Bicycle Trail Calculator
Bicycle "trail" is the most important part of a bicycle's geometry for determining how the bicycle will handle. Trail is the bike's caster force that keeps it going straight. Because of a fortuitous combination of factors in bike design, as a rider leans in a particular direction, the bike's trail will tend to turn in that direction, putting the bike back under the rider and keep him upright.
To imagine a bike's trail, draw a vertical line through the center of the front axle. It will hit the ground at the contact patch of the front wheel.
Draw another line through the head tube. It will (it better!) contact the ground some distance in front of the vertical line. The distance between the 2 is the bike's trail.
Bikes (like commuter bikes) that need to be very stable will have more trail. Racing bikes have less. How much trail is correct for a particular category of bike has been the subject of arguments since trail was incorporated into bikes over 100 years ago. When I was designing racing bikes, I liked the feel given by roughly 5.5 centimeters of trail. There are respected racing bike builders whose bikes have trail in the mid-4-centimeter range. It's really a judgement call. Here is a very good discussion of bike trail.
Please note in the chart below that reducing fork rake increases trail. This is entirely counterintuitive and makes some scratch their heads in disbelief, but that's the way the math and physics come out. Please note also that there are different ways to get the same trail. 6 cm of trail can be obtained with a 74.5 degree head tube and 3.3 cm of rake or a 71 degree head tube with 5.4 cm of rake. The best way to obtain a particular trail has been the subject of more than a few bar fights.
To use the BikeRaceInfo Trail-o-Matic find your bike's head tube angle in degreesbetween 70 and 76 degreesacross the top/bottom. Then go down the left/right vertical column find how much fork rakebend in the forkin centimeters your bike has. Most bike companies publish these head tube angle and fork race specs on their web sites. Where the 2 meet in the chart is your bike's trail in centimeters.