That is absolutely not correct. If the sides of the groove are tapered, then it's true there will be more contact area between the friction surfaces, . . . once the linings have worn down that far. It can take many months or years for that to happen. The bigger issue is even with multiple grooves in the rotor that the linings haven't worn down to match yet, that brake is capable of locking up the wheel causing the tire to skid. What's the advantage in having more contact area between the friction surfaces?
One of the main goals is to have balanced braking side-to-side. The only way to know for sure you've achieved that for your customer is to have two perfectly smooth rotors with no grooves, but that story gets even more involved today. I'll share that wondrous story in a minute. FAIRRACING31 posted a lot of good information about rotor thickness. To add to that for anyone else researching this topic, this is a federal law that all mechanics are very aware of. This also relates to my story later. There are two measurements published for rotors on 99 percent of all cars and light trucks. The first is the "machine-to" spec. If a rotor can be machined to no less than that thickness, it is legal to do so. Often a mechanic will leave a few very light scratches in the completed rotor to avoid having to force the customer to buy a new rotor. We can overlook that because tiny rocks and road dirt are going to cause those scratches to form in the rotors anyway. It can be a judgement call, but those few small scratches don't affect braking power or safety.
The second measurement is called the "discard" spec., and is typically .015" or .030" thinner than the "machine-to" spec. I have memorized a common Chrysler front-wheel-drive rotor from the '90s, so I'll use that for my example. The "machine-to" spec. was .830". As long as it ended up that thick or thicker, it could be reused. Once put back in service, it can be allowed to wear as thin as .815". It is legal to BE .815" thick, but it is not legal to MACHINE it to less than .830". That additional .015" gives the legal rotor room to wear under normal conditions.
In theory it is legal to put a rotor back on if it's, . . . say, . . . .820", if you don't machine it, but no professional would risk his reputation doing that. If the brake is apart because worn pads are being replaced, every brake specialist is going to insist on reassembling it with a smooth rotor surface to match the new pads, and if it's not legal to machine the rotor, he's going to replace it. Reinstalling that rotor in that under-size condition refers more to taking that brake apart to perform some other unrelated service, such as replacing a half-shaft. In that situation, you're simply putting back what was already there and was legal. The pads' friction surfaces are already worn to match any grooves in the rotor, so they're making full contact. If the mechanic observes the rotor looks to be under-size, he would be prudent to measure it, then inform the customer of the need for brake work, but if that work is declined, any future lawsuits where the brakes had a direct impact, it would be the person who did the previous brake work who would be involved. A mechanic and a shop owner normally can't be held responsible for safety-related work the customer refuses to have done. This is where documentation is critical. All reputable shops require their mechanics to document everything on the back of the repair order or enter it into their computer. How much of that ends up on the customers' final invoice varies between shops, but there is a record of it if legal trouble pops up. To add to the misery, we already have a bad reputation of selling unneeded parts and services. A mechanic is just as likely to avoid looking at the brakes so he doesn't have to tell the customer more work is needed. In the short run we avoid looking like we're trying to sell unneeded services, but that is not how it's handled when we have our customers' best interest at heart.
For the story I referred to earlier, cars have changed a lot in the last 20 years. They've become lighter and thanks to softer rubber control arm bushings that wear out frequently, they ride softer than before. This has caused a new type of braking problem to show up with an elusive cause. To start this exciting chapter, let me start by describing what I did on my 1978 Chrysler LeBaron a long time ago. The right front brake started grinding metal-on-metal for the second time. Since I had access to a brake lathe with no one looking over my shoulder, I ended up machining that rotor until it was .125" under the legal limit. That's 1/8th inch and terribly unprofessional, but I was broke, and I figured a smooth surface was better than leaving it unmachined and full of gouges. I drove with that undersized rotor for over ten years and never had a problem, but I knew if the other guy ran a red light and I piled into him, his attorney or insurance investigator would convince a jury that I was partly at fault for the crash because I was less able to avoid it because of that rotor, . . . and they would be right. (The same thing can happen to people with altered ride height, meaning lowered cars and raised trucks. That's a whole 'nother story for another day).
The point is, with a legal rotor on the left and a severely-undersized rotor on the right, there wasn't a single symptom or observation to make that known while driving.
Fast-forward to the late '90s. A coworker had been working on a full-size truck for over a full day trying to solve a severe brake pull to the right. On a test-drive, the truck would stop in a perfectly straight line three times, four times, five times, then, always coming down a steep hill near the end of that test-drive, on the eighth pedal application, with no warning whatsoever, the steering wheel got torn out of our hands and we just about ended up on the sidewalk! Once cooled down for about five minutes, that brake pull was gone again. The clue should have been the previous owner just had a perfectly legitimate brake job done. That is when the problem developed. I only learned of the recent history because the new buyer was an acquaintance who discussed the symptoms with me.
After that coworker worked the whole day trying to solve this, I started working with him to figure out what was overlooked. After another half day of frustration, we decided to switch the wheel bearings, brake rotors, and one-day-old new pads, side-to-side. Now, on that same test-drive, when the pull developed, the truck pulled real hard to the left.
All the steering and suspension parts had been inspected. I had already checked the alignment. The new pads hadn't changed anything the day before. Wheel bearings won't cause this. All that was left were the rotors. When we switched just them back side-to-side, the pull to the right came back! Two new rotors solved the problem. THAT is when we finally wised up and found there was a 32-page service bulletin for this exact problem. It pertained to just the 2500 and 3500 trucks, and mainly those with diesel engines because they're so much heavier. That makes the brakes work harder.
This bulletin covers checking for control arms with softer or harder rubber compound. It has nothing to do with the weight of the truck. All that's important is all eight bushings are the same. This only becomes a problem if you buy one used control arm from a salvage yard, and it has bushings that are different. The bulletin describes four different calipers. There's two different piston diameters and for each one there's standard calipers and low-drag calipers for better fuel mileage. Well, none of these things were changed between when the problem developed and before it developed.
There's a lot more to check, but the secret was found in the last few pages of that 32-page service bulletin. When the rotors are machined, they must end up EXACTLY the same thickness, and with the same speed of cut on the brake lathe. That's why the two new rotors solved the problem. When one rotor is thinner, it heats up faster because there's less material to absorb that heat. During the previous brake job, one rotor had been machined just a little more than the other one, a totally common, acceptable, and non-noteworthy practice, but it was enough to lead to this. At some magic temperature, the coefficient of friction changed to where one brake grabbed harder than the other one, and the pull developed. That's why the pull was never there at first when the brakes were still cold.
GREAT AND WONDROUS STORY, PART 2:
The district representative for our state visits every dealership once a month to meet with car owners with problems the dealer is not able to solve. One month he was driving a lemon-law buyback with that same brake pull problem. GM donates their lemon-law buybacks to schools, then they just wash their hands of the problem. Chrysler stipulates that any such vehicle be repaired, regardless of cost, then it goes to an auction. They use the documentation to develop the service bulletins that help their mechanics. I skipped most of that service bulletin and went right to the rotors. Both were way ticker than the minimum legal thickness, but one was .020" thinner than the other one. I used an on-car lathe to cut the thinner rotor very lightly, then I cut the thicker rotor multiple times until it matched the first one perfectly. Brake pull was solved and the district rep. thought I was a genius. I wasn't about to argue with him because I didn't get called a "genius" very often.
The next month he showed up with another truck with the same brake pull, but this one had only .007" difference in rotor thickness. That's the thickness of two sheets of paper on rotors that are 1 1/4" thick. The reason I shared my story of a rotor on my car that was .125" under the legal limit was to put in perspective why .007" and .020" would never be identified as a contributing factor under normal circumstances.
This time I told the district rep. that I was just following the service bulletin. Darn it; he never brought us another truck for that problem! The first mistake my coworker and I made was to not check for service bulletins. We solved this ourselves over a day and a half. Service bulletins are developed exactly so that's what we DON'T have to resort to.
About the same time, this intermittent brake pull was starting to show up on Fords and Chevys too. Brake system specialists and most other mechanics now automatically check that both rotors are machined to the same thickness, but today we are more likely to just install new rotors. Back in the mid '80s, a simple slide-on rotor for a GM front-wheel-drive car cost around $95.00 to $100.00, so it made sense to try to save the old ones. Today, it is common to find a new rotor for a front-wheel-drive car costs less than 20 bucks. Given you're paying for the mechanics time at the brake lathe, and the cost of cutting bits and other consumables, it is less expensive for the shop and much less expensive for the customer to just pop on a pair of new rotors.
One final word of wisdom refers to those new rotors. Many that are sold by the reputable auto parts chain stores are made in China and will develop a brake pedal pulsation in about three months. The proper repair is to go back to the shop that performed the brake service and let them machine those new rotors. There is nothing wrong with the quality of those rotors. When we make parts out of cast iron, we set them aside for 90 days to "age" before they get their final machining. The Chinese cast 'em, machine 'em, pack 'em, and ship 'em, then they age on your car. Minor warping is very common, and one light cut on the brake lathe solves that permanently. Reputable shops will perform that follow-up service at no charge. It's the people who become angry and insist on new rotors under warranty who are going to have the same problem in another three months.
Sep 29, 2019 at 6:48 PM
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