Steering wobble under breaking (again)

[MostlyMonki]

I've got the 410mm ap racing disks and calipers, and last year had a nightmare with a brake judder - long story short the place that tried to do a skim messed up all 4 discs - but I replaced the fronts with brand new discs and bells from bg developments in November... now after about 1500 miles my brake wobble is back! . Had a garage check it out and the odd disk is showing a very small run out -Theyve had REALLY light use though soni can only imagine pad deposits? But the AP bedding in process isnt clearing the wobble.

Any ideas? I cant keep throwing on discs once a year at nearly a grand a pop.

[welwynnick]

Brake wobble is usually caused by corrosion and contamination on the hub, underneath the disc. Everything has to be spotlessly clean, not even the tiniest spec of rust or grit.

Nick

[MostlyMonki]

Wouldnt that have happened as soon as I fitted them though? Not after 5 months of use?

[Jim Haseltine]

I understand that VAG now recommend that no anti-seize products are used between hubs and discs because brake heat can cause the stuff to liquify and flow. To my mind it's unlikely to be a problem unless the wheel nuts are not torqued correctly. There are 5 ball joints and a similar number of bushes in each side of the front suspension, a bit of uneven wear in such things can cause 'interesting' feelings at certain speeds. I used to have a car which would brake perfectly at any speed other than 65mph - brake at that speed and the wheel vibrated almost enough tear the steering wheel out of my hands - the problem was eventually traced to a single worn top mount.

[IanH755]

What I found with my AP 410 setup is that doing normal "A6-style" driving for a long duration without hammering the brakes once in a while "RS6-style", removes some of the pad deposits left during bedding in leaving an uneven surface, causing the wobble. I think this is due to the combination of a more race based harder-rated steel disc and more aggressive race based pad material from the DS25HP pads over the softer more "road based" OEM setup, so I treat these like the race setup they are based on and give them some abuse once a month and it cures the wobble temporarily for me.

What I do is 6-10 emergency stops back to back from 70mph to 10mph, so 70mph - brake as hard as you can to 10mph without hitting the ABS activation then immediately accelerate back to 70mph them immediately brake to 10mph again and repeat. After the last time where, between attempts 6-10 you will feel the car brake harder/firmer than before showing it's finished bedding in - DO NOT STOP driving - keep going for 15mins at least using the brakes as little as possible (slowdown using downshifts etc) to let the brakes cool naturally and, if you have to stop, immediately put the GB into neutral and get off the brakes otherwise you wreck the disc surface by adding a hot-spot.

If you've done this right, the hammering will have re-bedded the discs, removing any wobble, and left a dark matt grey surface finish on the discs rather than a bright shiny silver surface.

Unfortunately due to the "race based" combo, normal "A6-style" driving will always wear away the bedding over a short time so you'll end up doing this quite often. I do mine about once a month on a quiet country lane late at night without issue if I haven't been out for a "spirited drive" or a track day in a while. Sadly thats the downside to owning a race-based brake setup on a mostly road-based car.

Hope that helps.

Here's some pics which may help show the difference in the discs before/after -

Brand new AP 410mm top (new and shiny) - old AP 390mm below (darker surface with lots and lots of pad material bedded in)



New AP 410mm fitted before bedding in, nice and shiny -



Straight after bedding, just look at all that brake dust from the new DS25 pads



AP 410mm after bedding in, no longer shiny silver but darker matt grey - needed a wash TBF.



After a wash, the same AP 410mm discs, now clean but still darker matt grey.

[don0301]

I've had the 410 AP discs front and new AP rears 7 months now, no problems.

New DS2500 pads and standard calipers which were scrupulously cleaned and serviced at the time (if that makes any odds).

Surely, the disc grooves should clean up any "deposits"?

I've got the j hook pattern rather than the ones you have, AP told me no practical difference. Although their website states "Gives improved bite and debris clearance and reduces distortion / vibration"

[IanH755]

I've had the 410 AP discs front and new AP rears 7 months now, no problems.

New DS2500 pads and standard calipers which were scrupulously cleaned and serviced at the time (if that makes any odds).

Surely, the disc grooves should clean up any "deposits"?

I've got the j hook pattern rather than the ones you have, AP told me no practical difference. Although their website states "Gives improved bite and debris clearance and reduces distortion / vibration"

It's the pad material deposited into the disc face during bedding that slowly wears away, leaving the surface uneven, which the Grooves would do nothing to counter as they only keep the pad face clear of unevenness, not the disc face and why a re-skim also cures this wobble, for a short time.

I honestly don't know if maybe using the more "normal" bedding procedure that AP also recommend for their road setups (200+ miles of light braking getting slow harder over time etc) would give a better long-term bedding performance which is less likely to be worn away by "normal" driving as I've only ever bedded my various AP/DS25 discs/pads the same way following AP's "race" style bedding procedure.

What I find odd about athe wobbly only appearing after time is that, if the hub isn't cleaned the wobble should be there from the first drive, yet for me I only get the start of a slight wobble after a few thousand miles of normal driving unless I give the car a good hard drive and that can't be down to hub contamination/debris. Plus I can feel the slight reduction in brake performance vs pedal travel the longer I drive "normally" and, as my discs get shinier and shiner, the wobble now begins to appear, all of which disappear once I follow AP's race style bedding procedure which always led me to believe it's a rebedding issue.

[Brooner]

i always use the hard braking form 70-10 Mph 8 to 10 times on the new disks never have an issue with the disks very occasionally the judder will come back so i just re do the hard brake bed in procedure job done