new pads = new rotors? A rule? - 986 Forum - The Community for Porsche Boxster & Cayman Owners

FTD · 08-21-2008, 08:39 AM

I was just told that my front brake pads have about 1.5mm wear left before the sensor will tell me to get new pads. I was also told that I should replace the rotors whenever I replace the pads. I have dug around for similar statements or questions on this forum and didn't find answers, so pardon me if this is a redundant thread.
I challenged him as to why, but I don't remember what he said. It was something like seating and steering vibration, not that my rotors are worn.
The car has 22+ k miles, or something like that. Definitely less than 30k.
What say ye?

Chippy · 08-21-2008, 08:44 AM

dogs dangly bits!

The pads are softer than the disc so new pads will soon bed into the old discs.

However, if you replace the rotors you should always replace the pads at the same time.

Lil bastard · 08-21-2008, 09:31 AM

There is a minumum wear thickness to the rotors, after which they should be replaced for top efficiency. This wear limit is 22.0 mm (Fr) and 18.0 mm (Rr) - Base or 26.0 mm (Fr) and 22.0 mm (Rr) - 'S'.

You use a micrometer (or a vernier caliper - though less accurate) to measure the disc on several points and average the readings.

If you don't have these tools, a good rule of thumb is that the rotors will last through 2 sets of pads and should be changed for the 3rd set - pads usually last 30-35k mi. under normal conditions, so at 20k or so miles, you should not need them. This does not apply if you are tracking the car, or especially if you use more agressive street or track pads which wear the rotors faster.

But, whether replacing them or not, in an absolute world, the rotors should at least be machined at each pad change. This eliminates any warpage which may have occurred and provides a clean base for the new pads to bed-in and lay down a layer of pad material on the rotor face (less important if you are using the same type pads as before).

If you are feeling any pulsation in the brake pedal, this could indicate some warpage. You'd measure this as Lateral Runout with a dial gauge and should not exceed 0.06 mm. This will be machined away if you have the rotors turned (machined) which in this neck of the woods costs about $15/rotor at a machine shop, usually while you wait.

FTD · 08-21-2008, 09:40 AM

thanks, LilB.
Do I have to change all four pads if I change compounds? Can I try something different on just the fronts?

Lil bastard · 08-21-2008, 10:14 AM

Quote:

Originally Posted by FTD

thanks, LilB.
Do I have to change all four pads if I change compounds? Can I try something different on just the fronts?

You could try something different on 1 axle, esp. the fronts as they do most of the work.

You will notice a difference, but maybe not for the good though. Some combinations may work well, while others, not so well. Without a lot of study and information, it seems a little loosey-goosey, you may regret it. I'm not sure I'd want to be a guinea pig, but if you're willing, let us know how it works out.

FrayAdjacent · 08-21-2008, 01:36 PM

Quote:

Originally Posted by bmussatti

WOW! In over 153,000 posts on this Forum, the actual word "vernier" has only been used 5 times.

All by the same person!

Jim. (aka MNBoxster...aka Lil bastard)

1- by Lil bastard
4- by MNBoxster

When are you going to come out of the closet, Jim?

Probably because most people can't read a vernier scale. I had to learn when I picked up a Sharps type rifle with a long range sight setup on it. Minute of Angle on a vernier scale.

On topic -

I replaced my brake pads shortly after purchasing my car. Had 40k on the ticker, and the front pads were almost on the wear sensors. I did not replace my rotors nor did I have them turned. My brakes are fine, except for a little squeak here and there.

When I get closer to 80k and need to change the pads again, I will get new rotors.

It can't hurt anything but your wallet to change them prematurely.

jmatta · 08-21-2008, 03:06 PM

If you have base Box rotors, I think you're OK just to change pads, as the others have said. I wouldn't waste my time on having them turned, as new rotors are relatively inexpensive unless you feel pulsing indicating a warp. Box S has cross drilled; no turning, but look for spider cracks. The street is not hard on brakes at all and your factory rotors should give you long life. I prefer to change pads when they hit the halfway point, as they are cheap and a thicker pad keeps the heat out of the caliper.

NEVER mix pads on a track or DE car...the differences in grip will be exaggerated and the car will most likely be either off track or heading the wrong direction.

Rick V · 08-21-2008, 04:52 PM

Ok I wrench for a living so I will give you the work answer then I will give you the way I do my cars.
As many have stated you can "mic" the rotors and see what you have left. If you figure the amount of wear you inflicted on the rotors and that number is less than half of the wear, (from origional to min. thickness) keep your rotors and change them with the next set. If you don't have any pulsations when braking you shouldn't need to resurface them.
On My cars, I change the rotors every time I change the pads. I do this for several reasons,
1 I am very hard ( And I do mean very) on my brakes, I love to set the car late on entry, and drive alot of country roads
2 I run Hawk HP+ pads. They will kill a rotor.

RandallNeighbour · 08-21-2008, 05:00 PM

My factory rotors lasted 100k for what it's worth.

FTD · 08-22-2008, 06:25 AM

Thanks for all the info. Very helpful. I will replace the fronts with OEM, since the rears are OEM. I do track, but not often [enough]. I wanted to try some street/track pads but I don't need a squirrely rear on hard braking. The DIY on Mike Focke's link is sooooo well written, I will let you know how it goes for me.

FTD · 08-22-2008, 06:28 AM

Oh.....
and I also wanted to say

Vernier

JackG · 08-22-2008, 09:23 AM

Quote:

Originally Posted by FrayAdjacent

Probably because most people can't read a vernier scale. I had to learn when I picked up a Sharps type rifle with a long range sight setup on it. Minute of Angle on a vernier scale.

My dad, a machinist, taught me how to read vernier calipers when I was less than ten years old. I still don't have set of dial indicating calipers... mine is an old, but trusty Fowler vernier caliper.

There... I used the word "vernier" twice, err, three times.

FTD · 08-22-2008, 12:21 PM

One more question [I hope its the last one]
Can I use the PBR Metal Master pads on the front with OEM on the rear. Or is this too much a move away from similar compound? It has been referred to as an OEM-like pad.

FTD · 08-23-2008, 01:13 PM

bump bump

Rick V · 08-23-2008, 06:16 PM

JMO
You can run the metalmasters with the OEM stuff, but I would run the Hawk HPS pads.
They stop better than OEM but arn't an overkill. Also very low dust, no noise, and heat isn't an issue for them.
I realy like the Hawk line, just get the ones that are best for your driving, or you won't like them.

dmcutter · 08-23-2008, 06:20 PM

"All by the same person!

Jim. (aka MNBoxster...aka Lil bastard)

1- by Lil bastard
4- by MNBoxster

When are you going to come out of the closet, Jim?"

Originally Posted by bmussatti "

I don't care how many times you can work the word "Vernier" into a thread. If you forsake your Boxster for a Vespa scooter, you should not have the nerve to show your avitar around here.

Separate but unrelated, I see as I type this that the built in spell check has underlined "Boxster" as a spelling error. How ironic.

Tool Pants · 08-24-2008, 08:20 AM

I have a vernier caliper. But once I bought a digital caliper I did not use the old one anymore.

Also have an old micrometer that you look at the numbers on the barrel. Needed that years ago to measure valve shims. Now have a digital micrometer, so I don't use the old one anymore.

So I have worked in all the words.