I thought I had a fairly solid rack with the Megarack that I bought from L.L. Bean, however, it can't take the beating that I've given it on my Tidalforce bike. The screws that allow it to adjust to different frame sizes just won't stay tight. The two thin bars that attach to the frame by the brake mounts are also very flimsy and can't keep the 25 lbs of batteries I carry stable. I worry about the whole assembly wobbling free one day. So I asked around on the boards and found out that the Topeak Explorer MTX frame is an inexpensive but sturdy one piece frame. I bought it from my REI store and mounted it over the weekend. It's a much more solid rack. The two bars that attach near the brake mounts are twice as thick and much longer than the Megarack's bars. There are very solid large headed hex screws which mount to the frame. I put in a couple of lock washers and some Loctite to help hold the screws in so I don't have to worry about it. It seems the extra heavy duty welding and the three stays make a huge difference in the solidity of the rack.
Now when I go over bumps, the rack and my battery pack does not wobble one bit. It feels much tighter and more controlled. I still need to reduce the weight of the battery some since it still isn't great going over potholes or manhole covers, but the ride is much steadier now.
Thanks JD on Endless-sphere.com