GT3 cracks on steel discs

[cayman-black]

was looking at a GT3 today which had small cracks round all of the holes in the front discs. owner said this was normal. any one had this? car only had 3k miles on it.

[andy tims]

Absolutely normal.

Nothing to worry about unless the cracks get too big - circa 5mm long or more.

[cayman-black]

andy tims said:

Absolutely normal.

Nothing to worry about unless the cracks get too big - circa 5mm long or more.

thanks andy. whats the cause of this? do not remember seeing this before.

[Merritt]

cayman-black said:

andy tims said:

Absolutely normal.

Nothing to worry about unless the cracks get too big - circa 5mm long or more.

thanks andy. whats the cause of this? do not remember seeing this before.

Stress fractures caused by stopping a heavy car very rapidly lots of times!

I had new front discs on before Oulton on Friday and they are already cracked after one track day. (my Brembo floaters were binned recently because they had floated out of spec but thats a different story).

As Andy says, its normal but keep an eye on the crack lengths. Also - you might find that your MOT station will fail the car if they are cracked badly.

Steve

[cayman-black]

Steve how much is it for new front disc and pads? roughly.

[carylwills]

As Steve and Andy have said these cracks are quite normal. They are caused by the outer surface of the disc getting hot under normal braking where the disc core is not so hot. The outer surface expands more than the disc core and a surface crack is propogated from the cross drilling.

The rule is that the disc should be replaced if the cracks are more than 7mm long, or if 2 cracks join up linking 2 holes or if a crack reaches the outer rim of the disk. It is particularly dangerous to allow 2 cracks to reach the disk rim within about 50mm of each other as a chunk of disk can fly off.

If you plan to do lots of track work with the GT3, I would suggest you look at fitting discs that are not cross drilled from Alcon, Brembo or Performance Friction. BTW discs are only cross drilled to give the water on a soaked disc somewhere to go - they do not greatly improve cooling.

[Globulator]

carylwills said:

BTW discs are only cross drilled to give the water on a soaked disc somewhere to go - they do not greatly improve cooling.

The holes (slots on some discs) are to release gases generated when the pad gets hot - which prevents fading. Else the pads float around on the gas like a hovercraft and do not grip.

There is also IIRC a case of discs having the holes cast vs drilled, which are better at not cracking - or so I've heard

[964RS]

cayman-black said:

Steve how much is it for new front disc and pads? roughly.

£550

[carylwills]

Globulator said:

The holes (slots on some discs) are to release gases generated when the pad gets hot - which prevents fading. Else the pads float around on the gas like a hovercraft and do not grip.

Thanks Graham - another thing I've learnt today

[Hunter 66]

NOt a problem as said only at the MOT station where the tester was not that happy . I have a set of surpluss Mk 2 front pads (porsche ) if anyone needs.

[Merritt]

cayman-black said:

Steve how much is it for new front disc and pads? roughly.

Depends whether the car is MkI or MKII. MkII discs are £500 per pair for front motorsport discs or around £950 for a pair of OEM front discs. Pagid RS29 pads are around £350 list price but can be had for less if you look around. As Peter has posted above (Hunter66) he has OEM pads..

MKI discs and pads are significantly cheaper.

HTH

Steve