Kirk930

Could this problem be caused by improper bedding? I did follow the manufactures instructions, but maybe I should have waited a little longer before braking hard.

The brakes never pulsed prior to the rotor/pad change, and I highly doubt the rotor is warped.

I've double checked the installation and I don't see anything wrong. When turning the rotor without the wheel installed there is definitely a "sticky" area during the rotation. Visually I don't see anything on the rotor....in fact I can still see the machining marks. Without the pads in, the rotor spins freely.

Can the rotor be cleaned? Steel wool, sandpaper, etc??

ferraripete

yes it can but is more likely to occur in grey cast, ductile iron rotors whereas the 930 has a steel rotor. in my years in the friction materials industry i learned and saw alot.

again, any racer...especially the nascar guys ( they bed in multiple sets of brakes regularly but more and more nascar teams are buying pre-burnished pads) don't as a practice, bed in green pads on green rotors!!!

ferraripete

as far as cleaning the rotor surface, you can clean by spraying on brake cleaner and using a scotch birte pad to clean off pad material residue.

Kirk930

I'll give that a shot.

mark '87 930

Kirk,

Is the hub/bearing tightened to where it should be? have you checked it again since? I'm wondering if the hub/rotor assembly is not on as tight as it should be, thus some movement.

Kirk930

I had the wheel off AGAIN last night and checked the bearing tightness, scrubbed the rotor, tried switching the pads within the caliper, checked the hub/rotor mating (I haven't pulled it all apart yet).

With the wheel off and turning the rotor there is a drag, release, drag, release.......as if the rotor is warped. It does this with one pad removed also, and on both sides. I don't think a sticky piston would do this as it would just drag constantly?

Could the hub/rotor not be mated correctly, or something with the wheel bearings? Very frustrating considering I did this to IMPROVE the braking.

mark '87 930

Kirk, try switching them.. I know there is a direction they are supposed to be, but I would try switching the rotors and see if your problem shifts to the left side now. If so, you then knwo you have a bad rotor.

I take it you are not going to Sherway. I'll be in the your neck of the woods today if you want me to drop by.

graeme36s

I presume the Zimmerman rotors are direct replacement discs for the turbo, so one piece. First I would go and check the run out on the disk. From what you are saying the disc has warped. I've always used standard original Porsche Discs and Pads. I have seen a number of aftermarket discs warp. The only time I've seen a Porsche disc warp was on my own 964 3.6 turbo. I did two sets of front discs in a very short space of time. Turned out to be a caliper problem. The inside pistons were sticking and rubbing on the disk. Ended up buying new calipers from Porsche, problem solved.

nathanUK '81 930 G50

FWIW I bought some aftermarket discs for my 911 3.2 carrera, when they came & I saw them I hated them and they have sat in my garage for years. I bought some genuine ones.

Kirk930

I'm thinking it must be a warped disc. I took everything apart again, replaced the wheel bearings (lots of fun.....), cleaned up the hub/rotor interface, and triple checked everything else as it went back together including all the torque settings.

Still no change.......pull to right with no braking, and a pulsation under braking with a pull to the right. The rotor turns and there is a definite drag, release, drag, release.

Can the runout be measured on the car? Magnetic dial indicator? Can the disc be "cleaned up" with machining or am I looking at a new rotor?

69gaugeman

There are two factors in pedal pulse.

One is brake face runout and the other is thickness variation.

If there is thickness variation (measure with a micrometer the thickness all around the brake surface) of more than 10 microns (a little less than half a thou, in inches) it is too much.

You can and should measure runout while it is on the car. You should also do it with all 5 bolts torqued to spec. If you see more than 50 microns runout it is too much. (about .002").

If you have excessive runout then you can try to rotate one bolt and measure again. Getting high and low spot to line up may make the difference.

(spent the last 12 years working for a company that manufactures gauges for the automotive industry, which a large part of the market is disk brake rotors)

69gaugeman

If you need to machine 'new' rotors I would send them back.

spence88mph

I didn't know NASCARS had brakes, do they use them when they get too dizzy?

dholling13

I think they use them at the road corses 4 times a year

911rudy

In my long and messy carreer with brakes, I would say that you have a warped rotor. Or as was pointed out, you may have gotten a new one that had high spots on it. Either way its the rotor and not the pads. For about twenty bucks you can buy a cheap dial indicator and check the run out your self. If there is any found, send them back. Good luck.
Rudy