on my front rotors, i notice a 1 - 2 mm lip on the edge if i feel ...

how many mm till my rotors need replacing?.. whats the proper way to measure the thickness of the rotors?

Vinay, Bimmer rotors brand new are at their minimum thickness. If you have a 1 - 2 mm lip on each side of the rotor, that means that the rotor is already 2 - 4 mm below minimum. The proper way to measure rotor thickness is with a micrometer, measuring in several places across the face of the rotor. The minimum thickness is stamped on the hub of the rotor. Bite the bullet! Spring for new rotors!! Cheap insurance.

If the vehicle already needs rotors replacing.. wouldn't I notice reduced braking power ?

What are the negative effects of driving with a lip?... increased chances of rotor warping?

If the vehicle already needs rotors replacing.. wouldn't I notice reduced braking power ?
No, the caliper pistons will be just moving further out of the bores.

What are the negative effects of driving with a lip?... increased chances of rotor warping?
There's some chance of that, but a greater risk would be that of the pistons moving further out in the bores than designed. At the extreme that might result in the piston cocking in the bore and failing to retract, locking that wheel. On the front a single wheel locked while at highway speed is going to make things get really interesting and very exciting.

The rotors aren't that expensive and they are really easy to change. With that wear they are well past the point at which they should have already been replaced. Be kind and considerate to yourself and everyone else on the road with you and replace them.

hehe.. yes, I'll be ordering some brake parts.. and painting my calipers while at it !

I just bought this car a few months ago... I'm surprised that they werent already changed -- its only got like 30K miles on it... probably the same ones that were on the car at purchase time

30K is maybe a little early for needing pads, but it does vary with driving patterns. That seems way early to me to have that much wear on the rotors. I usually get around 40K miles out of a set of front pads and need new rotors every 80K.

I swapped out my stock rotors at 94K miles. They weren't too bad but did warant replacement. @ 30K I can't believe that you'd need new ones already. Uness you've been abnormally hard on the ol brakes. Could be, I guess.

Well, I dont know if i need replacing or not! .. all i know is that i have a 1 mm or so lip (approx, im just using my finger) on the front side of my rotors .. i didnt feel the back..

People I've talked to respond similar to Bobertsr so I figured I need to get them replaced

Ok, so lets do some math!

Bentley says:
FRONT: Vented rotor wear limit (min . thickness) 20.4 mm (0.803 in .)
REAR: Solid rotor wear limit (min. thickness) 8.4 mm (0 .331 in .)

to calculate how much mm loss means its time to replace.. i need the thickness of a stock rotor.. anybody have that number?
...

ok, just used a flashlight and looked carefully ...

looks like the lip is a little shy of 1 mm on the outer edge and less on the inner edge

from the numbers you guys are giving, it sounds like the bimmer brakes/pads need servicing at intervals similar to my old civic ...

I was always under the impression (from the forums) that bmw brakes usually need servicing more often than other vehicles ... maybe the forums have too many auto-x'ing enthusiasts and i've got biased information?

I'm just going to buy a tool to measure it .. lol, thats much better than eyeing ONE side :-)