V70 Brakes: Warped discs. Urban myth?

[ShadeTek]

After I had a front brake caliper sieze up on me and I repaired the seized caliper, I started to get wobbly braking at speed and grabbing at low speeds. Initially I condemned it as a warped disc as it and the wheel had gotten VERY hot. I was ready to replace it until I read up on the subject and seemingly it is not always the case it is a warped disc as the cause. It only happens in very few cases. So I decided to try some bush mechanic skimming. I got some flap discs for my angle grinder, Bosch 120 grit, and then got an old wheel bearing in a vise and bolted the disc onto it. I spun the disc by hand while lightly grinding the disc. It cleaned up a treat and took away all the blemishes. I put it back on the car and it had no wobble or grabbing at low speeds. So far so good! I forgot to take a picture after I polished it but it looks like a shiny disc! It is only 5 months old and i was loathed to replace it already

Blemsihed Disc



Old wheel bearing in the vise



Angle grinder with sand flap disc attached

[Brendan W]

Nice one. I suppose when they do get hot it's what happens in those minutes after the car stops that determines the fate of the disc.

[outnumbered]

nice one i do like it when people have a go rather than replace

[kebab10]

How strange. Getting the rear discs on my V50 ( I know this is the V70 forum), done tomorrow as have the same thing. They have been on since January and have done about 7000 miles. Dont really want to get another set of pads and discs yet, but when the time comes shall certainly spend a little extra on a "better" set.

[ShadeTek]

I'm planning on motorising my bearing so it will sin the disc faster to minimise any errors. Youtube has some good videos of professional skimmers.

[KBB]

The true test will only be possible after you refit them and test for runout with a DTI

[ShadeTek]

Originally Posted by KBB

The true test will only be possible after you refit them and test for runout with a DTI

Indeed, but I haven't got one but I guess all discs will be very slightly warped, it's when they start to annoy you when is when they need attention. I do own a micrometer but it is for measuring runout on video drum heads and I might try and adapt it to test discs.

[KBB]

All you will need is a magnetic stand, Max run out is 0.15mm, over that they should be changed or re-faced providing re-facing dose not bring disk below minimum thickness.

[TXC]

I realise your caliper was the original cause here, but gunning up or overtightening wheel bolts is another supposed cause of warpage over time.

[stuartggray]

I had the same grabbing after a rear caliper seized. The disk showed the same
marks as Shadeteks one. I polished it all off because I reckoned it was bits of the old pad baked into the disk. Disk worked perfectly with new pads and caliper after repair. No visible 'warp' before or after.
I did a bit of googling and found some info which supported my theory. Indeed, it was suggested that you don't sit at the lights with your foot brake on, especially if they are hot after a hard fast stop.