Brake shudder

[the duke]

I have seen several brake shudder TSB's for other vechicles but none for the 300. I have shudder when braking at speeds in excess of 80MPH. Anyone else have this problem?

[Northern Rider]

Originally Posted by the duke

I have seen several brake shudder TSB's for other vechicles but none for the 300. I have shudder when braking at speeds in excess of 80MPH. Anyone else have this problem?

Duke, you might want to give this a try. I do it routinely on all of my vehicles - saves me lots of $$$:

http://www.zeckhausen.com/bedding_in_brakes.htm

[DanRealtor]

Warped rotors perhaps?

[rmedina]

Is your steering wheel shaking when you are braking? If yes, you might want to get your rotors check. Sounds like warped rotors just like DanRealtor said.

Rmedina

[DZeckhausen]

Originally Posted by DanRealtor

Warped rotors perhaps?

Probably not. The diagnosis of "warped rotors" is almost always wrong, since the phenomenon rarely ever occurs. The vast majority of judder compaints are caused by a thickness variation in the rotors, which is the result of an uneven pad transfer layer. Brake judder can usually be avoided (and sometimes cured) by proper bedding. See: http://www.zeckhausen.com/bedding_in_brakes.htm for instructions. It's worth trying this technique before heading over to the dealer, since it's free and fun.

For those interested in the physics of what is actually going on, the following technical white paper may be of interest:
The "Warped" Brake Disc and Other Myths of the Braking System

[charlie D]

Originally Posted by the duke

I have seen several brake shudder TSB's for other vechicles but none for the 300. I have shudder when braking at speeds in excess of 80MPH. Anyone else have this problem?

Proper bedding wil eliminate this , as well as any ambiant squealing.

[DZeckhausen]

Originally Posted by charlie D

Proper bedding wil eliminate this , as well as any ambiant squealing.

Proper bedding will usually eliminate this unless the judder problem has gone unchecked for a while. The transition between bedded and unbedded portions of the rotor result in local hot spots that tend to accumulate more pad material and can eventually change the molecular structure of the rotor, resulting in cementite deposits. These deposits can't even be removed by turning, since the lathe's blade tends to skip over these hardened spots. When the problem has become this bad, rotor replacement is the solution of choice.

However, proactive bedding whenever you purchase a new car or install new rotors, should "inoculate" you from judder problems caused by transfer layer thickness variation.

Keep in mind that there are other root causes of judder that can be initiated by application of the brake pedal, but aren't directly caused by a brake problem. For example, the combination of old, weak shocks with worn control arm bushings and/or ball joints combined with a slight imbalance in the wheel/tire assembly can lead to a transient judder problem at certain speeds that is often triggered by a sharp input to the brakes. Diagnosis is often wrong, since attention naturally falls to the brakes as the culprit. Instead, it's the rhythmic input to a system (the car's suspension) that has a natural fundamental frequency. When the rhythmic input is a multiple (harmonic) of the suspension's fundamental frequency and the shocks are too weak to damp out oscillations, the "kick" from a sudden brake input may be enough to start an oscillation that continues until the vehicle speed falls outside a range of speeds. So, speeding up or slowing down may make the judder go away. And it's not necessarily the fault of the brakes. Most of the cars on this forum are too young for this to be happening yet.

[DanRealtor]

Originally Posted by DZeckhausen

Proper bedding will usually eliminate this unless the judder problem has gone unchecked for a while. The transition between bedded and unbedded portions of the rotor result in local hot spots that tend to accumulate more pad material and can eventually change the molecular structure of the rotor, resulting in cementite deposits. These deposits can't even be removed by turning, since the lathe's blade tends to skip over these hardened spots. When the problem has become this bad, rotor replacement is the solution of choice.

However, proactive bedding whenever you purchase a new car or install new rotors, should "inoculate" you from judder problems caused by transfer layer thickness variation.

Keep in mind that there are other root causes of judder that can be initiated by application of the brake pedal, but aren't directly caused by a brake problem. For example, the combination of old, weak shocks with worn control arm bushings and/or ball joints combined with a slight imbalance in the wheel/tire assembly can lead to a transient judder problem at certain speeds that is often triggered by a sharp input to the brakes. Diagnosis is often wrong, since attention naturally falls to the brakes as the culprit. Instead, it's the rhythmic input to a system (the car's suspension) that has a natural fundamental frequency. When the rhythmic input is a multiple (harmonic) of the suspension's fundamental frequency and the shocks are too weak to damp out oscillations, the "kick" from a sudden brake input may be enough to start an oscillation that continues until the vehicle speed falls outside a range of speeds. So, speeding up or slowing down may make the judder go away. And it's not necessarily the fault of the brakes. Most of the cars on this forum are too young for this to be happening yet.

So, if I am turning my rotors and on the first cut, parts of the rotor are being cut and parts of the rotor (as in whole sections) are being completely skipped over, this is not a warped rotor?

