Those who have been following my BMW repair and information site may have read about my recent attempts to track down and eliminate mysterious shuddering under braking that has continued unabated in spite of recent ball joint, tie rod, and tire replacements. I had last replaced the front pads and rotors roughly one year ago. Well, last saturday I removed the oem replacement Pagid front brake pads from my 1995 BMW 325is and replaced them with a set of Hawk HB136 ferro-carbon race pads I bought for a song on ebay. The Pagid pads had about 15,000 miles on them and looked okay save for some interesting angled wear towards the edges of all four pads, making the pad when placed face up look almost trapezoidal...not sure what that indicates.
Looking at the brake rotors, there was some minor scoring or grooves on the outer surface, but they were generally smooth and evenly worn with no apparent runout (checked with a micromemter).
I noticed that the new Hawk HB136 pads did not have the standard 3-pronged spring on the backside of the inner pads that center the pads and pop them into the brake caliper piston. It turns out that this is intentionally so, as to limit heat transfer from the pads to the calipers in high stress racing/track conditions. The first downside of this setup is that this kind of brake pads are awfully noisy! The click and clack and squealing of the pads is awfully irritating on a street car. The Hawk HB136 pads made an immediate and dramatic difference in braking power, but also made a ton of dark brown brake dust - the kind that you get when regular pads are so completely worn out that the rotor is touching the backing part of the pad. The braking vibration went away completely very quickly. Was it because the Hawk pads removed the high spots of brake pad deposits on the rotors? Or was it because the Hawk pads were so hard they wore away the outer material of the rotors themselves? After one week, I swapped the old pads back in (but not before 'bedding' the pads with 100 and 150 grit sandpaper to remove and glazing, then cleaning the pad surface with brake parts cleaner to get the sanded off bits off). The vibration is gone! I am not sure what to make of this other than the following conclusions:
Do not use track or race only pads on a street car. Unless properly warmed up, these pads bite so hard they can damage stock rotors. It is extremely difficult to get the dust and grime off of the wheels! This grime actually consists of rotor material that is now practically baked on my stock E36 wheels.
Always be sure to properly bed new pads before installing on any rotors, new or existing. Failure to do so can lead to pad deposits, glazed and ineffective brake pads. Use a sanding block and medium pressure when bedding brake pads.
Replace anti-rattle springs and backing plates when replacing pads if possible to prevent pad rattling.
The Hawk ceramic pads work yank-your-eyeballs-out great, but should not only be used strictly on track cars, but with better rotors than OEM-replacement ones. I am very lucky that the rotors appeared unharmed by the experiment and that old pads were still usable. Otherwise, it would have an expensive lesson.
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