Strange sensation and noise during braking

[Rus]

Hello,

I have recently changed my front rotors and brake pads. The rotors are stock replacement Brembo and the pads are Raybestos Quiet Stop Ceramic. The total mileage on the setup is not more than 100 miles yet and I'm experiencing a noise and response I haven't encountered before. When the brakes are cold, stopping with moderate pedal pressure produces a thumping noise and an accompanying slight shudder from the front end. Once the brakes are warmed up, much stronger pressure is required to reproduce the noise. I've double checked my work and everything is properly torqued, lubed, cleaned, etc. The pad material is evenly distributed on the disk surface and I experience no pulsation in the steering wheel or brake pedal. Since the Raybestos QS pads do not officially require a break in, I only did a couple of moderately strong brake applications from about 40-10 mph before proceeding to use the car normally. After a bit of use I also performed several 60-30 mph braking sessions. The thump became immediately apparent after the last 60-30 session which leads me to believe that it probably isn't a warped rotor issue. The general sensation before the brakes warm up almost feels like there is a ridge or a spot where the pads suddenly catch. The rotors are not cracked, which was my first suspicion. Has anyone encountered this before? I've come across one post about a similar problem with this same rotor/pad combination, but no resolution was posted

Thanks in advance for all input!

[DaveVoorhis]

Sure it isn't your ABS activating?

[billy]

Why is it that BMW have a little sticker after they replace your brakes saying to avoid sudden braking for xxx kms - yet others suggest 'bedding' them in requiring severe braking and heating up of the discs - im aware of the reasons for 'bedding' them in - however why do BMW recommend the opposite

with regards your problem, when you say 'the thump became immediately apparent after the last 60-30 session'
leads me to believe youve broken something...perhaps remove the rotor and make sure theres no runout ie its warped after the sudden braking

[brick8]

Could it just be bad bushings that you didn't notice before?

[BillionPa]

i would second a torn bushing, or a caliper guide bolt fell out.

[632 Regal]

the thud sound means you popped the thrust arm bushing. might have warped the rotors on you bedding procedure too.

enjoy the rollercoater ride I went through for about a year...

get M5 bushings and no more brake tests.

[Rus]

I sincerely doubt that I destroyed a thrust arm bushing because they are less than a thousand miles old (Lemforder w/ 750iL bushings). I am of the opinion that the "bedding" that I did accomplish was rather minimal compared to what is often recommended for bedding in the rotors and pads. As I mentioned earlier, I am well aware that these particular pads do not require a break in, and as such I didn't thrash the brakes from the get go. I'll see if I can get a micrometer and check the run out on the rotors. All hardware is properly torqued and installed, and I had no extra parts left Also, would excessive run out manifest itself when the rotor is rotated? As in uneven noise from the pad touching the rotor? When I inspected the brakes, both sides produced an unvarying drag noise (nothing seized, just the pad slightly touching the rotor) which led me to believe that there wasn't much variation in runout, if any. A warped rotor also wouldn't explain why the noise disappears after several applications (warming) of the brakes? Could it be that pad material is unevenly distributed on the rotor? The rotors have not yet achieved the machined appearance and I can distinctly see where pad material is still present in the rough surface of the rotors. Thanks for all the input and keep the thoughts coming

P.S.: I am also pretty sure its not ABS kicking in as the noise is definitely speed related (ie as I slow down to a near stop the noise is there until I stop....bad illustration: medium braking-thump.thump.....thump..........thump.............. ...............thump......stop)

[632 Regal]

trust me, check the bushings...

[Rus]

trust me, check the bushings...

Will do Jeff. I glanced at them when I yanked the wheels off, but didn't really do an up close and personal inspection. Come to think of it, I didn't even look at the driver side bushing. Sure hope they didn't croak on me. Thanks again.