924Board.org

[John Brown]

We have been running the purple #55 924 in 44Cup. Not competitive but fun. A data point for those interested in the performance with the 'little' stock 924 brakes. This at Summit Point. 2miles a long front straight (I dunno - say 110 at the brake point). 2 major braking zones and 2 taps. Weight 2400lbs with driver. The original 14 inch wheels with 205 Hoosiers.

Running in the 1:39-1:40 range the brakes are fine.

Today we upped it to 1:36.5-1:38. At this level the brakes go away after about 3 laps. It's pad fade from heat. Not fluid Using Carbotech 9 compound in front and 'Whatever' on the rear shoes. The rear shoes also cook pretty quicly showing up not as reduced braking but the come apart and so destroy the pedal feel.

We are interested in continuing with the 'light weight' setup. So the plan is to have rear shoes made up with a decent compound. Not for the braking but just to withstand some heat and stay together. And front cooling ducts. And, perhaps, different front compounds. I also plan to keep a close eye on the front wheel bearings and grease.

Another problem is the amount of play in the rear axles. I have NEVER found a 924 that did not have 'excessive' play. If someone knows a solution I'm all ears. This play causes accelerated wear of the rear brake shoes.

The upgraded brakes, obviously, would completely eliminate this problem. But they also entail a weight penalty in themselves plus the larger wheels and tires.
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John
80 931 - #931 44Cup
99 Escalade - tows track cars
gone but not forgotten: original 924.org car - 82

[NEMESIS]

John, have you tried the Porterfield shoes for the rear?

[924RACR]

Another option, if simply changing lining material doesn't do enough, might be to replumb the circuitry and use a prop valve for the rear, dial it back till they start to live. But yes, I'd definitely start with race linings in the rear first.
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Vaughan Scott
Webmeister
'79 924 #77 SCCA H Prod racecar
'82 931 Plat. Silver
#25 Hidari Firefly P2 sports prototype

[John Brown]

[John Brown]

[NEMESIS]

[924RACR]

Yes, big rear swaybar will make wheelspin much worse. I am running a 951 LSD. Go with the 400#/30mm setup - it'll definitely be better.

I was suggesting a prop valve not that you need less rear brake for balance, but just to keep the rears from doing so much (and thereby dying an evil death - if they still overheat with a race lining).

PS - you could also stop using the brakes so much... after all, they just slow you down anyway!
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Vaughan Scott
Webmeister
'79 924 #77 SCCA H Prod racecar
'82 931 Plat. Silver
#25 Hidari Firefly P2 sports prototype

[Cedric]

How long are you driving your races ?

3laps sounds like something´s wrong. Or maybe a crap shoes. I drove almost 2 seasons on my rear standard shoes, running on R compund tires ( i dont know what you call them over there),track days, autocross and some street. Not so much wearing until i drove 1h stints. But they never faded. Even when i drove tracks that are hard on brakes (170-180km/h braking). I actually got a stiffer pedal when heating things up. But that was after chaning to steel braking lines. The original ones got real spongy when things got hot. I´m running hawk hp plus pads in front.

But i think cooling ducts will be nice.

What dou you meen with "play in the rear axels" ?
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1980 924 Turbo
www.instagram.com/garagecedric/

[John Brown]

Well, the shoes are crap. Just whatever no-name VW/Audi shoes the import parts store had.

It wasn't until we really got to running the car hard this year that it developed the rear shoes were going to get beat this much. So only now did it become imperative to find a sturdy replacement. I appreciate the info on Porterfield. Interesting, appears Porterfield also makes Hawk, Raybestos, and PFC.

Every 924 with which I have been acquainted has had some (excessive) radial play in the axle arising from the loose fit of the outer roller bearing. Whether race to hub, to axle, race to rollers - actually I think all of the above. Some worse than others. On this particular car the right rear is worse and this exacerbate the brake wear on the that side.

The 3 laps or so has nothing to do with the shoes. Its the front pads & discs (which are doing all the work) that go away. This is with Carbotech 9 compound. Roughly equivalent to Hawk blue or black I think. Appeared only at the end of the season when the pace went from 1:40ish to 138 and below. The quicker pace simply requires later and harder braking at a couple of places. I actually sorta expected this.

