So I have been cycling about 8 months now. I live in southwest Florida so needless to say its very flat. I do have bridges in town. On the large one on the way to sanibel island (google maps sanibel island causeway) I can hit almost 40 mph on the descent. I have gotten much better at controlling the bike and when I'm not pedaling I rest a knee on the top tube to prevent a wobble. However and I can feel this in the car, in windy days especially the bridge moves and the wind blowing almost seems like its lifting the bike off the ground. Anyone else experience this on a bridge versus descending on a hill? What's ur fastest bridge descent?

I ride that bridge regularly, I have never felt it move on the bike. I know what you mean, feeling it move in a car. I don't think I have gotten over 40 going down. Is it possible the wobble is a product of your wheels? I know that is the case for me.

That's an awesome area to live in! I visit Ft Meyers a bunch and know that bridge.. next time i should bring my bicycle..
Bridges sway with wind, this is by design.

Similar in height and length to the Sand Key bridge in Clearwater that I ride often. No wobbling on the descent (even on my short-wheelbase light steering single-speed). Check your wheels for balance/true and make sure your headset is properly tightened. Also make sure to keep more of your weight over the front wheel.

No problems with wind, but I ride alloy rims instead of carbon specifically so I don't have to fight the sea breeze crosswinds.

Ah yes the Sanibel Causeway. I love it. Without it we’d have almost zero climbing around here. That bridge doesn’t sway. I think you’re just feeling yourself and the bike swaying from the wind. Your wheels could be contributing to that sensation. Also, when you say you rest your knee on the top tube do you mean you that your knee is parallel to the top tube or even with it? I’m a little fuzzy on that part because it seems like it would be difficult to actually put your knee on the very top of tube. That would throw me off balance and I’m not sure I can even get my knee up there. You’re getting some great speed on that descent.

I've vacationed there once, gorgeous place. 10/10 would go back. I think he means that he's tucking his knees as far to the inside as he can, which makes them touch the sides of the top tube.

The above post is correct. I tuck my knees far inside (usually just one). Also I'm hearing conflicting opinions does the bridge sway or no. I believe we can all agree on the wind being a part of the cause of a lack in confidence. And yes I just noticed my front wheel is slightly out of true.

Depends a lot of your skills as a rider and the bike. Some bikes no matter what they will handle like c r a p. Other ones will work fine all the time. Been in that bridge and at least the bikes I have never have handling problems.

I did some descending today and now I see what you mean by knees on the top tube. Yeah my knees are even with or above it. Technically all bridges sway but you're definitely not feeling the Sanibel Causeway sway. That's your bike and the wind swaying NOT the bridge.

How do you handle the switchback at the end of the descent?

I think every bike has an intended purpose. Most bikes handle what they were designed for very well.

A touring bike does a great job of toodling along at 25kph, but you wouldn't expect to ride one in a crit with great success. The same goes for my track bike. At low speeds, <30kph, it's twitchy as heck, but in a full sprint, it moves like its wired directly to my brain. OP probably needs a thru-axle to stiffen up the front end of the bike.