Vorshlag 2011 Mustang 5.0 GT - track/autocross/street Project

[kcbrown]

It doesn't absorb water. And that can be a problem. If water gets in the system, which happens when the hot brakes cool off, then the water pools in the low point in the system, the caliper. When it gets hot again, the water turns to steam and abracadabra! you have a soft pedal.

That is most interesting. And it raises the question: how can water be prevented from getting into the system?

It sounds like something like a desiccant barrier between the reservoir and the air would be in order.

This also sounds like it may be another variant of "there ain't no such thing as a free lunch". Is SRF's high wet boiling point what it is because it is so resistant to absorbing water?

 

[SoundGuyDave]

I'm positive that Terry and Jason know this, but just as info to those that don't: brake pad material is an EXCELLENT thermal insulator, and is what keeps the heat generated from a braking event from being transmitted to the brake fluid. Temps at the pad/rotor interface can easily reach 1000*F on an HPDE car, and race cars (with all the driving style change that comes with it) can frequently see upwards of 1600-1800*F. There is no way that a brake fluid that boils at 595*F can withstand that type of heat, which is what you get when you burn the pad material off, and have only a thin layer between the rotor and the backing plate.

Some have tried thin titanium heat shields between the pad and caliper, but from what I can tell, the jury is still out on that particular topic. BEST PRACTICE is to replace the pads when they reach 50-70% wear. I can certainly tell the difference between fresh pads and what's left four hours or so later (endurance racing) just by pedal feel. And yes, the wear rate is NOT linear. The thinner the pad, the faster it wears, simply from heat.

The S197 chassis is a very capable machine, but the factory brakes are absolutely MARGINAL if you push the car. The 14" Brembo setup is a rock-solid performer when coupled with brake cooling, proper fluid, and track duty pads. NOT "street/track" pads, "racing brake pads." The real issue is that we're still getting a 3500-3800 car up to 140+, then asking it to slow down, repeatedly. That generates a LOT of heat, and dual-purpose pads just aren't up to the task for intermediate level drivers or tracks that are known to be rough on brakes (Road America Road Atlanta, for example).

I was recently at a track day, and one of the students (first day evah!!) came out with a nicely done 2013 5.0 'vert. He had FRESH street/track pads with the stock 2-piston brake package. After five 20-minute sessions, he had a bit of an "off," and upon inspection, had not only burned the pad material off the backing plate completely, but had been standing on the brakes so hard that the inboard pad was warped, and had indents from both pistons. He turned his front brakes into a press, essentially. This, of course, extended the pistons far enough to in the caliper bores, and start to crumble from a combination of temp and pressure. This particular track is NOT known for being overly hard on brakes, either. Granted, his braking technique was really not up to snuff (nearly parked in every corner, excessively long braking zones), but the real culprit was the brake pad material.

I'm not trying to say that if you want to check out the corner-carving scene, that you need to immediately dump thousands into your brakes, not at all. BUT, if you're going to track the car on stock brakes, make sure the pads are absolutely fresh, flush the fluid so that you start on new, and then take it easy with the skinny pedal! Inspect the front pads after EACH session, and if anything feels "off" with the pedal, park the car. Now.

Terry, again, you have my sympathies, and I would like to personally thank you for bringing the tech on this failure out into public view. There's a BIG difference between a 15-20 minute TT session and longer, extended runs. When I started racing, one of the habits that I got into was a quick left-foot tap on the pedal to make sure it's up and firm before going into the big braking zones. That might be a habit you'll want to cultivate. I know, locking the barn door, etc... Oh, Castrol SRF is a seriously incredible brake fluid. At $70/liter, though, I just can't afford to run it. The price difference between RBF600 ($28/liter) is staggering, but if you're thinking of moving up to RBF660 ($56/liter) the price difference kinda starts to shrink a bit, particularly with the extended service intervals. I would look hard at the SRF if you're at the point of blowing through RBF600.

 

[Dubstep Shep]

Well said Dave.

This is a pic I keep around to show people how ridiculously thick F1 brake pads are, and how well set up those braking systems are.

 

[sheizasosay]

Can you feel when pad knock back happens?

 

[SoundGuyDave]

No, you can't. You CAN feel the results, though. Pad knock-back is caused by flex in the spindle and bearings, allowing the rotor to slightly out of true relative to the centerline of FIXED calipers. This slight mis-alignment causes the rotor to compress the caliper slightly, and once you get straightened out, and go to hit the hooks in the braking zone, the initial pedal application then has to push the pad back out to make contact with the rotor. Symptoms are excessive INTERMITTENT pedal travel on initial application, but otherwise perfectly fine brakes. Nice hard pedal, good bite, etc. Just a long pedal once in a while on initial application, usually after a fairly high-load corner, or where you hop the rumbles.

