Minimum Brake Rotor Thickness for CTT

[ChkbookMechanic]

I'm about to order new pads for a brake job and figured while I was at I'd also check the thickness of the rotors to see if I need new rotors as well. However, I couldn't find what the minimum thickness is for the rotors on a CTT. Does anyone know what it is?

Thanks in advance.

[ChkbookMechanic]

I got this from another forum, but here it is in case someone searches for it:

Brake disc thickness, new
front - 34 mm
rear - 28 mm

Brake discs minimum thickness
front - 32 mm
rear - 26 mm

[island911]

Wow, just a mm off of each side!?

Makes me wonder why they even have the wear sensors. --other than I notice that they likely get people in to the dealer faster and replacing more parts.

[ChkbookMechanic]

Yep, I was surprised it was that low as well. I'll probably have to replace the rotors anyway because they are scored pretty badly in the front... Now I just need to go measure the rotors.

[JHoffman]

If you have any lip on your rotor you "need" new rotors. If your in a crunch you can get away running rotors under spec. It's not recommended and chances are great that the rotors will warp and stopping distance with be drastically diminished after a few sudden stops.

[ChkbookMechanic]

I'm not really in a crunch in terms of having to get the brakes fixed asap, although I'd like to have them ordered while I'm on vacation and just replace them when I get back.

The rotor specs I posted are also good for the Cayenne S as well.

I think they use wear sensors just to force you to replace something else while doing the brakes.

[island911]

Originally Posted by JHoffman

If you have any lip on your rotor you "need" new rotors. If your in a crunch you can get away running rotors under spec. It's not recommended and chances are great that the rotors will warp and stopping distance with be drastically diminished after a few sudden stops.

hmmm... considering their design envelope (towing 7000_lbs down a Mt pass, or hammering a track) I really doubt that rotors will start warping once the 1mm wear is past. ...at least not the way most drive. (relatively gently)

Of course, if you regularly come down (hard) from 150mph, then yeah, you might want to keep in that factory spec. YMMV

[eric523]

Cayennes typically need the rotors replaced at every brake job...if you care about doing the job correctly. In my experience, the rotors are below the minimum or would be below after machining 90% of the time.

[dtxscott]

I measured my rotors at the last brake job at ~30k miles. I was at 33mm front and 27mm rear.

You don't need to replace the rotors if they are within spec. The service writers try to suggest it "for your safety". However, this is driven by profit and self protection.

I will replace my rotors when they hit the limits. If you think about it, tolerances have significant headroom built into them to avoid litigation. Keep it within the limits and you will be fine. Otherwise you are wasting money.

[eric523]

Originally Posted by dtxscott

I measured my rotors at the last brake job at ~30k miles. I was at 33mm front and 27mm rear.

You don't need to replace the rotors if they are within spec. The service writers try to suggest it "for your safety". However, this is driven by profit and self protection.

I will replace my rotors when they hit the limits. If you think about it, tolerances have significant headroom built into them to avoid litigation. Keep it within the limits and you will be fine. Otherwise you are wasting money.

When performing a quality brake job on ANY vehicle you should either replace the rotors OR have the existing rotors machined to a flat surface. Simply putting on a new set of flat pads onto a grooved surface is just asking for hot spots, lower friction which results in lower performance, and noise.

As a service writer myself, this is the ONLY way you can warranty your work! I can't have you come back to me in two weeks complaining that your brakes make noise after you REFUSED to replace your rotors. That's why I always require machined or new rotors to carry a warranty. It's not some profit driven conspiracy, it's doing quality, complete work. In my opinion, quality is not a waste of money...doing the job twice because you took shortcuts is.

[island911]

Eric, let me ask you; what makes rotors not wear evenly?

