs. now try and "age" a rotor before machining. Also most mfrs now use a higher nickel content in the iron, which makes a lot better more stable casting and is more resistant to corrosion.

Really, only a dial indicator will tell you. Also a lot of thickness variation is the result of 1) hub runout or 2) not cleaning the surface between the hub face and the back of the rotor when rotating tires or removing the rotor for service. Any "stuff" between those surfaces will cause rotor runout and eventual (several thousand miles) thickness variation. Gunk in there MAY cause a shimmy in the wheel as well.
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Old 10-30-2014, 07:29 PM   #46
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Bad Rotors

There are a lot of quality after market rotors with slot/kerfs or hole built into the rotors which are specially designed to dissipate a great deal of heat and reduce wrapping. The racing world has been using them for years.
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Old 10-30-2014, 07:34 PM   #47
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There are a lot of quality after market rotors with slot/kerfs or hole built into the rotors which are specially designed to dissipate a great deal of heat. The racing world has been using them for years.
The point is, if we as towers are using our transmissions and/or exhaust brakes properly, and driving at appropriate speeds downhill, heat shouldn't ever be an issue. (assuming a properly spec'ed TV for the trailer and GCW).
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Old 10-30-2014, 07:36 PM   #48
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Quote:
Originally Posted by dznf0g View Post

Rotors did have irregular patches of odd discoloration. That's a replace issue. Seem to recall lathe skipped and jumped all around the spots and damaged the cutting tips.

Rich,

Sight unseen but your description matches Martensite. Super heated, quickly cooled by it's surrounding thick metal transforms it into localized hot/hard spots. Typically can't lathe cut it tool bounces over it. Grinding can cut thru it but doesn't remove "roots" of it. We see it in flywheels. Grind it, reheat it and it grows back as a drum roll please, a high spot due to expansion. Can appear as a shadow in cast iron after grinding.
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Old 10-31-2014, 06:08 AM   #49
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When I first started to drive I was told to keep the car rolling after you get on the brakes really hard. Especially when it is cold and wet out. This would dissipate the heat around the rotor instead of keeping it on one spot.

Don't know if this is fact or fiction, but I've never had a warped rotor.
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Old 10-31-2014, 06:45 AM   #50
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Keeping the car rolling will also allow the rotors/calipers/ pads to dry, therefore better braking.
Apply brakes, let off a little, apply brakes, let off a little...
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Old 10-31-2014, 07:13 AM   #51
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Originally Posted by bel73 View Post
When I first started to drive I was told to keep the car rolling after you get on the brakes really hard. Especially when it is cold and wet out. This would dissipate the heat around the rotor instead of keeping it on one spot.

Don't know if this is fact or fiction, but I've never had a warped rotor.
Truth, as well as what Hony says above. In addition, I believe (and was trained somewhere along the line) that when descending long hills, it is better to periodically apply the brakes moderately hard than to ride them more softly and continuously. Allows for more airflow around the pad/rotor interface and allows for heat cycling rather than a continuous temp rise. The same amount of BTUs still have to be dissipated for a given descent/mass/speed, but it allows better cooling.

You could also make your stinky slinky do "double duty" (see what I did there) and store it in a fashion that they would also function as NASCAR style brake cooler hoses....
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Old 10-31-2014, 07:54 AM   #52
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Good driving is always a help. But the brakes are supposed to be able to stop the rig without overheating and shimmying. I worry about the trailer brakes when I tow and drive good to keep from overheating the marginal drum brakes on the trailer. The truck brakes are rock solid. Brakes might be an area where the Tundra is a little weak now.
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