Installing used rotors: does direction matter?
 

If installing a set of used rotors (that still have significant life; only faint hairlines), would it make a difference if they are remounted in the same orientation as previously (i.e., left on left, right on right) or not?
Just wondering if installing them in the opposite direction from the first use (so they would now be stressed in the opposite direction) would make them
a) more prone to crack (so less safe), or
b) less prone to crack, or
c) No difference

If b), then swapping rotors left-right at mid-life should extend their total life.

I'd guess c.

and isn't the answer on multiple choice questions always C?

some rotors have directional louvers, so cooling would be compromised in those,

stock S2K are unidirectional, right.

Well I've always just mounted them at random (stock are not unidirectional), so it might be c) or I've just been lucky to mount them in the same orientation :D . (...though I have had a rotor crack open, so maybe not.)

Where are the metallurgical engineers?
...or anyone with a rational theory :) .

I'm pretty sure the stock ones are unidirectional. Honda doesn't have 2 part numbers for them.

And the cracking is from heat. Why would direction matter?

Unidirectionbal means they can only be used on one side, but the OEM's can be used either side, and that's why when you swap sides the pads are pulling in the opposite direction with respect to the surface of the rotor. I expect the cracks are from a combination of heat and directional stress, right?

omnidirectional? :) the rotors are the same on both sides thats what both will and I meant.

no I'm pretty sure the cracks are just from the heat.

Me too.

With cast iron rotors if shouldn't make any difference at all (unless the vanes are directional). With carbon rotors it could make a lot of difference, depending on how they were constructed.

Yup. The outside cools faster than the inside of the metal, so internal stresses get created as the outside shrinks back down quicker than the inside.

I think he means once they've already been used. Yes, they are OMNIdirectional when new, but after they've been broken in, in a certain direction, do you have to continue using them in that same direction or would it matter?

I think that's what he means.

Warren