Why is my car where reaching 80km/hr starts shaking

@ayabongaphu2313 most common cause for this are tire/wheel issues. You will need to check your tires/wheels for being properly balanced. Check for uneven wear of the tread as well as damaged rims. Next are issues with the suspension i.e. worn ball joints or worn bushings of any suspension part. Check that after you rule out bad tires/wheels. Yes, bad brake rotors and wheel bearings can cause this as well but to a way lesser extend. Start with the easy things first.

Let us know what you find out.