beater_k20 said:
glazing. pretty much, with an old rotor you have a smooth slippery surface, and a freshly turned rotor has a smooth but rough finish. allows the pads to grab better.
And more importantly, allows the new friction material to break in properly. Like piston rings or any other machined surface, brake pads must break in.
Fierospeeder said:
you dont need to cut the rotors if they are within specs. It just needs to be scuffed up a bit with a scotch or sandpaper pad. Same rules apply after cutting the rotors.
That may be the case, but have you ever put a dial indicator on a used rotor? Usually the spec is about .003" runout is unacceptable. I've NEVER seen a rotor that was used, that wasn't out of spec from too much runout. Even grandma mobiles do not have totally straight rotors every time.
4X4HIGH said:
A brake rotor or brake drum should be machined EVERY time the brake pads or shoes are replaced. There are a couple reasons for it, first is to get rid of the glaze (shine), second is to make sure they are true without any runout, and third is to make sure both sides are paralell to one another.
Agreed, with one exception. New rotors should not be turned unless they're pressed on and off a hub. New rotors have a much better surface finish than can be made on most brake lathes, and this should not be interrupted if at all possible. I'd still check them for runout though if possible.
One other point I'm going to make is that new rotors must be washed THROUGHLY, especially if machined. Those tiny bits of metal you didn't think about get into everything, including your wheel bearings and brake friction material. This includes new rotors, they're coated with cosmoline for the boat ride from China and if not washed properly, the oil will contaminate the friction material.
They ALWAYS must be machined if they are removed from the hub and then reattached to either the same hub or a different hub because now they need to be made true to the new center, (which is the bearing races in the hub to which the rotor is attached).
Not only is this true, but the surfaces where the rotor meets the hub should also be cleaned up. All it takes is a tiny spec of rust to create runout in the assembly, which is a foundation for later problems.
Furthermore, make sure that they're using the correct cones to center your drum/rotor. Some shops I've worked in use the hub's outer surface to chuck up the assembly. This, IMO, is unacceptable. The drum/rotor should only be attached on the brake lathe utilizing the surfaces that the vehicle itself holds the rotor on the vehicle with, in our case, this is the wheel bearing race for full floating axles and in a rear semi floater, the center of the drum. This is just as critical as any of the other steps in the process because if the drum/rotor is not properly centered on the machine, you're going to machine runout into the assembly instead of removing it.
Every time I have ever replaced a drum or rotor on one of these vehicles, I've insisted on turning the assembly. The shop I previously worked in didn't do this (because their brake lathe was a piece of crap, it created more runout than it solved) but that doesn't make the practice acceptable.
If you never machine your rotors or drums when you change pads or shoes the only thing you accomplish is a pedal that doesn't go as far to the floor, you will not increase the stopping power one single bit since the rotors or drums have a glazed surface which will glaze the pads or shoes as well.
True. Sadly, even Mercades Benz does not turn rotors when you take your vehicle in for service. The replace pads only unless the rotors are badly damaged, and in that case they get thrown away and new ones installed.
Just because you guys have seen/heard of hack job brake jobs doesn't mean that nobody does it professionally anymore.
I'd also like to add--when you take your brake rotors in to be turned, ask to see their brake lathe. If it looks like hell, is dirty, never cleaned, etc., run away fast! If bushings/bearings are not replace regularly, and the machine is not kept clean, they'll likely produce a worse finish when done than the rotors/drums before you brought them in.
I hope I didn't write a novel on this but if you can't tell I've done this for a living as well. Honestly, most parts stores do not have the proper equipment to do this process correctly, and unless you're replacing rotors/drums, take them to a machine shop that does engines as well. Hopefully that means that you'll get an employee that understands how critical the small details of this process are.
In some shops I've worked, nobody even knew how to read a mic to check the rotors. 90% of the rotors from today's cars are going to be garbage. 60k+ miles on a set of ceramic brake pads will wear rotors quicker than the pads need to be changed--the pads literally outlast the rotors in most cases.
All of these things drive up the cost of a brake job, but its YOUR LIFE, and I don't take chances with any of it. Remember--when your car was new, someone took care of fine details such as this. When you repair your car, the object is to make it good as new. The fine details are the difference between a professional brake job and simply replacing friction.
