That's exactly how it works.

A similar example is oil change intervals. The OEM has already chosen a conservative number taking into consideration how the average owner uses his car. The published number is nowhere near any maximum or dangerous threshold. Owners choosing to chop that interval in half is similar to throwing away serviceable rotors.

 

Each to their own I say, although I do have to remind myself of this from time to time.

Its your car, your money, your safety and ultimately of course, your choice. Once again.......I just want to know where we are with the OP?

What have they decided, what have they done, if anything?
The suspense is killing me here..

 

The OP has not logged back on to the site since May 25th......

In my mind it's one thing to express a preference for one brand over another, whether it be beer, fuel or motor oil but it's a different matter when unfounded beliefs are being promoted under the catch-all pretence of 'safety'.

Many times back in my working days (seems so long ago) I'd run across a customer who had a seemingly high number of aircraft out of service or with outrageously high operating costs. With a bit of digging, we'd find that frequently the mindset was to second-guess the manufacturer's specifications and set their own more restrictive limits. We always did our best to make the manuals clear that the minimum limits already had more than enough safety margin built in.

 

So then assuming two sets of pads wear the disc down to 1 mm above minimum thickness you would put a third set on? Then when the fourth set comes up you would have a disc worn well below minimum spec.

One poster claims you replace the disc if it wears below minimum spec regardless of how much pad is left.

Which one of you is correct?

 

Rear discs on the XF are about 20 mm thick. Minimum thickness for the rears is 17 mm I think. Fronts are I believe 25 mm thick new and minimum would be around 22 mm. Both are vented.

 

Wait a minute. Oil change intervals are mandatory or your warranty is voided.

I agree that changing oil more frequently is a waste of money but the opposite isn't true, which you imply.

 

So if the disc is more than 50% worn after one set of pads then the disc should be replaced.

What I said.

As it happens new cars are fitted with discs intended to last for one set of pads with a margin of wear remaining. As it happens on my cars fitted with modern brake pads and the usual cast iron discs those discs are more than half worn after the first set of pads is worn out.

Since you need to machine used discs before fitting new pads even less thickness remains. Then there's the thrown away labour removing the old discs, machining them and putting them back, costs which are never recovered. How much do new discs cost anyway compared to those costs thrown away?

Finally, if you know the discs are going to be worn out you don't need to worry about pad remaining. Under 2 mm of pad remaining will still stop the car just as effectively, perhaps more so, than full thickness pads due to better heat conduction.

All these factors support what I am saying and what my mechanics are telling me.

EBC points out that brake bedding in mileage can be extended considerably if new discs are not used unless the old discs are machined, they of course recommend on the car brake lathes:

http://ebcbrakes.com/about-ebc/faqs/

Question 12 is the relevant one although question 9 is related. This is why I say you need to read the EBC information carefully to ensure you understand they are not saying it is safe to re-use old discs.

 

Both of us. We're saying the same thing. If the rotor is at or above, min thickness, it goes back into service.

I said the opposite. that's what 'owners chopping the interval in half' means.

I give up.

 

Mikey, I may be reading you wrong but everything I've ever read or heard is that min. thickness is when they should come off. I feel Jim is also right that there is a small amount of fudge factor so it's not that you're going to die immediately if it's less, but you would never want to our them back on if you were close to minimum thickness.

So if you've used one set of pads and you're less than 50% of remaining thickness between new and min. (if 25 is new and 22 is min, anything below 23.5 should be changed) you should be safe and not, as Jagular said, be wasting money on labor or underutilized pad wear.

 

I've had enough......
you can lead a horse to water and all that, everybody will do what they want to do, end of.
really just interested to see what has happened to the OP


I may spend some time, track their location down, find the mobile number and call them to ask, just so I can shut this thread down....lol

 

I can post as many links as is needed to clarify and provide proof, but the concept is that a rotor should not go back into service if it is at or below min thickness. On the other hand, if it is above this, the OEM has calculated that enough material remains for the rotor to go 'one more round'.

Here's an example

Brake Rotors: When To Resurface And When To Replace - Counterman Magazine

In specific:

"That’s why the thickness of the rotors should always be measured every time the brakes are serviced. If a rotor is worn down to the minimum thickness specification, or cannot be resurfaced without exceeding the dimension, it must be replaced. In some states, this is the law."

If it is legal to machine down to the min thickness and reuse, obviously there is enough material left to re-install.

As pointed out above, a mechanic cannot possibly know how many sets of pads have already been used on a given rotor, so saying '2 sets maximum' or similar would not be workable.

Similarly stating 'discard when 50% worn' cannot be a criteria as the original thickness is not known during servicing.

OEMs instead calculate the 'real' minimum safe thickness and add a decent fudge factor. That total number is what's written on the rotor. Much simpler system till people start second guessing.

