My car is about to have its 75k Kilometre service and the brake pad warning has just started, so that is listed to be done.

During an earlier service, the then service manager had said that when they do the brake pads they need to change the discs too. He mentioned that there is essentially equal wear and the discs normally need replacing.

Can anyone comment about whether this is necessary and common practice? What would be the criteria for having to change the discs?

Thanks for any helpful advice.

 

Your brake discs do wear they should be measured if your car is normally aspirated the minimum front thickness is 30mm and the rear minimum thickness is 18mm.

 

I've never had rotors machined. As long as brakes usually last, I justify the extra cost of replacing the rotors.

 

Your brake disks should be changed IF they are worn (grooved), warped or, near the minimal thickness. In my experience the disks tend to live much longer than most of the pads and I generally find absolutely no need to change them for at least the 2nd pad change cycle. If you're not one of those drivers who ride the brakes all the way down canyon roads, the discs should last a long time.

 

The discs / rotors do not need to be changed when pads are changed unless they are below minimum wear thickness / warped etc

Iirc I changed my discs on my A6 at around 90k but had perhaps 2 pad changes in that time.

Bear in mind that there is usually a "weak" point designed in, these are usually the cheapest and easist items to replace, in this case the pads.

 

I will agree with what those above had said. Must the rotors need to be changed? Not necessarily. Like was mentioned, if they are below a minimum thickness (stamped into the rotor in most cases), then YES, THEY MUST!!!! If you are stopping and the steering wheel is shimmying/wobbling, then odds are, the rotors will need to be replaced with how modern day rotors are made. But, there is a possibility that if you caught things early that you won't have to after getting the rotors machined true again. IF there is any significant grooving (rotor will look like a 33 LP where if you run your fingernail from center to outside, your fingernail will catch on one of the rings), then replacement is almost assured, but again is not always the case depending on how early you caught things.

I have found that a lot of shops will "mandate" the rotors be replaced for a few reasons. In a lot of cases, it saves the shop a lot of later heartache when someone comes back saying that their steering wheel shakes after the pad replacement and now they are going into the car a second time. Another motivating factor is that some shops will mark up the price of the parts compared to what they buy them at, so, they can make a few extra bucks on a job by selling you more parts than may be absolutely necessary. The final reason some shops will use is that while the rotors may be above minimum thickness when the new pads are installed, with use, the rotors will thin and they feel that it puts them in a bad position should something happen and it is found that the braking system had an issue and the problem is being blamed on the rotors being too thin (even though it may be 25000 miles/40000KM since the service). So, it is easier just to say "it is shop policy to always put on new rotors".

Not saying your shop is doing any of the above to get a few more dollars out of you, but they will try and justify what they are telling you.

 

Brake rotors warp because the monkeys don't torque the wheels properly.

If you have enough material to machine the rotors, and you torque the wheels properly afterwards (star pattern, in 3 increments with a good quality torque wrench, and never let an air impact wrench near the bolts), you won't have any issues reusing rotors that are warped after turning them.

If you put on brand new rotors, and they/you torque the wheels wrong, the new rotors will warp too and you will think they are inferior rotors, but it was just stupidity.

Ask me how I know this is true? The last time I took my white XF in for service, I forgot the wheels were going to be removed during the service. So I drove around for a couple of months without re-torquing them myself, and sure enough my brakes warped badly. I didn't have time to address the problem fully, so I just re-torqued them without fixing the warping. For some of the bolts, I had to stand and jump on my long wrench to remove them because they were on so tight. Others came off with almost no effort. After re-torquing them properly, the warping was noticeably less. After going around and around with the stupid service adviser that they didn't torque the wheels properly and it warped the rotors (and not caused by my EBC Red pads), I ended up just doing it myself. I took off the rotors, paid $30 to turn them at the local part store, and they have been perfect ever since, which was about 8 months ago.

Do yourself a favor, and when *anyone* takes your wheels off, re-torque them yourself as soon as you can before the warping sets in from the heat cycles of normal driving. I would say about 1 in 10 mechanics will take the time to do it right because they know you will struggle to pin it on their lack of effort to do it right. It's much easier for them to pretend they are Tony Stewart's pit crew and hit them with air impact wrench that can NEVER torque the wheels properly (that's not what it's for -- it's for breaking off bolts).

 

I will second the proper torquing of the wheels will lead to significantly longer rotor life. Even a cheapy "clicker" torque wrench is well worth the money if you plan on having the vehicle for a long time. If you can afford it, a dial style is much better as 1) you can see how close you are to the torque, and 2) if you happen to go beyond the desired torque, you know how far you went vice just getting a click.

