S4 Brakes again - a solution....

[DavidR]

I have lamented about the problems with my brakes here for a while now. Judder at moderately high speeds and fade. Having changed the discs about 2k ago and installed green stuff pads the fade and retardation issues were solved but the judder was still there and became worse with time until at 60mph, the brakes would require only minimal pressure to shake the steering wheel 1-2 degrees off centre!

I took the discs off the car and got them dialed, only 2thou runout on the outer edge of each disc. Hmm, not that then.

I have, however, discovered the pro cut brake lathe.

For £85 it machines the disc surface on the car to match what i have now discovered as an out of true left hub.

Brakes now perfect, no judder even from very high speeds and retardation etc as before.

Given that the bell of the disc has already deformed after the inital install and still is deformed, the amount of time before the contact surfaces become out of true again is increased so I'm hoping for many miles of use without further problems.

FYI, I used Dreadnought garage in Callander, Scotland for the work and am very impressed.

Cheers

Dave

[Golich]

David,

That's a very interesting post. Given that when I started reading it I was thinking about the stop tech article and the author talking about the irregular transfer of pad material to the disk and how in the author's opinion is responsible for most of the judder type problems he has enountered.

Excuse me for being a numpty [img]images/graemlins/spineyes.gif[/img] but I didn't understand what you meant by

"Given that the bell of the disc has already deformed after the inital install and still is deformed, the amount of time before the contact surfaces become out of true again is increased so I'm hoping for many miles of use without further problems."

Are you expecting the disk to settle back into a warped position?

[DavidR]

It has been suggested to me (an i'm not an engineer so cannot comment with accuracy) that some of the deformation on a oem s4 disc is due to iregularities in the casting of the bell (i.e. the bit where the bolts go through). If this deforms due to extremes of temperature, the contact surface will become squint and cause runout. Once the contact surface is machined true again, the deformity in the bell will remain (as this is not machined) and therefore cannot really deform further, therefore the disc contact surface should remain in true for longer?!?

[Golich]

Yes - now I understand. Yes that makes sense. I am a mechanical design engineer but that doesn't mean I'm clever or know much about brakes!!!

Generally it's standard practice with items that require heat treatment (generally to improve material properties) to machine them after the heat treatment. Thus any warping or relaxation of residual stresses from the manufacturing process is put right by the post heat treatment machining.

This general practice could easily be appropriate to disks but of course it pushes up the manufacturing costs and with the vast majority of cars on the market being relatively low performance in combination with cast irons inherent stability at high temps it is in general not necessary.

Having said that it's good that someone has recognised the need for the higher performance cars. I must admit I didn't even know this machining service was available until a year ago and that's when I saw some sales lit. at AMD.

Good post David with the name of the guys who can do the work too. [img]images/graemlins/jump_clap.gif[/img]

[Hinesy]

david
seeing as live about 10 mins from callendar how much was the cost mate ???

[DavidR]

Cost about £85