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E90/E91/E92/E93 (2006 - 2013)
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  #1  
Old 10-14-2013, 11:58 PM
jaj jaj is offline
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Brake Wear Questions

About 8k miles ago, dealer reported brake pad thickness of 9mm and 7mm, front and rear, respectively. I now have a warning light indicating rear pads need to be replaced while the dash computer shows 27k miles before front pads need servicing. Does this make sense? Can the rear pads have gone down so much in that interval so as to trigger the sensor? Is the front pad projection based on any actual measurement? I am now about 1k miles and two months out of warranty so repairs will be out of pocket. Please advise.
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  #2  
Old 10-15-2013, 03:37 AM
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BostonB6 BostonB6 is offline
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They can if the caliper is sticking. Best to have it checked. In any case, I'd complain about the previous measurement and wear reading. Maybe since you're so close to warranty, they might help you out a bit. Fanciful thinking I know, but stranger things have happened.
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Old 10-15-2013, 03:56 AM
hondo402000 hondo402000 is offline
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you can replace the pads without replacing the rotors if you can do it your self, that will save you a lot of money
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Old 10-15-2013, 07:42 AM
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Typical for the rears to wear first on this car. Is this the first brake job or were there previous under the maintenance plan at what mileage?

If this is the first you got pretty good wear.
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Old 10-15-2013, 07:53 AM
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Originally Posted by laser View Post
Typical for the rears to wear first on this car. Is this the first brake job or were there previous under the maintenance plan at what mileage?

If this is the first you got pretty good wear.
First work done on the brakes. Thanks for the reply.
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  #6  
Old 10-15-2013, 08:00 AM
fdriller9 fdriller9 is offline
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Originally Posted by hondo402000 View Post
you can replace the pads without replacing the rotors if you can do it your self, that will save you a lot of money
This is bad advice. Sorry to call you out but if the OP is asking the questions he is, I doubt he/she knows how to measure the rotor thickness with a micrometer.

You should replace rotors at the same time. There is no sense in risking the failure or for paying labor twice.
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Old 10-15-2013, 09:01 AM
surfcity335i surfcity335i is offline
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Originally Posted by fdriller9 View Post
This is bad advice. Sorry to call you out but if the OP is asking the questions he is, I doubt he/she knows how to measure the rotor thickness with a micrometer.

You should replace rotors at the same time. There is no sense in risking the failure or for paying labor twice.
If there is no vibration or other symptoms of rotor deformity, I'd just replace the pads and disregard what the micrometer might say at that mileage. The world will not come to an end because your rotors are a touch under the minimum spec. But that's just me.
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Old 10-15-2013, 10:23 AM
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Originally Posted by surfcity335i View Post
If there is no vibration or other symptoms of rotor deformity, I'd just replace the pads and disregard what the micrometer might say at that mileage. The world will not come to an end because your rotors are a touch under the minimum spec. But that's just me.
Yes, it is just you--this is also bad advice.

The micrometer trumps mileage. It does not matter whether the rotor has logged 1,000 or 100,000 miles of service; if it has reached minimum thickness it is scrap. A rotor that measures at or below the minimum when you install new pads will wear well below the minimum during the life of the pads. That's the whole point of the minimum specification.

There may indeed be no symptoms at the outset. They will certainly develop later as the rotor becomes more susceptible to warping and cracking.

To advise an inexperienced stranger to perform cheap, shoddy maintenance on a critical safety system is irresponsible. (Yes, leaving a worn-out rotor in service is being cheap.) Your decision to take shortcuts is your prerogative. Teach others to do the job right and let them acquire enough understanding to make their own decisions about cutting corners.
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  #9  
Old 10-15-2013, 10:28 AM
surfcity335i surfcity335i is offline
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Originally Posted by Zeichen311 View Post
There may indeed be no symptoms at the outset. They will certainly develop later as the rotor becomes more susceptible to warping and cracking.
I gotta disagree with you there. If it were even close to a certainty, or even a likelihood, I'd replace them.

