Help Needed!!! Rear brakes overheating on track

[gorrerjm]

So I have an e92 m3 with stoptech ST-40's up front only. Over the course of 4 track days, I noticed that my stock rears were eating up pads way faster than the fronts and then we realized that my rear rotors were as, if not more, blue than the fronts.

Fast forward to yesterday, and the guys at JPSS (highly recommend if anyone needs any work done in the carolina/virginia area) met me in the pits at VIR after a couple of hot laps. After 1 warm up and 3 hot laps on the full course, they shot my rotors.....464/450 degrees in the front and 566/537 FOR THE REARS! Temperature stickers on the rear calipers show max temps below 400, but not clue how the rear rotors are getting so hot! Any ideas what is causing this and if I should be concerned?

I am running PFC 08's on both front and rear. Most days are run at VIR and I haven't experiences any significant brake issues (fade, rear-end hoping out, nothing).

[peter de la mare]

Are you leaving DSC on?

[roastbeef]

DSC uses the front brakes around 95% of the time. It's not that.

Something is mechanically wrong with your rear brakes. Dust boots are probably ripped to shit and the pistons are somewhat sticking.

[peter de la mare]

Even the traction control element?

[gorrerjm]

No nannies regardles.

My thoughts exactly but there seems to be no visable issues with the rear brakes minus the pad wear. Car seems to respond as intended which would make me thing that they aren't sticking and/or dragging.....Just say screw it and put rear ST40's on?

[Inflame]

DSC uses the rear brakes for stability and understeer control. If you have it off, then you have a mechanical problem.

[gorrerjm]

Yeah that my thought - Any idea how I troubleshoot or just go right to new brakes?

[SYT_Shadow]

I wouldn't go straight to new brakes just yet

Are you running with DSC fully off?

[msan]

I see you have a thread about the DCT malfunctioning also. Maybe the rear brakes and DCT problems are related in some way. Just a thought.

[roastbeef]

Originally Posted by peter de la mare

Even the traction control element?

Originally Posted by Inflame

DSC uses the rear brakes for stability and understeer control. If you have it off, then you have a mechanical problem.

it has been well documented in dogbone's build journal that DSC uses the front brakes almost all of the time. it also uses the rears in some situations, but it is a very very heavily front brake biased system.
he used AIM data to see when brakes (and which brakes) are being applied when he is not pressing the brake pedal.

[roastbeef]

I have to clarify something, when I said DSC, I meant MDM.
I don't know exactly how DSC works, but if you're tracking with DSC on, you're going to be slow as hell because it is too intrusive for even cool down laps. My apologies.

[Inflame]

Originally Posted by gorrerjm

Yeah that my thought - Any idea how I troubleshoot or just go right to new brakes?

I'd begin by inspecting your rear brakes to see if they're sticking. If they're not, then you may be running the wrong brake pads and/or not fully turning the systems off.

[gorrerjm]

Yeah the system is completely off (Holding the DSC button until it turns yellow and I get teh DSC off message). In the beginning I ran with the MDM mode on, but that was long ago and have been running in this off manner only.

I have a vbox system that came in today, I am going to Atlanta this weekend and will pull wheel speed data to see if I can see anything being effected.

[pbonsalb]

Did someone install the rotors on the wrong sides so the venting is not as effective?

[M3SQRD]

You’re running a ST40 F brake kit with rotors that have better cooling, as well as more thermal mass, than stock rotors. I’m not surprised your R stock rotors are running hotter than the ST40 F rotors when using the same brake pad compound F and R. The pins in the stock rotors do a good job of blocking air flow to the rotor vanes whereas the ST40 floating rotors have no vane obstruction.

Does the stock brake bias change when running the ST40 F kit with stock R brakes?

[roastbeef]

i'm also curious- what tires are you running gorrerjm?

[pbonsalb]

Originally Posted by M3SQRD

You’re running a ST40 F brake kit with rotors that have better cooling, as well as more thermal mass, than stock rotors. I’m not surprised your R stock rotors are running hotter than the ST40 F rotors when using the same brake pad compound F and R. The pins in the stock rotors do a good job of blocking air flow to the rotor vanes whereas the ST40 floating rotors have no vane obstruction.

Does the stock brake bias change when running the ST40 F kit with stock R brakes?

Stoptech sizes the piston area in its kits to not change bias or to change it so minimally it does not matter.

[admranger]

Two things I would like to know: When and where are the brake temps being taken and how are they taken?

Also, take the temps after driving your car to the track and brake into your parking spot. What is the difference in temps when you do that?

[nrubenstein]

Originally Posted by M3SQRD

You’re running a ST40 F brake kit with rotors that have better cooling, as well as more thermal mass, than stock rotors. I’m not surprised your R stock rotors are running hotter than the ST40 F rotors when using the same brake pad compound F and R. The pins in the stock rotors do a good job of blocking air flow to the rotor vanes whereas the ST40 floating rotors have no vane obstruction.

Does the stock brake bias change when running the ST40 F kit with stock R brakes?

Yeah, this. Why is it a surprise?

[///Mobbin]

I'm curious what tires you are running?

I run ST-40 fronts and stock rears and used to run PFC-08's all around. I found that I was boiling my fluid up front with this setup. I now run Castrol SRF, Pagid RS-29 fronts, RS-19 OE shaped rears, and then throw 1mm Ti shims in the front after the first track weekend (they won't fit when pads are new). No issues at all with Cup 2's and RE-71r's though I suspect I will need to upgrade my brake system when I switch to slicks.

My theory on the PFC-08's is that they run hotter and transmit more temperature into the calipers than the Pagids. It doesn't sound like boiling fluid is your issue but perhaps try switching pads and see if that helps.

[gorrerjm]

So another weekend and another test. Here is what I did and my results - I switched to using a more aggressive pad upfront (had a half warn set of old PFC 01's). Brake temps flipped and I was at about 550 up front and 500 in the rear. I'm going to chalk it up to the car, even though it freaked the track side mechanics at VIR (who also do my work) out.

Temps where talked with a heat gun.

[gorrerjm]

Interesting about the boiling fluid....the only time I have had issues on a range of tires, was when some F'ing idiot (me) put in two much oil because we dont have a dip stick and the added pressure had oil all over the engine compartment, which found its way to my brake calipers and then caused my far other side to completely overheat stopping the car.