swanicyouth
Member
My Findings Regarding Pedal Pulsation and Brake "Judder"
So, I've owned multiple BMWs, and after a while - all of them have suffered from some degree of pedal pulsation, braking harshness, and brake judder. Nothing bad - and probably nothing a "normal" car driver would notice. In the past, I haven't worried too much about it I'd just hang new rotors when I replaced the pads and the problem would go away for quite a while. However, the BMW in question has M drilled rotors that cost $500+ a set... It's time for a permanent diagnosis and solution.
Interestingly enough, I've been using non-metallic (first organic / later ceramic) brake pads on all these cars. First thing I always did when purchasing a BMW was remove the dusty stock pads and replace them with low dust pads. Years ago, this mean using "organic" or asbestos type pads - today it means using ceramic pads. These type of pads are noted for being more "rotor friendly" than semi-metallic pads. So, they shouldn't be causing an issue - right?
These cars have always been driven with care. Since my experience tells me these cars are sensitive to pedal pulsation and braking roughness, I have always tried to using the brakes quite gently - "babied" the brakes. Conventional wisdom tells us pedal pulsation is caused by "warped" rotors that have excessive lateral runout. Shimmy of the steering wheel while braking also tends to point to the front rotors - assuming the suspension on the vehicle is tight and the wheels are fairly in balance.
It's commonly believed that rotors become warped from being overheated. The problem is, I've never come close to overheating the rotors on any of my vehicles. There is also another school of thought that says most pedal pulsation is caused not by warped rotors, but by brake pad deposits that have become embedded in the rotors. Obviously - replacing the rotors is a fix (possibly temporary) for either situation.
It's not uncommon for someone to experience this braking issue and take their car in for service. The most common fix is to replace the pads & rotors. Problem is fixed...Or is it? It's also not uncommon for the problem to return. I've seen dealers put 5 and 6 sets of rotors and pads on cars under warranty to fix pedal pulsation. New cars have been bought back by manufacturers in the Lemon Law due to pedal pulsation issues. While pad/rotor replacement fixes the problem temporarily, it seems to return for some drivers in a matter of weeks or months. So, why do some people have an issue with this and some don't on the same car? Why do some people go through multiple sets of new pads and rotors - only to have the problem fixed temporarily? What is the root cause of this?
Three well written articles on is subject are linked below:
http://www.centricparts.com/files/technical%20guides/CommonCausesofBrakeJudder.pdf
http://www.powerstop.com/what-causes-brake-pulsation/
http://www.racingbrake.com/v/main/pad_warning.asp?full=yes
First thing one needs to do is check the front end and balance/trueness of wheels. But, assuming the issue is being caused by something in the front end - this would not explain why many have had temporary fixes by replacing pads and rotors. All things being the same, if the pulsation is caused by something amiss in the front end - replacing/machining rotors and hanging pads should not improve the problem. But, any damaged or worn parts in the front end will need to be replaced first.
After checking the front end, you need to pull the wheels and check the front rotors for excessive lateral runout. To do this, you will need a mounted dial indicator. The procedure is quite simple - you just mount the dial indicator, zero it, and spin the rotor with the needle tip contacting the rotor.
If your rotors are slotted/drilled - you will have to stick to the inner/outer perimeter of the rotor to get an accurate reading. The holes/slots will make the gauge jump excessively when the measuring pin hits them.
As I suspected, my rotors have virtually no lateral runout. The runout reading was < 0.001". I'm not sure if BMW has an official "runout spec"...But, my experience tell me that once rotors reach about 0.004" of lateral runout - pedal pulsation may become a problem (depending on vehicle).
So, what gives? No runout. Good front end. No diagnosable issue... Well, once everything else proved to be good, you have to look at pad deposits on the rotors. You likely won't be able to visibly see them - but you sure can feel something amiss when you hit the brakes. The above articles tell use that pad deposits are responsible for most pedal pulsation - not rotor warpage.
Quite frankly - I'm not (wasn't) a believer. I'm from the old school with semi-metallic pads and warped rotors. But, since the "fix" is free - I decided to give it a shot. What is the fix? The fix involves warming your brakes up to operating temperature and making several (~10) panic stops from 60mph to 5mph (right before ABS kicks in), while letting the brakes cool a little bit in between. After that, you drive normally around trying not to stop to let the brakes cool. So, this is what I set out to do.
Another fix is to sand the rotor surfaces rotor down with garnet paper the. Clean them well with Brake Kleen. Garnet paper is old school "sand paper" that is used for wood working that doesn't have certain metals in it. Regular sand paper is not recommended to attempt to resurface rotors, as the metals in it can actually embed themselves in the rotors, possibly creating hot spots while braking - making braking roughness worse. I was skipping the garnet paper step - but will do it next time I have the calipers off.
After I buttoned it up - I realized it was going to be sort of hard to find an area to do these "panic stops" without getting killed or getting arrested. Basically, the highway is out unless it's 3am and your the only one on it. So, I went out to where there was a bunch of farms and stuff and started beating (gasp!) on my brakes. I probably did 15 panic stops. The pedal started to feel "different" - maybe a bit spongy at first. My wheels were filthy and trashed afterwards. Oddly, there seemed to be a bit of grease slung out onto the wheel. Not sure where it came from - nothing is leaking.
