That depend on the tuning frequency. Some can have over 2kg's of mass on them. The 4 I put in one of my DIY subs definitely would bottom out due to gravity if they were "downfiring".If weight is an issue I would expect the suspension would be fairly stiff to begin with. Gravity will have an equal affect whether it's facing sideways or down, wouldn't it? In any case, I'm fairly confident that Velodyne knows what they are doing.