What is a spar?
What is a spar that can at times “fatigue” or “go away” and in extreme cases even burst?
A longitudinal member is a rectangular tube (longitudinal beam) that runs across the front or rear of the car body (or through from the front bumper to the rear).
What is a longitudinal member?
Spars can be monolithic (manufactured as a single part by milling or stamping) and composite (constructed of several elements by welding). Usually the spars are paired: right and left. The left or right composite spar is conditionally divided into three parts: front, middle and rear. Moreover, the front and rear ones are higher than the middle one, which gives the spar the so-called S-shape.
In relation to each other, the spars can be arranged both parallel to each other (in spar frames) and at some angle (for example, in forked-spine frames).
Today, the side members are part of the design of any vehicle, regardless of the type of its load-bearing system
Spars are made of aluminum or titanium alloys as well as steels or composite materials. The basic cross-section is the channel (box-shaped U-section), and in the most stressed areas the section height is greater. Sometimes, at least in a small section, they may have a box (closed) section.
History of the spar
The history of the spar is connected with the history of the frame structures of the body, because originally the frames were the only supporting system of the car. Gradually frame structures were improved (at first they were wooden), and the frame began to include spars.
Around the 1920s, Auburn created a longitudinal frame with an X-shaped cross member. The longitudinal frames consisted of two parallel spars and several crossbars. In appearance, these frames resembled a ladder, for which reason they were sometimes called ladder frames. Today a ladder frame can be seen on any truck, such as a GAZelle.
Gradually, cars with integrated frames began to appear (the longitudinal members were integrated into the floor of the body, and became part of it).
Today the side members are part of the design of any vehicle regardless of the type of its load-bearing system.
Function of the spar
The longitudinal member is the vehicle’s main load-carrying element: in frame vehicles (trucks, heavy SUVs) the longitudinal member is part of the frame, and in frameless vehicles (cars) it serves as a reinforcing part for mudguards, underbody and luggage compartment floor.
The longitudinal member bears the loads of the body, engine and passengers. It is one of the strongest parts of the car.
In addition, the longitudinal members are the shock absorbers of the front of the body, as they are located in the deformation absorption zone. Too rigid and strong body structure can not absorb the energy of the impact, the body almost completely transmits the energy to the passengers. In order to minimize the deformation of the part of the body in which there are people, it is necessary to soften the impact.
For this purpose, deformable zones are organized in the front and rear parts of the body, their force structure in the longitudinal direction is deliberately weakened. In particular, the longitudinal members are weakened in various ways: their walls or section thickness is reduced (variable section of the longitudinal members), through cuts and special holes are made in the weakly loaded places, folds are created when stamping, which provide for crushing the longitudinal member into an accordion in case of an accident, etc. During a collision, these parts are subjected to considerable destruction, but at the same time they absorb most of the kinetic energy (energy of the moving body), thus reducing the impact force directed towards the passenger compartment.
Problems associated with spars
A longitudinal member can become deformed, fatigued and cracked under the influence of stresses over time. In addition, the spar may become severely deformed after an accident.
Important vehicle parameters may be impaired, such as the position of components (suspension, engine) in relation to the body, the parting lines of body parts, doors and wings, the parameters of the body’s power structure and suspension geometry, or unrecoverable cracks may appear on the outside.
If the side member bursts, the body structure is completely destroyed. That is why, after an accident, the first thing to check is how deformed the spars are, and in what position they are.
Statistically, about 90% of works on straightening and replacement of a spar account for a front spar.
Depending on the extent of the damage and the extent of the deformed area, the type of repair work is determined. A longitudinal member, which is part of the frame, is easy to change, but as for the longitudinal member of the body, sometimes it makes no sense to change it.
If the longitudinal member is not severely deformed, it can be repaired (pulled out). This is done on a special stand-caroliner, which allows you to accurately measure all its parameters.
If the longitudinal member is severely deformed, it must be replaced, since correcting such deformation will change the structure of the metal, respectively reduce its strength, which in turn will cause a change in the characteristics of the entire body.
In partial replacement of a longitudinal member, the connection is made by gas-welding butt welding, no couplings are used.
Since the longitudinal member is a reinforcing part of the body and in many ways determines its rigidity, the repair sometimes tries to strengthen (harden) the longitudinal member. However, if the side member is reinforced, it will no longer perform its shock absorbing function, and this can lead to more serious consequences.
What are and what are the spars in a car for?
The design of any car is clearly thought out and each element in it has a specific purpose. In the structure of the body, the spars play an important role. These elements are assigned not only a supporting function, but also protective and absorb external influences. In this article, let’s understand what a car’s longitudinal member is, its function, position and consequences of deformation.
