I'm gonna say it's mostly because you believe the bike will become unstable. It's a mental thing. You have this belief, so as you get to and surpass 25 mph, your body tightens up which causes the bike to overreact to any stimulus, such as wind, and you also magnify any movement in your mind.

I ride a bike with more side surface area (Felt IA) and have descended at 52mph in the aero bars in a race. I exceed 40mph almost every time I ride it. But I used to often feel like you.

When it's really windy, yeah the p5 will get pushed around more but in most conditions, it will be just fine and you need to just believe that and relax when you're on the bike.

Edit: also I would recommend not un-weighting the saddle in most descent situations.

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Ed O'Malley
www.VeloVetta.com
Founder of VeloVetta Cycling Shoes
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