As they say: “A building is only as good as its foundation.” If you start construction on a foundation that is not sturdy or is poorly designed, you are asking for trouble. The same goes for motorcycles. What is your riding style? Are you looking for form over function? Will this be a daily rider or a bike that does parade duty? These, as well as other questions, must be answered before an intelligent frame decision can be made.
As important as the driveline is for a motorcycle, your first concern should be the frame.
To get a better understanding of what makes a frame, a frame, we went to Daytec Center, located in Hesperia, California, where owner Phil Day brought out three popular frame styles — a rigid, a Softail, and a rubber mount. He then went through each design and explained the differences between them.
At this point, an explanation of pertinent terms is in order, see figure 1 for reference. A Softail-style frame is used here for illustration purposes only.
PLEASE REFER TO THE IMAGE AT THE TOP FOR THE FOLLOWING DEFINITIONS
A: Hidden axle adjusters: Mild steel plates, machined to accept a rear axle, are adjusted by means of an Allen screw in front of the plate.
B: Swingarm: The rear portion of the frame used to hold the rear wheel in place. The swingarm is attached to the frame by means of the pivot axle. A suspension system bolted between the frame and swingarm is used to cushion and limit the movement of the swingarm.
C: Side Plates: Cold-rolled steel pieces are machined to accurately hold the pivot axle. The plates (there are two of them) are welded to the wishbone tubes on the top and on the bottom rail tubes on the bottom.
D: Pivot Axle: A solid steel, round stock fastens to the side plates and, it provides a pivot point for the swingarm.
E: Transmission Plate: A machined steel plate to which the transmission is bolted. This plate may be bolted to or welded to the frame.
F: Oil Tank/Oil Bag: A sheetmetal tank that holds oil for the engine’s lubrication system.
G: Center Post: A tube welded between the backbone and a crossbrace to add rigidity to the frame.
H: Wish Bone Tubes: Tubes bent to create space for the seating area. They are welded to the top tube in the front and the side plates in the rear.
I: Seat Clip: A small bracket is used to anchor the front of the seat.
J: Top Motor Mount Bracket: A heavy steel bracket welded to the bottom of the backbone, to secure the motor.
K: Motor Mounting Plates: Two steel plates that allow the motor to be bolted to both the front and center of the frame, using a standard engine bolt pattern.
L: Neck Gusset: A flat steel piece is welded to the backbone, neck stem, bottom rails, and neck gusset tube. The neck gusset is used to add strength to this critical area of the frame by spreading the stress and load over a large area.
M: Bottom Rails: Bottom rails (there are two of them) are welded to the neck stem at the front and the side plates at the rear. The vertical portions are sometimes referred to as the downtubes.
N: Forward Control Adaptors: Threaded lugs that are used for mounting the forward controls.
O: Neck Gusset Tube: A frame strengthening member that ties the bottom rails to the backbone.
P: Gas Tank Mounting Plate: A flat steel plate that allows the gas tank to be secured to the frame.
Q: Neck Stem: A machined steel piece that is fit with bearings or bearing cups that will be used to support the frontend.
S: Top Tube/Backbone: A frame member that fits the space between the steering neck and the wishbone tubes.
Stretch and Rake:
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.
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).