But I will back off my "disregard what the micrometer might say" statement. Obviously, I'm talking about a rotor that is borderline. If I've got a rotor with a spec of 19.8mm and the mic says 19.75 I'm leaving it on. But again, that's just me.

Last edited by surfcity335i; 10-15-2013 at 10:40 AM.
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  #10  
Old 10-15-2013, 07:07 PM
jaj jaj is offline
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I appreciate all the input. I will go along with the recommendation of the dealership's service department re replacement of rotors in addition to the pads. Among the things I'm wondering though, is if it's possible that the displayed service message is in error, perhaps due to a faulty sensor. Assuming the pad measurement of 7mm 8000 miles ago was accurate, I would not have expected the pads to wear down to the point of tripping the sensor so soon. Any thoughts on that would be most welcome.
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  #11  
Old 10-15-2013, 07:08 PM
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My rear rotors went through 2 sets of pads before being replaced.
Indy last inspected and replaced rear rotor at 143k miles, he didn't say that front rotor needed to be replaced.
My original front rotor is at 150k miles. The 2nd pad went in at 102k miles.
I will definitely replace the front rotor on the next pad change.
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  #12  
Old 10-15-2013, 08:04 PM
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Originally Posted by jaj View Post
Among the things I'm wondering though, is if it's possible that the displayed service message is in error, perhaps due to a faulty sensor. Assuming the pad measurement of 7mm 8000 miles ago was accurate, I would not have expected the pads to wear down to the point of tripping the sensor so soon.
The warning light comes on when the brake rotor abrades through the tip of a sensor (front or rear) and severs the embedded wire loop. Of course, if anything else cuts or disconnects the sensor wire, you will get a BRAKE warning light even though the pads are fine. It is unusual but can happen.

Another possibility is that the guide pins on (at least) your right rear brake caliper became contaminated with dirt, causing the caliper to bind instead of floating freely on the pins. This can dramatically shorten the life of the inboard pad (where the sensor is located) and trip the sensor long before the other three pads on the axle are used up. In other words, it is indeed possible to chew through 5mm of friction material (7mm - 2mm min, at which sensor should trip) in only 8000 mi..though doing so may be a symptom of another problem.

The wear sensors--one at right rear, one at left front--only provide a signal that a pad has worn out. They do not provide any direct measurement to the CBS (Condition Based Service) system. The dash (& iDrive) display of mileage until the next brake service is an estimate. It is computed from braking force, braking duration, ambient temperature and other variables in a mathematical model of the thermal and mechanical properties of the OE pads, rotors and brake system.

It's pretty sophisticated but can still be wrong, especially if your driving patterns change over time. Mine was reprogrammed once by the dealer while under warranty, after it suggested my rear brakes needed service even though a physical inspection revealed about 50% usable material remaining. After that it was reasonably on target, until track days (with pad changes before & after) entered the picture. Now it is so confused it's been counting up for the past six months.
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  #13  
Old 10-16-2013, 02:42 AM
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My car has 53,600 on it and shows 3700 left on the fronts and 9000 left on the rears. At my 30,000 service the indicator showed it was time to get new rear pads which I thought was odd as fronts usually wear out first. The SA showed me that I had plenty of pad left and said the wear indicator was giving a false positive and they would reset it. 90% of my driving is all highway so this made sense to me although not to reassuring knowing that the indicator readings could be wrong. I guess nothing beats a visual inspection!
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Old 10-16-2013, 03:08 AM
hondo402000 hondo402000 is offline
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Quote:
Originally Posted by fdriller9 View Post
This is bad advice. Sorry to call you out but if the OP is asking the questions he is, I doubt he/she knows how to measure the rotor thickness with a micrometer.

