Toasted Brakes at a Lapping Event - Need Advice

[SoundGuyDave]

that is one technique, braking late and deep into the corner, past the turning pint can be used if to gain traction when your brakes are up to it. This will get you a faster entry speed and more cornering grip.

Not exactly. You're talking about trail-braking. Yes, it will gain you entry speed, however it will NOT gain you cornering grip. Basic "friction circle" theory tells us that we have finite mechanical/aero grip. We can use 100% of it for braking, or for cornering. When you do both, however, the sum of the forces cannot exceed 100%, thus any braking will REDUCE the net amount of grip available for cornering. TANSTAAFL. The way trail-braking provides gains is in the margins. A smooth corner entry doesn't suddenly transition to grip limit laterally, it does it over a finite amount of time, and during that time, you are applying less than 100% of the available grip to cornering. Trail-braking uses that marginal unused grip for additional braking, past the turn-in point, in exactly the same way that you start accelerating on corner-exit before the wheel is completely straight. A wee bit too much braking after turn-in, and your rear end starts to come around, though, since at that point, you've exceeded the total tractive capacity of the tires. Done judiciously, using the brakes to initiate rotation, then catching it with the gas pedal to maintain attitude, can and will allow you to increase your exit speed, however the point still remains that you only have so much grip available for cornering, and that WILL determine your mid-corner maximum speed.

Please note that this technique is seriously advanced-level, and cuts the margin for error right to the bone. A slight reduction in grip (sand/leaves/spit on the track, tires starting to "grease up") will cause you to either understeer past your apex or transition to oversteer too quickly. The first will result in a 2-off or 4-off situation, the second in a spin.

Done properly, trailbraking will result in the net fastest time through a corner. Roughly 90% of the braking is done in a straight line, with consistent force, then the remaining 10% (-ish) is done from turn-in to apex, with braking force being reduced at EXACTLY the same rate that cornering force is increased while the car takes a set, and rotation is begun. At the apex point, you will be at maximum lateral load, and no other forces can be applied. As soon as you pass apex, however, and start to open the wheel back up, you can then start to apply forward acceleration. The net effect is that there is a VERY brief (1/4 second?) period of time when the car is in pure cornering mode, but the rest of the time through the corner you're also either braking or accelerating. This does NOT work will with classic late-apex lines, you'll have to be running more of a geometric line (earlier apex).

In sum: Geometric line reduces your margin for error but increases speed at apex. Trail-braking on entry reduces your margin for error, but increases speed at entry. You can shave a few thousandths of a second per corner by doing this, but WHEN (not if) you bobble, it will cost you tenths of a second or more.

I'm not saying that you can't do it, but you need to think of when it's appropriate, and you need to be a VERY consistent driver before trying this on any sort of routine basis. If you're not clicking off lap times within a few tenths for an entire session, then you don't have the precision necessary to successfully execute this with a predictable outcome. Small changes (>3mph and 2-3' at turn-in, 1' at apex, 5% variation in braking pressure for example) can seriously impact the traction requirements for a given corner. Because you're operating right at the limit, it's difficult or impossible to recover from a bobble gracefully.

 

[kon5t]

Not exactly. You're talking about trail-braking. Yes, it will gain you entry speed, however it will NOT gain you cornering grip. Basic "friction circle" theory tells us that we have finite mechanical/aero grip. We can use 100% of it for braking, or for cornering. When you do both, however, the sum of the forces cannot exceed 100%, thus any braking will REDUCE the net amount of grip available for cornering. TANSTAAFL. The way trail-braking provides gains is in the margins. A smooth corner entry doesn't suddenly transit- SNIP .

Yep you are correct! You are also correct in pointing out that this is not something you should try unless you have know what you are doing.

I must however disagree about one aspect of your statement, if you are trailbraking, the center of gravity moves over the front wheels causing more more traction, and hence increasing grip (at the front). Admittedly it also unloads the back end, so you really do have to know what you are doing.

 

[martin_nj]

the center of gravity moves over the front wheels causing more more traction, and hence increasing grip (at the front)

this is true but also false because you're using that added grip to slow the car not turn it

therefore i would argue you don't gain any additional turning capacity

 

[kon5t]

this is true but also false because you're using that added grip to slow the car not turn it

therefore i would argue you don't gain any additional turning capacity

I wish i could argue with that

 

[Norm Peterson]

Actually, the CG does not move forward under braking at all (unless you're looking at the effects of forward fluid slosh in the sump, transmission, fuel tank, and parts of you in the harness).

It's the forward load transfer that dynamically adds vertical force at the front tire contact patches. As SGD has pointed out, trail-braking is not an inherently stable condition. It requires expert control inputs to stay in what looks nicely balanced from the outside.


Norm

 

[908ssp]

In any case weight transfer doesn't give you more than 100%. And Dave said 100% so whether you throw in weight transfer into the discussion is moot.

And to add to what Norm said trail braking is hard to master so it might not be the fastest way around if you keep ending up in the grass.

 

[kon5t]

Actually, the CG does not move forward under braking at all (unless you're looking at the effects of forward fluid slosh in the sump, transmission, fuel tank, and parts of you in the harness).

It's the forward load transfer that dynamically adds vertical force at the front tire contact patches. As SGD has pointed out, trail-braking is not an inherently stable condition. It requires expert control inputs to stay in what looks nicely balanced from the outside.


Norm

it is actually the Center of Mass that moves, thank you for correcting my terminology

 

[martin_nj]

trail braking ... might not be the fastest way around

regardless of difficulty it most certainly is not the fastest way around. depending on conditions and car setup however it may prove beneficial on a particularly tight corner

in a racing scenario trailing your brakes into a corner helps you to out-brake an opponent and get the car rotated but you sacrifice exit speeds which are where fast laps are set.

in a time trial or qualifying lap you will not see anyone trail brake unless their car is understeering too much... either from tire wear or poor suspension setup

 

[kon5t]

regardless of difficulty it most certainly is not the fastest way around. depending on conditions and car setup however it may prove beneficial on a particularly tight corner

in a racing scenario trailing your brakes into a corner helps you to out-brake an opponent and get the car rotated but you sacrifice exit speeds which are where fast laps are set.

in a time trial or qualifying lap you will not see anyone trail brake unless their car is understeering too much... either from tire wear or poor suspension setup

As Martin_NJ points out, it allows for quicker rotation of the car, which in some corners may be quicker than the pure race line, the classic one would be Adenauer Forst at the Nuerburgring in Germany (very complex corner, check out some of the great Youtube clips of it)

The main difficulty is pulling the technique off, get it wrong you will spin or plow understeer, or worse (don't ask how I know- 3 bent AC-Schnitzer wheels on my old M3)). Feels like a million bucks when you get it right.....

 

[Sam Strano]

The term trail braking gets tossed around a lot, and I find that it's used to describe very different things. What JAJ described is trail braking, or trailing off the brakes as you enter the corner. And if you are fast you pretty much will do this to some degree because if you come off the brake before turn in, you unload the nose and it won't want to turn very well.

That said, you can't just keep standing on the brakes like kon5t is calling trail braking. That's more what I'd call dive-bombing. Maybe he stated it poorly, but when he says "braking late and deep" I interpret that as doing a lot of braking from entry to apex. That you don't want to do with any regularity. While you will get to the middle of the corner faster, you won't carry more mid corner speed in fact you'll have less and less exit speed because you've been using traction to stop and turn vs. mostly just turn.

It's this simple. Tire can do X amount of work, if you force it to do X + Y, it's not happy. The more you turn, the less you can brake, the more you brake the less you can turn. If you can truly trail brake, you roll off braking force as you pick up cornering force. And fwiw, it's pretty famously known that this little trick is hard to get right... I think it was Andretti that said it's the last thing that people really learn to do. And for the love of God you don't want to "pop" off the brake pedal, because again you suddenly unload the front and that's not what you want. You just ramp up turning as you ramp down braking. If you are braking the whole way to an apex, it better be a pretty slow corner or you are wasting time.

Now, what this has to do with the OP cooking brakes, I'm not sure. I think you need to upgrade your brakes. I happen to know a guy that carries that kind of stuff (that's me). Because no braking technique will make the car weigh less, or not have 400 hp, etc. Maybe you are doing something that's a little harder on the brakes than normal, but still when you consider the power that the brakes generate (and that energy is turned into heat) it's not surprising you need to upgrade pads, fluid, etc. Car stops from 60 a lot faster than it can accelerate to it, and we aren't lacking HP.

 

[ddd4114]

Yeah, this thread got a little off topic, but I don't mind.

To beat a dead horse: kon5t, the center of mass/gravity never moves in a car except for fluid slosh/consumption (as Norm mentioned). The loads on the tires just change due to the moments created about that fixed CG. When you brake, a CCW moment is created about the car's CG, so a CW moment must react against it. That CW moment is what decreases read tire load and increases front tire load. It's not caused by any sort of CG migration.

