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Rake and Trail
Rake is the angle between the headset tube and a vertical line. Increasing the rake will move the front tire farther from the bike. Rake is measured in degrees, and is a frame specification. To visualize trail, draw an imaginary line along the path of the headset tube to the ground. Measure from this point to the center of the tire patch. This is trail, and is measured in inches.
The amount of rake is significant in determining a motorcycle's handling characteristics. In general, more rake provides greater straight line stability, less rake makes the bike more responsive. Larger values of trail also create more straight line stability. This is why the forks on a sportbike are more vertical than those of a cruiser.
Visualize the wheels of a shopping cart for an illustration of a machine with zero rake and a lot of trail. Exciting handling, but zero straight line stability.
People frequently inquire about using so called "raked trees", which don't actually affect rake at all, rake is built into the steering head angle, but rather it offsets the front axle and cosmetically gives the forks a raked look while reducing trail. For a good illustration and explanation of rake and trail, how to measure it and figure out whether you're going to get yourself in trouble, check out the Pat Kennedy Chopper site:
Explanations aside, the calculator works great: