Originally Posted by 50PlusCycling

On motorcycles its called “death wobble.” I’ve experienced it on both motorcycles and bicycles, and it can be a little hair-raising. There are some things you can do to minimize risk, make sure your headset isn’t loose, the axle bearings aren’t too worn, and balance your front wheel. Wheel balancing is easy. Put your bike in a stand and rotate your front wheel, let it turn freely, and see if the same side always comes to the bottom. Take a bicycle computer magnet, the kind which attaches to a spoke, and put it on a spoke on the opposite end. You can move the magnet up or down the spoke to change its effect on balance. The wheel is in balance when you can spin it, and it stops in any position randomly.

Originally Posted by 13ollocks

I think there are any number of factors that can initiate a speed wobble - road surface ruts, loose headset, unbalanced front wheel, etc - however, whether or not this relatively small input propagates/amplifies into a full-scale wobble is due to different factors, like F/R balance, "death-grip" on the bars, frame flex etc - factors that promote system resonance..