Older thread on this topic:

http://www.bikeforums.net/showthread.php/482881

I have since removed the rear rack on front because I found that when riding slowly up steep inclines with 15 or 20 lbs a wobble in the rack would begin with side-to-side play; this made steering difficult. However, at speeds faster speeds the rack handled well; never an issue with downhill rides. As noted earlier, I now use the Blackburn MTB front rack which seems to be very stable.