You should replace rotors at the same time. There is no sense in risking the failure or for paying labor twice.
well first I would not recommend using rotors that are below min thickness at the time of pad change out, so if they are above min, it would be fine to not replace them but I also know that BMW is not stupid and ALL manufacturers included a 10 percent safety factor in just about everything they make, so if min thickness is 19.8 mm you would still have 1.9 mm left before it got to the real min thickness. And its not like the OP is on a race track with hard braking where the rotors get cherry red,
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Old 10-16-2013, 03:12 AM
hondo402000 hondo402000 is offline
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Originally Posted by 3284me View Post
My car has 53,600 on it and shows 3700 left on the fronts and 9000 left on the rears. At my 30,000 service the indicator showed it was time to get new rear pads which I thought was odd as fronts usually wear out first. The SA showed me that I had plenty of pad left and said the wear indicator was giving a false positive and they would reset it. 90% of my driving is all highway so this made sense to me although not to reassuring knowing that the indicator readings could be wrong. I guess nothing beats a visual inspection!
I could be mistaken but the pad sensor only alerts you when the sensor wire is worn thru and you have about 1mm of pad left, I dont think it can calculate how many miles you have left on your pads.

and also if BMW recommends replacing rotors and pads all the time then who cares if you chew the rotors up with metal to metal contact.
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Old 10-16-2013, 12:05 PM
R1200 R1200 is offline
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Once a sensor trips it cannot be reset. It has to be replaced before the system can be reset, at least that was how it happened to me.

I too insist that new pads require new rotors. The old rotors would need to be turned down to provide a fresh and flat surface to the fresh and flat new brake pads. The wear pattern on used rotors will promote corrosion before the new pads take that shape. It's false economy, in my experience.

That said there is no reason to buy BMW pads, rotors and sensors. They are available elsewhere on-line and easy to replace.
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Old 10-18-2013, 03:19 PM
grazhoppa grazhoppa is offline
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Originally Posted by hondo402000 View Post
you can replace the pads without replacing the rotors if you can do it your self, that will save you a lot of money
I changed my rear pads (Wagner) and sensor out last month at 20k miles. The rotors are fine and did not need to be replaced or turned. BMW's standard is to change the rotors but I think it's unnecessary for every brake job. Don't let the pads wear out so that it grinds into the rotor. Also you can tell if your rotor is warped when you brake and feel the vibration. To the mechanically inclined, you can do your own brake jobs for less than $100 or take it to the dealer and listen to their recommendation, bend over and pay over $1000 if that's your cup of tea. Also, BMW OEM pads put out way too much dust and not necessarily better than aftermarket pads.

If you really want to replace the rotors, order the pads yourself from places like Rockauto and take to an indy shop. There's no magic to a BMW brake job. It's even easier to replace than a Honda or Toyota.
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Old 10-19-2013, 03:18 PM
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Originally Posted by grazhoppa View Post
I changed my rear pads (Wagner) and sensor out last month at 20k miles. The rotors are fine and did not need to be replaced or turned. BMW's standard is to change the rotors but I think it's unnecessary for every brake job. Don't let the pads wear out so that it grinds into the rotor. Also you can tell if your rotor is warped when you brake and feel the vibration. To the mechanically inclined, you can do your own brake jobs for less than $100 or take it to the dealer and listen to their recommendation, bend over and pay over $1000 if that's your cup of tea. Also, BMW OEM pads put out way too much dust and not necessarily better than aftermarket pads.

If you really want to replace the rotors, order the pads yourself from places like Rockauto and take to an indy shop. There's no magic to a BMW brake job. It's even easier to replace than a Honda or Toyota.
One thing to add here: a BMW dealer will anyways recommend replacing rotors with pads UNLESS the car is still in warranty(maintenance). Then they measure (with a loose fit I suspect as well).

Rotors are intended to be used up to a specified thickness. That is why they have the specs. Money is the only reason to replace without measuring the thickness, and the fact that they don't replace when the svc is covered under maintenance says a lot.
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Old 10-19-2013, 07:54 PM
Pilgrim Pilgrim is offline
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Originally Posted by hondo402000 View Post

and also if BMW recommends replacing rotors and pads all the time then who cares if you chew the rotors up with metal to metal contact.
Because any metal to metal contact makes the brakes MUCH less effective than pad-to-rotor contact. Under any heavy use with metal to metal contact, you could get severe brake fade and have a collision as a result.
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Old 10-20-2013, 06:58 AM
R1200 R1200 is offline
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I still feel that renewing rotors and pads at the same time is proper to ensure that the flat surfaces mate and wear correctly. Rotor corrosion will eat away at pads and it won't be long until another set of pads will be required.

