Cracked rear rotors

[Ross Nicol]

Here's proof how hard I was pushing last weekend. That's the 2nd set of rear rotors I've gone through. Ambient temp wasn't very high either. You can see the big crack in the photo but there are lots of little ones all around the disc.Just as well these are coming off in favour of bigger ones.Free to a good home if anyone wants them though.
Ross

 

[CliffBeer]

Ross, yeah, looks like you'll need some bigger rotors for track work. It appears you have some good ducting to the rotors and plenty of swept area with the four pistons so really all that's left is increasing the thermal capacity. If you can find it, you might just try thicker rotors as then you wouldn't have to change the attachment points of your calipers. Going from say a 20mm thick vented rotor to a 24mm vented rotor can make a big improvement (that's 20% more thermal capacity) - might be all you need to avoid cracking.

Did your pedal get a little soft? Typically, if the rotor is seeing temps high enough to crack then the caliper is getting warm enough to boil the fluid. I've had good luck with the Ate blue fluid which typically gives an extra 20-30 degrees of max temp before boiling.

 

[JohnC]

Ross do you cryo your rotors? Some racers here in the US swear that cryo treatment prolongs their rotor life and keeps them from cracking.

I run drilled & slotted cryoed rotors on my 2 fairly high powered street cars and after 8-10K miles, I've not had any signs of cracking whatsoever. Granted these aren't racecars, but I've had to do a few ABS assisted 90-0 stops and they're giving good service so far.

John

 

[Malcolm]

I was wondering how rear disc failure makes the car behave? At the front it screws your steering but never had a rear failure yet. What does it feel like?

 

[Ron Earp]

Here's proof how hard I was pushing last weekend. That's the 2nd set of rear rotors I've gone through.
Ross

I see a hose there that looks like a duct hose. Do you have hats that channel flow on both sides of the rotor? And a duct that gets air down to the caliper pad area, straight in the top (a bastard to make, making two now for the TR8)? I'd think with a couple of well placed ducts it'd help.

We've not suffered any rotor failures but everything is ducted up well, knock on wood! As mentioned a lot of racers use the cryo treatment and report good effects, but we've not tried it yet.

R

 

[Ross Nicol]

I will check out the cryo treatment guys. Malc sorry to hear your race drive was cut short. Sounds like you went ok though. I didn't feel any problem with the brakes while driving as the pedal was bouncing under my foot due to rotor warpage. I suspect now, it was the rears that are warped. It may also be the cracks appeared on cool down after the last race. I don't really know because I only spotted them yesterday. Ron the hoses are a crude duct I set up to try and help brake cooling. I just trained the outlet to the centre of the disc but I'll improve it after the brake upgrade.
I always thought these rear rotors were too small and I have to admit I have a fear amount of bias on them as you can imagine with all the weight of the motor and transaxle you can bias a lot of rear brake without locking before the fronts.
Here is my set up at the moment, calipers are road type with seals
Front rotor- Dia 330mm/ 13" Thickness- 28mm/ 1 1/8" (1 piece)

Caliper- AP Racing 6 piston- CP5555 816s4/817s4

Rear rotor- Dia 280mm/11" Thickness- 20mm/ .75" (1 piece)

Caliper- AP Racing 4 piston- CP5100 806s4/807s4

My upgrade will include changing to 2 piece hats and rotors all round. I know I can fit all my wheels over the front brakes so I'll fit 330mm/13" rotors all round and change the rear calipers to the same as the front. These calipers will accept a max rotor width of 32mm so I hope to obtain this size. I should be able to crank rear bias on then until the tail begins to squirm under max braking and then back it off a little. Even with the speed the car is now doing I would be hoping to brake at 100m or a little less. The fear factor sets in at about that distance from the corner.
Ross

 

[Clayton]

Hi Ross
As an intermediate upgrade you could retain the rear calipers and upsize the disc to a 295mm x 25mm (the maximum size for that caliper) and use a DBA 5000 series rotor replacement from a Subaru Impreza, p/n DBA 5000.1 and have a hat machined up.
Or you could put the 330mm x 32mm on : )
Just my 5c worth.
Regards

Clayton

 

[Ross Nicol]

Hi Clayton
Thanks for that info, I'm impressed you've spent time thinking and researching for me. That's what this forums all about. The only thing is the machining of the hats and the fact I like to do things once. To get on par with the GT3 which appears to have 18" wheels and 350-380mm rotors with 6 piston calipers, I'll go all the way now. The brakes are the final weakness in my car and I'm sure they were the reason for my slipping lap times. I didn't recognize this fact until the new motor/more power highlighted it. If you have any other ideas Clayton let me know as I'm still in research mode.
Ross

 

[Garrett Reid]

Have you considered fogging the rotor (distilled or demineralized water)? I've used a heat sensor to activate a small pump(windshield washer pump) and a spray bar in the front of the cooling duct. Worked well for me on keeping down brake fade. Cheap setup too.

 

[Ross Nicol]

I've seen something like that on racing trucks but not cars. Actually I have not had any brake fade at all but obviously the temp on that small rear rotor is high. I have recently started using the Castrol SRF fluid with 590 degrees f
dry boiling point too, however I never had fluid boiling with the Penrite super dot 4 I was using. Of course more power/ higher speed and the brakes are having to work harder than before so everything has changed.
Ross

 

[Jac Mac]

Ross , I think you will find that the one piece rotor/hub setup was primarily responsible for the large crack/failure. Heat soak while stationary in pit lane, while waiting to rejoin after an off track excursion, or even in the pits at the conclusion of a race will crack rotors. With the amount of rear bias you have been running it was only a matter of time. Going to seperate hat/rotors will help, also a recirculating fluid check valve system would help a lot also.

Jac Mac

 

[Clayton]

Hi Ross

What did you end up coming up with ??
I'm using a HSV rear disc with internal park brake so there’s only one caliper, AP of course : )
regards

Clayton

 

[roaldin]

2 piece is the only way to go; good decision Ross.
Floating rotors are the answer if problems persist :-

Tim.

 

[Phil]

Ross
We had a problem with cracking on a 962 porsche..pedal would get really spongy also. It was a baffling problem because we thought we had sufficient ducting, which was the case, but it turned out the air to the rotors was too cold, a simple fix was routing the air past the radiator, which solved the problem, since the rotors were way hotter than than that air anyway. Just a piece of information for the mix.
Good Luck
Phil

 

[roaldin]

Jac Mac's comment about heat soak in the pits is a very valid concern.
I have seen disc's crack in the pits because the brakes come in hot and the car is left standing. The area under the pad is insulated to some extent and cools slower than the uninsulated area. A sensible cooling down lap and the occasional push/roll when the car is stationary will help in this scenario.

2 piece is the only way to go for track work though.

Do you have any measure of rotor/caliper temps? I find temp' sensitive paint and indicator strips to be very useful.
The DBA5000's mentioned above should come with three temp' sensitive paint stripes on them.

Tim.