Mercedes Benz Warranty

[gasman2]

Just had my first encounter with Mercedes benz
Recently purchased a c63 amg and had a approx half inch rust line on outside of both front discs,tried a few hard stops etc but it didnt clear so contacted local well established dealership
45mins trying to get through to book car in for inspection
my understanding of brake function is as follows
Apply pressure
Fluid moves piston
Piston applies even pressure to back of pad
Pad applies even pressure to disc
No ime wrong apparently lack of use means the brakes are so clever they can decide to apply different pressure to the disc all on their own
Just reaffirms my view that all dealers are more slippery than a bucket of eels

 

[SLKGirl]

"lack of use means the brakes are so clever they can decide to apply different pressure to the disc all on their own"

Hows that work then?

 

[MancMike]

What you're referring to is corrosion encroaching. It works like this;

Edge of disc starts to rust, rust wears away pad, but pad wears away rust. Rust returns, pad doesn't, so there's some encroach.

This process repeats and the band gets thicker, it's like a disease.

You can't just grind/sand the rust away, as it'd be pointless without also changing the pads, as there'd be no pad to contact that fresh metal, and so it'd rust again.

The only way to get rid is a new set of pads on a freshly ground disc, or replace both disc and pads which is obviously very costly, especially on a C63...

Braking efficiency isn't affected that much by it, because as disc to pad surface area reduces, the force applied per cm/2 increases, so the friction produced remains similar, and you've still got the same brake disc mass to absorb that heat generated. You'll experience brake fade on the pad side sooner though, as the heat in the pad would reach the pad's limits sooner at that concentrated, smaller area.

It only becomes risky when the band of brake material in contact with the disc narrows to the point where the "meat" can't take the pressure and so it crushes (like cake) all the sudden during hard braking, that puts the worn back section of the pad right back onto the rusted area of the disc which goes very grabby and your brakes feel very strange, but then a few more hard stops and it sorts itself out again. It'd be very rare for it to get that bad though, especially on a high performance car as you'd replace the components before it got this bad. It can get this bad on cars where their wheels or wheel trims obscure the view of the disc.

 

[B-BOY]

Hmmm..... Different pressure on the two front discs during braking....surely that would mean pulling to one side or the other!!!! Which last I read would be a M-o-T failure.

 

[tec]

Sounds like lack of use to me.

Half inch of rust on the edge should not cause issues.

If the calipers are seized that is another issue.

My rarely used Volvo, rear brakes have an inch of rust on inner and outer edges and still passes MOT and the brakes work fine.

 

[reason42]

What you're referring to is corrosion encroaching. It works like this;

Edge of disc starts to rust, rust wears away pad, but pad wears away rust. Rust returns, pad doesn't, so there's some encroach.

This process repeats and the band gets thicker, it's like a disease.

You can't just grind/sand the rust away, as it'd be pointless without also changing the pads, as there'd be no pad to contact that fresh metal, and so it'd rust again.

The only way to get rid is a new set of pads on a freshly ground disc, or replace both disc and pads which is obviously very costly, especially on a C63...

Braking efficiency isn't affected that much by it, because as disc to pad surface area reduces, the force applied per cm/2 increases, so the friction produced remains similar, and you've still got the same brake disc mass to absorb that heat generated. You'll experience brake fade on the pad side sooner though, as the heat in the pad would reach the pad's limits sooner at that concentrated, smaller area.

It only becomes risky when the band of brake material in contact with the disc narrows to the point where the "meat" can't take the pressure and so it crushes (like cake) all the sudden during hard braking, that puts the worn back section of the pad right back onto the rusted area of the disc which goes very grabby and your brakes feel very strange, but then a few more hard stops and it sorts itself out again. It'd be very rare for it to get that bad though, especially on a high performance car as you'd replace the components before it got this bad. It can get this bad on cars where their wheels or wheel trims obscure the view of the disc.

This is a perfect and reasonable explanation.

 

[Rory]

Just had my first encounter with Mercedes benz
Recently purchased a c63 amg and had a approx half inch rust line on outside of both front discs,

Where did you purchase the car - MB dealer, indie, private?

My car is doing this too - I don't use it much but do try and do a few hard stops when I'm out in it. However rust is gradually creeping across the disc.

 

[gasman2]

purchased from jaguar dealer
ive had more cars than ford in my time and my last car bareley moved and had ap racing brakes and it was never an issue with the discs returning to new after the first couple of stops

 

[Rory]

Mine have been on 4yrs and until recently cleaned up perfectly, but I noticed over winter they weren't looking so uniformly shiny after a run and now are looking pretty bad.

Wife's cars do it as well, and they're used more frequently - years ago she had a Peugeot 106 which has pretty tiny front discs to start with and it ended up with a very narrow shiny band and the rest of the disc rusty. Honda just changed the rear discs under warranty on her current Jazz for the same reason, but Jazz with rear disc brakes are noted for doing that as there's very little braking load on them.