Windshield on a bike I'm doing a bunch of work on my bike today, and had an idea. My biggest speed killer especially downhill is air resistance. I have a mountain bike but I only ride on the road.

I've been looking at my bike and it would be fairly easy to attach a piece of curved perplex from above the front tyre to a few inches above the handle bars. Motor bikes have little screens like this above the handlebars which seem to deflect the air over the riders head if he puts his head down, so for example I could smoke a cigarette on my last motorbike without the wind bothering me much.

Has anyone done this? Or is there some reason it would be a bad idea?

Update:- After quite a bit of messing around and testing it coasting down a hill with and without. I found that going downhill it does increase speed and comfort. However it's in the bin as totally unsuitable for the tropics since I am an aircooled gadget and when cycling hard into a headwind when I need airflow the most I don't get the same cooling as I do without it.

So it was a total success in terms of the question (downhill) but no good for other reasons.
Windscreens are more about rider comfort than decreasing air resistance. I would think particularly with the upright riding position of a mountain bike that your hypothetical installation of a windscreen would increase air resistance, unfortunately, by expanding the area of mass that would push against the air.

The common advice for getting more aerodynamic on a bike boils down to get low and small. This is why cyclists trying to hit speed records always duck down while gripping drop-style handlebars. They also wear tight-fitting clothes, use specially-shaped helmets, get a bike that is engineered specifically to cut through the air quite efficiently, etc.

So I'd conclude that if what you're looking for is not so much maximizing your possible speed but rather getting the wind out of your face, there's nothing inherently wrong with installing a windscreen. But if you try pushing into a head-on wind, I'm sure it would take a lot more effort from you.