Your completely wrong here bill. The whole purpose for turning the rotors on the vehicle is to ensure that the rotors match whatever run-out may be existant in the hub itself, when you are buying new rotors they may be flat and true to the machine that they are manufactured on, or re-surfaced on a brake lathe. But they will NOT be true to the runout in your own hubs. Remember, your own vehicle hub is the base pioint of rotor action, and is therefore the perfect reference point when machining rotors, and this is accieved only with on-the-car lathes

How does the on car machine attach to the car to machine the rotor?

Toasty, if your runout error is at say, 3 oclock postive on the RF rotor, and the machine corrects for it, then you rotate your rotor at ANY time in the future, you could have DOUBLE the error. This is for a car that will NEVER remove the rotors for any reason, and for a car that better not have a bad weheel bearing when they do the work.

By turning them off the car, you get a mechanical 'compromise' to prevent doubling the runout error by turning them on the car then forget which hole went over which stud.

There are plates that lug nut onto the rotor, then you set it and forget it. It will stop when its finished.

turning the rotor on the car does in fact incrase the runout on the rotor VS another device, but by using the hub as a center point, you are putting an opposit runout on the rotor than the hub, and therefore making it smooth on vehicle axis, where it actually matters... if it is flat on a machining device axis, it may or may not be flat on the vehicle axle axis

the runout will be existant and cause excessive runout wear on the rotors no matter what happens unless the runout is compromised by on-car-lathe. yes, if the rotors are removed and re-installed in a different position, it will be a larger increas in runout, but there is a deeper issue causing run-out that will cause perfeclt machined (off the vehicle) rotors to wear unevenly aswell

...

to answer your question, yes, the rotors should always either be changed or machined

I deleted your new topic specifically because this one spawned it. Your question would have been better asked here, as it wasn't related to performance and was general in nature, and this topic is what spawned your question in the first place.

Splitting replies between two topics get us nowhere fast.

Any time the pads are changed, the rotors should be turned as Toasty replied. In the reverse, if you're going to replace or turn the rotors, the pads should be changed, but if there'* alot of material left on them, some people will 'scuff' them. It'* generally not the preferred practice.

me too

rotors are so cheap these days