Auto Brake question

[clare]

Obviously spoken by a non-mechanic.

[clare]

Not always, by a long shot. Rotors can be worn so rough (grooved} that new pads would not have a 5% contact and you would not feel ANYTHING on the pedal to indicate there was anything wrong..

They can asl be so badly corroded that you only have a ring 1/2 inch wide where the pads are doing anything - with the OLD pads, and not have ANY vibration..

Brake pu;sation is not the ONLY reason brakes need replacing - and rotors too.

[Stormin Mormon]

I've had some experience with worn rotors. Like you write, sometimes the rotors groove, and then the pads don't make complete contact. And they can do that (incomplete contact) and still not be pu;sating.

[clare]

Used to be I could do that too - but the rotors on the mystique were bad - all 4 of them, at less than 120,000km. The rotors on the PT were bad at 115,000km. The rotors on the TransSport were bad every 70,000 kms, and before I caught on to the carbon metallic pads, the Aerostar needed rotors every 2 years ( pads were still about half lining, but hard as glass and had "eaten" the rotors). With the carbon metallic pads I got 3 years out of them (both pads and rotors) and they still looked like new when I sold it.

[Ashton Crusher]

Nope. Years of experience on my own cars. Apparently you don't bother reading the genuine repair manuals put out by the automakers.

[AZ Nomad]

When you replacing pads, you'll see the grooves. If there's no vibration and no visible grooving, then the rotors are fine.

[clare]

Did it for a living for over 20 years. Have bought the manual for every vehicle I've owned in the last 30.

[clare]

Group 06- Section 00, pages 17 to 19 of the Ford PG2040 manual for the 1996 Mystique /Contour states "each time the brakes are serviced, the front and rear disk rotors should be checked for scoring, runout, parallelism and thickness" Runout of over 0.006" is unacceptable, and parallelism (variation in thickness) must not excede 0.0006" and there is half a page dedicated to how to refinish a rotor. They also stipululate that you NEVER machine to miinimum thickness spec. Front rotor discard thickness is

22.2mm, or 0.87". Original spec is 25MM (0.984"). That leaves 0.114" MAXIMUM cut/wear to scrap - aprox 0.040" per side to the service limit - and .057" to scrap. So it is OBVIOUS they intend/expect that the rotors will often require refinishing or replacement.

[clare]

Correct - or at least very close. - but what percentage of brake rotors, at service, have no visible grooving or pitting, and do not pulste or thump????? From my experience, which covers THOUSANDS of vehicles, the percentage is VERY low. I'd say on vehicles where the brakes have NOT worn to metal to metal, around here the numbers would definitely be 15% or less. Better than 30% have touched metal to metal on at least one wheel, which means BOTH sides need to be either refinished or replaced - so only 15% of 70%, or 10% of brake services would NOT have visible grooving or pitting, or be causing a thump by the time they came in for brake service.

In areas with no salt, the numbers would be higher.

[Jim Yanik]

[e-mail] wrote in news: [e-mail]:

.057 is a lot of material. 1/16" is .0625".

"often"?? that's just your assumption.It's not so obvious to me.

I doubt many brake techs actually measure runout,parallelism,or rotor thickness;they just quote new rotors,and add the cost to your bill.

How much rotor material wears off if the car's brought in before the pads reach minimum thickness? How much does a ordinary refinishing job remove? (On a rotor that is not grooved.)

Any idea? (I don't know.)

[clare]

Brake techs, mabee - but mechanics DO.

That depends on the pads, to a large extent - and also on the rotor material I've seen rotors worn to below the limit, with NO grooves, without ever being machined. I've seen rotors that were not machinable any more with half the frictionnmaterial still left on the pads - after having pads and rotors replaced at the same time less than a year before.

That depends, but you won't get away with ten thou per side - almost guaranteed. This is assuming there is a requirement to machine them in the first place.

If the rotor is dead smooth, but badly glazed, an abrasive disk will do the job at about one thou per side. If a cut is required, (shop doesn't have the "grinder pucks" generally closer to five thou per side. Any less leaves a bad finish because the cutter needs to get UNDER the glaze to remove it. It's too hard to cut the glaze itself. Kinda like being case hardened.

Prep discs on a die grinder can take off the glaze without removing much metal - spin the rotor up (car running in gear) and run the prep disk on the grinder untill the glaze is gone - generally 80 grit does the job, but 120 could also be used