Taken from the vehicle Arena example/demo:
if upkey ( ) = 1 then phy set vehicle motor force 2,300 : key = 1 : phy set vehicle wheel multiplier 2,50
if downkey ( ) = 1 then phy set vehicle motor force 2,-200.0 : key = 1 : phy set vehicle wheel multiplier 2,-50
As you said, you need to apply the force using
phy set vehicle motor force. For the wheels you use
phy set vehicle wheel multiplier to rotate the wheel, with a positive integer for forward and negative for reverse.
By any chance, are you setting up the vehicle as automatic. If this is the case, then you need to switch it off, as there is no reversing.
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