Brake discs warped AGAIN !!!!!!!

Pads can't create wheel wobble no matter what condition they are in. The disk has to be warped (or incorrectly fitted) for this to happen. Presumably you understand that as a matter of basic common sense something that rotates has to be creating the wobble rather than something which is stationary.

Some cars are prone to warped disks and some aren't. Fiestas with non vented disks used to suffer from it a bit as I recall. In fact I imagine vented disks are less prone to it in general due to their thickness. OE disks may well be made from different material and heat treatment than aftermarket ones so you might do better to stick to those.

The answers are exactly the same as before: fit new disks, have runout checked, fix as necessary. This series of steps are the only way forward. Using genuine parts is usually the best option if available.

Apparently, doing a quick google it appears that Kia Sedonas (the car in question) has a problem with warped discs.

There appear to be many reports of Sedonas with discs that need changing every few thousand miles. Looks like a major design flaw with the Sedona.

In fact, in the US, there is currently a class action lawsuit outstanding.

Yep. Insisted they check the run out last time they fitted the disks in January.

To be honest, I've agreed to do what they want this time (i.e. let them bed the disks in), but if they go again, I'll be demanding a refund and going to Kia to get genuine parts instead.

From an engineering point of view the reason why things warp is because there are stresses locked into the material from the manufacturing process which haven't been properly relieved by heat treatment. The first few heat cycles under braking then starts to release them. Once those stresses have come out they can't recur so in fact the best thing to do is have the disks ground true again after they've warped. Fitting new ones of the same make time after time is just going to repeat the unhappy process.

I agree, OE ones will be much more likely to have been made to a very specific quality that suits the application.

As to the fitting centre claiming the problem is unduly heavy braking or not running them in properly that's clearly bollocks. Any heat cycling of any sort will start to release locked in stresses so simply using the brakes at all will suffice - assuming of course the brakes on this car obey the normal laws of physics and turn kinetic energy into heat. Gentle usage might take longer to fully warp them but the end result is going to be the same in time.

the disk manufacturers claim that disk thickness variations (giving juddering brakes) are always caused by excess runout

I think that's a truism because the pads can only machine the disks thinner in some areas than others if there is some disk runout. I'm not sure what it has to do with the instant question.

I was trying to point out that it may not be a warped disk giving the problem, but a disc with thickness variations caused by runout.

It is my experience that when a car comes in with juddering brakes it is always a question of DTV causing the problem, ie the disc is not wavy through heat distortion, but through initial malfitting which has lead to DTV which gives juddering.

I cannot remember seeing a car with the original (from new) set of discs with judder, it always seems to be on the subsequent set of discs that judder develops, a run out check invariably shows a problem either with dirt on the mounting surface or a hub that no longer runs true. Fords are the worst offenders for this, especially fiestas which frequently need a new hub to fix the fault.

Mrcheerful

The cure is re-machine in situ. Then they match the hub.

Just a thaught. There is advice to not keep your foot on the brake pedal after stopping, especially after heavy braking when the discs are very hot., as it can cause discs to warp. Ignoring the warping factor, maybe the pad contact causes local chilling and hardening. If this did occur, the disc could wear unevenly. The hardened area probably wearing less. If so, this would inevitably lead to juddery brakes. As I said, just a thaught. Could be false reasoning.:-) Mike.

(a) Take your foot off the brake at lights and use the handbrake.

(b) What the f*ck do you expect? It's a Kia.

apparently some pad material may transfer to the disc under these conditions, again giving juddering brakes

Presumably though, any pad material transferred that way would wear off fairly quickly, and not cause a permanent problem. Mike.

Apparently it bonds to the disc so well that it is a problem.

Are they giving you all these new disks free? Because if not, and you're paying, it might be better not to keep going to the place which keeps fitting bits which go wrong ...

Ian

Although you do have to wonder why old Citroens never seemed to suffer from it.

Duncan Wood ("Duncan Wood" ) gurgled happily, sounding much like they were saying:

Especially with front-wheel handbrakes to exacerbate the localised heat transfer.

I certainly never had warped disks on any of my CXs, although the disks on the XM would start to wobble slightly after about the second set of pads.

That's nothing compared to the pattern disks on the front of the Saab at the mo (with a rear handbrake) - they've started to warp slightly after only three months.

maybe it is the lack of asbestos in modern pads giving higher brake temps?

The garage have tried to accuse me of that one.

Its an automatic - generally you have to stick your foot on the brake when you're stopped !!!!