[eric523]

I'm not a physics expert but anyone that's seen a rotor after 30K miles of use can tell you that the surface is not flat. Whether it is from metallurgy, debris, or excessive heat I cannot say with certainty, and it can be different on each vehicle. You will end up with a rotor that has "peaks and valleys" across the surface and is very common to have a big "dish" or lip on the outer edge. This outer lip is also the area where the rotor cuts into your sensors. Simplest answer for the lip is that the pad does not contact the outermost surface. If you run your finger across the rotor and can feel bumps, ridges, grooves etc. then you should NOT use it again without machining it to a flat surface. However, when machining the rotors you have to shave off material to the lowest point to get a flat surface, which is often below the minimum thickness. It takes at least 1-2mm to properly machine a rotor, and with cayenne tolerances from max to min only 2mm apart, that often leaves you with the only option of replacement.

The pad and rotor surfaces start as flat as possible and then mate to each other over time. The flatter the surface, the greater the contact patch, the more friction and stopping power you will have. The more thickness on the rotor and pad the more heat they can absorb. Running rotors below their minimum thickness reduces the heat that they can handle and causes them to operate above their normal temperature which can lead to cracking or ultimately failure in extreme cases.

Think drag racing tires and track...
You want wide flat tires and a track that has been swept of all debris, sand, etc. This will give you the most friction possible.
Then think all season radial with grooves in the surface and a freeway with grooves in the ground for rain.
There will be MUCH more friction with the two flat surfaces mated to each other than the other with high and low spots.

[island911]

Originally Posted by eric523

I'm not a physics expert ...

.... The flatter the surface, the greater the contact patch, the more friction and stopping power you will have. ...
...
There will be MUCH more friction with the two flat surfaces mated to each other than the other with high and low spots.

This is where your physics model breaks down. Friction is not a function of surface area. I know, it seems so intuitive that bigger brakes must produce more friction, but it's not so simple. The thing is, bigger brake pads simply allow for lower pad stress. That is, the contact materials have more volume and thus will last longer. --there are some other effects like minimizing the pulsation hot-spots on the rotor, but that's more advanced. Just know that you are not losing friction by having a smaller contact area. ...which, btw, self-machines away, any contacting high spots.

Just think for a moment, that if surface area was key, just how much smaller brake rotors could be made with actually building in ^^^^^ (ridges) to the rotor and corresponding pads.

[eric523]

a flat on flat surface will make better contact, dissipate more heat, run at a cooler operating temp, be quieter, and be more efficient than a flat surface mated to your old ^^^^ridged rotor.

[dtxscott]

Great discussion. Eric, my point about re-using rotors is that most writers state that rotor replacement is a must. In my life's experiences, few things are absolute (except death and taxes.... Unless you are a congressional member )

Quality is a constant tradeoff between cost and practicality. Is it practical to replace rotors every 30k miles? Some say yes, some say no. I chose to re-use my rotors and have not had squealing or performance issues in the ensuing 25k miles. Granted, I don't drive the truck hard so the YMMV disclaimer applies.

As a service writer, I can see where you don't want to see an angry customer return for nuisance issues. However, for those that do our own simpler tasks, the replacement often does not hold water. Statistically speaking, I'm one sample. However, my experience tells me its better to maximize the $$ and reuse the rotors. The only gotcha is to properly bed the pads.

[eric523]

Cheers Scott.
I definitely see your point that yes they can be reused, yes it is MUCH cheaper, and yes your car will still stop just fine under normal street use.
On my side of it, the Cayenne is one of the highest performance trucks of it's size and weight ever produced. Customers that come to me for the work on their Cayenne expect it to perform flawlessly and that's the quality I strive for. Spending over $1000 on brake job that squeals as you pull up to the county club is absolutely unacceptable, even if it happens to one of every ten cars.
If a customer just wants pads, that's fine, I just won't warranty the work. I spend the next ten brake jobs trying to make up lost money from one comeback, and I don't like having ANY comebacks in the first place.

[eric523]

You can all be lucky that you don't have the Turbo S brake package...big $. The rotors retail at over $700 each. These are going on a Turbo S with 60k miles.

[island911]

U-oh . . .that last pic really shows some machining ridges. You had better have those polished out before installation.