When building a bike engine car, one of the things that gets overlooked is the reverse. In our cars, we currently use an electric motor to drive the prop shaft in the opposide rotation to ultimately drive the car backwards. One of the questions I get asked a lot is how to wire in the electric motor to prevent accidental use - If this isn't setup correctly, you could strip all the teeth from the ring gear and pinion which is a consequence of activating the reverse while the vehicle is currently in the forward motion.
To prevent this, the reverse electronics need to be connected to the neutral switch of the bike engine.
Wire the switches on the dashboard like this to prevent accidental activation.
In this scenario, there are two switches on the dashboard.
Switch 1 - latching switch, which when activated will illuminate the reverse light (so you can see at night).
Switch 2 - momentary switch, which when activated will drive the vehicle in the reverse direction.
Note : the electric motor is very binary and is either on or off, so only a light touch on the momentary switch will be sufficient to drive the car backward.