TL;DR Rotor mounting direction does matter – but only because your rotors get hot! 🙂
Here's a quick visual representation of the stresses in a disc brake, to show why if the rotor specifies a mounting direction, you should follow it.
There's 4 combinations shown in the graphic: you can see that for cold rotors, mounting direction doesn't have any real effect. But, once the rotor gets hot, the outer track expands, putting a LOT of tension on the rotor arms. Mounting it the wrong way adds more tension when you brake; mounting it the correct direction puts the arms in compression, partly counteracting the heat forces, and makes for less stress overall.
Taken to the limit, a hard braking action + very hot rotors could overload the design and cause the rotor to fail if mounted the wrong direction. When in the correct orientation, these extreme forces partly counteract each other and the rotors remain within design limits.
I was finding a lot of "just follow the mounting instructions, it was designed like that for a reason", but I hate those kind of explanations – so I hope this post was interesting / helpful to some people!
by _brkt_
2 Comments
Don’t get me wrong, I am all for mounting parts the right way but doesn’t this graphic show pretty much the same stress on the Correct / Cold mount as on the Wrong / Hot mount?
This might be due to the legend not being consistent (and the scale not being readable) but assuming the color pallette stays the same, the difference seems to be barely there but non critical.
Interestingly the new Brembo rotors look like they reverse the long standard angle of the support/spoke direction from leading to trailing.