On the subject of discs.

On Sat, 05 May 2012 09:21:35 +0100, A.Lee wrote:


There's a somewhat controversial theory that transfer of microscopic
particles of friction materiel from the pad to the disc can build up
unevenly, causing judder. This is said to be more likely when non-OEM
materials are used.

If the discs are 'scoured' with a Scotchbrite pad, the material can be
removed, and smooth braking returns.

It appeared to work for me on the one vehicle I tried it on, although I'm
also willing to accept that removing/replacing the pads as part of the
process might also have done the trick.

Either way, it's a virtually cost-free thing to try first.

Chris

--
Remove prejudice to reply.

A.Lee wrote:

If the discs do not run exactly 'true' when new the pedal will feel fine at
first and they will stop properly, but over time they will develop disc
thickness variations (DTV)  The only real cure is new discs and when you fit
them do a run out check and fix the problem (sometimes by replacing the hub.
A common cause of excess run out is insufficient cleaning when fitting new
discs, another is when a wheel is removed some dirt falls between disc and
hub (common on things with loose discs, unlikely on vehicles where the disc
is secured)

a good write up here:
http://www.powerbrake.co.za/downloads/tech_01_judder.pdf

ebc have a good page too:

http://www.ebcbrakes.com/troubleshooting.shtml

wrote:


I can't help feeling that if I'm trusting you to make my brake discs then  
I'd want you to turn them more accurately than I could manage with  
something bolted to the caliper.
But the biggest cause of repeated judder, IME, is the ARB bushes going  
soft and exaggerating any slight issues.

Duncan Wood wrote:

On car rotor machining is something the yanks have done for years, nearly as
common as getting a four wheel alignment done over there.  But here it has
definitely been rare.  If the disc is well bolted on, then machining from
the caliper mounting points should be very accurate , they are pretty solid.
But it is another thing like on vehicle wheel balancing, not done very much
in this country.

On 5/05/2012 6:21 PM, A.Lee wrote:

My thoughts!

Some of the common problems - some have already mentioned in this thread.

On long descents the rotors will heat and hot spots will occur. (This is
somewhat prevalent when a trailer is attached.)

Heated rotors will distort. (hard pad material)

Dirt on the rotor mating surfaces when the pads are replaced the rotors
become loose dirt can drop between.

On VW's recommendations over tightening of the wheel nuts/studs will
cause the judder.

I have found that it has become more critical for the pads and rotor
materials to be compatible. Such, genuine rotors and pads are optimal.
One of the reasons they get changed together.

Just a question have the callipers been serviced as they could be
sticking causing the pads to rub on the rotor creating heat, is there
one rotor giving off more heat than the other after a drive?