why are my disc brakes shuddering I have just replaced my disc brakes with avid bb5s. The braking is fine but there is a severe shudder, especially when braking hard. Any ideas?

Thanks for your responses but I have solved the problem. The online parts store provides only one bracket for front or rear. The front brake actually requires a shallower bracket, so the pads were skipping across the cutouts in the disc. The LBS was able to supply me with the proper bracket and the shuddering went away.
Pick up the shuddering wheel(s) and spin them to make they are "free", i.e., no brake contact when the lever is not depressed.
Check that your wheel axle is securely seated in the dropouts. 
Check that your hub is tight -- pick up the end of the bike with the shuddering brake, push side-to-side on the top of the wheel. You should feel no play or a minuscule amount.  
Check the brake rotor for trueness. Spin the wheel, looking in where the rotor goes between the pads. Is the rotor staying in pretty much the same point between the pads, or going from side to side? Alternatively, or additionally, you can spin the wheel and slowly depress the brake, to see if the pad-rotor contact happens first at just one "high" spot or if it happens more or less evenly. 
Center the caliper. Loosen the caliper position bolts, grab the brake lever, and then tighten the position bolts. 


That's what I'd do to start. With more information I (or someone else) might be able to help more.