How to check & replace your BMW front disc brakes and rotors

Just keep em. "By the book", they are within spec.

And, lconsider this: rear rotors see MUCH LESS stress than fronts.

And (number two): When was the last time you saw a caliper crush through a rotor?! It takes a SEVERLY deteriorated rotor to have this happen.

The real risk of a rotor below the spec thickness is that it would be more suseptable to warpage, which is not a life safety issue, and hardly ever happens to rear rotors.

Leave em on, replace the pads.

Originally Posted by dvsgene

Donna,

I am not sure how many people on this forum use micrometers to measure runout. Nor do I know of many indys or dealers let alone DIYers whom by nature are trying to save some money would not go out and buy expensive micrometers and digital calipers. Most will take a look at the rotors and if the feel a significant lip on the edge of the rotor (running your fingernail along the edge and getting caught) will replace it or if DIY will usually replace them after going through 2 sets of brake pads(depending on type of pads being used- more abrasive metallics vs less harsh ceramics)

The question really becomes:Will .004 to .010 last before the next brake pad change? If not, you'll find yourself either below recommended limits before the next pad change or replacing the rotors before the new pads you just put are worn out. Thus requiring disassembling the brakes a second time and then replacing the pads again for a third time shortly thereafter. My recommendation would be to just go ahead and replace the rotors since you are putting new pads that are a different composition than the previous set of pads. This is because new rotors and pads will help with bedding in pads. A new surface will bed in easier to bed in than an older rotor with a different pad material embedded in the rotor. And since the measurements indicate differences ranging from .004 to .010 in various parts indicates a bit of uneven wear. To even it back out, turning the rotors with a lathe may be necessary and by turning the rotors you'll wind up removing more material bringing you closer to the 0.72 limit anyway.

To make along story short, I personally don't know how much of a difference .004 to .010 makes or lasts but if I think through the logic in the previous paragraph, I would just replace the rotors. The other piece of information that's missing that I couldn't find was what is the measurement of a brand new rotor. If a brand new is say 0.74, then it's safe to say the current one needs replacing from a measurement standpoint. Hope that logic helps.

Replace the rotors.

Originally Posted by bluebee

Thinking more along these newly logical lines ...

The rear rotors start at ~20mm (0.787 inches) and the wear limit stamped on the rear rotor is 18.4 mm (0.720 inches in the Bentleys).

If the wear limit is 0.720 inches for a rear rotor, I'm assuming that means the wear limit really isn't a wear limit, it's a pad-replacement limit. The true it's-too-thin-now wear limit would be, I'm logically assuming, 0.720 inches minus whatever is normally worn off the rotors during the lifetime of a single set of pads.

Given I have 60K miles, I'm assuming that's two sets of pads in the rear, and given the rotor started off at 0.787 inches and it's now roughly 0.727 inches, that makes it about 0.030 inches per pad set.

That would mean the TRUE wear limit for these rotors is 0.720 minus 0.030 inches, which is 0.690 inches.

Does this make logical sense to you that, in summary, each set of pads eats up about 0.030 inches of rotor, and that you can put pads on the rears at 0.720 inches, so, the true minimum thickness, after those pads are worn, would be 0.690 inches?

Actually that doesn't make sense as the wear limit of the rotor stamped with 0.72, as I understand it, indicates the minimum recommended width before replacement of the rotor becomes CRITICAL as there may be minimal metallic material left on the rotor making the rotors prone to cracking or warping. Now in today's litigious society, I am not sure if that 0.72 limit takes into account a buffer to prevent possibly being open to lawsuits for too thin a specification. But as I stated before how much wear is definitely depending on the type of pads used. Generally, more metal formulation=more wear. In any case, with a average .030 wear rate you would definitely be below the 0.72 limit. Had the data said average wear rate is .010, then with .0724 you would be at .0714 that is a lot better than 0.69. After all, the brakes combined with proper tires are the only thing keeping you from winding up rear ending the car in front of you. I, for one, have never compromised on tires or brakes. It's not worth trying to save on those two parts. Other parts maybe but not brakes and tires.

