Buddy cop movie, city stalker with arm-length retractable blade?

I saw I movie when I was a kid back in about 1990 on tv. What I remember was it was about to guys, a white cop and a black cop who were partners tasked with tracking down a killer in a city who had a arms-length retractable blade who went about the city stabbing random people. The end of the film they somehow catch up to him and he dies after a grenade goes off in his bike helmet and the black cop replies that 'his head is gone, or what head's? Anyway it was a dark gritty film like so many from what must the late 70's early 80's. Hope someone can help identify it.