I just did an HPDE at Road America, and I'm fully convinced that I need a front brake upgrade for exactly the reasons Sam mentioned: 1) my car weighs 3770 lb with me in it, and I'm not going to lose a significant amount of weight while keeping the car streetable, 2) I'm not decreasing power or increasing drag, and 3) I'm sure as hell not going to drive slower.

This time, I used 0.040" Ti backing plates on the front pads, and I turned TCS/ECS completely off. I was expecting to wrestle with the car a lot more, but if anything, I think that made the car easier to drive. Kudos to Ford for making a behemoth of a car with a live rear axle handle so predictably off the showroom floor.

The end result was cracking brand new front rotors, destroying brand new front calipers, and going through two full sets of DTC-60 brake pads. I also had lots of pad fade, and I'm pretty sure I started boiling the brake fluid (ATE Superblue). More cooling will certainly help, but no matter what, I will need to upgrade the front brake eventually if I want to continue tracking this car. I don't think there is any way around this. Time to look for a big brake kit...

 

[SoundGuyDave]

Interesting... I just ran a 3-day weekend at RA: one HPDE day, one day of W2W racing, and one day with two race sessions and a 3-hour enduro on roughly 1/2 set of DTC-60 pads. Granted, there are LOTS of differences between our setups, but I strongly suspect lack of proper cooling to be the main culprit in your issue. Cracking rotors comes from heat differentials unevenly contracting and expanding the rotor surface until the casting lets go, so it sounds like you got them WAY hot, further supported by your comments of boiled fluid with a race-grade fluid, and pad fade with a friction material rated to 1600*F. That all points to excessive brake temps.

We all agree that if you don't dump weight, won't decrease power, and can't slow down, you will need to get rid of that heat somehow. Cooling ducts, man, it's the answer! And before you say it, yes, a BBK of some sort will certainly also help, but only by increasing the thermal capacity of the front rotors. Without cooling, you're still going to be cooking things.

 

[ddd4114]

Yeah, I thought about that too. The BBK is basically delaying the inevitable.

I already have 3" brake ducts, so I'm not what else I can practically do to reduce brake temperatures. Putting decent fans into the ducts might help a bit.

I think two-piece rotors will really help deter cracking in my case. However, they're mad expensive, and I'll probably still have to replace them at regular intervals, so I'm not sure if they're really worth the extra cost considering they're also harder to get in a rush. Calipers that can be easily rebuilt will also considerably reduce my maintenance costs, so that's very appealing to me.

What's interesting is that the only times I really noticed the fade was entering turns 1, 5, and 12. On most other corners, I didn't have much of an issue. Turn 1 was ~145-65, turn 5 was ~135-45, and turn 12 was ~140-55. No matter what, that's gotta be hard on the brakes when I'm doing it lap after lap.

The brakes would start off feeling fine, but just before turning in, the pedal would sometimes start to sink, and the car would start to decelerate less. Of course, because I was experiencing fade, I started braking earlier for corners and using less pedal force, and that probably wasn't helping. I'm wondering if a BBK will reduce the temperature rise enough to keep things from overheating, and then (ideally), the brakes would cool down enough by the next corner to keep the temperatures in check. That's my goal, but I'm just speculating at this point.

Dave, I'm amazed you ran that 3-hour enduro without going through even one full set of brake pads. What are you doing for cooling? I spent the first hour of your endurance race replacing my front rotors...

EDIT: For what it's worth, I put 95 min of track time on the first set of DTC-60 pads and 85 min on the second set. I noticed that each set had turned white when I pulled them out of the mounting brackets.

 

[SoundGuyDave]

Hell, if you were up there, you should have stopped by and said HI!!

My cooling package is really pretty simple, but it works. 05-09 front fascia, cut out the outboard bits of plastic on the fascia, add Butler Bilt PVC ducts inside the fascia. 3" hose STRAIGHT back along the frame horns, smooth 90* transition outboard to FullTiltBoogieRacing.com duct plates. These seal the inner area of the GT500 rotors perfectly, directing all of the cooling goodness into the vanes, with nothing wasted, and absolutely no coverage of the inner face of the rotor.