However, given what you said about 60K miles and having gone through 2 sets of pads, I would replace them if I was in that situation. Besides given the variance of .004 to .010 almost necessitating a need to turn the rotors, and the cost to turn each rotor at a cost of $10-$20 each and the cost of new rotors around $60, you're already at 1/3 to 1/6 the cost of new rotors just to turn them. Why not just replace them then. If the rotors were like $150 each, then turning is more cost effective. My 2 cents.

Originally Posted by larobj63

Just keep em. "By the book", they are within spec.

And, lconsider this: rear rotors see MUCH LESS stress than fronts.

And (number two): When was the last time you saw a caliper crush through a rotor?! It takes a SEVERLY deteriorated rotor to have this happen.

The real risk of a rotor below the spec thickness is that it would be more suseptable to warpage, which is not a life safety issue, and hardly ever happens to rear rotors.

Leave em on, replace the pads.

I agree with this statement.

Rears do wear much less than fronts because fronts take the brunt of the braking force when the car's weight shifts forward during braking and why you'll usually go through two front pads before rear pads. However, when you start having warped rotors you start wasting time wondering if it is the wheel bearings, the rear ball joints or rear rims. Why not eliminate one less diagnostic point for about $120? IF you are tight on a budget than it may be okay to just replace the pads but not something I would do if in that situation.

Originally Posted by dvsgene

I agree with this statement.

Rears do wear much less than fronts because fronts take the brunt of the braking force when the car's weight shifts forward during braking and why you'll usually go through two front pads before rear pads. However, when you start having warped rotors you start wasting time wondering if it is the wheel bearings, the rear ball joints or rear rims. Why not eliminate one less diagnostic point for about $120? IF you are tight on a budget than it may be okay to just replace the pads but not something I would do if in that situation.

lol - I agree with your statement(s), as well.

If you leave them on, you'll likely not reget it, but, you're most of the way there (labor wise), may as well spend the cash and be done with it. BUT from a technical point of view - they ARE within spec - that number is THE threshold, so by rights you can leave them on and the lawyers can all sleep at night.

The OP will have to do a time vs cost vs anxiety analysis, not excluding inflation, the environment (throwing out the old rotors), or the cost of money / avoided interest on the $120 bucks.

Three spreadsheets, two pie charts, a gnant chart, and 5 pages of reports later, she'll wind up replacing them.

Usually the machining limit is .030" greater than the wear limit

This is interesting in that it looks like our rough guess that each set of pads takes off 0.030 inch seems just about right given that is the same as the machining limit.

Do we agree that 30 thousandths of an inch seems to be the magic number here?

IF you are tight on a budget than it may be okay to just replace the pads but not something I would do if in that situation.

Just to be clear, I have all the money I need to buy the right tools and replace the right parts. The cost of good tools and parts is nothing compared to the cost of having the local dealership do the job. So, cost isn't a factor except (and this is a biggie) I don't want to WASTE money and I want to LEARN the right decisions.

You see, you guys intuitively know these things but I don't. I have to learn how to make good decisions.
So, I ask questions for those two reasons, not pure cost per se!
- KNOWLEDGE
- DECISIONS

As long as total cost (for good tools and parts) is below dealership prices, it's "free" in my mind.

Originally Posted by bluebee

Just to be clear, I have all the money I need to buy the right tools and replace the right parts. The cost of good tools and parts is nothing compared to the cost of having the local dealership do the job. So, cost isn't a factor except (and this is a biggie) I don't want to WASTE money and I want to LEARN the right decisions.

You see, you guys intuitively know these things but I don't. I have to learn how to make good decisions.
So, I ask questions for those two reasons, not pure cost per se!
- KNOWLEDGE
- DECISIONS

As long as total cost (for good tools and parts) is below dealership prices, it's "free" in my mind.

Sorry, I should've realized money was not an issue. Hence probably one of a handful of people here on the forum with a Micrometer, brake pad spreader, and myriad of other tools most of us don't have.

