Brakes Tutorial/Bedding Procedure
Why rotors warp |
Brake rotors warp because the temper in one area of the rotor is different then the rest of the rotor. When subjected to thermal stress (or mechanical stress from unevenly or over-tightened lug nuts) the rotor warps. Turning or truing the rotor will alleviate warps that exist when the rotor is cold, but will not solve the problem when hot, the uneven temper still exists within the substrate. The only permanent solution is to replace the rotor. The manufacturing cost of brake rotors is mainly related to the length of the cool down time during the casting process. Cheap rotors are removed from the sand molds too soon and generally develop uneven temper. Expensive rotors are left overnight to cool in the molds, allowing them to temper very evenly. This extra time in the molds eats up the foundry's production capacity, so the parts are more expensive.
How do you know if you're getting good rotors? That's a tough one. Several aftermarket rotors are available for the SHO, the SHO Shop and Donelson Motorsports come to mind, these guys have better quality rotors that are available cross-drilled or not, Donelson claims that theirs are preheated. Preheating, performed properly, will anneal-out the uneven temper in the casting. |
Lug nut torque |
For Pete's sake, don't let the garage jockeys set your lug nut torque with an air wrench! Request that they hand torque, watch them like a hawk, if they comply, hand the technician a $5 tip, he's done you a nice favor. If they balk at your request or look at you like you've got an arm growing out of your ear, take the car somewhere else. The preferred hand method is called stage torquing; first to 60 or 70 ft-lbs, then to the final torque of 90 to 100 ft-lbs. According to Ford, those new-fangled torque sticks (ACCUTORQ tool, 100 ft-lb) are OK as well per TSB 95-6-4. |
Rotor seasoning |
No, this isn't done with salt and pepper, seasoning is how you prepare a new rotor for service. Seasoning will bake out the oils and solvents used to manufacture the rotor and will also even the temper of the rotor, increasing its life, improving its wear characteristics, and lessening its tendency to warp. Seasoning is performed by GENTLY heat cycling the rotors several times. For street cars, this is best done over a 2 or 3 day period. Drive the car normally, avoid panic stops, get the brakes warm but not smoking hot. Allow the brakes to cool to ambient temp to complete each heat cycle. Avoid holding the brake pedal down or setting the parking brake when the rotors are hot, this promotes uneven cooling of the rotor surface which can lead to uneven tempering.
Again, the key phrase here is 'gentle heat cycling'. If you go to the trouble and expense to put on new pads and rotors and then go out and immediately HAMMER the brakes, you're wasting your time and money. Now that the rotors are seasoned, the brake pads, if they are new, can be bedded. |
Bedding Brake Pads |
What will my spouse think? Since pad bedding (sometimes called burnishing) can be done on the public roads, your spouse shouldn't mind all that much. Bedding is necessary to get the maximum performance from any metallic compound brake pad. During bedding, the pad releases excess gasses from its hardeners and organic bonding agents. Bedding also tempers and cures the friction material so that it will withstand future agressive use with minimum fade.
Find a location where you can accelerate up to 60 MPH, brake heavily to 5 MPH, and then repeat this cycle several times (usually 5 to 7) until the brakes fade significantly. If done properly, the brakes will smell and may smoke somewhat, that's why bedding is often referred to as "letting the smoke out". Now, drive for 10 minutes or so, at highway speed, utilizing as little braking as possible. Park the car overnight, allowing the brakes to cool to ambient, again, don't set the parking brake, especially if you've put new pads or rotors on the rear. BTW, if you bought cheap rotors, with poor temper, they will probably warp during pad bedding, so sorry. Some pads come pre-burnished, the manufacturer has baked the pad to drive off the organics and cure the bonding agents. These pads don't need to be bedded, but it won't hurt anything if you do it anyway, bedding also helps to carbon-load the rotor surface, improving brake effectiveness. Note about carbon-loading: if you change from carbon-compounded brake pads to pads with no carbon content, you'll need to change the rotors. The surface carbon loading on the rotors can make non-carbon content pads somewhat ineffective, i.e. NO STINKIN' BRAKES! |
Cross-drilling |
Nearly everyone on this list has seen a cross-drilled rotor. They can be seen on most street-roaming exotica such as Porsches and Ferraris. They can be purchased from several aftermarket vendors found in Car and Driver, Motor Trend, Auto Week, etc. They are almost always on anything that is raced professionally, so they must be good for
your street car, right? WRONG. ('scuse me while I put on my Nomex undies...) OK, wrong, maybe, application- and a whole lot of other things-dependent. Here's the breakdown of "whole lot of other things-dependent". You be the judge. |
Why are rotors cross-drilled? |
Primarily to reduce the chance of brake fade during heavy brake use, such as while circle track or road racing. Cross drilling will also improve brake cooling, provided there is a steady supply of cool air forced to the center of the rotors. This means brake cooling ducts. |
Why do brakes fade? |
Brakes fade because the brake pads get so hot that the organic binders holding the pad's friction materials together begin to boil and liberate large amounts of gaseous by-products. This gas forms a layer between the brake pad and the rotor that drastically reduces the coefficient of friction between the two surfaces. Cross-drilled rotors give this gas layer an easier escape path than just past the edges of the pad. The holes allow the gas to vent through the internal cooling slots of the rotor. |
What about grooving rotors? |
Grooving rotors accomplishes much the same thing as cross-drilling; it's just not as efficient at allowing gas removal. Grooving also provides a slight scraping effect to the pad which can break a glaze buildup. Glazing is a hard, glass-like layer that forms on the surface of a brake pad that has gotten WAY too hot, molten actually. Needless to say, glazed brake pads have a rather poor coefficient of friction and should be replaced.
