Brake pads + area swept

[zcacogp]

Chaps,

First post in this forum ... I'm in need of some technical advice.

This is a photo of the front brakes from my car.



It's not a great photo, but you can see that the disk isn't shiny all the way to the edge - there is a band around 6 or 7 mm wide all the way around the edge which is cruddy (and slightly rusty). The pads are more worn in this area - as in, they don't actually touch this part of the brake disk. They used, to, but don't now.

What causes this? I can see that grot build-up on the surface of the disk should be wiped off by the pad, but grot like this will be missed by the pad. The question is whether the pad became worn away in this area first, or whether the grot built-up which caused it to wear away.

What is the solution? (Do I need one?) New pads will surely just be worn away on the grot, and not solve the problem. Is the only way forward therefore new disks and pads? (A very expensive caper, and not one I would consider lightly!)

The car is a 944 S2. The brakes work and feel fine - lots and lots of stopping power from any speed you care to mention. It was mentioned (verbally) at the last MOT by the tester, who pointed it out but said that the braking test performance was 'excellent'. That was about 9 months ago.


Oli.

[maniac0796]

It's essentially where your pads didn't bed in properly when they were first installed. Also, rust generally spreads from an exposed area. As the outside of your disc has no contact and there isn't a coating on the disc, it soon spreads to the outer edge.

You can either grind down that edge on the disc with a flappy grinding pad or just go round with a hammer and knock some of the rust off, and use a screwdriver to ping some off. Then use a bit of sandpaper (something coarse) and rub the pads down till they're flat.

Or leave it as it is.

[ian_uk1975]

Discs are worn and have 'lipped'. The braking force applied by the caliper piston to the pads isn't 100% evenly distributed across the pad. The clamping force applied by the caliper is at its weakest at the edge of the pad, hence the rusty band on the outer edge. No real solution other than to replace the discs.

[New POD]

New Discs Every 2nd or 3rd pad change along with new fluid every 3 years should help

[zcacogp]

Ian, the photo is slightly deceptive and there is just about no lip at all around the edge of the disk. They are 30k miles and three years old, and the pads were new and the fluid changed at the same time. The calipers are four-pot and in good condition (no 'plate lift', as affects porsche calipers of this age - I rebuilt them a year ago to a high standard.)

POD - thanks, that's the sort of ratio of pad:disk that I work to as well.


Oli.