The danger of this is your rim may begin to rub the brake pad – especially when you put in hard out-of-the-saddle efforts.

In theory, if your caliper has become off-centre then one pad will have come closer to the rim and the other will have come further away. You should simply be able to push and rotate the caliper by hand back into a central position without loosening (or tightening) the retaining nut. Pull the brake lever a couple of times and if the caliper does not return to a central position, your brake requires balancing i.e. one pad is moving more than the other towards the rim and causing them not to touch the rim at the same time. The other effect of this is it may cause the caliper to rest off-centre.

There is a small balancing (tensioning) screw on top of the caliper. Turn this left or right until both pads move the same amount to the rim.

How important is centring dual-pivot brakes?
I've noticed that Wizards, Clerics, Druids, and most other spell-casting classes (With the exception of Sorcerers) spend at least 1 hour Studying or Meditating to regain their spells.

What I want to know is that if I (As the GM) used an encounter to interrupt the preparation time of the party halfway through, will the party have any spells available to utilise?  If they do, is there any RAW method to restrict which spells they will have? (Such as no spells requiring an attack roll, or spells that deal damage.)

Is there anything I can do to force a Sorcerer or other spontaneous caster to contend with the same issue?

Backstory:

I want to introduce a recurring villain, who will be involved as a plot device for quite a while before eventually transforming into a BBEG.  Said villain will have quite high DR-which will prevent melee characters from being able to kill him, but looking for a method to surprise the casters, forcing them to look for alternate methods to escape. (Particular villain has yet to be decided, but I don't think it would help the answer at all.)

I'm not looking to kill the PCs, at all.  Just want a way to scare them a bit, making them think about what they are actually doing.

I have considered an anti-magic field already, but I think this would lead the party to kiting, rather then looking outside the box.
