Two reasons:
Rear wheel steering actually turns your vehicle by wider angles. That’s why it’s easier to parallel park in reverse. Never forward. If you’ve ridden a bicycle, you know how easy it is to turn it by moving it backwards.
When we drive, we don’t hold the steering wheel 100% still. It doesn’t really have an impact when the vehicle moves forward because it is “forgiving”. But when you drive in reverse, every little steer of the wheel gets amplified by the process described above. You try to correct it by steering in the opposite direction, which makes it even worse.
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