Brake Fade/Boil Q&A
>I've got an S4 with the stock flats and its an A/T. I haven't experienced
>either of the above at a DE yet. I believe the reason for this is that
>I've been reasonably easy on the brakes, and never engage the ABS. I've
>heard a lot about the stock S4 rims not allowing enough cooling air to pass
>through, making the above symptoms common on the track. My questions are:
>1) When the brake fluid boils, are there any tell tale signs that it is
>occurring, other than the pedal going to the floor?
None visible from the driver's seat. Fluid will often be forced from the vent hole on the fluid reservoir. Fluid boiling can (and often does) come as a complete surprise.
>2) Does it typically boil mid-turn?
No. Brake fluid pressures during heavy braking may be 500 psig. This pressure will help keep the fluid from boiling. As soon as you release the brakes and the fluid pressure drops, the fluid boils in the calipers. It is normal for drivers who suspect that they are having a boiling problem to tap the brakes BEFORE they need them. If there is normal resistance, you brake at the normal point. If the pedal drops when you tap the brakes, you still have a chance to scrub off some speed before the corner.
>3) Once the fluid boils, are your brakes gone until the fluid is replaced?
No. Once the fluid condenses, you will have brakes. BUT - if the boiling has forced enough fluid out of the system, you may have air in the system, and you are in serious trouble for the rest of that race. If the fluid boils, your sense of self-preservation should encourage you to correct the situation before the next race. Finding no brake pedal as you go past your braking point into a hairpin at 130 mph tends to get your attention.
>4) When brake fade occurs, do you lose all braking, or just some.
Yes. Fade is not usually an "all or nothing" situation. Fade is a loss braking effectiveness due to a reduction in the coefficient of friction between the pads and the rotors. The surface of the pads may get slick because the binder in the compound melts, or because of severe outgassing from the binder. In extreme cases, you can put 300 pounds of pressure on a rock-hard, high pedal with virtually no braking effect - BTDT! In mild cases, you may just notice a slight reduction in braking effectiveness.
>5) I believe that the majority of you guys drive in hotter climates, here
>in Seattle the typical driving weather I would say is 60 degrees. Could
>this be a factor in why I haven't had any problems with fade or boiling.
Perhaps, but probably not a major factor. The difference between 60 deg and 90 deg doesn't mean much when brake temps are 1200 deg at the pad surface. The major factor is driver aggressiveness, with course design having a somewhat lower influence.
>6) Can anyone that has tracked the stock S4 give any type of objective
>opinion as to how hard you have to be on the brakes before they start to
>give you problems?
With good pads and the right fluid, most amateur drivers will not have problems with the 928 brakes on most courses. Luckily, as you get better, thus more aggressive, you also should learn how to conserve the brakes better.
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