So back to the question at hand: As others have stated,you are TECHNICALLY within limits so you can keep them on and by the time the rear pads have worn out, the rotor should be below the .72 threshold or if you decide to turn the rotors to get a clean surface, you will also be below the threshold or the mechanic you bring the rotors to may decide they don't want to turn the rotors so close to threshold. Again, if it was me I would spend the $100 or so and be good for the next 60K miles.

In the end, I may be a waste of money (not much since it is almost completely near the limit) but the alternative is a waste of time as the wheels are already off, a complete analysis has been done, opinions have been gathered. For some people time is money and that may wind up being more than the money saved by keeping it. As you can see, ask 20 people on the forum and you'll probably get 8 different answers and if we were there with you looking at the rotor in person, we might even answer differently. In the end, it is nothing more than opinion and what you learn is based on whom you decide has sufficient knowledge to make the right recommendation.

As others have stated,you are TECHNICALLY within limits

I thank everyone for the answer to the rotor-minimum-width conundrum.

To summarize, there's "no need" to replace the rotors as they are "within spec". Performance or safety is, therefore, "within spec" if I simply replace the rear pads and sensor. It's not a "bad idea" to replace the rear rotors (while I'm in the vicinity); but it's not something that must be done right now. So, I won't likely replace them. (Although I was kind'a looking forward to seeing what that parking brake looked like inside.)

Of course, the new pads will chew off their requisite 30 thousandths of an inch of rotor, so, the NEXT pad replacement will REQUIRE new rear rotors; but who knows if I'll even have the BMW thirty thousand miles from now (I tend to drive cars into things way before their time is up).

As for tools, I was taught, ever since I was little, to always buy the right tools for the job. Whenever I do a job, I ask what tools are needed and, unless they are thousands of dollars (like for alignments), I simply buy the necessary tools. I think this job would cost about $1,200 at the local BMW dealership and I wouldn't learn anything if I did drop it off. I'd have nothing but new brakes for my twelve hundred dollars.

Here, if I do it myself, with the right tools, I'll have the tools, the knowledge, the new brakes, and still lot'sa money left over. The only thing I lose is time and effort but that's the fun part!

I even bought a handy torque wrench that helped me do the torques right on the lug nuts. One problem is that it's a half-inch square on the torque wrench so I couldn't do the 7mm caliper bolts because I only bought a 3/8 inch square hex fitting so I will head out to the auto-parts store for an additional 3/8 inch torque wrench.

It's fun doing things the right way; learning from all of you; and teaching the next person what we've learned together!

Why does it called MicroMeter if you give the measurements in inches?

mw

Egads?! Is the bluebee BMW not on the road?!

We need to have a progress report!! Including percent complete and total earned value!!

What is your SPI? CPI?

C'mon, PM, give us the answers!!





J/K, hope all is well with Bluebee.

Originally Posted by wingspan

Egads?! Is the bluebee BMW not on the road?!

We need to have a progress report!! Including percent complete and total earned value!!

What is your SPI? CPI?

C'mon, PM, give us the answers!!





J/K, hope all is well with Bluebee.

Oh, and, erm, from the latest picture, Bluebee said that he/she wanted to know if you were going to go by the store to get some P21S wheel cleaner?

get some P21S wheel cleaner

OMG. You guys really hate my dirty wheels huh! Where'd you growed' up? In Kalifornia? A car isn't an extension of your personality; it's just wheels! Ooops. Did I say that? I must clean 'em up for the next photo shoot!

Since you guys seem to have such a visceral dislike of my wheels, I guess I'll haf'ta study this science a bit more and experiment with one side on P21S and the other side on Sonax for a long-term report, making sure to take bluebee up in the sandy, salty snow again like I did last week (see pic below).

BTW, I had to look up what P21S meant and I learned a bunch of new terms in doing so, such as "surface safety" and "sensitive wheel coating" and "acid free carnauba wax", etc.

