Rake and Trail
How To Measure Correct Trail:
Using a tape measure, or if not available a piece of string, raise the bike to
the upright position.
Start by holding the tape measure straight down from the front axle to the floor. Put a mark on the floor at that point. Then, place the tape parallel to the steering axle, following the angle of the steering head all the way down to the floor. Put a mark here too. Now all you have to do is to measure the distance between the two marks and you have your trail figure. It should read between 2 and 4 inches.
Note: If your bike is equipped with rear suspension, have the rider sit on the seat when you make these measurements, to simulate the actual riding conditions.
Click on the links to view examples.
Too Little Trail:
With too little and in extreme cases, negative trail (steering mark behind the front axle mark) the bike will handle with unbelievable ease at low speeds, but be completely out of balance at high speeds. It will easily develop a fatal high-speed wobble.
Somewhere between 2 to 4 inches the bike will handle easily both at high and low speeds. Flowing smoothly thru curves without swaying or wobbling. If you use a very fat rear tire, you should keep the trail as close to 4 inches as possible. A slightly large trail is also practical for touring.
Too Much Trail:
If the trail is more than 4 inches the bike will handle sluggishly at high speeds. It will seem almost too steady. You will have trouble balancing your bike at lower speeds, or on winding roads it will feel generally sluggish and clumsy.