If you don't have a torque wrench, you can make your own. All you need is a tape measure, a wrench with the correct size socket on the end and a spring/weight scale. You will need to measure from the center of the socket to the point at which you are going to attach the scale to the wrench. For sake of argument, lets say that the wrench is 18 inches (1.5 feet). If you need to torque to say 90 ft-lbs, you simply divide the 90 by 1.5 (the length in feet that the wrench is) and you come up with 60. So, while keeping the scale perpendicular to the wrench, pull on the scale around the wheel til it reads 60 pounds. TADA!!!! You have now torqued your wheel to 90 ft-lbs. This same process works with the metric system, you just have to measure everything in meters and when it talks about newtons, that is really kilograms (newton is a kilogram acting under 1G of gravity). This works for any torque you need. If you need ounce-inch torques, you simply measure in inches and have a ounce scale (or a pound scale that you divide your ounce inch torque by 16 which will convert it to inch pounds). It is all unit conversions (remember those days sitting in math class wondering why do I need to know this? Guess what.......)

Any questions on torque, let me know. One of my jobs had me way deeper into how you torque, why you torque, and how you figure out what to torque things to than I care to admit to.

 

In my experience as a shadetree mechanic, assuming the pads are in good shape and everything is installed/torqued correctly, what folks perceive as "warped" rotors are actually spots of pad material deposits embedded into the rotor faces in a non-uniform distribution. This is most frequently due to pressing the pads hard into the hot rotors during full stops as party of stop and go driving.


The gentleman who suggested the Italian tune-up was correct in that in most cases pad deposit can be burned/abraded off by getting the rotors HOT and then aggressively braking. The key here is to always keep the rotors moving and not static with the pads pressed into them.


A proper Italian tune-up usually involves the sort of driving that can earn you a traffic ticket, so choose the venue wisely!


FWIW, on customer's cars I always change the rotors when doing brake work if there is any braking feedback in the steering wheel as they are very inexpensive (I generally get new rotor pairs for under $75 total). It only adds about 20 minutes total onto the job, and it assures they will not come back and complain afterwards. Most customers will not do a proper Italian tune up nor do they want you doing one with their car! Also, just as a side note, I always index the wheels to the rotor and use a calibrated torque wrench on lug nuts and other sensitive hardware.

 

Yes, it is necessary on all modern iron brakes, modern brake pads are very hard compared to the old days and rotors are thin for heat dissipation and weight reasons.

Your rotors will be within spec when the new pads go in but wear too thin before the second set of pads is worn out.

Wheel nut torque cannot warp rotors, this is a myth. If you ever did your own brakes you would quickly understand this. The rotors actually don't even need to be fastened in place though often there is a small machine screw or stud that locates the rotor for convenience of inserting wheel bolts. For studded hubs the rotor usually just slips over the studs. The wheel bolts or nuts act on the wheel hub which contains the wheel bearing. The rotor just floats between the hub which holds the wheel studs or bolts and the wheel itself. If wheel nut or bolt torque could warp the hub to which the rotor is attached then the hub itself would have to warp and the bearing would go out of round. Impossible.

 

Dude, that was HILARIOUS! I seriously laughed out loud! You are a riot! Thanks for that!

 

Isn't this the case though? I hardly think those tiny little locating screws are responsible for holding the brake disc on. I've seen plenty of cars in my time that once you remove the wheel at least one of the screws (sometimes both!) is missing, with no ill effects at all, obviously, because the wheel bolts go through the disc.

 

No, it's not the case. Make a note that Jagular acts like he knows what he's talking about, but he doesn't.

He clearly has never done a brake job on an XF. I have. When you remove the wheels, the only thing holding the rotors on is a couple of little washer locks that are put on by the factory to hold the rotors in place until the wheels are bolted on to hold it properly.

When you do a brake job, you don't even need to replace those washer locks. The rotor does not "float." It's sandwiched securely in between the hub and wheel. Whatever torque you put on the wheels bolts is transferred right to the rotor. That's why it warps if you don't torque the wheels properly.

He says that's a myth, but he's flat out wrong. It's well known. If he knew how to google, he would know also. Lots of articles like these are all over the web:

Raybestos Brake Tech School, Part One: Rotors Don't Warp | Hendon Publishing

"The root cause of the uneven wear is one of two things: either the rotor was installed out-of-true with the hub, or the tire was improperly torqued to the hub during the last tire change."

I know this is the case from first hand experience. The next time your rotors warp, loosen the bolts and see if they are all tightened the same amount. They won't be, and that's what caused the warp.

 

What's so funny. As one who has personally driven Crown Vic Interceptors at extra legal speed with operating lights and sirens, on official business, on a regular basis and maintained a fleet police cars, quoting a police car fleet repair article is narrowly focused. Comparing law enforcement and fire apparatus braking use to your average Jaguar driver on his way to the golf course is questionable. They're selling their brake seminars. "Warping" a cast iron rotors vs. uneven wear or run-out. Especially listening to Police car fleet mechanics... you get guys talking like these are F22's. Obviously your torquing lug nuts on a steel police car rim is one over simplification of the many variables involved here.

Here's another article for you which includes a little more detail besides lug nut tightening patterns. Google it... everything on the internet must be true. If it was all so simple. 10 of Your Top Brake Pulsation Questions

#8 illustrates Jagulars point and even GM is quoted saying correct torque of lugs is important.

The forum is better when we respect opinions. Sarcasm and insults rarely solves problems and results in a bunch of smart a$$ come backs from a bunch of smart guys.

