Frame is the mounting base and skeleton of motorcycle, which connects the engine, transmission system, operating braking system and electrical system into an organic whole, and bears the quality of vehicle parts and various external forces during driving. The frame can be divided into bridge rhombic frame, U-shaped frame, double-tube cradle frame, combined rocking frame, sitting frame and so on.

Figure 7-27 Jumping Diamond Frame

① Jump into the diamond box. The straddle-type diamond frame is welded by two groups of symmetrical steel plate stamping parts, as shown in Figure 7-27. It uses the machine body to connect the lower part of the frame into a whole. XF 125, JH 125 and other domestic motorcycles all adopt this type of frame.

② U-shaped frame. The U-shaped frame has simple structure, low strength and rigidity, and weak bearing capacity, and is only suitable for mopeds such as Jialing JH50 and Chongqing JT50. The shape of the U-shaped frame is shown in Figure 7-28.

③ Double-tube cradle frame. The double-tube cradle frame is composed of two steel pipes side by side from the frame to the riser under the engine, as shown in Figure 7-29. The structural feature of the double-tube cradle frame is that the engine is mounted on the frame, just like putting a baby in the cradle. This frame is used for high-power racing cars and off-road motorcycles.

Figure 7-28 U-shaped frame

Figure 7-29 Double-tube cradle frame

④ Combined rocker arm frame. In the combined rocker arm frame, the engine is not completely fixed on the frame, but the engine, the rear wheel fork, the driving device and the rear wheel axle are combined into a whole, and connecting points are arranged on the engine to make it hinged with the frame to form the combined rocker arm structure, as shown in Figure 7-30.

⑤ Sit on the frame. The advantages of sitting frame structure are that the driver sits in the car instead of riding on the frame, and there is enough space at the front end of the frame, so the whole car is easy to shape. Suitable for riding motorcycles and mopeds. The shape of the seat frame is shown in Figure 7-3 1.

Figure 7-30 Combined Rocker Frame

Figure 7-3 1 sitting frame

(2) Deformation of the frame

Several deformations of the frame are shown in Figure 7-32.

Figure 7-32 Deformation of Frame

① The frame bends vertically and horizontally. The frame bends and deforms in the longitudinal and horizontal direction. In the top view, it can be clearly seen that the driver's seat, tail and fork deviate from the longitudinal plane O-O. As long as these parts are deformed and deviate from the central vertical plane, the front and rear wheels will not meet the flatness requirements, and the center of gravity of the whole vehicle will deviate.

② The steering seat is twisted. On the front view of the frame, you can see the twisted steering seat, and the steering axis M-M deviates from the vertical plane O-O of the whole vehicle, so that the front and rear wheels do not meet the requirements of curved surface.

③ The central axis of the rear fork is twisted. The central axis M-M connecting the flat fork and the symmetrical central axis of the frame is perpendicular to the longitudinal vertical plane O-O of the frame. After the collision, the thin plate frame structure of the frame may change. The central axis moves from M-M to M'-M' and is not perpendicular to the vertical plane. The rear wheel deviates from the longitudinal vertical plane of the frame, and the front and rear wheels lose ride comfort.

④ The frame is bent vertically. After the front wheel is impacted in the forward direction, the cross beam will bend and deform in the vertical plane, so that the steering shaft will move from the original C-C position to the C'-C' position and the wheel spacing will be reduced. Vehicle overload or pit, frame deformation, increased wheel spacing, and decreased speed-up performance. This situation has a great influence on maneuverability, so serious that it is even impossible to drive.

Figure 7-33 Correction of Longitudinal Bending of Deformed Frame

(3) Correct the longitudinal and transverse bending of the frame.

After the frame is firmly supported on the workbench, the bending part is acted by the pressure N, as shown in Figure 7-33, so that the bent beam is restored to its original state. Different support points and pressure points should be selected for bending in different parts.

(4) Correction of steering seat deformation

After the frame is firmly supported on the workbench, clamp it with pressure N. When clamping, the support block must be padded to prevent the frame from bending. When straightening, twist the car beam with the hand force P through the straightening rod, as shown in Figure 7-34, so that the steering axis returns to the vertical plane of the longitudinal center of the frame. Because the torque required for rectification is very large, reaching above1000 n m, if the workbench is not fixed on the ground, it is necessary to add 1 ~ 2 anti-overturning bars with the length of 1.5m on the workbench, or take other measures to prevent the workbench from overturning during rectification.

Figure 7-34 Correction of Steering Seat Deformation

(5) Correcting the deformation of the shaft in the frame.

The skew of the central axis on the frame is the result of the torsional deformation of the frame structure of the frame. During rectification, some auxiliary tools can be made and put on the workbench, as shown in Figure 7-35. Under the action of pressure n, the frame-shaped car body will be twisted and restored to its original state, and the central axis will then return to the vertical position of the longitudinal center of the frame.

Figure 7-35 Correction of deformation of central axis of frame

(6) Correct the longitudinal and vertical bending of the frame.

There are two different cases of vertical bending of the frame: the wheel pitch increases or decreases. During correction, the correction force should be applied at different positions according to different situations, and auxiliary tools with different structures should be adopted according to different frame structures of different vehicles, so that the frame will not be damaged or damaged during correction.

Figure 7-36 Correction after Forward Collision

As shown in Figure 7-36, when the vehicle frame is hit forward, it will bend and deform to reduce the wheelbase, and then correct it. The auxiliary tools are clamped on the car beam with steel plates, and the straightening rod is inserted into the steering seat, and then corrected with a jack.

(7) Frame calibration standard

A complete frame can be divided into two parts which are basically symmetrical left and right by a plane. According to this structural feature, the center line C-C of the driver's seat, the longitudinal center line A-A of the frame, the straight line B-B passing through the transverse midpoint of the flat fork shaft and in the same direction as the symmetrical axis of the flat fork, and the straight line D-D passing through the rear of the frame and in the same direction as the symmetrical axis of the flat fork must all be on this plane. So the framework must meet the following conditions:

① The projections of A-A and B-B on the horizontal plane coincide.

② The projections of C-C and D-D in the front (back) view coincide.

③ The front and rear wheel tracks meet the size requirements.

④ The horizontal fork axis (central axis) line is perpendicular to the vertical plane of the frame.

⑤ The structure of each part of the frame can meet the requirements of installing related parts, such as the lug of the engine and the shape and position of the shock absorber strut after installation.