Recent legislation limits g forces to 6 g's on roller coasters, for safety reasons. It makes no difference if it "negative" (up), down or sideways.
See http://www.iaapa.org/industry/funworld/2003/Mar03/Features/G%20Forces/G%20Forces.html
This is not an answer that can be derived using physics alone. It was established by committee and legislated.
What is the normal maximum amount of negative "g" forces allowed for a roller coaster ride?
1 answer