Hi personally i put force on Y axis depending of velocity and adding less friction to body
as my code is "almost" working as i wanted, here it is:
local roue
local air = true
speed=nil
for roue=0,3 do
if carobject.vehicle:TireIsAirborne(roue)==0 then
speed = math.abs(carobject.model:GetVelocity(0).z)
air = false
break
end
end
----si vh sur terre on freine
if not air then
chassis:SetFriction(0.3,0.2)
chassis:SetForce(Vec3(0,-300+speed,0))
end
hope it helps