Originally Posted by
CyclingVirgin
So I was thinking maybe a couple things can be the problem but please, you guys be the judge.
3) The wheel was not on tight enough? (How do you know what is too tight?)
If you mean the bearings are loose, I've had that cause shimmy. Lift the bike (or put it on your car rack), and try to move the wheel side to side in the frame or fork. If there's appreciable wiggle, take it to the LBS's mechanic.
4) The wheel not being true? (But when I was riding it, it looks straight and smooth riding down the hill)
Hard to say, have the LBS check it out.
But the most important question, is it SAFE for me to ride it?
Read the shimmy article in dsbrantjr's post, #2, above. Re-read it, and again again, until you understand all the possible causes, and can eliminate them, or deal with the shimmy. Shimmy can cause a crash at high speed, which is unsafe (obviously). Sometimes it's a bike problem, sometimes it's a rider problem. Learn how to deal with it, or fix it, before
you experience the crash.