Multiple cracks on rear disks.. what causes this?

[ivan]

Had a set of these rotors once on my MX6.

 

They cracked inwards from the weakest point - the dip at the top of each groove.

[Nic]

I was a long way from easy on my cars brakes.

 

There's your answer, it was Terry's fault

[Guest Terry S]

There's your answer, it was Terry's fault

 

it's just like being at home LOL

[dandan]

Same as at my place then!

[Chris Wilson]

Erm, I *THINK* I may have mentioned discs which start off as plain castings that have holes nailed in them being prone to cracking one time? Oh, no, my mistake, it was dozens of times... Even a proper "drilled" disc will crack, especially in the wet. F3 drilled discs are sacrificial, they get binned after each wet race. Buy plain discs!!! 280 quid brand new AP Racing drilled disc after a single track day at Donington on my old GTR Skyline below. Note, this is a proper "drilled" disc, see just how many more holes are in it? That's `cos it was cast with vanes arramged to allow a lot of holes, not cast as a plain disc and then someone tries to find a way through the casting with the drilling machine without hitting a vane rib, which is the process used on discs that are more show than go. Drilled discs are used in racing to save weight and in the wet to give a more immediate pedal response. IMO they totally inappropriate on heavy road cars, these days.

[Mike B]

I can see how the wet would cause them to go sooner - holes like that are not going to stand up to heating and quenching repeadtedly. Donnington was very wet.

I hope ap were kind to you after you destroyed them in one outing, and I like the photo taken on the bonnet of the car..lol

Do you also find that grooved disks give you a lot of noise and vibration under heavy hot loads...?

[Chris Wilson]

AP don't change cracked discs, it's the nature of drilled discs to crack. A small number of radiused grooves shouldn't add much noise, and no vibration. You NEED a flaoting disc set up if you intend to get into track days, it'll save you a lot on discs, in the long term. The front uprights on 90% of road cars are like rubber, and bend when you take them on track. A floating disc will allow the disc to stay upright between the caliper pistons and not be deflected. They are more expensive and rattle a bit on the road.