E39 touring 530d, 2003: Rusting brake discs at 22000 miles, and again at 53000 miles


Thread: E39 touring 530d, 2003: Rusting brake discs at 22000 miles, and again at 53000 miles

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  1. E39 touring 530d, 2003: Rusting brake discs at 22000 miles, and again at 53000 miles 
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    Carl Haworth's Car Details
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    Front discs changed at 22,000 miles. Second change needed at 53,000. Inner face of discs develops wide rusted band at outside. Braking efficiency affected, and can cause brake vibration.

    Possible explanation: ATE swinging caliper poorly designed, and needs high pedal pressure to force inner (piston-side) pad flat against disc. If hard braking is not performed regularly, the rusty band builds up. After this, hard braking does not clear the rust, but erodes the edge of the pad.

    What comments are there around on this problem, and on avoiding it? I'm unwilling to throw more money at what may be an intractable problem which is bound to recur!

    Can I upgrade to either single pot calipers which SLIDE instead of pivoting on rubber bushes, or to fixed two pot calipers? This option is not available via a BMW workshop as they have to order parts by chassis number, so would fit same OEM calipers.

    ADDED SUNDAY 26 MAY 2013

    I may be wrong in describing these callipers as of the "swinging" type. My 5-Series Haynes manual describes them as "sliding". If they SLIDE (using the "guide bolts") the fact is that the guide bolts slide inside RUBBER bushes pushed into bores in the calliper body.

    This is no doubt a cheap way of avoiding the possibility that the calliper slides will stick. On the French Bendix single-piston callipers fitted to three Citroens which I owned before this BMW the slide did not use rubber bushes, but the callipers did NOT stick.

    The use of rubber between guide bolt and the calliper housing must mean that the callper is to a small degree "flexibly mounted". The fact that the rubber is not in torsion or shear, but acts as the bearing surface for a sliding metal rod (the guide bolt) means also that it must wear. As it does so, the calliper alignment relative to the disk seems likely to become increasingly sloppy (in relative terms).

    Is this the reason why the calliper twists out of parallel with the disk when the brakes are applied, and does not align itself fully unless quite a high pedal pressure is used?

    As the piston-side pad contacts the rotating disk, friction will try to pull this pad, and so the calliper, round with the disk. If the pedal pressure is light, and especially if the rubber guide bushes (wrongly called by ATE "dampening bushes"!), are worn, or have gone slightly soft with age/use, this may tend to tilt the piston-side pad. It can't tilt as it is trapped between the face of the calliper and the disk, so the whole calliper tilts slightly.

    Possilby the reactive (passive) pad does not tilt because it is mounted, I am told, with a degree of flexibililty, and so can align itself flat on the disk surface when/if the calliper tilts. This may be why the outer and inner rusty bands on the OUTSIDE of the disk are normal (very thin). So, on the outside of the disk, the width of the area polished by the pad is the same as the contact dimension of the pad, but not on the inside of the disk.

    Tilting of the piston-side pad under low pedal pressures, and when the rubber guide bushes are worn or soft, may possibly explain why this pad does not contact the outside of the disk properly, with the result that the excessively wide rusty band already mentioned develops at the disk perimeter.

    However, I am told that a similarly excessively wide rusty bank appears also at the INSIDE of the disk area which ought to be contacted by the piston-side pad. I don't understand that, at all.

    Please help me to understand why I am getting this tiresome and expensive problem, and how I can avoid it in future if my only repair option is to fit the same (OEM) ATE callipers.
    Last edited by Carl Haworth; 26-05-2013 at 10:52. Reason: correction/additional information

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