 

actually what you said was this:



it sounds like you're advising otherwise now.

 

It's my understanding that the min thickness, aka discard thickness, is what you check when replacing pads (or I suppose if skimming). At or below it, discard.

So, this does mean that with a tiny amount above the min you're good for one more set of pads.

This means that the "minimum" is not the "safe minimum" or the "absolute minimum", it's the "when to replace when changing pads (*) minimum".

(*) or skimming or similar

I think this is what various others have also said.

That said, it's not a lot of money to change them if they're getting close to the min so if you like to change them then do - it's your money!

 

More sources:

http://www.tirerack.com/brakes/tech/....jsp?techid=80

http://www.juratek.com/support-discs.php

http://cbsparts.ca/admin/bulletins/B...nd%20Rotor.pdf

http://brakeperformance.com/inspecting-brake-rotors.php

http://www.tirereview.com/brake-roto...en-to-replace/

I can find no reputable source that says anything about it being ok to leave worn discs on the vehicle if the pads still have wear left. Ergo, if one set of pads wears the disc halfway or more then you fit new discs when you fit the second set of pads even if the disc is not yet worn out. The last link even confirms that newer discs are thinner to begin with. Also, pre metallic era pads used asbestos and hardly wore discs at all. Another clue comes from service shop manuals for my SAAB and my Alfa which prescribe a minimum grinding thickness as well as a minimum service thickness. The discs are cast for 2 mm of wear. The replace thickness is slightly more than 2 mm less than new. The re grind thickness is more than 1 mm thicker than the minimum spec. So, no, the minimum spec is not the number you use when deciding on new discs with the new pads. The minimum spec is the discard spec regardless of the state of wear on the pads.

For the cars I have owned the discs are always more than half worn after one set of pads. The XF certainly wears discs that fast.

Pre 1995 approximately, you could routinely get two sets of pads for each set of discs. My 1991 Alfa Romeo just had new front rotors fitted for the third set of pads (at 230,000 km and the second set of pads lasted about 80% of the life of the first) whereas my 1997 SAAB Aero turbo requires new pads and discs each time, both ends of the car. Pad material has changed that much.

If you don't use your brakes hard maybe you will experience less wear.

At least one poster suggested that disc thickness should be checked routinely and discs replaced if they reach minimum thickness.

None of the mechanics I use are of the view that you should not refit worn discs that will not remain above discard thickness during the expected pad life. My experience has been that they are correct.

Bear in mind the worn out discs will still feel fine until you ask the maximum from the brakes. It is when you really need the full braking performance that the thin discs will overheat and the pads fade. Trouble is you can't know when that once in a while event might occur, maybe never but maybe this morning.

My main point is you should follow the practice of replacing discs and pads together if you do not know much about brakes. If you drive enthusiastically you need the new discs whether you think you can get away with the old ones or not. Bottom line it isn't worth even thinking about, change them.

 

That's a good way of putting it. The go no/go decision for reusing the rotors is put in the hands of the OEM and not in those of the mechanic/hobbyiest who may not know the wear rate or number of pad sets used.

I found a separate paper discussing situations where rotors are found to be worn below the stated min. thickness after just one set, and suggesting that cars be 'recalled' for interim inspections. The end of the lengthy back-and-forth was with an interpretation similar to what you've stated above. There was a side discussion about garages doing self serving while-we're-at-it inspections. That's one of my other favourite rants, but we'll leave it for another day.

 

The answer to the original post is "yes ".

I gave that answer in post number 2.

 

all the links you have posted above counter your initial advice on changing rotors with every pad change and demonstrate your advice as being wrong.

they all suggest periodic inspection and replacement of braking components where necessary.

if a vehicle is well maintained and serviced in accordance with manufacturer specifications there will be service records to show when the pads/rotors were replaced and how they are wearing.

it's not only wear that affects rotors, heat, defects and physical damage can render a rotor unserviceable well before pads are due which makes periodic inspections even more critical.

 

In your opinion the answer is yes?
I haven't seen any definitive evidence to support that view. 9And I have read several of the links posted)

 

But the original poster has the answer she asked for. Whether she accepts the advice or not is surely up to her.

I know I am correct. I don't need anyone to agree with me.

The technicians I deal with are up to the minute dealer trained with decades of experience. I accept their advice over anything posted here.

 

Without prejudice...
Jagular - you say "you know you are correct" and accept the word of dealer trained techs, usually the same people who are taught to maximise each and every sale btw, on everything they say, that's fine, and your viewpoint.

I think it's safe to say that, we all know we're right too, so probably best to leave it at that, as I don't need anyone to agree with me either, probably most others feel the same way I guess.