My Q45: 154 mph, heavy car, stock brakes, had "Warped Rotors" on several occasions (also boiled the brake fluid numerous times). Each time I cut the rotors (until they got too thin, then new rotor time) whole sections were skipped over on the first cut.

The reason I ask is the article from stop tech you linked says that the author has never seen warped rotors, it's always a misdiagnosis!

Not arguing, just want to learn.


Dan

[DZeckhausen]

Originally Posted by DanRealtor

So, if I am turning my rotors and on the first cut, parts of the rotor are being cut and parts of the rotor (as in whole sections) are being completely skipped over, this is not a warped rotor?

My Q45: 154 mph, heavy car, stock brakes, had "Warped Rotors" on several occasions (also boiled the brake fluid numerous times). Each time I cut the rotors (until they got too thin, then new rotor time) whole sections were skipped over on the first cut.

The reason I ask is the article from stop tech you linked says that the author has never seen warped rotors, it's always a misdiagnosis!

Not arguing, just want to learn.

I totally understand why seasoned mechanics believe in warped rotors. Everything they see and do reinforces their misconception. For example, if they turn the rotors, the judder goes away. If they replace the rotors, the judder goes away. In both cases, if you believe the root cause to be warped rotors, the "fix" you just did will cure the problem. The catch is, the "fix" also cures pad deposition!

As for placing a rotor into a lathe and watching the blade cut into one section and not the other, that can also be explained. If you are making an aircraft quality part on a lathe, you don't use a cone shaped mounting fixture. Instead, you have a mounting surface that is completely perpendicular to the cut and you measure it with a dial indicator prior to mounting anything on it. If it's not within specification, you cut the mounting surface with your lathe. (Have you ever seen a Pep Boys employee measure and adjust a brake lathe?) And you use a 4-jaw chuck with a flat shoulder that holds the piece perfectly flat. If the piece has a section that is not going to be cut (analogous to the rotor's hat portion), you use a dial indicator to make sure the piece is being clamped perfectly flat. Only then do you begin to make the cut.

An automotive brake lathe is a terribly inaccurate device that, when used without meticulous setup technique, will usually induce several thousandths of an inch of runout into your rotor. This is runout that wasn't there to begin with. That perfectly flat rotor will appear warped because it isn't laying flat against the mounting fixture.

Back to diagnosis of warped rotors. Often, a mechanic will call and tell me he's just taken measurements of a rotor on the car and insists there is at least 8 or 9 thousandths of an inch of runout. So the diagnosis is warped rotors. I ask how the runout measurement was taken. Were the wheels simply removed and the dial indicator placed on the face of the rotors? If the only thing holding the rotor to the hub is a rotor retaining screw (or the metal clips on the 300C studs), you will get a false runout reading. You need to use washers and wheel nuts (or wheel bolts) torqued to the same setting (it doesn't have to be all the way to 100 lb-ft, you can torque them to 40 lb-ft as long as they are all the same) and only then will the dial indicator give you an accurate measure of rotor runout. If there is real runout on a newly installed rotors, I usually find a chunk of rust or debris (e.g., a metal shaving) between the hub and the rotor hat. Properly clean the hub mounting surface and reinstall the rotor and the runout goes away.

The problem is, the mechanic is expecting the rotors to be warped as an explanation for the customer's symptoms. So when he sees excessive runout during an on-car measurement, he doesn't question his measurment technique. And when he slaps the rotor onto his lathe and sees the blade only cut one section of the rotor on the 1st pass, he doesn't question his set-up. When you know the true cause of most judder problems to be pad deposition, you go back and question everything and that's when things start to make sense. For example, you can set up a fixture and take careful thickness measurements at about 20 evenly spaced locations around the rotor (same distance from hub) and you will find regions of thicker transfer layer in excess of four ten-thousandths of an inch.

If the problem was really warped rotors, then it wouldn't be cured by any of the following techniques:

1. Re-bed the brakes.
2. Install Hawk Blue race pads and drive around normally for 30 minutes of stop-and-go traffic.
3. Remove all pad transfer with a Flex-hone tool.

Once I've cured a "warped rotor" problem with brief use of an abrasive track pad, my customer becomes a true believer.

[DanRealtor]

Thanks for the explanation. It does seem to make sense. Next time I have "Warped Rotors" I will be sure to carefully evaluate before turning them.

BTW: I know I am repeating myself somewhat but, in my Q45 I was able to boil the brake fluid and eventually have the pedal going down to the floor (a bit scary), in my 300C SRT-8 I tried really hard to get 'em to fade at Sears Point, 12 turns for 30-45 minutes a session 0-110mph down to 30 or 40 back to 110, all day long. NO Brake Fade, NONE! Aside from the squeal and dust, these brakes are amazing!


Thanks again for the explanations.



Dan