Q: How can we find out if the front or rear rotors are warped?
A: There are two methods and both are imperfect, because they don’t make a scan tool for brake rotors!
The first method is to test drive the vehicle and feel the shaking when braking. If the seat or pedal shakes first, you likely have warped rear rotors. Now, when the steering wheel shakes, the front brakes probably need the work.
This method is imperfect for several reasons. First, you might feel shaking in both or the shaking might be ambiguous. You see, what if the vehicle shakes in the seat at 40 mph, but in the steering wheel at 55 mph? It would be hard to be confident in your diagnosis merely by feeling. Second, there will be times when customers complain, “When I drive 85 mph on the freeway, it shakes like crazy.” Granted, if Sammy Hagar can’t drive 55, he should not be doing 85 mph, but the customer brought the car to your shop to fix the problem. What do you do?
This is when a dial indicator becomes handy. After you attach the dial indicator and spin the rotor with a screwdriver or pry bar, the needle will tell you how warped the rotor is by thousandths of an inch. Most manufacturers (excluding the European ones) will publish specifications on how much “run-out” is acceptable, usually within 0.002” to 0.005”. You can easily look up this information in shop information systems.
Sometimes, you cannot easily measure run-out on the backside of the rotor or even after measuring it, you cannot find anything. In this situation it would be best to try to see how uneven the rotor is going latitudinally instead of around, or simply compare the rotors. If the left rear rotor is 0.004” while the front are both “0.0015, rear rotors should do the trick.
Q: What’s with all the hype I hear about brake lathes?
A: Due to the increasing amount of brake pulsation comebacks, most OEs require that their dealers use an on-the-car brake lathe. The benefit of an on-the-car brake lathe is that it can compensate for run-out issues that may be caused by a hub. For example, if you replace a hub assembly, the run-out on the rotor might change because the angle of the rotor may be altered by hub placement. Now there is nothing wrong with a bench lathe, because it requires less finagling (moving it around the shop, adjusting it for different hubs, etc). However, you just can’t compensate for hub run-out.
If you don’t have a brake lathe, you’re probably used to a degree of pulsation comebacks and have learned to live with them. However, if you work in one of the shops that still “cuts” brakes, you know why they come in real handy. Rotors can be really messed up. Factory machining is often substandard, and the only way to have a true comeback-free brake job with the vast majority of aftermarket rotors is to cut every rotor.
The problem is, in the real world, cutting rotors on the car can be a 40-minute process for many technicians. A lot of guys can do pads and rotors that quickly. If you work on a lot of vans (whose rotors are very expensive) or have customers willing to pay for a superior brake job, an on-the-car brake lathe is the way to go.Q: What’s the best way to prevent premature wear?
Q: What’s the best way to prevent premature wear?
A: Procedure is very important when it comes to brake work. Take a look at if the shims are lubricated (uneven brake wear could be from a lack of lubrication), and if not, see if one side is wearing differently than the other.
Granted, if workmanship is fine you can have a brake caliper acting up and you can diagnose this with a brake caliper pressure gauge. To use it, put the block that the gauge is attached to in place of the brake pad and step down on the brakes. Simply compare the questionable caliper to the good one and see if the pressures are different. If there is a difference greater than 10 percent, you can sell calipers along with pads and rotors.
Q: What can I do to stop a squishy brake pedal?