Now, I'm the biggest skeptic when it comes to stuff like this and the first person to call BS. But, it actually worked. The slight pedal pulsation that was so annoying was 98% gone. I mean it was a marked difference. I was quite stunned - as this went against my gut feeling - which is usually right. So, after going through this and my experience with brakes in general; I've come up with a few THEORIES:
1. Performance type cars today that are experiencing pedal pulsation likely have a pad deposit issue.
2. Rotor deposits are more likely from "rotor friendly" ceramic / low dust pads - since these pads don't have the aggressiveness (or abrasive friction) of the older semi-metallic like pads to self-clean the deposits off the rotor(s).
3. Most new cars that have had multiple rotor/pad replacements under warranty due to "warped rotors"; only to have the issue return - likely have pad deposit issues. Obviously, new rotors have no deposits - so the issue is temporarily fixed. However, since the driver's driving habits have not changed - the issue will return in weeks to months.
4. "Babying" brakes (especially on performance cars) is probably a big contributor to this phenomena. Occasional aggressive use of the brakes actually helps to clean the deposits off the rotors. Performance cars braking systems are designed to run hotter, cool better, and take more abuse. If the brakes are "babied" the rotors may never get hot enough for the pads to self clean the deposits off the rotors via abrasive friction.
5. Pad deposit issues are less likely if you keep the more aggressive / factory dust-maker BMW pads. This makes sense, as the more aggressive pads clean the rotors better / easier (more dust) and likely just generally run hotter. This also makes sense as to why is issue has not been caught by engineering. Although, pad deposits can occurs with factory pads which is likely due to driving habits.
6. Ceramic pads tend to work more by adherent friction (leaves layer on rotor) than abrasive friction (like sanding) opposed to semi-metallics which are likely vice-versa at normal operating temperature (hence more dust - but less deposits)
**** One practice I am now avoiding, which I learned from the above articles, is keeping my foot tightly on the brake at stop lights. Apparently, this is a big cause of pad deposits. At the stop light, when the rotor is hot - and brakes are held on with a lot of force - pad deposits can occur from transferring friction material to the rotor unevenly. So, it is recommended to just keep your foot on the brake as light as possible to keep the vehicle from moving. Something I used to do the opposite of.
I'm really curious of others' experiences with pedal pulsation, the fixes, and what you think of my "theories".
So, I've owned multiple BMWs, and after a while - all of them have suffered from some degree of pedal pulsation, braking harshness, and brake judder. Nothing bad - and probably nothing a "normal" car driver would notice. In the past, I haven't worried too much about it I'd just hang new rotors when I replaced the pads and the problem would go away for quite a while. However, the BMW in question has M drilled rotors that cost $500+ a set... It's time for a permanent diagnosis and solution.
Interestingly enough, I've been using non-metallic (first organic / later ceramic) brake pads on all these cars. First thing I always did when purchasing a BMW was remove the dusty stock pads and replace them with low dust pads. Years ago, this mean using "organic" or asbestos type pads - today it means using ceramic pads. These type of pads are noted for being more "rotor friendly" than semi-metallic pads. So, they shouldn't be causing an issue - right?
These cars have always been driven with care. Since my experience tells me these cars are sensitive to pedal pulsation and braking roughness, I have always tried to using the brakes quite gently - "babied" the brakes. Conventional wisdom tells us pedal pulsation is caused by "warped" rotors that have excessive lateral runout. Shimmy of the steering wheel while braking also tends to point to the front rotors - assuming the suspension on the vehicle is tight and the wheels are fairly in balance.
It's commonly believed that rotors become warped from being overheated. The problem is, I've never come close to overheating the rotors on any of my vehicles. There is also another school of thought that says most pedal pulsation is caused not by warped rotors, but by brake pad deposits that have become embedded in the rotors. Obviously - replacing the rotors is a fix (possibly temporary) for either situation.
It's not uncommon for someone to experience this braking issue and take their car in for service. The most common fix is to replace the pads & rotors. Problem is fixed...Or is it? It's also not uncommon for the problem to return. I've seen dealers put 5 and 6 sets of rotors and pads on cars under warranty to fix pedal pulsation. New cars have been bought back by manufacturers in the Lemon Law due to pedal pulsation issues. While pad/rotor replacement fixes the problem temporarily, it seems to return for some drivers in a matter of weeks or months. So, why do some people have an issue with this and some don't on the same car? Why do some people go through multiple sets of new pads and rotors - only to have the problem fixed temporarily? What is the root cause of this?