Think of it the other way... would you use half worn pads on new rotors? Not likely, unless it was an emergency like metal to metal contact and you happen to have saved old pads from a previous brake job.

Rotors are manufactured now so that turning them down is not an option. The idea is that both pads and rotors wear out at the same time (unless you have a sticky piston or guide pin. BTW, BMW guide pins are stainless steel so not a likely problem).

It's false economy to not change both rotors and pads when doing a brake job, in my opinion.
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Old 10-20-2013, 03:12 PM
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Originally Posted by R1200 View Post
I still feel that renewing rotors and pads at the same time is proper to ensure that the flat surfaces mate and wear correctly. Rotor corrosion will eat away at pads and it won't be long until another set of pads will be required. ...

BTW, BMW guide pins are stainless steel so not a likely problem
This is easily prevented by properly "bedding" the new pads to the rotor. This is a process whereby you quickly (matter of minutes) mate the pads and rotors to a perfect fit. It does not take thousands of miles; the job is easily done in a bit over ten. Most people with more than a few track days under their belts are familiar with the procedure, as it is a critical final step when switching from street to track pads and back on the same rotors. For the reasons you noted, it's also necessary on a routine brake job that does not include rotors. Basically, it's a controlled series of increasingly hard stops to get the brakes just shy of fade temperature and develop matching profiles on the pads & rotors.

Regarding guide pins, crud can accumulate on stainless steel even if the pins themselves have not corroded. Here in the snow/salt belt I have yet to complete a regular brake service that did not involve polishing accumulated, rock-hard gunk of unknown composition off of every guide pin (even when all the protective caps are still present ). In at least one case it was responsible for exactly the sympton I described (accelerated inner pad wear, resolved by cleaning the pins). Beats me how common it may be but it does happen.
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Last edited by Zeichen311; 10-20-2013 at 03:14 PM.
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Old 10-20-2013, 05:31 PM
R1200 R1200 is offline
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I agree with your method to burnish the pads and rotors. It's required even for new brakes and rotors before full braking power can be achieved. I have in the past used new pads with old rotors and the profile does wear in eventually but the risk is high that corrosion will build on the rotors before the pad surface wears into place. I've seen it too many times especially on rear brakes in snow country. The inner surface of the rotor will show only 50% contact with the pad and this gets lower and lower as corrosion builds on the rotors.

My only point is that rotors are not made with enough material these days to be turned down so expecting them to last through two sets of pads may not be realistic. After 120k km my front pads tripped the sensor and at that point I renewed rotors, pads and sensor. One of the front rotors was slightly warped as well. The rear pads tripped the sensor at about 140k km.

Regarding guide pins... I have cleaned too many over the years and they are a constant source of problems on other makes of cars. I was pleased to find that the BMW pins were stainless and clean when I pulled them but old habits die hard and I still check them once a year when mounting snow tires.

You really don't need new rotors with new pads, I agree but it's just something that I do.

As a side note, consider cold galvanizing compound as used by the US Navy. It's great when applied to the non contact surfaces of rotors and to exposed brake parts to fight corrosion. The surfaces remain light grey in colour and heat does not affect the coating. Even if you get a little on the working surface of a rotor the pads will scrape the zinc off.

Enough on this topic from me.
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Old 10-20-2013, 07:30 PM
Pilgrim Pilgrim is offline
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Part of the reason that BMW and a few other manufacturers recommend replacing the rotors at the same time as the pads is that they don't make rotors as thick (and heavy) as they used to.

Reducing the thickness of the rotor also reduces unsprung weight. This has very desirable consequences in terms of the suspension performance. That's a reason BMW has gone this way. The downside is that rotors reach minimum thickness more quickly and need to be replaced more often to prevent warping.

On a practical basis, for our personal use I generally figure that I can get away with replacing one set of brake pads without replacing rotors. The second time around I replace rotors. However, this is on my wife's BMW which is not driven hard or aggressively, and which does a great deal of highway driving.

Which reminds me - I need to get a new micrometer for rotors....
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