Assemblymass not doing what it's supposed to with body force

local magnitude = upe.Body.AssemblyMass
magnitude *= gravity
local mover =, 1, 0) * magnitude
upe.VehicleSeat.BodyForce.Force = game.Workspace.Compass.CFrame:vectorToWorldSpace(mover)

This gets the assemblymass of the main body. All parts of the ship are welded to it. The bodyforce of the ship is set to assemblymass. This comes out to about 9.73m. The ship falls at a slightly slower rate than when I comment out the lines above, but it still falls. Clearly everything is welded. What am I missing?? This works for all other ships, but not this one