Purpose and Construction
The longitudinal member is a longitudinal profile or channel, which is located in pairs at the front and rear of the car body.
In the structure of the support body, they are at the level of the underbody, parallel to each other. In some models, they may be positioned at a slight angle. If the carrier part is frame, the parts are located along its entire length as one piece. In the picture you can see the position of the parts. They are marked in red.
The safety and drivability of the vehicle largely depends on the integrity and strength of these elements. The longitudinal members are designed to withstand various pressures and loads. This is the weight of internal units, passengers, cargo, as well as external shocks and vibrations during driving. As you can see, these elements perform an important load-bearing function in the body structure.
Functions of the side members
Thus, the spars perform the following functions:
- Connecting. Connect the various elements of the body and chassis into a single structure.
- Carrying. One of the main functions. The elements are able to carry a lot of weight and loads. Static and dynamic forces are constantly acting on them during the movement.
- Dampening of vibrations. Parts also act as dampers, taking a certain share of kinetic energy during shocks. They are a kind of connecting elements between the body and the suspension.
- Passive safety. Some designs have special programmed deformation zones that dampen the impact energy during a collision.
Design features and types
Heavy-duty vehicles and SUVs use a frame design of the support system. This is due to the increased load. In such designs, the longitudinal members are arranged parallel to each other along their entire length. Whole elements are connected to each other by special crosspieces. Because of the peculiar shape of the frame, drivers often call this design a “staircase”.
In modern passenger cars, the load-bearing part is the body itself. Therefore, the side members are already integrated into the body structure and are part of it. They can be one-piece or separate and are necessarily installed at the front and rear, although there are designs without rear load-bearing elements. Rear spars do not carry as much load as front ones.
One-piece spars can also be used in a load-bearing body. This has its advantages. The structure becomes stiffer and more reliable, but has a complex shape. This has an impact on the manufacturing technology.
Thus, spars can be divided into two types:
If the advantage of one-piece is stiffness, the composite ones are more practical. In a collision, the longitudinal members suffer most often. They bend and break. Composite elements will be easier to replace with new ones.
In its turn, a lot depends on the geometry of the elements. Additional bends allow increasing the structural strength in case of an impact from the side or front. Also the bends allow to increase the maneuverability of the car and the wheel turning angle.
Position of the side members in the body
The position of these elements will depend on the body design and car model. There are several options:
- parallel to each other or at a slight angle;
- vertically at an angle;
- horizontally at an angle;
- with a horizontal bend.
The first variant, as already mentioned, is found on the frame construction of heavy-duty vehicles. The other three variants are found on the load-bearing bodies of various brands.
Possible malfunctions
Only special aluminum alloys or titanium are used to manufacture the spars. The strength of the structure is very important since the elements are subjected to constant stress.
Such exposure over time can lead to their deformation. During use, microcracks may appear in the metal. This effect is often referred to as metal fatigue. Damage, bends, and cracks can affect body geometry and many other vehicle parameters. For example, wheel alignment will change, there will be an improper gap when closing the doors, all sorts of creaks in the cabin and so on.
Also, these important elements can become deformed during an accident. These are the most common cases. Bending of the spars threatens serious repairs, if not even the disposal of the body. There are gaps and looseness, body geometry is disturbed. It is not safe to drive a car in such a case. Even repairs are not always successful. Disturbance of geometry will most likely cause increase of tires’ wear and break of factory settings of steering.
On the other hand, spars also refer to the elements of passive safety. They have the programmed deformation zones. In case of an accident, the part will be bent in this very place, taking a part of the impact energy.
Peculiarities of repair
In most cases, it is the front spars that are addressed with problems, the rear ones are deformed not so often. The bent elements are “pulled”. This is done with the help of a special installation. The complexity also lies in the fact that it is necessary to disassemble a large part of the car. Remove the engine, suspension elements, the body and so on. Not every technician takes on such a complex repair.
If the spar is broken, but the main part of it is in normal condition, the repair can get by with welding work. In this case it is worth bearing in mind that even after quality welding it will be impossible to achieve the previous state of these elements. The strength will decrease precisely in the place of the weld. In frame structures it is much easier to replace these elements.
In some bodies, the spars are attached not by welding, but by brackets. This also makes the repair easier, as it is easier to replace the part. But if the elements are badly damaged or bent, there is often no point in repairing them.
Some drivers try to strengthen the spars in advance, adding additional stiffening ribs. In this case, you should understand that the part loses its shock-absorbing properties.
Longitudinal members are one of the most important parts not only of the body, but also of the whole car. They may be hidden in the depths of the body, but their importance should not be forgotten. They play a big role in the safety and drivability of the car. If these important elements are deformed, you should definitely contact an auto repair shop to avoid possible expensive repairs in the future.
