Why is the disc thickness so critical??

Joined
Jul 17, 2007
Messages
471
Location
Alabama, USA
On disc brakes I read that the disc should not be thinner than a certain dimension. If the disk is thinner than that dimension is the piston going to pop out of the caliper?
 
Joined
Jun 25, 2009
Messages
4,065
Location
Chicago, IL
the minimum thickness spec is entirely to ensure enuf material remains on the disk to absorb and dissipate heat to ensure safe braking. last winter I did repair an Isuzu that had a frozen caliper and had actually worn the back side of the disk all the way down to the cooling fins. at that point the piston came out of the bore and jammed. the owner brought it in because it was leaking brake fluid - never mind all of the noise it made.
 
Joined
May 29, 2009
Messages
610
Location
Afghanistan
she will no longer be safe. the most important part of the car is the brakes, that is what makes sure you won't hit anyone, like a child running out after a ball. there is a margin there, for safety, but discs can be obtained cheap enough now, i routinely replace them instead of resurfacing, i prefer the extra margin given by a brand new disc, rather than pushing the other end.
 
Joined
Jun 3, 2005
Messages
16,226
Location
Santa Barbara, CA
i have seen rotors wear to the cooling fins, the pads worn past the backing plate and the piston fall out of the caliper. the customer complained of a squeak... i have seen rotors that were undersize explode when the brakes have been applied, literally explode and blow the wheel apart. luckily it was at a HPDE and there were safety crews right there. i always replace rotors on my own cars. makes the job go a lot faster. if the ones i take off still have plenty of life i will machine them and save them. if they are below ill take them to the shooting range.
 
Joined
Mar 14, 2003
Messages
8,711
Location
Nothern USA
Every stop you make puts a given amount of btu's into the rotors. The more iron, the less the temperature goes up.
 
Joined
Dec 2, 2002
Messages
6,388
Location
Washington St.
Brake discs (rotors) are a wear part. Too many ignore that fact. Yes, they might be good for more than one set of pads, and maybe can be resurfaced with good results, but someday they're worn below the spec and must go into the scrap bin. I agree with all the comments about the needed mass of the rotor to absorb heat.
 
Joined
Jun 5, 2009
Messages
965
Location
San Antonio, Texas
 Originally Posted By: labman
Yes, I think they have cheapened them, giving you fewer pad changes before they are too thin.
The metal is not of the quality it used to be IMO, they're softer. On my last car I had cryo treated rotors with some hardcore racing pads and after almost a year they didn't really show much wear at all. I plan on picking up a set for the Stang next time I need to change pads.
 
Joined
Sep 23, 2007
Messages
8,598
Location
Florida
 Originally Posted By: AdRock
 Originally Posted By: labman
Yes, I think they have cheapened them, giving you fewer pad changes before they are too thin.
The metal is not of the quality it used to be IMO, they're softer. On my last car I had cryo treated rotors with some hardcore racing pads and after almost a year they didn't really show much wear at all. I plan on picking up a set for the Stang next time I need to change pads.
Every German or Swedish car I ever worked on needed new rotors at each pad change. On other cars, it varies. Mopar rotors tend to have enough material to be turned, too bad they quickly warp. Toyota rotors seem to last quite some time.
 
Joined
Jun 5, 2009
Messages
965
Location
San Antonio, Texas
I agree, some tend to last longer than others. My opinion on this though is it's the difference in the material used in the pads that has the biggest effect on the life of the rotors. Some manufacturers use a more aggressive pad than others.
 
Joined
Oct 18, 2007
Messages
3,756
Location
CA
My TL has the worst rotor I've ever experienced. One stop braking is very good. But as soon as you get some heat in them, they warp badly yet return to normal when the cool. On a 100 degree day the shaking is only two hard stops away. I have the 13" Wilwoods ready to go on, just got to find the time.
 
Joined
Apr 17, 2006
Messages
19,528
Location
Lake Forest, CA
 Originally Posted By: bdcardinal
... I always replace rotors on my own cars, makes the job go a lot faster. If the ones I take off still have plenty of life I will machine them and save them. If they are below I'll take them to the shooting range.
bdcardinal, I have a question for you: Why Mercedes Benz disc can not be re-surfaced even if it still has more than minimum thickness ? My mechanic said that MB disc can not be machined, either replace with new disc or do nothing.
 
Joined
Jun 3, 2005
Messages
16,226
Location
Santa Barbara, CA
 Originally Posted By: HTSS_TR
 Originally Posted By: bdcardinal
... I always replace rotors on my own cars, makes the job go a lot faster. If the ones I take off still have plenty of life I will machine them and save them. If they are below I'll take them to the shooting range.
bdcardinal, I have a question for you: Why Mercedes Benz disc can not be re-surfaced even if it still has more than minimum thickness ? My mechanic said that MB disc can not be machined, either replace with new disc or do nothing.
i dunno, dont have a lot of experience with MBZ products. they might be a pain to mount on a lathe.
 
Joined
Sep 24, 2004
Messages
1,571
Location
Thousand Oaks, CA
The Mercedes practice is not unique. Many if not all European auto makers do the same thing, Volvo and BMW included. It seems to be part of the design intent. The rotors are a bit soft to enhance braking bite. It is intended for the rotors to wear instead of warp (sort of machining on the car). The pads are also very abrasive to enhance braking bite and with the intention of wearing the rotors. They allow the replacement of pads alone (no machining or no rotor replacement) if there is still enough rotor thickness remaining. I don't like the concept, but it is what it is.
 
Joined
Apr 17, 2006
Messages
19,528
Location
Lake Forest, CA
I always buy rotors whenever I buy pads for my '00 E430, if the rotors are not needed to be replaced when the pads are replaced then I keep it for the next time. The front pads (Pagid) are replaced every 10-12k miles but the breaking is very very good compares with other brands even they all made in Germany, the other problem with Pagid pads is it is very dusty, the wheels gets cover with brake dust in 2-3 days driving.
 
Joined
Jan 3, 2004
Messages
6,122
Location
northern Alabama
Big Jim, So, what is the surface finish requirement for Volvo & BMW as well then? What am I missing or do they just now want the typical rotor resurface so they just say don't do it all?
 
Top