what year is the Hardrock, what size and is is a 26" or 29", how much seatpost do you have extended, are you using a stock aluminium or carbon seatpost, all these are factors.

From googling images of the rack, as long as the area you are clamping onto is above the level of the tire, then it will fit. Would not want to be using one with a carbon seatpost though.

for Topeak's specs' look here http://www.topeak.com/products/Racks