There's a particularly long straight decent on a fav riding course that allows one to get and hold speeds of 50mph+ for some time. Little traffic, good shoulder, and the posted I believe is 45, 50 once it feeds into, and becomes the Pallisades Parkway (which is not lawful for bicycles). OP, these speeds may not apply. I had a blast the first time down, at 50mph the bike tracked true and confident the whole way. The second time down was a whole nother story. Wobbles and terror at anything which seemed like 40+. I was too terrified to glance at my computer at that point. What was the difference in the two rides? First time was on my budget alu Cinelli with some cheap Mavic hoops. 2nd time was on my Tarmac SL3 superbike, with the Dura-Ace 7850 deep section carbon/alloy clinchers. It wasn't the headset (which had 2 adjustments) and I'm loosey goosey on the bike. It was either the frame, the wheels, or a combination of both. Since a number of riders had probably exceeded that speed in the recent TDF on their Tarmacs I surmised it wasn't the frame. Although now as I type I reckon it is very likely that those bikes were "team" geo, and not regular geo. That leaves the wheels - my theory, and the fella who owns my LBS figure in this case it may have been the deep section's long inner-tube valves, which at normal speeds, even pro TT speeds, wouldn't hit the threshold required for wheel-balancing, but what is the threshold for bike wheel balancing anyway? Apparently Fulcrum factor the weight of the valve in when milling their alloy rims, maybe they are onto something. Unfortunately never got to troubleshoot further, sold the bike on ebay when I had to choose cannon fodder from the stable while I was unemployed. It was a case of the reluctant trophy wife vs. the eager butterface mistress. An easy choice, the bike that shimmies at 45mph goes. The bike that handles any input without question stays. But like mentioned above, many things could be the culprit. And beer...