A US man is suing BMW after he claims a motorcycle ride gave him a "20-month erection".

Henry Wolf alleges a four-hour trip on his BMW bike in September 2010 has given him a persistent problem with his member, so much so he has filed a lawsuit against the company, USA Today reports.

In the lawsuit, it claims Henry now has a persistent erection as a result of the bike's "ridge-like seat" and he is "now is unable to engage in sexual activity, which is causing him substantial emotional and mental anguish".

He is suing BMW for medical expenses, lost wages, emotional distress and "general damage".