There are some strategies, like anti-knockback springs inside the caliper to keep the piston out and the pad in contact with the rotor, but that also causes other issues. The simplest, easiest, and IMO best solution is to move your left foot onto the brake pedal and GENTLY press on it. You're not trying to slow the car, since your right foot is trying to push the gas pedal through the floorboards, all you want to do is give a "confidence check" to the brakes. Is the pedal hard and high? Cool, good to go for the braking zone. If the pedal drops on you (due to knockback), tap it again with your left foot; you'll have already taken the "slack" out of the system with the first tap. Good now? Cool. Not Good? You've got issues, but now you have time and distance to work them out. If you're late-braking into a corner, you simply don't have the time/distance to get on the pedal, realize you have a problem, then pump it to actually slow down. Far too late. The confidence check simply puts you ahead of the car, rather than reacting to it.

 

[csamsh]

Well said Dave.

This is a pic I keep around to show people how ridiculously thick F1 brake pads are, and how well set up those braking systems are.

I have looked and looked for this picture. Saving for future use! Thanks!

 

[csamsh]

That is most interesting. And it raises the question: how can water be prevented from getting into the system?

It sounds like something like a desiccant barrier between the reservoir and the air would be in order.

This also sounds like it may be another variant of "there ain't no such thing as a free lunch". Is SRF's high wet boiling point what it is because it is so resistant to absorbing water?

Yarp. I'd be interested to see a structure of SRF. I'm going to guess that water is either miscible with or soluble in "regular" brake fluid, and has some sort of differing polarity or what have you from SRF, and instead of the water being diffused through the fluid, there's a bolus in the SRF. I'm also going to guess that brake fluid is organic and has a relatively low specific gravity (unless it's halogenated), so the water pooling in the bottom of the caliper makes sense. You only need 100C to boil a drop of water...so while SRF has a high wet boiling point, that's not the worry, as it doesn't get wet.

Water is unfortunately in the air. I would be interested to see a "desiccant cap," but there would need to be a rock-solid system for not letting solids get into the fluid. Anybody got any magnesium sulfate lying around???

 

[Sky Render]

Well said Dave.

This is a pic I keep around to show people how ridiculously thick F1 brake pads are, and how well set up those braking systems are.

Dude, if you're ever at a Le Mans race, check out the brake pads on those cars. They're even thicker, because they weigh substantially more than a formula car!

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[Dubstep Shep]

Dude, if you're ever at a Le Mans race, check out the brake pads on those cars. They're even thicker, because they weigh substantially more than a formula car!

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I bet. Those calipers must be MASSIVE.

I would also gamble F1 has some tech that allows them to use a thinner pad, but I'm not sure.

 

[Bob]

Thanks for the braking update... perhaps I'll be more careful when starting a track day with 50% pads.

 

[claudermilk]

I'm not trying to say that if you want to check out the corner-carving scene, that you need to immediately dump thousands into your brakes, not at all. BUT, if you're going to track the car on stock brakes, make sure the pads are absolutely fresh, flush the fluid so that you start on new, and then take it easy with the skinny pedal! Inspect the front pads after EACH session, and if anything feels "off" with the pedal, park the car. Now.

Taking car of that this weekend. Vorshlag lines, RBF600, and Carbotech Bobcat pads going on. XP10/XP8 pre-bed pads waiting in the wings for track duty. Spare Centric rotors on deck just in case. I think I'm about $900-ish into brakes right now. Next up will be cooling ducts.

I am definitely filing all this info away for reference, and am off to educate myself some more on knockback.

 

[SoundGuyDave]

Knockback: You won't generally get it with floating calipers (like the stock 2-piston set, that rides on slide-pins), but you might on fixed calipers, like the Brembo, Wilwood, StopTech, etc.

Assuming you have fixed calipers, try this trick at home: Take off a front wheel, then grab the bottom of the rotor and pull outwards. You won't be able to move it much, but that angle that it's creating inside the frame of the caliper will push both the outboard and inboard pads outwards just a little bit. Now, push the rotor back into normal position, and slide a few pieces of paper between the pad and the rotor, or use a feeler gauge to get an idea of how much the pad moved. Next, hop in the car, fire it up (vacuum for the booster) and step on the brake pedal in one, fluid stroke. You'll notice that the first application of the pedal will be VERY long. While this is happening, you've applied 100% braking force to the rear brakes, but almost nothing to the fronts, since the fluid displaced by the piston in the master cylinder has been used just taking up the slack up front... Finally, hit the brakes again. Notice how your pedal is back to normal? THAT is what pad knock-back is all about.

