Disc Problems

[Alistair E]

I currently have Boxster "S" calipers with 314mm x 30mm AP drilled discs fitted to my Avant. The same discs were previously fitted with 928 calipers.
Inspection of the brakes the other day revealed that the inner pads were wearing very quickly compared to the outers. I found that the inner surface if the discs had rusted and it appears that the rough surface is wearing away the pads.
I didn't have any problems like this when I ran the 928 calipers and can't understand why this is happening now. The only thing I can think of is that I had the discs skimmed before fitting them with the new calipers.

Any suggestions?

Alistair

[twoqu]

Alistair,

I would think that it indicates that the disc face is not being swept by the pad for some reason. Either it's the disc or the pads. Put a straight edge (steel ruler) across the disc face. It looks as though the edges of the discs are higher from the pic as they are shiney. If they were, then it would explain the problem.
Can you post a pic of the pad? It should have a corresponding wear pattern.
Another thing that springs to mind is that the AP discs have much less friction area than an oe disc. I have some 323 AP discs and an RS2 pads just about covers it. If your previous calipers were 928, I guess they used similiar sized pads. On pics I've seen of other peoples installs, the Boxter S pads covers all the friction face of an oe disc.
For comparsion, the AP discs have 52mm whilst a 314X30 oe disc has 64mm!
As the only other thing you have done is to change calipers/pads, the there might be something about the combination.
Anyway, try the straight edge first. That will tell you if the disc is flat.

[meep]

Could be that the 928 pad is smaller? And that thoose shiny bits on the disk is where the 928 pad didnt reach so to speak

[90quattrocoupe]

I would have to agree with TWOQU, it looks a though the pads are only contacting the outer and inner areas of the rotor. That is why those areas are the only ones shiny. Did the company skin both sides of the rotor?

Yea, check the straight edge.

Another reason I like 323X30 stock rotors. More pad on the rotor. It seems to me that you are going to have more braking with more pad area.

Greg W.

[Alistair E]

The outer pad completely covers the available disc area. The inner pad run right to the outside of edge of the disc but it overhangs the inner edge of the disc by about 4mm. I would agree that it appears from the photo that the pad is only contacting the inner and outer edges of the disc. However, the inner pad has less than half it's friction material remaining. This means that it's been in contact with the disc. The problem seems to be that the rusted surface is wearing away the pad more quickly and therefore the pad is only really braking properly on the edges.
Once the pad is partially worn, I would have thought it would mould itself to the shape of the disc and therefore contact all across all of it's surface.

Any comments?

Alistair

[meep]

Well i belive the rust would be worn away pretty rapidly. But i agree with teh others. Check that there is edges on the disk. And check that the disk is level so to speak.

[twoqu]

Alistair,

I would have thought that if the pad had been worn out on the disc, the disc wouldn't be rusty like that. It might have some surface rust. After all, pads are pretty abrasive, so whatever is wearing the pad ought to be worn too. To be frank, that looks more like corrosion. What Greg and I are trying to establish is whether the rust is above or below the shiney bits on the circumference of the disc.
The only other time I've have seen something like that was when a caliper had seized pistons.
As you say, if the pad was only pushing against the circumference then it would have two grooves in it. It does look like you what would expect from a well worn disc, with the unworn portions at top and bottom. How long ago was the disc skimmed?
So the pad overhangs the disc. Does that have an unworn portion at the bottom then?
Try to establish whether the rust is above or below the ridges. It's a start!