This evening my road bike went into a frightening wobble on a fast, straight downhill. I have been down this hill dozens of times with no problem. I usually top out at 40 mph in the aerobars, 42 when I crouch down, roadie style. Today a strong wind was at my back and I topped out at 48 crouched down in the drops, which the fastest I have gone on this hill.

That was when the wobble got going: It scared the shit out of me! The guy I was riding with was a few yards behind me and said at first he thought it was the wind but he said the wobble was so 'quick' and my bike was 'flicking.' It was frightening because it started wobbling at the apex of speed and the increased over the course of several seconds. I got up a bit, braked lightly, but the wobble didn't go away until I was back below 40. I stopped at the bottom of the hill and my training partner was wide-eyed.

The wobble definitely felt like it came from the rear wheel or triangle. After I stoppped, I cranked the QR a bit tighter and made sure the headset wasn't loose. I continued my ride which included another short downhill of about 38 mph and never felt the wobble again. I have felt slightly wobbly a couple times decending last summer, but thought it might have been my imagination. I am a wuss at descending.

Questions: At the time, I didn't think to press my knees against the frame. Does that help? Would it have been worse had I been on my aerobars? Any ideas on where a wobble comes from? Should I worry about my rear hub? I had in my skewer for my trainer, is that bad?

I have a 3-year-old Fuji aluminum bike, BTW with late 90s Rolf vector wheels. I've had the BB checked out recently (due to creaking) and my trusted bike tech said no problems with that.