Help with my brake dance

October 15, 2009 11:20 AM

Give me a brake!...I'd like to have a clue about brake rotors. I'm getting my brakes repaired today, and don't want to replace brake rotors if I don't need to. Thing is... I don't know squat about brake rotors so could be easily taken for a ride on the rotormobile. How can I ask the right questions to make sure I really need rotors if the mechanic recommends they be replaced?
posted by ecorrocio to travel & transportation (10 comments total)

There are two things that concern someone regarding brake rotors: runout (that is, the degree to which the surface of the rotor is not flat) and thickness (literally, how thick the rotor material is). The manufacturer publishes specifications for the maximum runout and minimum thickness for rotors, and most shops can measure thickness very easily. Runout is a touch more specialized, but excessive runout can be felt as "vibration" under braking. You want to ask about each of these, and ask whether they're within manufacturer tolerances.

Thin rotors are caused by mechanical wear, and the only solution is to replace them.

"Warped" rotors (that have excessive runout) can be caused by two factors - uneven wear due to over-torqueing the wheel nuts/bolts, or deposits of brake friction material on the rotor surface ("glazing"). If thickness is okay but runout is excessive, aggressive refinishing with a powered rotary sander can occasionally restore the rotor surface if glazing is a problem, but will likely not work in the long term unless you also replace pads (replacement necessity also determined by physical thickness) and follow a proper bed-in procedure. If the runout was caused due to warping from improper nut/bolt tightening, the only solution is to replace them and find a shop that will use torque sticks or a torque wrench when putting your wheels back on.

If I had a dollar for every time I saw a shop rip the lugs back on a car with 300lbft air wrench, I could afford the legal fees to hit them in the head for doing it, but I digress...
posted by TheNewWazoo at 11:31 AM on October 15 [1 favorite]

TheNewWazoo has it right. One additional thing to note is that resurfacing ("turning") the rotors will always remove a certain amount of the surface. You may want to turn the rotors, but the process of turning them will automatically put them beneath the minimum thickness. So the shop will just recommend that they be replaced, even though technically they're still "good" with regard to thickness alone.
posted by Cool Papa Bell at 11:39 AM on October 15

The reality is that many shops will want to sell you new rotors, they make a tidy profit from the sell of them. If you really want to know that you need them, request that they show you your rotor and the measurements they are taking and the published specs showing the exact numbers.
posted by bprater at 11:41 AM on October 15

I can't add a whole lot to TheNewWazoo's description, aside from the following:

There are times when I'm inspecting brakes before replacing the pads that I'll find that the rotor is within its spec as far as minimum thickness goes but the pads have worn the rotor such that there's a substantial lip around the outside edge of the rotor. That is, the pads generally contact a surface area that's just shy of the edge and center of the rotor so they create somewhat of a groove that's the width of the brake pad. Although no harm will be done by putting new pads on those rotors, a great deal of vibration and noise will be generated as the new pad fits irregularly in the old pad's groove. For customer satisfaction and passenger comfort, I typically recommend new rotors in that circumstance, even though it's not a safety issue. I find that most people don't want their new brakes to sound like a garbage truck screeching to a halt.

I would also not recommend "cutting" rotors as a way to resurface them or correct any runout or irregularity. Old-style rotors are generally more substantial and made of heartier materials. New style rotors are designed to be lighter and reduce what's called "unsprung weight" which is the amount of weight not carried by the suspension springs. Generally, the more unsprung weight, the greater the mass in the wheel area, and the more harshness and suspension reciprocation over bumps and dips in the road.
So, new rotors are lighter, cheaper, and are made of different alloys than old rotors. Old rotors could be cut on a lathe to eliminate irregular surface texture, glazing, or runout. New rotors generally don't have enough material to be cut more than once and, in my professional opinion, the last thing you need to do is make brake rotors thinner. The thinner they are, they more susceptible to heat warping and brake fade they are. New rotors should be as cheap as cutting them, anyway. I don't see any value in cutting late model rotors. So, avoid it if they give you the option. Although we do make a few bucks marking up the price, I've never made more than 20% on brake parts. The money in a brake job is primarily labor.
posted by Jon-o at 11:58 AM on October 15 [2 favorites]

I think another question is why are you shelling out your cash to a mechanic that you don't trust? If the shop is that untrustworthy you should not be letting them touch your car anyway.
posted by COD at 12:25 PM on October 15 [1 favorite]

I replace my brakes myself every 30 or 40k miles, and I just replace the rotors. They're really not very expensive, and no amount of machining will give you a surface that will last as long as when the rotor was new. Resurfaced rotors will start shuddering in half the time that the new ones did.

Given the cost of machining, you're only looking at maybe $20 difference. If you're worried about getting taken for a ride, skip the ripoff brake shop chains, and instead, take your car to your personal mechanic. If you're still worried, go to NAPA yourself, buy the rotors and pads there, then take them to a trusted mechanic for installation.

Finally, brake rotor and pad replacement is easy, even for someone without much mechanical know-how -- here's a good step-by-step guide. The rotor itself just comes off the lugs once the brake caliper is out of the way, and you can take the old ones to your local metal recycling center.
posted by M.C. Lo-Carb! at 12:26 PM on October 15

First you need to spend a couple of hours tracking down the specifications of maximum runout and and minimum thickness. Say you have a Toyota so these will be measurements of distance like 1.5mm and 10mm. Then when you go into the shop ask, "What's the runout and thickness of those roters?" They will respond with, "Well the rear left was 5 hundredths runout and the right was 6." Don't let them throw you off this this car mechanic technobabble. "5 hundreths" means five hundredths of an inch, that's a measurement of distance. What you need to remember to be prepared for this is that one hundredth of an inch is 0.0254 mm. So then ... oh, never mind.

If you don't trust the guy you should find a new mechanic before you need another repair. Ask around before you have leaks and grinding metal sounds.
posted by bdc34 at 12:34 PM on October 15

If your mechanic is not willing to show you the rotor then get a new one. M yrotors had to be repalced and the mechanic actually showed me the router and the thickness. The funny part was this was at midas . Whic hyou dont expect to have good customer service because they are a chain.
posted by majortom1981 at 1:21 PM on October 15

"The reality is that many shops will want to sell you new rotors, they make a tidy profit from the sell of them. If you really want to know that you need them, request that they show you your rotor and the measurements they are taking and the published specs showing the exact numbers."

Any dishonest mechanic can screw you about 60 ways from sunday without having to play games like replacing parts that are in working condition. Are you going to be able to tell your rotor from any other one they happen to have stacked on their scrap pile? Do you really want a mechanic you can't trust changing your brakes? Just find a shop you can trust and let them do their jobs.
posted by Mitheral at 4:21 PM on October 15 [1 favorite]

Thanks for the detailed info! I now know 1000% more than I did. Yes, a mechanic I trust would be the best way to go... and I have no reason not to trust the one I'm using. This is a place a mile from my house, so convenience is a factor. My go to guru for bigger jobs is all the way across town.

Thanks again!
posted by ecorrocio at 8:49 AM on October 16