Police and an ambulance attended the accident scene.
By Democrat reporter
Police Scotland are appealing for witnesses following a serious road crash which occurred on the A82 Balloch to Arden Road on Monday 31 August 2020.
Around 5.10pm two men were travelling on a black Suzuki motorcycle northbound on the A82 near to its junction with Auchenheglish Lodges.
The motorcycle slowed to turn right and was at this point involved in a collision with a silver Volkswagen Golf Estate, driving behind them.
The driver and the passenger were thrown from the motorcycle onto the northbound carriageway.
Emergency services attended and the 21 year old passenger was taken by ambulance to the Queen Elizabeth University Hospital where medical staff describe his condition as critical.
The 23 year old driver was taken to the Royal Alexandra Hospital in Paisley where he was later discharged. The 46 year old driver of the Volkswagen Golf Estate was not injured.
Inspector Archie McGuire from Argyll and West Dunbartonshire Road Policing Unit said: “We are appealing for anyone who was travelling along the A82 around the time of the incident, and is yet to speak to police, to please get in touch.
“It is imperative that we speak to all witnesses in order to gain a full view of what happened. Motorists with dashcams who were in the area are also urged to check their footage and contact us if they have captured either vehicle on the road, or anything else which could be significant.
“Anyone with information is asked to contact the Divisional Road Policing Unit based at Dumbarton through 101, quoting incident number 2478 of Monday 31 August 2020.”