We have some good tuts on bike shadows, but I'd like to offer a down-and-dirty simple way to make a drop shadow.
First, select the bike layer and make a duplicate copy. Layer> Duplicate Layer.
In the layers palette, drag and drop the duplicate layer behind (below) the original bike layer.
With the move tool (Arrow, upper right in tools palette) pull the duplicate layer down where you can see it.
Next, make the duplicate layer black. Image> Adjust> Hue/Saturation> Slide the lightness control all the way to the left to get black.
With the Transform tool, (Edit> Free Transform) grab the top bar and pull down to shorten the height of the shadow.
If you pull far enough, you'll flip the shadow upside down and it will appear to be coming toward you. If the light source is behind the bike, this is the way it would look. If the light source is in front of the bike, you'd want the first version with the shadow behind the bike.
Still using the Transform tool, move the cursor just outside of the square and you'll see a curved, double-ended arrow. That's the rotate tool. Use that to rotate the shadow to align with the tires.
Use the Move tool again to bring the shadow to the bike.Also, if you first move your cursor inside of the box and next outside of the box, it will change from the rotate tool outside to the move tool inside so that you can rotate and move while using the Free Transform tool.
Now if you want a gray shadow, go back to Hue/Saturation and use the Lightness slider.
If you want some color in the shadow, use Image> Adjust> Color Balance.
Add some blur to suit. Filter> Blur> Gaussian Blur.
If the bike is on a textured layer such as concrete or grass, sometimes it looks good to lower the opacity of the shadow and let the texture show through. In the layers palette, select the shadow layer and slide the opacity control to the left to suit.
This shadow is pure black with a little blur.
Sometimes it's fun to skew the shadow. Edit> Transform> Skew and/or Edit> Transform> Perspective.
1badnt did a great tut on putting a gradient into the shadow. That tut is here:
http://www.hdforums.com/m_2127743/tm.htm
And here's and example of what you'll see:
Simple bike shadows 101