Three well written articles on is subject are linked below:
http://www.centricparts.com/files/technical%20guides/CommonCausesofBrakeJudder.pdf
http://www.powerstop.com/what-causes-brake-pulsation/
http://www.racingbrake.com/v/main/pad_warning.asp?full=yes
First thing one needs to do is check the front end and balance/trueness of wheels. But, assuming the issue is being caused by something in the front end - this would not explain why many have had temporary fixes by replacing pads and rotors. All things being the same, if the pulsation is caused by something amiss in the front end - replacing/machining rotors and hanging pads should not improve the problem. But, any damaged or worn parts in the front end will need to be replaced first.
After checking the front end, you need to pull the wheels and check the front rotors for excessive lateral runout. To do this, you will need a mounted dial indicator. The procedure is quite simple - you just mount the dial indicator, zero it, and spin the rotor with the needle tip contacting the rotor.
If your rotors are slotted/drilled - you will have to stick to the inner/outer perimeter of the rotor to get an accurate reading. The holes/slots will make the gauge jump excessively when the measuring pin hits them.
As I suspected, my rotors have virtually no lateral runout. The runout reading was < 0.001". I'm not sure if BMW has an official "runout spec"...But, my experience tell me that once rotors reach about 0.004" of lateral runout - pedal pulsation may become a problem (depending on vehicle).
So, what gives? No runout. Good front end. No diagnosable issue... Well, once everything else proved to be good, you have to look at pad deposits on the rotors. You likely won't be able to visibly see them - but you sure can feel something amiss when you hit the brakes. The above articles tell use that pad deposits are responsible for most pedal pulsation - not rotor warpage.
Quite frankly - I'm not (wasn't) a believer. I'm from the old school with semi-metallic pads and warped rotors. But, since the "fix" is free - I decided to give it a shot. What is the fix? The fix involves warming your brakes up to operating temperature and making several (~10) panic stops from 60mph to 5mph (right before ABS kicks in), while letting the brakes cool a little bit in between. After that, you drive normally around trying not to stop to let the brakes cool. So, this is what I set out to do.
Another fix is to sand the rotor surfaces rotor down with garnet paper the. Clean them well with Brake Kleen. Garnet paper is old school "sand paper" that is used for wood working that doesn't have certain metals in it. Regular sand paper is not recommended to attempt to resurface rotors, as the metals in it can actually embed themselves in the rotors, possibly creating hot spots while braking - making braking roughness worse. I was skipping the garnet paper step - but will do it next time I have the calipers off.
After I buttoned it up - I realized it was going to be sort of hard to find an area to do these "panic stops" without getting killed or getting arrested. Basically, the highway is out unless it's 3am and your the only one on it. So, I went out to where there was a bunch of farms and stuff and started beating (gasp!) on my brakes. I probably did 15 panic stops. The pedal started to feel "different" - maybe a bit spongy at first. My wheels were filthy and trashed afterwards. Oddly, there seemed to be a bit of grease slung out onto the wheel. Not sure where it came from - nothing is leaking.
Now, I'm the biggest skeptic when it comes to stuff like this and the first person to call BS. But, it actually worked. The slight pedal pulsation that was so annoying was 98% gone. I mean it was a marked difference. I was quite stunned - as this went against my gut feeling - which is usually right. So, after going through this and my experience with brakes in general; I've come up with a few THEORIES:
1. Performance type cars today that are experiencing pedal pulsation likely have a pad deposit issue.
2. Rotor deposits are more likely from "rotor friendly" ceramic / low dust pads - since these pads don't have the aggressiveness (or abrasive friction) of the older semi-metallic like pads to self-clean the deposits off the rotor(s).
3. Most new cars that have had multiple rotor/pad replacements under warranty due to "warped rotors"; only to have the issue return - likely have pad deposit issues. Obviously, new rotors have no deposits - so the issue is temporarily fixed. However, since the driver's driving habits have not changed - the issue will return in weeks to months.
4. "Babying" brakes (especially on performance cars) is probably a big contributor to this phenomena. Occasional aggressive use of the brakes actually helps to clean the deposits off the rotors. Performance cars braking systems are designed to run hotter, cool better, and take more abuse. If the brakes are "babied" the rotors may never get hot enough for the pads to self clean the deposits off the rotors via abrasive friction.
5. Pad deposit issues are less likely if you keep the more aggressive / factory dust-maker BMW pads. This makes sense, as the more aggressive pads clean the rotors better / easier (more dust) and likely just generally run hotter. This also makes sense as to why is issue has not been caught by engineering. Although, pad deposits can occurs with factory pads which is likely due to driving habits.
6. Ceramic pads tend to work more by adherent friction (leaves layer on rotor) than abrasive friction (like sanding) opposed to semi-metallics which are likely vice-versa at normal operating temperature (hence more dust - but less deposits)
**** One practice I am now avoiding, which I learned from the above articles, is keeping my foot tightly on the brake at stop lights. Apparently, this is a big cause of pad deposits. At the stop light, when the rotor is hot - and brakes are held on with a lot of force - pad deposits can occur from transferring friction material to the rotor unevenly. So, it is recommended to just keep your foot on the brake as light as possible to keep the vehicle from moving. Something I used to do the opposite of.
I'm really curious of others' experiences with pedal pulsation, the fixes, and what you think of my "theories".
