Time for Rotors?

[marticus24]

Hi All - I'm no stranger to doing brakes, but have never done so on a performance car that I track. I'm prepping for my next track day in my '12 ZL1 with OEM rotors (11,500 miles, 4 x HPDE days and 1 x day at the strip) and see that they have what looks like of like scales. (see pic) I'm wondering if this is an issue that indicates they're ready to be replaced or not. I don't see any cracks and will be measuring their remaining width once I get the micro-meter that I just ordered.

So, overall, as long as they measure OK, I'm not experiencing any vibrations or pulsations, and I haven't experienced any loss of stopping power, do you see any reason to replace or get machined?

Thanks for your input.

[jamestown]

They look normal. Just check the thinkness imo.

[marticus24]

My original photo - doesn't quite do it justice, could be the lighting. Looks like a little bit less extreme version of this:



I just read that these are surface cracks and may be able to be machined out as long as there is enough meat left... anybody else have thoughts or experience here? Does that make sense?

[jamestown]

I see what you mean even in the original photo - I still think it is normal for track used rotors to look like that and isn't an issue unless the micro cracks get significantly worse.

[jamestown]

From a quick search:

This was a TSB from GM regarding Corvette rotors.

Bulletin No.: 05-05-23-007

Date: August 03, 2005

INFORMATION

Subject:
Information on Visual Brake Rotor Surface Cracking

Models:
2005-2006 Chevrolet Corvette

with Heavy Duty Brakes (RPO J55)

Important: DO NOT replace OR resurface rotors for small surface cracks that are described below.

The pass criteria for thermal fatigue testing specifies that the rotor is allowed to have small thermal cracking up to 2/3 of the brake plate surface. The brake plate surface for the front rotor is 50 mm (1.96 in) radially which means cracks are permitted up to 33 mm (1.29 in) radially across the rotor brake face in length. This would be the pass criteria for the end of severe testing. To have a large safety margin, a rule of thumb - small thermal cracks are allowable and the rotor does not need to be replaced unless the cracks connect from hole to hole or the crack reaches the OD of the rotor.

[SPCBA]

Looks like a lot of pad on the rotor too.

Where you using pads designed for the proper heat range that your car on that track would put on them?

[marticus24]

Hi, yes. These are OEM pads. Only the first pic is my car, the second is one from internet just to show what I'm talking about better. I do tend to get a lot of pad stuck though after a wash and it sits overnight. I'll usually do few hard stops to help clear it off the next day.

So, if that TSB can be used in my context (don't see why not) then it appears this is normal for this kind of rotor as long as it doesn't get out of hand. Thanks all for your responses.

[marticus24]

So I've measured everything.

Front rotors are both averaging about 33.8 mm (average of 4 spots 90 degrees apart) while the front pads are measuring between 4 and 5 mm. I know the minimum rotor width is 32mm, so I'm safe there, but guessing getting them machined is not going to be an option.

The rear rotors are both averaging about 27 mm with pad thickness at between 5 and 6 mm.

These are all OEM parts and I'm heading to Gingerman (South Haven, MI) in a few weeks. Safest bet is to put new pads and rotors on before that day, however, is there enough life left for one more HPDE? I tend to brake late.

Thanks for your help.

[jamestown]

How comfortable are you swapping the pads and/or bleeding the brakes track side? You could consider buying pads and then just check the remaining thickness between sessions.

I just measured a new front SS pad to be 9.5mm thick, so you might have half of your pad remaining? How thick are the zl1 pads new? It should be enough for a day.

Would you replace again with oem?

[marticus24]

Originally Posted by jamestown

How comfortable are you swapping the pads and/or bleeding the brakes track side? You could consider buying pads and then just check the remaining thickness between sessions.

I just measured a new front SS pad to be 9.5mm thick, so you might have half of your pad remaining? How thick are the zl1 pads new? It should be enough for a day.

Would you replace again with oem?

I'd have to guess the ZL1 pads new are at least that thick new, I can't find the official specs. But if that's any indication, then I'd say we're looking at about 1/2 of their life left or just under... seemingly plenty for one more HPDE.

I also found out that the front rotors new are 34mm so they have a ton of life left. I think I'm going to buy a new set of OEM pads, bring them to the track and change out if necessary, track-side. I don't want to do it there while the brakes are piping hot and lose track time, but will have the tools and can if I need to. I plan on using the OEM brakes for this round of replacement. I do several driving events per year, but am not serious enough to start dropping a lot more $ when the factory setup is pretty darn good and reasonably priced (respective to some of the after market racing setups out there).

Thanks for all of your input.

[ZL1-V]

You also need to change your brake fluid to either Motul 600/660 (more affordable) or Castrol SRF ( more expensive) for heavy track usage.

[h018871]

marticus24 My rule of thumb is if you can feel the cracks with your fingernail, you should replace them before a track day. they will last a while on the street though.

something else IMHO, thinner pads will keep more heat in the rotors.........causing more cracks. this is much more prevalent when you late brake. again IMHO

i agree, go with a good quality DOT4 fluid & bleed before and after each HPDE event

[jamestown]

Originally Posted by h018871

thinner pads will keep more heat in the rotors

You mean fluid right? If the pad is thinner it isn't providing thermal insulation to the caliper / fluid. How would thinner pads heat the rotor?

[h018871]

my observation has been that the "shim" on the back of the factory Brembo pads does a really good job of blocking a lot of the heat from getting into the caliper pistons. I've noticed that thinner pads tend to fade and that my cracking worsened. i surmise that the pad material adds a bit of heat sink or that the pad back is so close that more heat stays in the rotor. i have not seen any boiling of the fluid in my car, granted the track i mostly run at you don't have to stab the brakes at the end of the straights, just a light tap to load the front tires for the turn in.
attached are some pics of a front shim, the pad side was noticeably hotter than the piston side.
i could easily catch my fingernail in the cracks