Vibration under braking, rear rotors warped

[Mr. Hemi]

Had an issue with vibration under braking that has developed. Would not transmit through the steering wheel yet the bed would rattle so I knew it must be rear brakes. Pulled rear wheels and the LR had . 011"TIR and the RR had . 006 TIR. Question is why? Rear still have original pads, front replaced at 40K with EBC Greenstuff, they have . 00025 run out, truck has 51K on it. Had ordered EBC rears but want to turn the rear rotors tomorrow and either re-install old pads or pads from Autozone. Is this kosher? OR just wait for the EBC pads. Iwould rather not sue the truck with the rear getting beat up by rotor runout.

 

[jelag]

If the co-efficient of friction is a longs way off from the front friction to the rear friction you could have a problem... the friction with the higher co-efficient will have more braking power...

What I'm trying to say is that the rear friction, which is not designed to do more work than the front, might after the brake pad change force the rear to be doing more work... this overload so to speak could cause overheating and warpage of the rear rotors...

We see this all the time in truck friction where a company will change one wheel or one axle of a tandem set... . this 25% or 50% of better friction will cause the new higher co-efficient material to wear faster, on the other hand if the new brake shoes installed are of a lower co-efficient they will not work as hard and prematurely wear out the other brakes...

Its a shame that in the real world, some friction manufactures may market their product as "highest quality" or any other number of buzz words, that might not be the case... the industry has a non-profit branch FMSI = Friction Material Standards Institute which sets standards and does testing... . and keeps some brands off the market for cars... .

In class 8 truck brakes there are at least 5 different quality ranges you can buy that will fit and meet DOT standards for these trucks, some will stop you faster, and last longer, than others... .

As I remember my GL1800 has EBC pads on it... they were all changed at one time... . and its fine... but from the onset it stopped differently than the stock pads...

I know that the 2 lower ranges of friction available for my 3500's I don't bother to carry in stock... as I hate to do the labor so I only sell and carry premium pads... the manufacture that I use actually makes 4 grades of quality.

I can sight several tests of friction on very controlled fleets if you are interested... .

So I suggest go back to OE pads on the front and turn the rotors on the rear, or change the rear pads to EBC after you turn the rotors...

The above is the most common answer but not always... . as an example if someone drove your truck with e-brake on that might cause it... . if the boots on the pistons are damaged and dirt is behind them not allowing the piston to retract, this can cause it... there are others... . but a qualified technician who understand friction and the co-efficient of friction should be able to help... .

 

[JHMiller]

Could it be crud

After spending a very educational hour with a brake man of 25 years I came away with a new understanding of some brake issues. He told me rarely are rotors really warped, he's seen them blue from heat but still ok runout wise. His contention is crud embedded in normally the pads and some in the rotors is usually the real culprit. The rotors are turned(cleaned) and new(clean) pads installed. He indicated he cleans his brakes occasionally, sands his pads. Said he extends his brake life and maintains smooth brakes to the end of parts useful life. Haven't got the NAPA yet to see what is available but he stressed using protective gear around brake cleaner chemicals and using them with caution. He also suggested not turning rotors on a routine brake job unless absolutely necessary, to extend their life and just letting the pad brake(form) in to the rotors by about 500 miles without hard braking. Need to see what NAPA has that will actually clean out material embedded in pads, we'll see. You might inspect your pads with magnification to see if there is anything to this. Best of luck. John

 

[PLaFrombois]

Have you checked the u-joints? Had the same problem, and found a bad u-joint on the rear shaft.



Paul

 

[Mr. Hemi]

Warped, and pitted rear rotors

No problem with the ujoints it was only under braking and when you would slow down you could count the revolutions of the rear wheels/tires with each thump. There was not a hint of vibration on the steering wheel which led me to the rear as well as the bed rattling at every stop.

Checked TIR at the rear rotors, as per post, and they were warped 6 to 11 thousands TIR. After two hours with a can of Gibbs, a 3 lb engineers hammer and an 8 lb sledge backing up the outside they came loose. Back of one rotor had heavy pitting in one spot. Guy that turned the rotors said that moisture from the vented area gets through to the face and causes blistering and pitting. To me more like a poor casting with impure or contaminated base metal. Per the few posts on warped rotors the latter seems the case, Chrysler's suppliers quality control, lacking at times.

Re-installed after polishing the contact areas and coating with anti-seize which I strongly recommend, but take car not to be too liberal or it may contaminate the brakes.

Everything works fine now.