Here is a good article on the how and why of wheel cleaners where they tested P21S in the real world versus Sonax.
http://www.guidetodetailing.com/arti...p?articleId=41

Originally Posted by larobj63

Three spreadsheets, two pie charts, a gnant chart, and 5 pages of reports later, she'll wind up replacing them.

LOL, Nope she decided to keep them! What's next on the list of things to do for your car? I'm looking forward to the DIY write up to this and the next project.

Originally Posted by dvsgene

LOL, Nope she decided to keep them! What's next on the list of things to do for your car? I'm looking forward to the DIY write up to this and the next project.

Right - go figure! I thought maybe she could use the old rotors as paper-weights to keep things in order...

BlueBee - you summerized things well, and I'm glad you've made a sensible decision. Don't let my or anyone else's smart a$$ comments bother you - like you say, this info is great for someone else down the line.... You analyse the snot out of things, but we all learn in our own ways, so

Originally Posted by bluebee

I thank everyone for the answer to the rotor-minimum-width conundrum.

To summarize, there's "no need" to replace the rotors as they are "within spec". Performance or safety is, therefore, "within spec" if I simply replace the rear pads and sensor. It's not a "bad idea" to replace the rear rotors (while I'm in the vicinity); but it's not something that must be done right now. So, I won't likely replace them. (Although I was kind'a looking forward to seeing what that parking brake looked like inside.)

Of course, the new pads will chew off their requisite 30 thousandths of an inch of rotor, so, the NEXT pad replacement will REQUIRE new rear rotors; but who knows if I'll even have the BMW thirty thousand miles from now (I tend to drive cars into things way before their time is up).

As for tools, I was taught, ever since I was little, to always buy the right tools for the job. Whenever I do a job, I ask what tools are needed and, unless they are thousands of dollars (like for alignments), I simply buy the necessary tools. I think this job would cost about $1,200 at the local BMW dealership and I wouldn't learn anything if I did drop it off. I'd have nothing but new brakes for my twelve hundred dollars.

Here, if I do it myself, with the right tools, I'll have the tools, the knowledge, the new brakes, and still lot'sa money left over. The only thing I lose is time and effort but that's the fun part!

I even bought a handy torque wrench that helped me do the torques right on the lug nuts. One problem is that it's a half-inch square on the torque wrench so I couldn't do the 7mm caliper bolts because I only bought a 3/8 inch square hex fitting so I will head out to the auto-parts store for an additional 3/8 inch torque wrench.

It's fun doing things the right way; learning from all of you; and teaching the next person what we've learned together!

You don't need to buy an additional 3/8 torque wrench. Just buy something like this adapter set: click here .

I would agree with pretty much all you said above. The thing is that you can't go wrong any way you choose with the rotor. If you replace it now, you are right. If you replace it during next pads, you are right.

Bear in mind that even at minimum thickness the rotor still have a safety factor built in, and you can still go thinner before it is "too thin".

Also, there are grooves on the side of the rotor to show you the minimum thickness. When you reach the groove, you reached the minimum. (turn the rotor around and look for it. Its about 1/2" long.). So you don't have to have a micrometer, since you can have a very clear visual of where the thickness is. (Sorry, I don't have a picture).

In addition to the thickness with the micrometer, also factor in how badly the rotor is 'grooved' or smooth. If it is smooth, and you going to install pads that are not very abrasive (like the Axxis for example), than you are in a better shape. If your rotor, for the same thickness as above, is very scarred and grooved (like a vinyl record), and you are going to use abrasive pads like the OEM, then you are in worse shape then the one above.

Installing new pads on grooved rotor will initially give you smaller contact area, and less effective brakes. Thats until the pads match the shape of the grooves on the rotor. Then you have grooved pads and rotors... It just gets worse from that point.

Otherwise, if it was me, and the rotors are smooth, and I am not racing/tracking my car, I will go a little below the recommended thickness and keep an eye on it from time to time.