What was the question? Should I replace my rotors with the pads when the wear indicator light appears? Sometimes yes and sometimes no. If they are flaking apart or heavily grooved, yes. Sometimes its less costly to replace the rotors than pay for the labor and machine time. Replacing the rotors with the pads is much simpler and less time consuming. You can do this entire job yourself then. There is a detailed brake service article in the HOW TO SECTION. . . If you need help let me know.

Regarding torquing lug nuts, in my experience star... pattern is good, I had more issues with wheels almost falling off because of hand tightened lug nuts plus distractions that caused the mechanic to neglect to torque to spec. Never had it happen on a steel wheel with an impact wrench.

 

+1 on the above!

 

Actually it doesn't, because his claim was that under or over torquing does not cause 'warping'.

From the page you cited:

Lug nut torque is also extremely important. Under-torquing only a single lug nut may create as much as 0.003-inch of lateral runout in a rotor!

The difference is semantics. In particular the constant arguments caused by the distinction between warping and disk thickness variation, trueness, parallelism and perpendicularitry. Lateral runout in one particular direction certainly seems like warping to me. I suspect many people feel the same.

As far as the accuracy of the Raybestos information is concerned, while it may be true that they are interested in selling their courses, which is more accurately gaining friendly eyeballs, I would say it is unlikely that they would deliberately lie in their course materials or handouts. That would be a public relations disaster and completely counter-productive.

At least two currently practicing brake system engineers use the word "warp" in reference to brake rotors. One of them was first at ATE and now at Ford. ATE is the OEM for Jaguar brakes including the ABS system.

The other problem with the post you are trying to defend is the statement that the rotor will "wear too thin before the second set of pads is worn out." Sorry, but that's not how the rotor thickness minimum service specification works. It is a statement of how thin a rotor may be at the time the rotor is placed back in service after machining. Therefore, it is anticipated that there will be some wear during the lifetime of the pads to which they are mated and that in the intervening period no one is going to be measuring the thickness. Accordingly, there is an allowance for the additional wear caused by one set of pads.

 

Perfect example above of what I'm illustrating. Another hijacked thread. .003" is actually splitting hairs. It was a simple question.

After 75,000 kilometers or 46,000+ miles is it reasonable to believe that the rotors should be replaced with the pads? Do I believe the dealer? Yes or No.

Yes and No

 

Front and rear wear differently. The only way to tell if the rotors need to be changed is to turn (remove material to make flat again) them and see if they are still within spec. It's not something you can eye-ball unless you are just trying to make a buck. If you have significant shaking during braking that may be an indication of warpage. Most mechanics will try to turn them unless they see major grooves that indicate turning would be a waste of time. Turning is actually very easy to do. I am not sure why some shops shy away from doing it.

 

Thanks to all for their advice and comments. I had not intended to create such a discussion.

I have rarely worked on my own car since I had a Mini-Minor, and then a Unipower GT with a 1275 Cooper S engine. My efforts were mainly in balancing twin SU carbs by ear and setting mixture with a Colortune spark plug. So whilst I accept that many people prefer to do their own work, it is not something that I wish to do.

I had a "free inspection" at a local aftermarket brake muffler etc shop and they suggested that the rotors were near the limits, but that they too recommend replacing them mainly. In their parts book "base" rotors are not expensive and they said it avoids issues of vibration etc after changing the pads.

So I ended up with the work being done during my 72k service at the dealership.

Again, thanks to all for the comments.

 

glenonoka, a lot of shops make money on selling you parts. They may get the rotors from your local auto parts store for say $50 a rotor, but they will turn around and sell them to you for the retail price of say $80. I am not saying that all shops do this, but it is a fairly common practice. They can easily make an additional $100 on a job by getting you to buy additional parts. Why do you think they almost always will tell you that you need your air cleaner replaced because it is "dirty"? Just saying.

The other thing is when you are doing a brake job and you are faced with the fact of taking the 20 minutes to turn the rotor (time to remove, walk over to the lathe, set it up, turn it, walk back) or to take the 5 minutes to remove the rotor and pull the new one out of the box, which way would you go if you weren't the one paying for parts and getting paid by the job, not by how long it takes you to do the job?

I agree that turning a rotor is a very easy thing to do. In a lot of cases it can solve a lot of the minor issues. So, if you can do it, by all means. But then, I am also one that subscribes to the philosophy of "if the steering wheel doesn't wobble when you brake and the fingernail doesn't catch on the rotor when testing during a pad replacement, then don't worry about turning the rotors".

Edobernig, the need to turn the rotors has a lot of factors that a simple mileage rating can not take into account. Someone that drives in say the desert where it is dry a lot, if they had warped rotors after 50K miles, I would be asking them what the hell they were doing to their car as the rotors may get warm during repeated braking, but they are also allowed to cool at a fairly slow rate and it happens at a fairly even rate. Where you take someone from say Seattle and hearing them talk about warped rotors after 30K miles would not surprise me as they would be very likely to have hot brakes that would get splashed by water from a puddle which would then cause their hot rotors to be cooled unevenly, resulting in the rotor warping due to thermal stesses.