tom many to list so i copied and pasted from http://www.slash5.net/org/Tank%20Slapper.html Some causes and or contributing factors (sometimes a combinations of these), but by no means all of them: *Out-of-spec bearings (wheel, steering head, swing arm, ...) --bearings too loose --worn bearings --improperly shimmed --spun outer race --improper alignment (deficiencies in the rear of the bike CAN be telegraphed to the front) -- etc. ... *Out-of-spec wheels/tires --out of true --out of round --improperly balanced --improperly seated/inflated --unusual wear pattern -- etc. ... *Poor suspension --improperly adjusted --overloaded --worn/defective components -- etc. ... *Improperly loaded bike --too much weight on the back and not enough on the front tire -- etc. ... *Bad steering dampener ... *Wind turbulence (fast wind and or highway speed) --handlebar mounted windshield, jacket or anything else the wind can catch --exposed pillion back rest --saddle bags, especially wide ones on naked bikes --mis-aligned or damaged fairing -- etc. ... *Frame flex --any change in the bike's geometry that induces an undesirable effect ... *Road grooves --any road characteristic that induces bike instability (not to be confused with a bike's natural tendency to follow a groove or path of most traction) -- etc. ... *Anything else that can lead to or build on a harmonic oscillation affecting the bike's steering which can also be a permutation and or combination of factors that add up to a wobble... http://en.wikipedia.org/wiki/Tank_slap