A sub-frame is a structural component of a vehicle. Sub-frame is a separate structure to carry the engine, drive-train, or suspension system. The sub-frame is bolted to basic structure of the vehicle along with rubber bushings or springs to dampen vibration. A sub-frame reduces forces generated by the engine and transmission and it can be damped enough that they will not disturb passengers.
The various components of a vehicle are bolted directly either on the main frame members or cross members. However, sometimes the engine and gear box are carried on a sub-frame supported by the main frame usually at three points. This arrangement helps to isolate the components from the effect of twisting and flexing of the main frame. Rubber mountings are used between the engine gear box unit and the frame to reduce the vibrations. For the same reason the body is also mounted on rubber blocks on the frame.
Sub-frames are mostly used on Cars which having frameless or unibody construction. The various chassis components are mounted on sub-frame. These sub-frames are supported by main frame or unibody.
The principal purposes of using a sub-frame are, to spread high chassis loads over a wide area of relatively thin sheet metal of a monocoque body shell, flexibility & simplified production or construction and to isolate twisting, flexing, vibrations and harshness from the rest of the body or main frame.
Sub-frames are prone to misalignment, which can cause vibration and alignment issues in the suspension and steering components. Misalignment is caused by space between the chassis-sub-frame mounting bolts and the mounting hole.
A sub-frame is usually made of pressed steel panels that are much thicker than body-shell panels, which are welded or spot welded together. The Hydroformed steel tubes may also be used.
The sub frame, or stub frame, is a boxed frame section that attaches to a unibody. Generally seen primarily on the front end of cars, it is also sometimes used in the rear.
Application of Sub-frame:
The best example is Austin Mini (1959) - monocoque transverse engine front wheel drive.
The most prolific example is the Chevrolet Camaro (1967–1981).
Another good example of such sub-frame usage is the BL Mini.
Separate front and rear sub-frames are used in modern vehicles to reduce the overall weight and cost.
The sub-frame saw regular production in the 1960s and 1970's General Motors X platform and F platform bodies.
Benefits of use of Sub Frames:
Sub-assemblies can be introduced to the main body-shell when required on an automated production line. Hence simplifies assembly line and facilitates subsequent overhaul or repair.
Three basic reasons for the sub-frames are employed:
The first is to isolate the high frequency vibrations of, for example, an engine or a suspension assembly, from the remainder of the structure. In this case, rubber or other resilient mountings are interposed between the sub-frame and main structure.
Secondly, a sub-frame can isolate an inherently stiff sub-assembly such as the engine or gearbox from the effects of the flexing of the chassis frame. This is done generally by interposing a three-point mounting system between the sub-frame and main frame, one of the mountings being on the longitudinal axis about which the main frame twists, and the others one on each side.
Thirdly, a sub-frame may be used to carry, for instance, the front and rear suspension sub-assemblies, where to utilise the front and rear ends of the body structure for this purpose would increase unacceptably its complexity or cost, or introduce difficulties in either manufacture or servicing, or both.
Types of sub-frame:
A simple "axle" type which usually carries the lower control arms and steering rack.
A perimeter frame which carries the above components but in addition supports the engine.
A perimeter frame which carries the above components but in addition supports the engine, transmission and possibly full suspension. (As used on front wheel drive cars).
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