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More brake cooling questions...


wvumtnbkr
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Okay, I am finally getting brake pads to last an entire race weekend (JUST).

 

I redid the brake coolign ducts (moving the inlet closer to the center of the car) and have rotor sealing plates where the air is directed to the center of the rotor.

 

I think I need some caliper cooling now....  Calipers are still getting hot enough to boil Motul 660.  Not hot enough to have NO brakes, but hot enough that travel goes up and feel goes down.  The fluid comes out black after 1 day.

 

So, help me figure out how to do this!  I run 15" wheels (and i will NOT change).  I am using the stock rx7 brake rotor and caliper (4 piston aluminum caliper).

 

Is it as simple as stacking another hose at the front of the car that directs air to the inboard side of the caliper?  Should I figure some way to blow air on TOP of the brake pads and caliper too?  Is that even possible?

 

Then, I am also thinking about titanium shims between brake pads and caliper pistons.  I can get thin sheets and cut out the rough shape of the pads myself.  My concern is that this would heat soak after about 2 hours and not help anymore.

 

So, I am thinking belts AND suspenders.  Ti pad shims with air directed at inboard side of caliper.

 

Whatcha think?

 

I am running carbotech XP24 pads in front and XP12 in the rear.

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consider this.    is your problem getting cold to the caliper, or removing heat from the wheel well area?

 

you've probably seen teams that cut behind the front fender and less obviouly running more open wheel design etc.   my point is to make sure you're moving air through the area and not just into it.  Your ducts that you have now are already getting fed / impacted in temp by the veins in the rotor.  Consider drivers also.  the better drivers get, the less abusive they become on brakes.  I used to destory brakes when I was starting track days, now I'm a lot faster and more gentle on them

Edited by theblue
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5 minutes ago, theblue said:

consider this.    is your problem getting cold to the caliper, or removing heat from the wheel well area?

 

you've probably seen teams that cut behind the front fender and l less obviouly running more open wheel design etc.   my point is to make sure you're moving air through the area and not just into it.

Good thought.

 

We cut and bent the fenders to force air out the back of the wheel well.  I am thinking I am going to make fender vents like Red0's CRX.....

Edited by wvumtnbkr
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  • Technical Advisory Committee

 

19 minutes ago, Team Infiniti said:

Weight?

this.

 

Seems to me that with all the extra torques you have from the soccer-mom power, you are requiring much more braking capability.  Increased braking = more heat that has to go somewhere.  While cooling airflow helps, you probably need more base thermal mass.  More rotor mass gives the heat somewhere else to go other than into the caliper/fluid.

 

So, how big of a rotor can you fit under those wheels with a spacer for the caliper?

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5 hours ago, wvumtnbkr said:

Should I figure some way to blow air on TOP of the brake pads and caliper too?  Is that even possible?

Blowing directly into the pads would be in conflict with your center out venting.

 

Vent, tuck, blow like mad.

 

Eventually, you will be forced to make the changes that you want to avoid.

 

 

 

2 minutes ago, NigelStu said:

So, how big of a rotor can you fit under those wheels with a spacer for the caliper?

Based on our starting point, 15in wheels, 11 in rotors... he is maxed

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5 hours ago, wvumtnbkr said:

Okay, I am finally getting brake pads to last an entire race weekend (JUST).

 

How many hours, 14 or 24? Pretty impressive with the xp24's & fitting everything into a 15" wheel package/small rotors.

 

With our improved cooling, I am curious if we'd be back in bidnizz with the xp24's up front, but the st45's work really well and are hard to beat for the monies. 

 

 

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Rob, 

 

Verify your caliper temp with temp stickers. You might be seeing fade that isn't from fluid boil over. What is the wet boiling point of that fluid? 

 

It is often the high caliper\pad temp is caused by a lack of rotor cooling making the whole system hotter. If your caliper width would support it a wider rotor (like common 1.25") would give alot more cooling area and lower temps. If you don't want to go this route adding forced cooling to the rotor from your current duct is a great option. Remember last week the avg speed was what, in the 70s. With the fans on you will probally see a flow increase up to 90ish mph. Adding a duct that forces air to blow over the caliper surface can help, but many oem calipers lack the holes and passages racing brakes do which drive up the surface area. Pad\caliper cooling would be my last addition, it may help but isn't the bag for the buck these other mods are.

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Lemme hit yall with some anecdotes to maybe help with this issue...

 

1) We ran these brakes for YEARS!  Our car was not slow before (wins at Lime Rock Park, impounds at WGI, win at Summit Point Shenandoah, impounds / podiums lots of other places, top 6 at VIR 24....  etc...).  We used to run Hawk Blues with NO cooling ducts or anything.  They lasted 2 races....

 

2) These brakes are a common UPGRADE for E30s.  Similar power and similar weight to our current car.

