The Motorcycle Frame / Chassis: Part II
Engines are usually attached to the frame in at least three places, and sometimes in as many as six places. Instead of bolting the engine directly to the frame tubes, there are metal tabs or mounts either bolted or welded to the frame. Usually the mounting bolts pass completely through the engine cases, adding stability to both the frame and engine.
Controls and Pegs
Another important set of mounts found on your motorcycle frame are those for the pegs and rear brake pedal. Often the foot peg mounts are adjustable for height or fore and aft positioning. Sometimes the rear brake pedal is incorporated right into a cross brace of the frame for added strength to this important section.
There are two important mounting points on the tail section of the frame. They are the swing-arm pivot mount and the top shock-absorber mount.
The swing-arm mount is usually bushed with a bronze, iron, or special rubber bushing, but some use tapered roller bearings. This mount is very important because rear-wheel stability is almost totally dependent on the condition of the swing-arm mount. If the swing arm is free to wander laterally and flex on its pivot, dangerous handling will result.
Tank, Fender, Seat
The frame is usually the base mount for other components such as the gas tank, fenders, seat, side covers, battery, oil tank, electrical components, center and side stands. You will find a myriad of engineering approaches to mounting these items, but most frames have them all.
The frame, hiding behind all the paint, glitter, and noise of the components it supports, is too often ignored. Give this base component the attention it deserves by periodic inspection and maintenance.
Frame Service Inspection
Certain areas of frames are more susceptible to fatigue and damage. A common one, especially on well-flogged dirt bikes, is the area between the front-down tube and the steering head. Another area to keep your eye on is the section just above the swing-arm mounting.
Any place where you can detect chipping, flaking paint that hints of bending or flexing frame tubes, be cautious! Your frame tubes might be getting ready to break. Keep checking the suspicious section and reinforce or repair it as soon as you determine that bending or flexing is actually taking place.
Shake and Hold
Another way to pinpoint frame flex problems is the shake-and-hold method. With the bike off its kickstand, stand to one side of the machine, face it, and grip one end of the handlebar. Give it one violent shake, returning firmly to the original position. Watch carefully to see which joint bushing, axle, or mount continues to oscillate for a moment after you've shaken the machine.
Cracks and Breaks
Cracks and breaks in the frame are sometimes "painfully obvious" because the problem is detected only after the faulty section has completely separated. This sort of catastrophe can be avoided by frequent frame cleaning, inspection, and correction of frame weaknesses at the first sign of chipping paint.
Frame Repair Straightening
Bent tube-frame sections can often be repaired by heating and bending them back to the original shape. Since the reshaped frame may not be as strong as it was originally, some bracing and gusseting or tube sleeving may be necessary. Use a neutral flame on an acetylene torch and heat a large section of the area to be strengthened. Apply gentle, even pressure to the tubes until the original shape is attained.
Bracing and Gusseting
Sometimes a specific frame junction on a certain model machine may demonstrate a tendency to break after a period of hard use. If you have a machine that has such a symptomatic problem, correct it before it starts by reinforcing the weak area. A triangular plate of steel welded into the junction of tubes at the steering head, or a plate welded between the two tubes at the rear of an MX bike, can prevent any mishaps that might arise. However, you should take this step only if the particular model you are working on has gained a reputation for a specific frame weakness, and you'd be surprised how many have!