Hi,
Thanks for the reply. I'm convinced that they are warped as under braking even from 70mph the car judders with each wheel rotation. As the car slows, so does the judder. I'll go for MB discs and pads all around, hopefully not too expensive.
Are the Pagid offerings at Euro Car parts not worth a shot?
The discs aren't warped. As said the judder will most likely be caused by uneven pad deposits or uneven disc wear. Either of these result in DTV; disc thickness variation
"Warped" suggests physically distorted like a buckled bicycle wheel.
Grey cast iron has a lot going for it as a material for brake discs but it isn't a material that's known for it's ductility i.e. try and physically bend/distort it and it virtually always cracks
Excessive axial (side to side) run out has a pretty similar effect to physically warped, the disc wobbles from side to side as it spins. As with a buckled bicycle wheel this alone doesn't cause a brake judder because when applied the brake pads will simply follow the disc as it wobbles i.e. as one pad is moved back into the caliper the other is free to move out until it contacts the disc
Excessive disc runout (due to dirt or rust on the hub face etc) does mean that the pads are gonna contact the disc in a couple of spots every time the disc rotates even when the brakes aren't applied. If the run out is bad enough then over time this can result in a miniscule variation in disc thickness due to uneven wear i.e. the disc is fractionally thinner in a couple of spots. This WILL cause a noticable judder, it's akin to riding a bike down a flight of stairs instead of along a smooth road
Uneven pad deposits cause the same result but for kinda opposite reasons... instead of there being several low spots on the disc there are high spots due to uneven transfer of friction material to the disc. Uneven friction deposits can sometimes be 'fixed' simply by using the brakes hard a good few times but it depends some on the pad compound as brakes work with both abrasive friction and adherent friction. More abrasive (dusty) pads are probably better in this regard?
There's also a metallurgical cause of DTV which we don't need to go into but basically it also results in uneven wear and (along with pad deposits) is the reason why manufacturers advise
not keeping your foot on the brake when stationary after hard use of the brakes. Especially when bedding in new pads and/or discs
This crude sketch shows the difference between "warped" / axial run out (on the left) and DTV on the right. The white lines represent a disc with zero run out and thickness variation, the red is a gross exaggeration of run out and DTV
In reality we're talking about tiny fractions of 1mm, a generic spec for max runout of a newly installed disc is typically 0.8mm while a decent quality disc will usually have a claimed thickness variation of less than 0.01mm! A thickness variation of 0.05mm can result in a judder felt through the pedal according to many sources for example
http://europe.delphi-dso.com/downloads/private/techical%20bulletins/Brakes%20Tech%20Bulls/32%20-%20BRAKE%20JUDDER.pdf
Note that a single dial indicator, while useful for checking disc run out when fitting new discs, cannot differentiate between run out and DTV which may be part of the reason why the myth of "warped" discs refuses to die?
