Stretch is a word that gets tossed around quite a bit when it comes to custom motorcycles. Although stretching a frame is a very simple concept, it is widely misunderstood.
Simply stated, stretching a motorcycle is changing the length of the backbone and bottom rails (the downtube portion of them) to move the steering neck to a more desirable position.
This repositioning may be done to change the handling characteristics of the motor-cycle, fit the frame to the rider, or more often than not, to give the motorcycle a particular look, i.e. long and low or a high neck.
If a customer orders a frame with 4 inches of backbone stretch, the frame builder will use a longer backbone and modified bottom rails.
There is a common misconception that there are pieces of tube welded into the existing framerails -- this is not the case. It is easier and more precise to calculate the new tube lengths and build the frame to its desired dimensions from the start.
In the case of a 2-inch upward stretch, the steering neck would be raised 2 inches from its stock position. Daytec's frame jigs are designed with a 15-degree rail used to locate the steering neck. When the shop raises the neck 2 inches, the measurement is taken along this 15-degree angle. This means that 2 inches will need to be added to the bottom rails in order to reach the new position of the steering neck.
When adding backbone stretch of 2 inches, a measurement is taken from the centerline of the rear axle (in the case of a rigid) or the pivot axle (on a bike with a swingarm). The neck is then moved 2 inches away from and perpendicular to that centerline. Next, calculations are made to determine the length and any bends that may be needed in the new backbone and bottom rails. From there, the frame is welded up in a jig as usual.
While we're discussing how to move the steering neck, it would be appropriate to talk about rake angle. Rake is expressed in degrees based on the measurement between the line perpendicular to the bottom rails and the line extended through the axis of the steering neck. As this angle is increased, steering response is slowed down and stability at higher speeds is increased. Conversely, as neck rake is decreased, steering becomes more responsive at lower speeds, but high speed stability is adversely effected.
Trail
Trail is a measurement of the relationship between neck rake, fork length and style, wheel diameter, and triple trees. This measurement is an important aspect in the handling of a motorcycle. For the type of bikes that we feature on the pages of HOT BIKE, a trail measurement between 2 and 4 inches is the norm.
To measure trail, start by holding a tape measure straight up and 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 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, then have someone sit on the seat when you make these measurements, in order to simulate actual riding conditions).
To measure trail, start by holding a tape measure straight up and 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 measure the distance between the two marks, and you have your trail figure. It should read between 2 and 4 inches.
Somewhere between 2 to 4 inches, the bike will handle easily at both high and low speeds, flowing smoothly through 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 larger trail is also practical for touring.
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.