 

3) Other RX7's (with even more power) are running stock calipers and stock rotors.  (Like Sahlens).  I think @FlorahDorah and her team are running lesser pads than we currently are and are not having these issues.

 

I have changed calipers (twice) and changed master cylinders (3 times).  I have installed brake pressure gauges in the cockpit and KNOW that there is no residual pressure in the system.

 

WTF?

 

There is a way to get caliper spacers and possibly run wider brake rotors.  

 

I know NASCAR runs 15" wheels, so there MUST be a way to get brakes that work under a 15" wheel.  I have 20 or 30 points to play with if needed.

 

Thanks for all the input so far.

 

P.S. The brakes worked well with Carbotech XP24 pads.  We did have a long pedal for the second half of both days.  The fluid is black after being replaced both days.  The brakes did start to "shudder" at the beginning of the day on Sunday (which was annoying, but didnt seem to change braking distance).

 

 

 

 

 

 

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11 hours ago, Team Infiniti said:

Eventually, you will be forced to make the changes that you want to avoid.

 

 

 

Based on our starting point, 15in wheels, 11 in rotors... he is maxed

What changes do I want to avoid?  The 15" wheel situation?  That is the only change I am NOT willing to make.  E30's (similar power and weight), other RX7's, NASCAR all run15" wheels.

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6 minutes ago, wvumtnbkr said:

What changes do I want to avoid?  The 15" wheel situation?  That is the only change I am NOT willing to make.  E30's (similar power and weight), other RX7's, NASCAR all run15" wheels.

Rationalize any way you like, but you are no longer working the same as with the rotary, you new program is less momentum, more brute force. The 15in wheels will keep you from solving the real issue of inadequate rotor mass as we are not allowed to have massively wide nascar calipers/rotors.

Edited by Team Infiniti
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I agree with @Black Magic.  You're just spitting in the wind if you don't have any idea of your brake temps.  Temp strips on the calipers and temp paint on the discs/pads will give you peak temps.  Strips aren't too bad, but the paint is pricey.  It goes a long way (several sets of discs/pads) so maybe go in with another team.  The best investment will be to get a temp probe that is calibrated for brake temperatures.  Take temps on pit road if possible (closest to pit-in) which will give you more of an average temp.  Brake temps cycle pretty dramatically so the average is really what is important when trying to lower temps.

 

Make sure your ducting has minimal bends/direct flow and the biggest hose you can cram in there.  Volume is key.  I'm curious if inline fans will help with OE style discs and our lower avg speeds.  I'd guess they will but you should really start with taking temps to see what works and what doesn't before making changes.

 

Don't relate brake fluid color to good/bad.  Racing brake fluid can look dark and nasty, but still test fine.  Maybe try another brake fluid if you're getting a spongy pedal.  Like engine oils, not all racing brake fluids are the same.  My only disagreement with BM is that I don't think wet boiling point really has much relevance if you're bleeding regularly and flushing annually.  It takes a long while to get enough moisture in the system to reach WBP.

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54 minutes ago, Team Infiniti said:

Rationalize any way you like, but you are no longer working the same as with the rotary, you new program is less momentum, more brute force. The 15in wheels will keep you from solving the real issue of inadequate rotor mass as we are not allowed to have massively wide nascar calipers/rotors.

 

And we run 17s on our Mazda.... not sayin', just sayin'....

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Bremsen,

 

Wet matters if you are trying to extend fluid life to save cost....i get used srf other people are tossing out. New it is about $100 a bottle but i run the same stuff for a half dozen races or more. The wet to dry spread is nothing on srf, so your only concern is particles in fluid (i haven't had much issue here). I have thought of filtering the srf, but 2 years on the same fluid seems wrong even to me.

 

Rob for perspective i have burned the pad material off backing plates and had the brakes hot enough to bend the backers and still had reasonable pedal feel. With srf i was not temp limited on fluid. I would look hard at caliper temp stickers, your heat might be another source (header near hardline). I also ran forced cooling (fans), but either way i think you can make your setup work with some tweaks. I needed rotor mass because of the cracks i would get thru both sides of the rotor cheeks, if yours are still one piece post race your are pretty close to have this sorted.

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Oh yeah, rotors are not cracked and look pretty good.  There is some discoloration on the rotor face "below" where the pads ride (closer to the hat of the rotor).

 

I run Motul 660 fluid.  I bleed the brakes after every race day or practice day.

 

I have brake temp paint and stickers.  I bought them last year when this all started.  I havent re- painted any of the parts because at the time, everythign was sooo hot that it was above teh temp that the stickers or paint went to....

 

I will repaint and re-sticker for the next track day.  Thanks for reminding me that I had that stuff!

 

DATA is GOOD!

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2 hours ago, Team Infiniti said:

 as we are not allowed to have massively wide nascar calipers/rotors.

 

Why?  Can't I get a wilwood setup for 5 pts per corner?  Do the rules specify which caliper I have to use?

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