Any brake engineers have any critique of our intended plan?
_________________
John
80 931 - #931 44Cup
99 Escalade - tows track cars
gone but not forgotten: original 924.org car - 82

[NEMESIS]

I know it's considered cheesy, and others may disagree with my thinking, but have you given any thought to trying drilled rotors? I can't find the link but I know Brembo does make them (or did a year or so ago)

Just trying to think of any little advantage you might be able to pick up in the cooling department up front. Ducts will of course help a lot.

[Paul]

From an old post by 924RACR

http://www.924board.org/viewtopic.php?t=5155&highlight=drilled+rotors+engineer

Posted: Wed Nov 06, 2002 11:23 am Post subject:

--------------------------------------------------------------------------------

Not a racing engineer - unless you mean an engineer that races - but rather a brake engineer. Work for Bosch. Lizard's pretty much given the executive summary of what's been said in the past.

Of course, I don't stand to make any money off of them, so I'm not going to be all hot for them.

The claim is generally that they improve cooling, if anything is said. From what I've seen recently, they don't even make any much more claims like that. Anyway. There is no improvement in airflow for cooling with cross-drilled rotors. They don't make enough of a diff in mass to improve rotational inertia.

Alright, down to the bare bones, I worked on a project for a car that was supposed to have cross-drilled rotors stock. They finally had to toss the idea (late enough in the project to really screw us up in ABS/TCS/ESP development) because they had suffered too much of a loss of brake output (Newton-meters of torque per bar of brake pressure aka Nm/bar) with the cross-drilled holes. Another lower trim level of the car had the identical brakes (and otherwise identical car) with the same rotors, not drilled. Same pad material in both cars. You can't get a more back-to-back comparison. That lower level car had no brake output issues, it had a higher output with no holes.

In driving the car for 2 years in development, we ate pads, they groaned and grumbled horribly, never really stopped well (and was scary when we took it on track and got the brakes hot - horrible brake fade)...

They won't help you any. Your money is better spent on good quality pads.

If you can list more detailed reasons why you might want them, I could perhaps tell in more detail why they're no help (and what you should do), but the usual explanation is fade resistance, to which I answer - stock rotors with pads appropriate for your application are all you need. I run old stock rotors with full race pads, and have never experienced brake fade on any track. Without having to add cooling ducts. A good "semi-met" autox type pad works great for the street; I use the PBR/Axxis Metal Masters for my street cars and really like the pedal feel and bite, and they warm up pretty quickly.
_________________
Vaughan Scott
Webmeister
'79 924 #77 ITB racecar

[Paul]

Here's some more info:

http://www.924board.org/viewtopic.php?t=4694&start=10
_________________
White 87 924S "Ghost"
Silver 98 986 3.6l 320 HP "Frank N Stein"
White 01 986 "Christine"
Polar Silver 02 996TT. "Turbo"
Owned and repaired 924s since 1977
Porsche: It's not driving, it's therapy.

[924RACR]

Yeah, just say no to drilled rotors!

Regarding options; I think you're basically in the right track (was just offering up other options on the rears). Let's review the symptoms, just to make sure we're tackling it properly (though I have a hard time believing you wouldn't need front cooling ducts).

If the fronts are fading from heat, you've got the same solid pedal, but you just don't have any decel to go with your pedal apply. Does this match what the driver saw in-car?

Because if you're getting a long pedal, but still get decel, you've either got knockback (not likely in this case as a function of increased pace) or you're boiling the fluid.

Also, as was raised, you are using stainless braided brake hoses, right? I would never hit the track without them.
_________________
Vaughan Scott
Webmeister
'79 924 #77 SCCA H Prod racecar
'82 931 Plat. Silver
#25 Hidari Firefly P2 sports prototype

[924RACR]

PS - this is a classic example of how a car must continue to develop as the driver develops; a setup that works at 1:39 turns to crap as you go to a low 1:38! I've been working through the same issues on my car; used to be fine, but now that I'm pushing into the 1:19's at Waterford, inadequacies come to light.
_________________
Vaughan Scott
Webmeister
'79 924 #77 SCCA H Prod racecar
'82 931 Plat. Silver
#25 Hidari Firefly P2 sports prototype

[John Brown]