Simple physics based vehicle simulation

I’m building a physics based vehicle system.
So far the vehicle is a simple rectangle with four cylinders attached with physics constraints.
The vehicle moves forward by setting “Angular Velocity Target” for the physics constraints of the two back tires.
If the vehicle is already moving then this approach seems to work fine.

The problem:
If the vehicle starts by standing completely still it will not start moving no matter what angular velocity
is set on the back tires. It is only if something hits the vehicle (the player nudging it, for example) that it starts moving.
Does anybody know why this happens?

Thanks for your time!