But if I am replacing the rotors, then I MUST replace the pads as well. Because I want them both to start from perfectly smooth. As a poster above me said, I have the Axxis down to half their thickness and the rotor looks new.

mw

Originally Posted by MatWiz

You don't need to buy an additional 3/8 torque wrench. Just buy something like this adapter set: click here .

Actually, Adapters and extensions will throw off the torque reading. At least that's what it says on the instructions that came with my torque wrench. More importantly, bluebee is not concerned with costs of the having the right tools as it is "free" considering the cost at the dealers to do these things. Not being sarcastic, just re-stating what I've been told by bluebee. AND also learned bluebee likes to do it the "right" way. So adapters and extensions are out unless you can provide a documented formula indicating the adjustment needed for using a 1/2" to 3/8" adapter with the torque wrench. I think I'm beginning to understand you Bluebee...

Originally Posted by dvsgene

Actually, Adapters and extensions will throw off the torque reading. At least that's what it says on the instructions that came with my torque wrench. More importantly, bluebee is not concerned with costs of the having the right tools as it is "free" considering the cost at the dealers to do these things. Not being sarcastic, just re-stating what I've been told by bluebee. AND also learned bluebee likes to do it the "right" way. So adapters and extensions are out unless you can provide a documented formula indicating the adjustment needed for using a 1/2" to 3/8" adapter with the torque wrench. I think I'm beginning to understand you Bluebee...

Yeah it will throw it off. Especially if you put a really SKINNY and LONG flexible extension.

Torque wrenches need to be professionally calibrated before first used, and then periodically sent to recalibration.

With all that in mind, and seeing what a long extension he is using in his picture, I don't think he will be too concerned about 1/2" rigid extension.

mw

So - what's the latest on Blubee BMW??

Would I be rude to say wash your damned car and change the @*%) brakes already?
Maybe its me, but this whole thread was a frustrating read. I take the following steps:

1) Call my parts guy "Autopost? Send me the most expensive pads you have for a 97 540i"
2) Wait 30 mins for parts guy. Sign the bill, pay next month. (Drink a 6-pack)
3) Do brake job in 20 minutes
4) Drive.

Seriously... gah

While watching Terminator 2, I noticed Ah-nuld using a torque wrench on a starter.. makes sense for someone with the strength of a terminator. If you are of superhuman strength, then okay...
otherwise if you're using a torque wrench on the lug bolts...... you probably shouldn't be working on a car.



that's my 2¢


That being said, my father lives in California, I'm about ready to send him to your house to do the brakes for you. hah

Originally Posted by Fragzem

if you're using a torque wrench on the lug bolts...... you probably shouldn't be working on a car.



that's my 2¢

I've been working on my car for a loooong time, and my lug bolts/nuts are ALWAYS tightened using a torque wrench. The last time I trusted someone using a torque stick, I went through 3 busted impact gun sockets before it came off.

Originally Posted by Mr Hyde

I've been working on my car for a loooong time, and my lug bolts/nuts are ALWAYS tightened using a torque wrench. The last time I trusted someone using a torque stick, I went through 3 busted impact gun sockets before it came off.

Plus not using a torque wrench is a good way to distort/warp your rotors!!

If you can't feel for when something is tight enough, that's sad.

Some people do use gureilla strength to tighten wheels, and that's not necessary.. hand those ppl a torque wrench.

Originally Posted by MatWiz

Why does it called MicroMeter if you give the measurements in inches?

mw

The dictionary says that the etymology of the word comes from the Greek "micros" meaning small, and "metros" meaning measure. Makes sense.

The extension is too long my friend, you won't get to 89 ft/lb with that long extension, it won't be accurate. Part of your force to torque your lug bolts went to the twisting of metal on your extension tube.

It's a very interesting thread in that you value the correct knowledge and decision over your valuable time (that most of the people with a full time job and family don't have). Most people just won't bother when the rotor nears min thickness, they just replace them since they are replacing the pad also. I look at it as a value added activity, not good plating the project. But then, it's your time, you do what you like.

Originally Posted by bluebee

It's fun doing things the right way; learning from all of you; and teaching the next person what we've learned together!