how important is the proper torque on a disc brake?
 

i got my first set of disc brakes (front only really) for christmas and i was installing them yesterday and i've never seen a part or instructions push the proper torque as much as putting the disc on the wheel. it was in the instructions 3 times and even printed on the disc. is it THAT important to have it tightened to that amount? or is the point just to get it really tight? i can't see overtightening being a problem... not by hand anyway

oh, it's an Avid bb7, btw

I think the recommended torque is 10Nm or something, iirc. What's important is to tighten the 6 screws in a crossed pattern (1, 4, 3, 6, 2, 5), while holding the rotor and forcing it clockwise with your other hand, so it sits with no play in the direction of the braking. I tighten all the screws little by little, until they're tight enough that using a short torx or allen wrench hurts my hand a little, and I feel the wrench has a safe marging before rounding the head in the case of allen screws (I recommend you get torx screws anyway if you don't have them, they're a lot better). Also, a good thing to do is to drop a little blue loctite on the threads before installing the screws.

Have fun with your new disc brake :)

I think PPC is right on the money.

Although proper torque (bolt preload) is important, the torque sequence might be more important on a rotor. It's important for the rotor to sit flat against the hub. I always tighten a little at a time in a "cross Pattern" as suggested by PPC. First, I'll snug the fasteners down until they all contact, finger tight. Then I'll use a tool and turn each 1/4 turn in the cross pattern. When all is done, I usually go once around to make sure all is tight. Also, this prevents rotor warpage.

Installation torque helps keep prevent the fastener from vibrating loose and maintains friction between the rotor and hub. Depending on the design, the proper torque ensure the interface is clamped and held by friction, not shear in the bolts. But too much torque can strip the threads in the hub.

So, yes installation torque is important, but given proper torque sequence vs installation torque, I'd take torque sequence.

BTW, you can always estimate torque - remember, 1 ft-lb means: a 1 lb force acting at 1 foot lever arm, or a 1/2 lb force acting at 2 feet lever, or 2 lb force acting at 1/2 foot lever....

Actually, it doesn't matter in what pattern you tighten them. Sometimes people think it is like a car wheel. If the wheel is not centered properly on a car it will make the whole car bounce around, especially at high speeds. A bike rotor will only deviate a few mm from round which does not cause any problems what-so-ever.

Not so sure: I had a super-cheap wheel once, a shimano "mach-1" or whatever was written on the sticker, that had the disc holes out of true. Not by much, maybe a couple mm or so: I installed the rotor, put the wheel on the bike, spun it and it went "thunk" in the calliper. The wheel simply wouldn't turn :)

The rotor may have been out of true to begin with. I have seen many cases when the rotor needs to be trued on a bike right out of the box, especially cheap bikes.