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    [ Brakes ] Plain vs Slotting vs Drilled Rotors

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    [ Brakes ] Plain vs Slotting vs Drilled Rotors

    Post by dp on Sun Mar 22, 2009 6:48 pm



    Once upon a long time ago, rotors were originally drilled to eliminate something known as "green fade". The best way to explain "Green fade" is to relate it to an air hockey table. The puck is suspended on a cushion of air that prevents it from touching the table, this reduces the friction between the puck and the table.

    A long time ago, pads were made with the best resins we had available. Many of those resins would produce gas as they cured. When a pad was used the first few times, the heat would "cure" the resin which would cause it to produce vapors. This was known as "out gassing". The vapors would build up between the pad and the rotor and lift or "force" the pad away from the rotor (like the puck in air hockey). This caused the brakes to be very ineffective, even though they were not yet at the maximum rated operating temperature. [when this happens, you'll retain your brake feel, i.e. the pedal pressure will be firm, but the car just won't stop! this led to alot of accidents on the track in the earlier years of motorsport] The holes were drilled to allow that gas a place to escape. So, it is correct to say that rotors were cross drilled to eliminate fade, but not for the reasons you would think. The good news is that today's resins no longer suffer from these problem and the modern race pads are so good that this is really no longer an issue. So, by cross drilling rotors, you will only manage to shorten the lifespan of that rotor (it now has less surface area to wear against the brake pad and will wear more quickly as well as a reduction in weight that will cause the brakes to operate at a higher temperature).

    Another problem with cross drilled rotors is the potential for cracking around the holes. The holes become a stress point in the cast iron that can more readily allow cracks to form in the rotor surface. This requires that you pay close attention to the rotor surface for signs of cracking. Some small cracks, known as "surface checking" are acceptable, but anything that resembles a crack would be a reason to replace that rotor. When looking at slotted rotors keep in mind that the slots should not be milled off of the edge of the rotor. This is a great place for cracks to form, and they will. The slot should be ball milled in the rotor face and originate and terminate on the surface of the rotor without exiting the rotors edge. The goal is to eliminate sharp edges that cause stress risers on the rotor surface. This will reduce the possibility of cracking. If you see slotted rotors with slots that are milled off the edge of the rotor, shop for another brand. Slots that are not cut through the edge of the rotor are a good sign that the manufacturer of that rotor knows what they are doing. This is a good indicator of parts made by a brake company and not a machine shop that happens to drill and slot rotors.

    If you must go bigger, look at any of the various brake upgrades available from many major manufacturers. Brembo, Baer, and many others will have what you need, if you need to upgrade. There are larger kits and they increase in both cost and braking ability. Only your needs and your budget are the limit.


    AP Racing wrote:
    Disc grooves and sometimes cross drilling are frequently used on racing brake discs to clean the surface of the pad and allow gases produced to escape. In doing so the friction characteristics are modified, different groove and & drilling patterns affect the friction characteristics in different ways, some affect overall friction and others the bite or release characteristics and therefore the best solution is not necessarily the same for each application.


    Baer Brake Systems wrote:
    What are the benefits to Crossdrilling, Slotting, and Zinc-Washing my rotors?
    In years past, crossdrilling and/or Slotting the rotor for racing purposes was beneficial by providing a way to expel the gasses created when the bonding agents employed to manufacture the pads began to break down at extreme temperatures. This condition is often referred to as “green pad fade” or “outgassing”. When it does occur, the driver still has a good firm brake pedal, but simply little or no friction. Since this normally happens only at temperatures witnessed in racing, this can be very exciting!

    However, with today’s race pad technology, ‘outgassing’ is no longer much of a concern. When shopping for races pads, or even ultra-high performance road pads, look for the phrases, “dynamic surface treatment”, “race ready”, and/or, “pre-burnished”. When these or similar statements are made by the pad manufacturer, the pad in question will likely have little or no problem with ‘outgassing’. Ironically more pedestrian pads used on most streetcars will still exhibit ‘outgassing’, but only when used at temperatures normally only encountered on the racetrack.

    Although crossdrilling and/or slotting will provide a welcome path to expend any gasses when and if they develop, it is primarily a visual enhancement behind today’s often wide-open wheel designs.

    Crossdrilling offers the greatest gas relief pathway, but creates potential “stress risers” from which cracks can occur.


    Brembo wrote:
    Why use drilled or slotted discs?
    Drilling or slotting discs aids the disc in several ways:
    The edges of the slots or holes continuously clean and refresh the pad surface as well as providing increased brake "bite". Additionally, they prevent gasses from collecting between the pad and disc interface.
    The disc is lightened, thereby decreasing its rotational inertia. Improved ventilation increases the disc's ability to shed heat, resulting in cooler operating temperatures.



    From Integra Sg Forum


    Last edited by dp on Sun Mar 22, 2009 6:50 pm; edited 1 time in total
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    [ Brakes ] Slide-rail vs Piston Calipers

    Post by dp on Sun Mar 22, 2009 6:50 pm



    Floating "slide-rail" calipers are a fact of life. They are OE on practically everything now and as usual the marque fans will yell "The brakes on my [fill in the blank] are great!" Sorry. If they are any type of sliders, they are not "great."

    Simply, slide rail calipers work by moving a single large piston -- and the caliper frame -- to press the pad against one side of the rotor, while in opposition pulling the other side of the housing and pad to the other side of the rotor. The whole 5-6 lb. assembly moves. In addition to that, the piston covers just a portion of the pad, while leaving the rest of the pad unsupported by anything but its backing plate. The motion of the rotor causes the pad to flex in on one unsupported side, and out on the other, leading to pad taper. (See drawings below of slide rail and 6-piston versions of Wilwood calipers. The dotted-line circles show how much of the pad is used by the respective pistons.

    The biggest problem with slide-rails centers around the most important issue in braking: release. That you can "lock them up anytime I want" is NOT a good test of a braking system. On the contrary.

    The average driver is right in thinking that weight (unsprung weight in calipers and rotors on four corners) and temperature are the critical factors in braking. The article here in the University on brake pads looks at some of those considerations. But the real issue in performance braking is not the clamping force but release and how quickly and precisely you can take the force OFF the rotors to keep that knife edge of balance. Slide rail calipers just don't -- can't -- release rapidly and smoothly enough.

    With the application of pressure through the hydraulics, slide rails must move the caliper against both sides of the rotor. To release the clamping force they must move the same distance, back. On the other hand, pistons move a fraction of that distance to clamp and unclamp the pad to the rotor. The result translates into lots more finesse.

    Most of the time, slide rail calipers contribute to significant pad taper, again due to the basic geometry of the system. The inherent flex of the rail system causes the pads to interface with the rotors unevenly --'tilting' or torqueing, rather than clamping the rotor with consistent force along the face of the pad and the rotor. As you will read in the article on brake pads, this has a dramatic effect on performance and feel -- another element in that 'release' issue we're discussing.

    Fixed piston calipers -- with pistons on both sides -- are designed to deliver more consistent force across the brake pad. That's why six-piston calipers are more precise than four-piston ones, given the same installation design.

    Here, I need to note that radial-mount calipers enhance this intent compared to lug-mount calipers. First, because the radial mounts, that are perpendicular to the diameter of the rotor allow for more leeway in where/how you need to mount them; and second, because the wider the gap between the mounts, the stiffer the caliper -- some of the lug-mount calipers have a 3 1/2" gap, some have 6" -- which again affects how evenly the clamping force is delivered to the pad. But some wheel configurations don't allow for radial mount. And lug-mounts are definitely less expensive.

    Piston calipers equal more braking force controllability, which equals greater ability to feather your brakes.



    From Integra Sg Forum
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    [ Brakes ] Wearing Out of Rotors

    Post by dp on Sun Mar 22, 2009 6:51 pm



    If you suspect your rotor is worn out.. feel the edge of the rotor, if you can feel a depression where the rotor surface meets the edge, then the disc might be nearing the end of its life.. another way to tell is to use a vernier caliper and measure the thickness of the rotor and comparing it to the manufacturer's specs.. once the rotor is thinner than the manufacturer's minimum thickness, then it needs to be replaced.

    Discs are usually damaged in one of three ways, warping, scarring, cracking. The useful life may be greatly reduced by excessive machining.

    Warping is caused by excessive heat build up, which softens the metal and can allow it to be disfigured. This can result in wheel shimmy during braking. The likelihood of warping can be reduced if the car is being driven down a long grade by several techniques. Use of a lower gear to obtain engine braking will reduce the brake loading. Also, operating the brakes intermittently - braking to a slower than cruising speed for a brief time then coasting will allow the brakes to cool between applications. The suitability of this is of course, dependent upon traffic conditions. Riding the brakes lightly will generate a great amount of heat with little braking effect and should be avoided.

    Scarring can occur if brake pads are not changed promptly, all the friction material will wear away and the caliper will be pressed against the metal backing, reducing braking power and making scratches on the disc. If not excessive, this can be repaired by machining off a layer of the disc's surface. This can only be done a limited number of times as the disc has a minimum safe thickness. For this reason it is prudent to periodically inspect the brake pads for wear (this is done simply on a vehicle lift when the tires are rotated without disassembly of the components). When practical they should be replaced before the pad is completely worn.

    Cracking is limited mostly to drilled discs, which get small cracks around the drilled holes. These cannot be repaired.

    Resurfacing or "skimming" as it is more well known here.. has three purposes; to remove warps, to remove scoring, and to remove a glazed surface when new pads are installed. Brake shops will often resurface through a machining operation regardless of the need to do so due to warping or scarring. This can reduce the useful life of the rotor in cases where only a light glaze removal (using emery cloth) would suffice. Reducing the life of the rotors is of little concern to many brake shops as they can make money on replacing rotors worn (or machined) below the manufacturer's minimum specified thickness.



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