My uneducated guess is that it's not the strength of the vibration that does it. It's the fact that a motorcycle will vibrate at a specific frequency for tens

My uneducated guess is that it's not the strength of the vibration that does it. It's the fact that a motorcycle will vibrate at a specific frequency for tens of minutes or hours at a time while driving. On a bicycle it's more random and not uniform, unless you drive on a completely uniformly covered gravel road for an hour at a constant speed =)