Something bears further explanation here: two characteristics that I've mentioned so far, pad outgassing and glazing, are normal characteristics of the older style, high-organic content brake pads, the kind with lots of asbestos in them, the kind that aren't generally available anymore. Why would anyone want to use a high-organic, asbestos brake pad when the new generation of metallic, carbon, and kevlar-based linings have higher coefficients of friction that are more consistant versus temperature, lower organic binder content, and substantially longer life than their asbestos counterparts? I'm not saying that the newer pad materials won't outgas and fade. They're far more resistant to this behaviour than the older materials because they are higher temperature materials to start with and they have much lower organic content. I won't go any further with this now, wait for Part 4: Sorting out the different brake pad materials. OK, so you're still salivating over that cross-drilled rotor ad in the latest issue of Motorhead Fanatic magazine. Go ahead, order 'em. In fact, while you're at it, order several pairs; you're going to need some spares. These guys are just taking standard rotors and drilling them full of holes. The structure is weakened and the mass of the rotor is reduced, therefore limiting its ability to deal with heat buildup. I can almost guarantee that a set of these drilled rotors will be warped and dangerously cracked within 12 months, 2 or 3 months the way I drive. :-) (I road race my SHO, that's why I have the 13" Baer front brakes, with undrilled rotors.) |
But Porsche and Ferrari do it! |
Yep, and their rotors are designed at the casting stage to be cross-drilled. They have thicker rotor surface sections and more material in the cooling slots. They are stronger to begin with so they will hold together longer despite the cross-drilling.
Next time you're at the race track, get a garage or pit pass and spend some time talking to the mechanics. Ask them how often they replace the drilled rotors on their cars. The answer you're likely to hear is "every couple of races", assuming that they can afford it. Ask to see a used-up rotor. It will have radial cracks around nearly every hole. On holes near the outside edge of the rotor, the cracks may extend to the edge of the casting. Get more than a couple of these major edge cracks and the rotor is likely to self-destruct by throwing large hunks of itself at the insides of your wheels. Stopping is difficult when this happens... At my last visit to the SHO Shop, I spied a pair of SHO front rotors that had been drilled. They were so badly cracked and heat checked that they would have shattered from a drop to the concrete floor. According to Vadim, they had been on a car for about a year. |
Advantages and Disadvantages |
For cross-drilling:
Against cross-drilling:
OK, so you still want drilled rotors. (I want to be locked in a closet with Gillian Anderson of the X-Files but that hasn't happened yet...) May I suggest the following:
Let me say a bit about grooved rotors before I conclude. Grooving is not nearly as hard on the rotor's structural integrity as cross drilling. If you must do something visually appealing to your rotors, get them grooved. Grooving will accelerate brake pad wear, be prepared for this. Also, get in the habit of inspecting your rotors periodically, especially if they are drilled or slotted. Or, get some brake pads with gas grooves in them. Gas grooved pads look like they have a saw kerf across their narrow dimension. Typically, the kerf depth is about 2/3rds the total depth of the friction material. Racers have been known to groove pads with a hacksaw blade, although I don't recommend that you try this at home; "standard disclaimer applies". |
