Rake and Trail Basics
Although the front end of a bicycle is quite simple compared to many other types of vehicles, there are certain subtle angles that can affect the way a bike or trike steers. A few extra degrees of head tube angle can make a ride feel "twitchy", or too much rake can create something called "wheel flop". When building a bike from scratch, you may have to mess around with these angles in order to get your ride tuned exactly the way you like it, and you will quickly find out that there is no magic rule to getting it right without a little experimentation.
Figure 1 - A typical mountain bike
Figure 1 shows your typical hardware store mountain bike, which has what I consider "typical" front end geometry that often works well for many home built projects. Although a more relaxed head tube angle is better for a longer cycle, you can't go wrong using typical upright bicycle front end geometry as a starting point when creating your own frame from scratch. Your first test ride will give your more useful data than any technical frame building guide or chart ever will.
Figure 2 - Rake and trail defined
There are 3 angles of concern when working with bicycle steering, and they are shown in Figure 2. Head tube angle is shown (A), and is usually measured from a line taken parallel to the ground. A 90 degree head tube angle would be straight up and down, and a 45 degree head tube angle would be very relaxed, pointing your forks way out front like a chopper. Typical head tube angle on most upright bicycles is between 60 and 70 degrees.
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