on a motor bike a bicycle helmet will not cut it, you need a DOT or SNELL rated helmet, a bicycle helmet is designed for an impact at pedal speed, 7-15 mph. motor bikes go upwards of 30. you can trust your life with a bicycle helmet, i will continue to wear my motorcycle helmets
I think a lot of people do not realize something about bicycle helmets. The same thing applies to motorcycle helmets as well. That point is this; Helmets that you and I can buy today are based on helmets designed for racing. The tech gets passed down to commercial grade helmets. And if you buy one from a reputable manufacturer then you have a good helmet if it's a bicycle or motorcycle helmet.
Anyone that lives in a mountainous area can tell you, going downhill on a pedal bike you can reach some really fast speeds. I once hit 42.5mph on a full suspension TREK Y-frame, a Y-22. The speedo I had was well calibrated and accurate. As tested by a police officer with a laser radar gun. Heck, on that same bike, right before I got the speedo, I hit 35mph on mostly level road, told to me by the guy behind me in his work van. He was fixing to pass me when he looked at his speedo and saw he was doing the speed limit which was 35. So he just stayed behind me till we got to the red light a half mile away. This was on a pedal bike, no motor.
The riders in pro bicycle races wear bicycle helmets. If it's good enough for them then it's good enough for me.