It happened at 2.25pm on Wednesday 3 June 2015 at the traffic lights for Morrisons Supermarket.
The two vehicles involved in the incident were a yellow Kawasaki motorcycle and a black Suzuki XS4.
The rider of the motorcycle, a 35 year old man from Ripon, was taken to Harrogate District Hospital with serious injuries.
The driver of the Suzuki, a 63 year old woman form Knaresborough, was uninjured in the collision.
Police are appealing to anyone who witnessed the collision to come forward. In particular, anyone who has any information about the colour of the traffic lights at the time is urged to contact police.
It is believed a car was waiting to turn left at the traffic lights, into Morrisons, at the time and we would like to speak to the driver of that vehicle as they could be a key witness.
Anyone with any information is asked to contact police. Dial 101, press option 2 and ask to speak to TC1295 Rob Roberts.
Please quote reference number reference 12150092152 when passing on information about the incident.