Oh, remember to put the wheel back on and torque your lugs!

Again, the cause of knock-back is flex in components that are supposed to be rigid. In particular, the spindle and the wheel bearings. To get the spindle to flex requires some severe loading, like very high-G cornering loads, or a pretty stout berm strike:

The other common culprit are the wheel bearings. As they start to wear, they allow the hub and rotor to kind of flop around, but the caliper doesn't move, and as a result, you can get knock-back. If you experience knock-back on a fairly regular basis, it sounds to me like it might be time to replace the hub assemblies. And yes, those are known maintenance items on the S197 racers.

 

[Sky Render]

Did the factory "GT500" 4-piston Brembos experience knock-back?

 

[SoundGuyDave]

Absolutely. The GT500 Brembos are nothing more than a custom casting of one of the F-series 4-pots, that incorporated the radial mounting bracket into the casting. The aftermarket calipers use a stock, off-the-shelf 4-pot caliper bolted to a vehicle-specific mounting bracket that in turn bolts to the stock caliper bracket holes in the knuckle. All Ford did was have Brembo make a special casting that combined the caliper and the bracket into one piece of metal.

Knock-back is NOT common, at least on HPDE level S197s. When you get to TT or in the race groups, it becomes more common, but it's still not something that you'll see with any regularity unless your hubs are toast or you're REALLY whacking the berms. Trust me, this is a RARE thing to experience. In my years of TT and racing, so far I've had it happen exactly once, and that was due to failing wheel bearings, NOT berm-hopping (which I do) or high-load cornering (which I also do). It STILL makes sense just to give the brakes a left-foot tap before getting to the big braking zones, though.

 

[Sky Render]

Ah, I see. I always thought it was more of a problem when using aftermarket brakes on the rear axle.

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[csamsh]

Ah, I see. I always thought it was more of a problem when using aftermarket brakes on the rear axle.

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It is. The c-clip axle has play in it, and will knock any fixed setup literally every time you'd turn.

 

[Dubstep Shep]

It is. The c-clip axle has play in it, and will knock any fixed setup literally every time you'd turn.

Would that cause a wobbling noise when cornering with track pads and factory calipers?

 

[Vorshlag-Fair]

I thought the whole point of SRF was that it needed to be flushed/bled less frequently, due to being a lot less hygroscopic.

Well yes, it is less hygroscopic, but... that means that when water does get into the system (and it will) it separates and falls to the lowest point in the system, which is right at the calipers. Many race teams that use SRF will push a little fluid out after every single session on track, as there's going to be a little water at the bottom of the system each time. The more hygroscopic fluid absorbs this small amount of water vapor into the fluid, and eventually it needs to be flushed out. The Motul 660 is more hygroscopic than their RBF 600, so the maintenance schedule is higher for flushing. But we already flush and bleed the brakes a bit before every event, so its not a big deal to us.

• Street/track cars: Use Motul 600
• Track only cars: Use 660
• Professional race teams with a crew that attacks the car every time it comes off track: Use Castrol SRF

That's my story and I'm stickin to it.

Then.... Captain Obvious chimes in....

My orange silicone hoses are smooth bore on the inside....and corrugated on the outside. I thought they all were constructed that way?

Flexible brake duct hose isn't even remotely smooth bore, even the ones you think that "have the wire on the outside". Anything flexible is going to make for turbulent, restricted air flow compared to smooth bore rigid tubing/pipe/hose.

We have upgraded the front of our car to 4" hoses and ducts (today) which, while still having 100% flex hose, will flow more cooling air than 3" flex hose. Going to aluminum tubing wasn't practical in the space constraints we had on this car, and the time available to do the work this week.

And due to the data we've seen directly (see temp strip indicators on calipers) we know the rear calipers aren't seeing anywhere near enough heat to need brake ducting, as of yet. They are getting to barely 190°F, whereas the fronts are seeing upwards of 475°F.

I think you said you are using 4 piston brembo's with 14" rotor's ? Dunno if 6 piston caliper's would fare any better. 15" rotor's are used on the newer GT-500's.

You don't say????

Look, I know this - I own a a freagin shop that works on Mustangs!!! Everyone here on this subforum knows what brakes are on a GT500. We're not some bunch of noobs on some scab forum. But the kicker is this: 15" rotors don't fit inside almost any 18" wheels. I have many reasons NOT to "upgrade" to 19" wheels, which I have posted about dozens of times.