They're not easy to see, but if you look at the extreme corners of the lower fascia opening, you can see the ducts there.

http://www.butlerbuilt.net/products/air_mgt_systems/BBP-7022_Headlght_Mount_Brake_Duct.jpg

I'm 99% sure that's them. I think one of the overlooked keys to cooling is in managing the airflow properly, not necessarily getting enough to start. If you have multiple bends in your hose routing, particularly "S" bends, you're shooting yourself in the foot. Now, granted, I don't have a washer bottle, AC Condenser, or any other generally useful street equipment in this car, but I'm pretty sure you can find a routing that doesn't compromise the straight-line shot too badly.

 

[ddd4114]

I would have stopped by, but I was too busy fixing my brakes!

I'm using the Laguna Seca brake duct kit on my car, so there is basically just one ~70 deg bend. The washer fluid tank and horn do put some small kinks in the ducting though.

I think I'm going to start staking a thermocouple into one of my calipers next season to see what's really going on temperature-wise. Then I can start playing around with ducting to see how much I can improve it. Otherwise, I'm more or less shooting in the dark.

 

[OkieSnuffBox]

I started braking earlier for corners and using less pedal force, and that probably wasn't helping.

No, it wasn't.

On brakes, threshold, off brakes. Using the brakes to slow down on the track, like you do on the street increases the time you are using the brakes and raises the temps.

Could also do some rotor paint to judge temps.

 

[jymontoya]

Seriously?? Still?

Why go through so much trouble trying to get the stock setup to work, when there are proven setups available?

Just get the Brembo kit and be done with it! $999 for the FRPP Brembo Upgrade kit from: http://www.gefracing.com/

or one of the rebranded Stoptech (Saleen, Roush, Steeda)...

 

[DUFUS]

I think two-piece rotors will really help deter cracking in my case. However, they're mad expensive, and I'll probably still have to replace them at regular intervals, so I'm not sure if they're really worth the extra cost considering they're also harder to get in a rush. Calipers that can be easily rebuilt will also considerably reduce my maintenance costs, so that's very appealing to me.

What's interesting is that the only times I really noticed the fade was entering turns 1, 5, and 12. On most other corners, I didn't have much of an issue. Turn 1 was ~145-65, turn 5 was ~135-45, and turn 12 was ~140-55. No matter what, that's gotta be hard on the brakes when I'm doing it lap after lap.

What lap times are you running with speeds like that? Just curious. I struggled with the stock brakes on a 2011 GT (using DTC-60 also), but never to the extent of what you're experiencing. I just got tired of the nannying/maintenance required, but never had pads wear quite that quick or a cracked rotor. I don't think I ever was going that fast at RA though. Not at that time at least. I think it's a losing battle considering the weight of these cars, and potential of the 5.0. Even with the bump up to 13" rotors, you're just asking too much of the stock brakes at the track.
I've had the GT500/Brembo stuff for this year, and I can say that you won't experience ANYWHERE NEAR the wear rate. That will be a non-issue. Unfortunately, I'm experiencing bad vibration/shudder after a few sessions every damn HPDE though, and am considering 2-pc rotors. Maybe I'm special and unique, just like the doctors always told me, because I don't find too many people with the same problem. So I'll continue to experiment with other brands of 1-piece rotors, tweaking here and there, and different braking technique, etc. to see if I can't cure that before the expense of 2-pc. We'll see what happens at Road America this weekend to close my season.
BTW, if you shop around you can find the calipers and rotors for much less than the "kit" prices.

 

[martin_nj]

I think a big question everyone should be asking is which tire you're running.

Compound and width will play into how much heat you're able to generate

Your braking technique will also dictate what is going on down there..


For perspective -

Ran my 3800ish car on 245 Dunlops (200 treadwear summer tire) on a cold semi wet track with DTC 70s

Then I ran the same car on 285mm conti slicks for over 130 laps

Then I ran the car on the 245 summers again but only for 37 laps before I noticed that the pistons in the calipers were crumbling and the DTC70s were down to their backing plates.

Besides the pads and Motul600 fluid the brakes were factory OEM.


For my next outing the plan is Ducts, motul600 fluid, 1mm Ti plate and raybestos st43 pads

 

[ddd4114]

jymontoya, I replaced the parts I toasted with OEM components because I couldn't afford to get a BBK and new wheels/tires right away. I know how ironic that is. I won't make the same mistake twice.

My fastest Road America laps were in the 2:46.x range. The kink is scary, so I pussyfooted that turn. This was with Nitto NT05 tires. Other than that, the car is basically stock.

I have the whole winter to save money for new wheels, tires, and front brakes, so that'll give me a chance to shop around and find deals when I can.