My seat post mounted rack doesn't move around at all because I have it clamped tight. The back section will bounce if you hit a bump but no sideways movement.
Once because of the bounce a lens fell of one of my blinkies.


I'm not sure if I've ever carried 25 pounds on it. My panniers weight about 4 pounds (per manufacturer) and I usually carry work clothes (trousers, shirt, socks, underwear, belt), rain gear (just in case), toiletries and towel, shorts and jersey for the ride home, lunch, and a magazine to read at lunch. Is that 25 pounds?