rotor question

I'm helping a friend this weekend with brake pads on her 1998 E328i sedan.
I believe there are about 90,000 miles on the car. Some have recommended
hanging the rotors.

How do you tell if a rotor is bad and needs changing?

Which rotors tend to fail first, front or rear?

Thank you.

"Franz Fripplfrappl" > wrote in message
news
> I'm helping a friend this weekend with brake pads on her 1998 E328i sedan.
> I believe there are about 90,000 miles on the car. Some have recommended
> hanging the rotors.
>
> How do you tell if a rotor is bad and needs changing?
Deep scratches such that rotor can't be resurfaced.
>
> Which rotors tend to fail first, front or rear?
>
> Thank you.
>
>
Jim

"Franz Fripplfrappl" > wrote in message
news
> I'm helping a friend this weekend with brake pads on her 1998 E328i sedan.
> I believe there are about 90,000 miles on the car. Some have recommended
> hanging the rotors.
>
> How do you tell if a rotor is bad and needs changing?
>

You have to measure them with a micrometer or suitable calipers. The Minimum
thickness specification is stamped on the rotors, and you must be thicker
than the min spec after the machine work is complete.


> Which rotors tend to fail first, front or rear?

Front, by a wide margin. With 90k on the car, there is a good chance the
rear brakes need attention too, but the rotors ought not be worn too badly.

Franz Fripplfrappl wrote:

> I'm helping a friend this weekend with brake pads on her 1998 E328i sedan.
> I believe there are about 90,000 miles on the car. Some have recommended
> hanging the rotors.
>
> How do you tell if a rotor is bad and needs changing?
>
> Which rotors tend to fail first, front or rear?
>
> Thank you.
>

There is a minimum thickness specification. It's probably stamped in
the rotor. If it's worn beyond the spec or it's deeply grooved, it
needs to be replaced. (BMW rotors generally cannot be resurfaced and
stay within the minimum spec.)

JRE

In article >,
Jeff Strickland > wrote:
> You have to measure them with a micrometer or suitable calipers. The
> Minimum thickness specification is stamped on the rotors, and you must
> be thicker than the min spec after the machine work is complete.

Since they only last for two sets of pads, the chances of turning them and
having enough left for the second set without going below minimum is
remote. Just change them for new - they're not expensive.

--
*Wrinkled was not one of the things I wanted to be when I grew up

Dave Plowman London SW
To e-mail, change noise into sound.

Jeff Strickland wrote:
>
> "Franz Fripplfrappl" > wrote in message
> news >
>> I'm helping a friend this weekend with brake pads on her 1998 E328i
>> sedan.
>> I believe there are about 90,000 miles on the car. Some have recommended
>> hanging the rotors.
>>
>> How do you tell if a rotor is bad and needs changing?
>>
>
> You have to measure them with a micrometer or suitable calipers. The
> Minimum thickness specification is stamped on the rotors, and you must
> be thicker than the min spec after the machine work is complete.

Further, BMW does not recommend resurfacing rotors at all (ever). If
the rotors are still thick enough you should just put on replacement
pads and leave the uneven surface of the rotor alone. The pads will
take on the shape of the rotors in very little time, you'll actually
have slightly greater contact area with the pads(though probably
insignificant) and you will save a lot of mileage on the old rotors.
After machining the rotors tend to warp due to loss of mass (heat
dissipation).

Machining old rotors flat is highly over-rated.


--
-Fred W

In article >,
Fred W > wrote:
> Machining old rotors flat is highly over-rated.

Seems to be a US thing as I doubt you'd find anywhere that offered this
service in the UK. Drums, on the other hand, often benefitted from a skim
to get them round again.

--
*With her marriage she got a new name and a dress.*

Dave Plowman London SW
To e-mail, change noise into sound.

Here's a discussion on measuring brake rotor runout:

http://web.archive.org/web/200012151.../bbrunout.html

"JRE" > wrote in message
...
> Franz Fripplfrappl wrote:
>
>> I'm helping a friend this weekend with brake pads on her 1998 E328i
>> sedan.
>> I believe there are about 90,000 miles on the car. Some have
>> recommended
>> hanging the rotors.
>>
>> How do you tell if a rotor is bad and needs changing?
>>
>> Which rotors tend to fail first, front or rear?
>>
>> Thank you.
>>
>
> There is a minimum thickness specification. It's probably stamped in the
> rotor. If it's worn beyond the spec or it's deeply grooved, it needs to
> be replaced. (BMW rotors generally cannot be resurfaced and stay within
> the minimum spec.)

IMO it's never worth having the rotors skimmed, even if they would still be
in spec, as the cost of grinding them is likely to be near the cost of
replacing them with new.
Mike.

Mike G wrote:

>
> "JRE" > wrote in message
<snip>
>>
>> There is a minimum thickness specification. It's probably stamped in
>> the rotor. If it's worn beyond the spec or it's deeply grooved, it
>> needs to be replaced. (BMW rotors generally cannot be resurfaced and
>> stay within the minimum spec.)
>
>
> IMO it's never worth having the rotors skimmed, even if they would still
> be in spec, as the cost of grinding them is likely to be near the cost
> of replacing them with new.
> Mike.

I rarely have rotors ground either.

Oddly enough, my wife's brake indicator came on today, so she stopped
for the parts on the way home and I did them this afternoon after work.

The rotors, after one set of pads' worth of wear, were .010" from the
machining limit (not counting the depressed parts of the grooves, which
I couldn't measure with the micrometer I used, but which I estimate were
close to .005-.010" deep or so per side). While they were not deeply
grooved, and though they were still pretty smooth, I replaced them anyway.

Incidentally, neither the old nor new rotors, despite what the TIS says,
were stamped with the minimum thickness.

Oh, the car? 2002 E46 325i.

JRE