CONTENTS OF THIS SECTION
The frame forms the basic building block of the vehicle. In addition to carrying the vehicle load, it furnishes support and maintains alignment for the body, power train and other units.
The Chevrolet frame is of the box girder type of construction, incorporating a flanged channel section front cross member and a channel type rear cross member. The suspension systems, body, body mountings and power train mountings are so designed to add strength and rigidity to the assembly, making an integrated structure which uses the strength built into each unit. This type of design permits a maximum of accessibility for servicing the various units.
The front cross member is rigidly riveted and welded into a split in the frame side rail, allowing the side rails to assume part of the loading. A radiator-support is part of the cross member and supports the radiator and the front of the front end sheet metal.
The rear cross member is riveted and welded to the frame side rails.
The full length frame side rails form a rigid support for the front and rear bumpers and a full length body support.
Body brackets and engine rear, support brackets are welded to the frame side rails. The short brackets provide ample clearance for service operations.
Convertible models have an I-beam X-member welded inside the frame side rails to add the necessary rigidity for this type of flexible body.
Changes for 1956
The only variation in the chassis frame from the previous model is a relocation of the number three outer body support brackets on the frames of the new four door Sport Sedan. The new location of the body supports was incorporated to make necessary rigidity points for the cantilever type body which has no center pillar post above the belt line level of the body. These supports are rearward to a point directly under the center pillars.
Additional support is coordinated at the stub center pillar by the reshaped body cross member Fig. 1 which arches rearward to meet the numberv three outboard body support. This body cross member is an integral part of the underbody.
Specifications for all the frames except the four door Sport Sedan remain unchanged. The change in specification for the frame of the four door Sport Sedan is the measurement between the second and third outer body support bracket. On the Sport Sedan the distance is 6 41/64 inches longer than other models.
Numerous improvements have been made on the frame and shock absorbers for 1956 but only changes mentioned in previous paragraph affect service procedure.
Vehicles which have been in a collision, upset or an accident of any nature which might result in "swayed" or "sprung" frame should always be checked for proper frame alignment in addition to steering geometry and wheel alignment.
When checking a frame for alignment in case of damage, the most efficient method is "X" checking with a tram from given points on each side member.
In figure 1, reference points are indicated "A", "B", "C", "D" on each side of the frame.
Frame alignment checks on all models should be made with the tram points set at the center of each locating point indicated and the cross bar level to insure accuracy.
When "X" checking any section of the frame, the measurements should agree within 3/16"- if the measurements do not agree within the above limit, it means that corrections will have to be made between those measurement points that are not equal.
The dimension between spring hangers is shown in figure 1 and in addition, the spread at the front and rear of the side members is also shown. The dimensions for the front and rear frame kick-ups are given for additional checking purposes.
If a tram gauge is not available, the "plumb bob" method of checking may be used. To assure any degree of accuracy when using this method, the vehicle should be on a level floor when dimensions are checked.
By using this method, it is only necessary to have a piece of cord attached to an ordinary surveyor's plumb bob. When measuring the distance between two points, the free end of the cord should be placed on the reference point allowing the plumb bob to hang just off the floor.
A check mark should be made on the floor just under the tip of the plumb bob. This operation should be repeated at all reference points. With these points located on the floor, they may easily be measured with a rule.
Any damaged frame brackets or hangers and the rear cross member may be replaced. The procedure is to cut off the old brackets at the welds, clean off the old welds, and weld on the new brackets. On the rear cross member, the rivets must be drilled out in addition to cutting the welds.
Shock absorbers both front and rear on all passenger car models are the non-adjustable direct acting type, consisting of two concentric tubes known as the pressure tube and the reservoir tube. A dust shield or outer tube is used on rear shock absorbers because of their relatively exposed position. These shock absorbers are permanently sealed and require no maintenance other than replacement if necessary.
Front shock absorbers are stem attached at the top directly to the top of the front spring housing and at the bottom are eye attached to a retainer bolted to the lower control arm.
Rear shock absorbers are stem attached at the top to the body floor shock absorber reinforcement channel and at the bottom are eye attached to an anchor pin in the rear spring "U" bolt and shock absorber anchor bolt plate.
The shock absorber (fig. 2) consists essentially of a cylinder and piston, the piston being attached to a steel rod which extends through a rubber seal in the top of the cylinder. The cylinder, or piston tube, is surrounded by a reservoir tube, the chamber between their walls serving as the fluid reservoir. A baffle ring in the reservoir prevents turbulence. Attached to the top of the piston rod is a third tube, called the dust shield, which is designed to protect the highly polished piston rod from dust and flying stones, thus preserving its smooth surface and insuring long life to the rubber seal.
The piston tube, or working cylinder, is divided into an upper and lower chamber by the combined piston and rebound valve assembly, which is attached to the lower end of the piston rod. The compression valve is fitted into the lower end of the piston tube, allowing the fluid to be forced back and forth between the reservoir and the piston tube.
When the spring is compressed, the shock absorber starts on its compression stroke, and the piston moves downward in its tube, displacing fluid in the lower chamber. Part of the fluid is forced upward through the outer holes in the piston, lifting the intake valve plate, and entering the upper chamber (fig. 3). Not all of the fluid that is displaced by the piston can pass into the upper chamber since the rod takes up part of the volume. The remainder is forced out of the lower chamber through the compression valve orifice into the reservoir. As this opening is always below the reservoir fluid level, no emulsion of air and fluid can take place. Thus the amount of compression control is governed by the volume of fluid displaced by the piston rod. On fast or extreme movements when the fluid flow exceeds the capacity of the orifice, fluid then flows through the compression relief valve. The resistance to the vehicle spring travel is determined by the diameter of the orifice and the strength of the compression valve spring.
On the rebound stroke, when the car body moves away from the car springs, the resistance of the absorber is instantly effective. As the piston is pulled upward, fluid in the upper chamber is forced through slots in the intake valve plate and through holes in the piston, building up pressure against the orifice plate. As the pressure builds up, fluid is forced through the rebound orifice, bending the orifice disc downward, and compressing the rebound relief valve spring, letting fluid pass into the lower chamber. Again, because of volume taken up by the rod, the displacement of fluid in the upper chamber is not as great as the displacement in the lower chamber. Therefore, as the piston moves upward, an additional amount of fluid is drawn into the lower chamber from the surrounding reservoir, through the compression valve assembly. The compression valve orifice plate is lifted from its seat, allowing the fluid to enter the chamber freely.
Since both front and rear shock absorbers are permanently sealed, service operations are limited to replacements only. Shock absorbers may be replaced on the vehicle as follows:
Removal
Installation
Since the upper stem of the rear shock absorber extends up through body floor, the upper retainer nut is reached through the trunk compartment.
Removal
Installation
Symptom and Probable Cause
Probable Remedy
Hard Riding
a. Shock absorber broken or seized.
a. Disconnect shock absorber and test action, replace if necessary.
b. Rear spring leaf inserts worn.
b. Replace inserts.
c. Excessive tire pressure.
c. Check tire pressure, maintain at 24 pounds.
Vehicle Too Flexible
a. Faulty shock absorber.
a. Disconnect shock absorber and test action, replace if necessary.
Shock Absorber Noisy
a. Faulty shock absorber.
a. Disconnect shock absorber and test action, replace if necessary.
b. Loose dust tube on rear shock absorbers.
b. Replace shock absorber or refasten tube.
c. Improper grommet installation or loose retaining nuts.
c. Inspect and correct as necessary.
Leaks Fluid
a. Faulty shock absorber.
a. Replace shock absorber.
