Four teenagers are believed to have thrown a piece of metal at a biker as he rode along the street.
The victim, a man in his 30s, was riding a motorbike when he was allegedly struck by a metal object in Thong Lane, Gravesend.
He had seen four teenagers, aged between 13 and 16 years old, shortly beforehand, and one looked as if he was throwing something.
They then disappeared into surrounding fields.
Officers are keen to speak to anyone who can help.
It happened between 6pm and 6.30pm on Friday, August 11, but details have only just been released.
Witnesses are asked to call the appeal line on 01474 366149 quoting reference number XY/42447/17 or call Crimestoppers anonymously on 0800 555111.