I know you're new here, but you keep posting the most obtuse things and its getting a bit ridiculous. My suggestion is this: READ MORE, POST LESS.

What about the rear brakes ? With heavy springs etc, minimizing nose dive vs oem suspension, on paper, more of the load should transfer to the rear brakes. The oem 11.8", 1 piston brakes are probably not up to the task. Some folks have installed 14" rotor's on the rear....

Kid, we kind of pioneered the 13.8" GT500 swap on these cars. We were one of the first shops to offer this as a kit for the cars with the 11.5" rear rotors - and of course we tested it on our car first.

Again, don't take this the wrong way, but: READ MORE, POST LESS.

 

[Vorshlag-Fair]

Knock-back is NOT common, at least on HPDE level S197s. When you get to TT or in the race groups, it becomes more common, but it's still not something that you'll see with any regularity unless your hubs are toast or you're REALLY whacking the berms. Trust me, this is a RARE thing to experience. In my years of TT and racing, so far I've had it happen exactly once, and that was due to failing wheel bearings, NOT berm-hopping (which I do) or high-load cornering (which I also do). It STILL makes sense just to give the brakes a left-foot tap before getting to the big braking zones, though.

Good tips and data from Dave, as usual. I was wondering if this was knock-back that caused the TOTAL loss of pedal, as I had a firm pedal braking into Turn 7 right before the L-O-N-G back straight. Pro racer Jason Saini suggested the same cause (knockback), and the same cure: a little tap of the brake pedal with the left foot on long straights just to re-seat the pads. Its a trick I'm going to employ in the future, for sure.

But as Jason mentioned the brake pedal was still MUSHY AS ALL HELL when we drove the car a week after the accident. The guys here flushed out all of the Motul 600 and replaced it with 660, and the pedal is perfect again. And yes, the front pads had less than 1/4 of the material thickness left (see picture below!), and the rears were just as bad.

That didn't help matters, of course. And while I have used SRF on some customers' cars, its not the right fluid for me. I'm a Motul dealer, and I try to race what we sell, and I kind of like a little hygroscopic aspect to the fluid. The DOT5 silicone stuff has never really been my favorite brake fluid.

The titanium shims between calipers and pads have been suggested by many, and I've seen all sorts of tricks like this over the past 27 years of doing track events... but none of it works all that well, in my opinion. It might barely slow down the heat transfer but once that heat passes into the calipers they'd act like an insulator. Just like the phenolic upper intake manifold spacers people used to use on 2-piece aluminum intake ... it worked great for a very short period (drag racers), but then it became an insulator, trapping heat where you didn't want it. No, I've just got to go out on track at these brake intensive tracks full thickness, and leave those 1/2 and 1/4 thickness pads for "less critical uses" like autocross or whatnot.

And while I did jump up pentalab's ass for his silly observations (he has been doing this all over this forum, and I'm not the only one noticing) I do appreciate all of the tips, suggestions, tricks and advice from the more experienced folks. And anyone is free to ask questions of us here anytime, in this thread or elsewhere. That's why we're here. I don't hold anything back, and this accident was totally avoidable. So please, learn from my mistakes!

We're testing the 4" brake ducts (fix #1, see below) tomorrow at a brake intensive track - Eagles canyon Raceway. Amy is driving, as my broken bones are still healing for at least 4 more weeks before I can get back in the car. I'm hoping by NASA @ Hallett in late June that I'll be able to drive, to defend both of my track records there (TT3 going CW and CCW)! We'll see. I'm going NUTS not being able to race... or go to the gym, or lift more than about 5 pounds, or lay down flat or bend over without serious pain. GRR!

I'm also going to be sticking temp strips on a number of other Mustangs' Brembo calipers tomorrow at ECR, to check their temperatures vs their pad material vs their track times. Could be some good data to share!

Mustang is all fixed now: it has a new bumper cover, one new plastic side skirt, brand new XP20 pads and 660 fluid, and the bigger brake ducting. It is all loaded up and ready for track abuse once again (we will be at ECR at 7 am tomorrow). The splitter took almost no damage, and with a new bumper cover and some mounts + some patches to the flares it is all good to go.

Cheers!

 

[Dubstep Shep]

Got two questions for you real quick:

Isn't turbulent air flow better in terms of air resistance? Hence why golf balls have dimples? Laminar flow is quieter and more predictable, but I thought for maximum air flow turbulent was better?

Why not use the Wilwood six piston brakes? They advertise that they fit under a 18" wheel.